CA1091965A - Self-tapping screws - Google Patents
Self-tapping screwsInfo
- Publication number
- CA1091965A CA1091965A CA280,378A CA280378A CA1091965A CA 1091965 A CA1091965 A CA 1091965A CA 280378 A CA280378 A CA 280378A CA 1091965 A CA1091965 A CA 1091965A
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- Canada
- Prior art keywords
- thread
- fastener
- self
- crest
- screw
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Abstract
ABSTRACT OF THE DISCLOSURE
A threaded fastener of the self-tapping type is described having the conventional head, shank and pilot end. The shank includes a helical thread which has been reinforced by a deposit of, usually accompanied by impregnation with, a carbide or other substance harder than the metal from which the fastener is made.
A thread-cutting fastener, having its pilot end reinforced in this manner is also described. A thread-rolling machine combined with electrode-deposition equipment is disclosed. The electrode of the equipment is caused to engage the fastener during a produc-tion phase immediately following the thread-rolling operation itself. A method of manufacturing or forming the threaded fastener is described whereby a tough, ductile fastener is pro-duced having superior self-tapping characteristics.
A threaded fastener of the self-tapping type is described having the conventional head, shank and pilot end. The shank includes a helical thread which has been reinforced by a deposit of, usually accompanied by impregnation with, a carbide or other substance harder than the metal from which the fastener is made.
A thread-cutting fastener, having its pilot end reinforced in this manner is also described. A thread-rolling machine combined with electrode-deposition equipment is disclosed. The electrode of the equipment is caused to engage the fastener during a produc-tion phase immediately following the thread-rolling operation itself. A method of manufacturing or forming the threaded fastener is described whereby a tough, ductile fastener is pro-duced having superior self-tapping characteristics.
Description
DA~
This invention relates to self-tap,ping screws, as well as methods and equipment for producing ~uch screws, The fastener manufacturing industry has had the problem of making threaded metal fasteners possessing at once charac-S teristics of ductility and toughness, yet having relative threadhardness, The problem is particularly acute in connection with self-tapping screws, though it may be encountered in different environments, too, By being ductile or tough, a metal i9 considered herein as being pliant yet strong, as having the ability oE being bent and/or twisted without tearing or br0aking. Antonymously, the term "hardness" is u~ed herein to connote pronounced suscepti-bility to breaking, i.e, brittlene~, in addition to resistance to penetration, In the pertinent industry, it is customary to use the Rockwell testing devices for measuring hardness. Using such devices, a metal is penetrated by a probe of known hardness, such as a diamond, with a predetermined force. The depth of penetration gives an indication of hardness. One o~ a series of nur~bers is assigned directly to every penetration depth. The results of such tests are ordinarily expressed on the scales designated Rockwell A, Rockwell B and Rockwell C. The higher the number given as a result of such a test, the harder the metal, Thus, a metal having a hardness corresponding to a reading of Rockwell C = 30 or less would be considered ductile and is used in the manufacture of relatively tough, ductile fasteners, A screw made from such tough, ductile material generally fails as a self-ta~ping ~crew. A self-tapping screw is one which forrns its own mating thr0ad, or threads (if there are two parallel threads), in a material into which the screw taps DA-ll 109~l965 itself, when rotated under the application of an axially oriented ~orce, In one form of self-tapping screw, a previously drilled hole initially receives the more or less pointed, so-called piIot end of the screw, and then the thread on the self-tapping screw forms a mating thread by partially displacing material around ~he hole, substantially without removing any such material. This type of screw is frequently referred to as a thread-forming screw, as di~tinguished from the thread-cutting screws which, mostly by virtue of axially oriented cutting edges acro8s the thread, particularly in the pilot end, actually cut their mating thread by removing material in which the initial hole has been drilled. As a specific form of thread-forming screws, the so-called self-drilling variety should also be men-tioned, This type of self-tapping screw has a pointed pllot end whose thread drills its own hole, thus avoiding the need for a previously drilled hole in the material into which the screw taps itself, Any one of these classes of self-tapping screws i9 fre-guently subjec~ to fai~ure, inasmuch as such failure occurs when the ductile or tough thread on the fastener collapses within the unthreaded hole.
In general, the hole for thread-forming self-tapping screws should be sized to produce approximately 8~ thread engagement, This value can vary, however, with the flexibility of the materials used, Thread-cutting self-tapping screws, on the other hand, are manufactured to function as a tap to cut threads into the material being joined, Usually, as a thread-cutting screw cuts into the mate~ial, chips are generated and pushed ahead of the screw, For further explanations and illus-trations, see, for example, "Solutions to Plastic Fastening Prob-lems" in Assembly Engineerin~, September, 1971.
DA-ll ~9~ti5 Various methods are presently used to make the fastener harder only on it~ surface while maintaining toughness by virtue of the unchanged ductility of the internal part of the body of the screw, descriptively referred to in pertinent literature as a core, The most common, presently used process is a three-step process, First, the fastener is heat-treated to approximately 1700 F or(more, The fastener is then, in a second ~tep, sub-jected to a treatment commonly and descriptively designated ~Ica~e-hardening~, inasmuch as a hardened shell, i.e~ case or casing, is produced around the core, which is the interior o the body of the screw which retains its ductility. In this second step, the astener's surface i9 thoroughly cleaned, heated to approximately 1250 F to 1300 F and placed into a carbon-rich atmosphere, i,e, an atmosphere containing a large proportion of hydrocarbons. In such an environment, the metaI has a tendency to attract the carbon. The usual result is a carbon layer of approximately 0.004 of an inch to 0,006 of an inch in thickness deposited upon the fastener' 5 surface, The third procedural step is an induction heating treatment, In this step, the pilot end of the threaded fastener is placed in a rapidly changing electro-magnetic field, Upon cooling, it is found that the molecular structure in a surface layer of at least approximately 0,06 of an inch in thickness of the pilot end has been changed. The result of such treatment, unfortunately, hardens the fastener and makes it more brittle, In fact, such an induction heating pro-ce~ure with the carbon layer, known as "carburization", has made test fasteners possess a hardness in the order of Rockwell C = 45 and higher.
To manufacture these relatively hard fasteners in -the currently known manner, a premium alloy steel containirlg an 10~1965 DA-ll additional metal component is required, Such additional me-tal could be manganese, chromium, nickel and the li~se, which makes these alloys expensive, Furthermore, the three-step procedure outlined above requires continuous attention and careful handling.
Prior to case-hardening, for example, the fasteners, and parti-cularly the thread root areas between the threads, must be thoroughly cleaned to remove the lubricating oils and other grit of manufacture, so that the carbon will become evenly and finely deposited upon the surface of the fastener~ The composition of the carbon-rich atmosphere must be precisely controlled and require~ constant attention, Additionally, the induction heating step requires meti-culous positioning of the fasteners with only the pilot end exposed to the field, in order to prevent that the entire fastener be 90 treated. Moreover, the equipment for the induc-tion heating~step is complex and very costly.
The result of all of these manipulations and attendant expenses is a Eastener which has substantially lost the desired ductility and toughness, and in some instances has not even achieved the desired hardness. For example, a fastener having a Rockwell C = 45 measure of hardness would be too hard and brittle in certain applications where predictable shocks require the fastener to have a certain resiliency, ductility and toughness.
Yet, the thread~ of such a fastener, even after case-hardening and induction treating, have collapsed when attempts were made to produce and then use such fasteners in the form of self-tapping screws.
A self-tapping screw or other threaded fastener made of comparatively tough, resilient material possessing hardness characteristics necessa~y for successful use as self-tapping screws has been long sought but was heretofore unavailable~
~091965 This invention is based on the recognition that this long-felt need could be filled, specificallv with respect to self-tapping screws, by producing them from comparatively tough, ductile material, and providing a com-paratively hard substance deposited at, and bonded to, points of maximum wear, ~.e. on the thread crests exclusively.
To achieve this resuIt, t~e threads ma~ be subjected to an electrode-depOsitiOn treatment ~h;~ch preferahl~r advantageousl~
immediately folloT.rs, in one s~ngle pass, the thread-rolling step wherein the threads are rolled onto cylindrical blanks.
S~rews with the resulting thread structure are particularl~
useful in the case of seIf-tapping screws having self-locking features, as described below, and for high strength fasteners, norm~ formed o mater~als wh~ch are adversely affected bv conventional hardening methods, such as case-hardening.
~ ne aspect of a specific method resulting in a self-tapp~ng screw of the invention involves the eIectrode-deposition, which is assumed to be associated with impregnation, upon t~e screw threads or other metal parts as a production phase o~ the machin~ng, i.e. thread-rolling, process. Unlike other coating processes which reauire a high degree of clean-liness, the car~ide deposition, i.e. reinforcing by coating, process su~gested here~n can tolerate the precence of coolant, lubricating fluid and machining grit, and thus can be incor-porated into manufacturing metal-working processes with little additional expense.
As used herein,~the phrases "eIectrode-deposition"
and "electrode-deposition technology", described and illu.s-trated further beIow in more detail, are intended to denote a class of methods, and the relevant technology, wherein the suhstance of an eIectrode is deposited from the electrode upon a workpiece under the application of electric power between the electrode and the workpiece. T~e depositi~n mb/ )~7`~ 6 -.. . . .
19~S
effect is. achieved under the formation of an electricalarc, resulting in a strongly adhering deposit constituting a coating on the workpiece, so th.at re.ference to impreg-nation is justified, inasmuch as penetration of the elect-rode material into -the material of the workpiece, as it is deposited, may safely be assumed to occur. The strong bo~d, thus achieved, may be explained by the molecular forces between the mutually penetrati.n~ materials of deposit and workpiece along the interface.
~hus in accordance with a broad aspect of the invention, there is provided a se.lf-tapping screw having a relatively ductile core and carb.on-containing material substantially harder than the relatively ductile core deposited on to the crest, or crests, of its thread in at least the area of the pilot end of the screw, wherein the relatively ductile core extends to, and is defined by, surfaces pertaining to grooves consti.tuting the root of the thread ~etween relatively hard crest areas.
In accordance with specific features of an embodiment of the invention, a self-tapping screw has relatively hard sur~ace areas of the crest, or crests, of the thread only in the area of the pilot end of the screw.
There may be discrete, relatively hard sur~ace areas of the crest, or crests, disposed along the circumference o a turn, or turns, of the thread. Alternati~ely, a relative-ly hard surface area of the crest, or crests, along the entire circumference of a turn, or turns, of the thread - may be provided. In the case of self-tapping screws of the thread-cutting variety, the. relatively hard surface areas of the crest, or crests, suitably include cutting edges o-f axial notches in the thread. ~s to useful mater-ials, generally any metal or alloy having.the des:;re~ ~uctility and toughness may be selec-ted and gocd results have been.a~e~ed when the core was made oE lo~-cæbon or a.luminum. With ~ .
-- pg/~ - 7 -s DA-ll respect to the relatively hard surface area, or areas, of the crest, or crests, a deposit of titanium carbide or tungsten carbide has been found particularly suitable, as such deposits lead to successfully tested reinforcement effects.
The invention will become better understood~from the following detailed description of several emhodiments thereof, when taken in conjunction with the drawings, wherein:
Figure 1 illustrates, in a side view, one embodiment of a self-tapping screw in accordance with the invention' Figure 2 is a partial, relatively enlarged cross-sectional view taken al.ong an axial plane through the pilot end of the screw of Figure l, Figure 3 schematically illustrates, in.a perspective view, one emhodiment of a combination of a thread-rolling machine with electrode-deposition equipment in accordance with the invention, Figure 4 is a schematic top view of an al-ternative embodiment of a combination of a thread-rolling machine with electrode-deposition equipment, Figure 5 is a schematic, partial side view of a different embodiment of thè electrode-deposition equipment, Figure 6 is a cross-sectional view, taken along line 6-6 of Figure 4, Figure 7 is a fragmentary perspective view of yet another alternative embodiment of the combination of thread-rolling and electrode-deposition equip-ment, and Figure 8 is a cross-sectional view taken along line 8-8 of Figure 7, 1 ~9 ~ ~ ~ S DA-ll To satisfy the aforementioned, seemingly contradictory needs in a single threaded fastener, particularly a self-tapping screw, a new construction for such screw has been developed which is shown in Figures 1 and 2 of the drawings. As a representative example of possible applications, a self-tapping screw of the thread-forming variety having a head 12 and the conventional pilot end 14, described in more detail below, is shown. The fastener has a shank 10 with a helical screw thread 22 thereon, The shank 10, head 12, pilot end 14 and substantially the entire thread 22 are made from a relatively ductile, possible resilient, metallic material which is ductile and tough enough to stand up to the stresses under which the fastener could be -foreseeably placed. The pilot end 14 of the screw of Figure 1 is shown in enlarged cross section and more detail in Figure 2, The thread 22 of the pilot end has applied to its outside edge, conventionally referred to as the crest 24, a small layer 26 of metal, or other relatively hard substance such as, in some successfully tested examples, tungsten carbide or titanium carbide~ A~ such substance is harder than the ductile material used when initially making the body of the screw, such as the shank 10 and other parts, the illustrated screw has a relatively ductile core and relatively hard surface areas of the crest, or crests, of its thread, and the relatively ductile core extends to, and is defined by, surfaces pertaining to grooves constitut-ing the root of the thread between relatively hard crest areas 26A preferred construction i3 ko produce the deposit only upon the crest 24 of the forward turns of the thread 22, i,e, at the pilot end 14 of the screw. As shown in Figure 1 and producecl suitably as described further below, there may be discrete, relatively hard surface areas 26 of the crest disposed along the circumference ,. .
,. ~.
19~5 of a turn, or turns, of the thread. When the screw is forcibly introduced into a non-threaded or poorly grooved hole, the reinforced hardened crest 24 at the pilot end 14 will form a correctly shaped, mating thread therein. The following non-treated thread 22 of the shank portion closer to the head 12 will then fit within the mating thread formed by the self-tapping pilot end 14 so treatedO No undue axial stress will be placed on the base,'i.e.- ~he root, of the thread 22 where it joins the shank 10 in such a construction.
Such an impregnated construction of a screw, wherein the word "impregnated" connotes the penetration of mater-ials when the layer 26 is deposited upon the crest 24 at the pilot end 14, is especially useful when employed in combination with the recently developed self-locking, ribbed screw thread 23 described in United States Patent No. 3,517,717 and in Canadian Patent Application Serial No. 157,345 filed November 21, 1972. The thread in such a screw, known under the trademark ORLO ~, has a resiliently bendable rib which protrudes outside the boundary of the flank of a normal thread. The thread 23 in Figure l r'epre-sents such thread shaped in accordance with this patent, or patents. The rib of the thread 23 is forced against the remaining thread when the screw is inserted into a conventional mating internal thread in a nut or a tapped hole. A ~ery superior grip is maintained by the outward resilient tension exerted by the rib of the thread 23. It can be seen that a rela'_ively ductile, tough and non-brittle material is preferable for the success of such a self-locking screw thread. Moreover, a precise, correctly formed mating thread in the nut or hole is highly desirable, in order to supply the surface against which the resiliently bendable rib will be forced.
)~
.
~V~ 5 DA-11 A self-tappincJ screw having its thread 22 reinforced by a hardening deposit ~6 at the pilot end 14 of the shank 10, as described herein, will self-tap a precisely mating thread, which then will constitute a nut or a tapped hole. The portion with thread 23 formed beyond the pilot end 14 and thus beyond the hardened crest portion will accurately fit into the thus sel~-tapped nut or hole and the resiliently bendable rib will provide the desired self-locking ef~ect.
Self-tapping screws o~ the thread-cutting variety, not shown, frequently have their pilot ends notched, i.e. grooved, in the axial direction. The axial groove has an axial edge which, when the screw is forcefully rotated in a hole, cuts its mating thread into the material around the hole. In the past, such cutting edge necessarily had to be of material which was harder than the material around the hole which was being cut. It has now been found that a hardening, thus reinforcing, deposit upon the edges provides sufficient hardness to the cutting edges to permit them to cut the hole itself and to cut the matiny threads in the material around the hole. Such a satisfactor~ thread is cut despite the fact that the main body, and therewith the core, of the screw is made oE a tough, ductile material which may even be softer than the material surrounding the hole and to be cut.
Other resilient, self-locking screw thread arrangements have also been proposed in the fastener industry. For example, one such arrangement involves the displacement, at regular inter-vals, of a portion of the entire crest along several turns of the thread of a fastener. The so-called "impregnation", i.e. appli-cation, of hardening material on these modifications of screw threads avoids the need ~or sur~ace-hardening by heat treatment or case-hardeningj thus preventing the otherwise resultant DA-ll degradation and embrittlement of these types of self locking structures when used on self-tapping screws.
It has been found that the resultant screws possess a hardness at the hardened crest 24 which is comparable to a metal having a hardness Rockwell C = 70, or more A screw having its thread so treated, it has been found, does not require case-hardening nor induction heating treatment to possess the required self-tapping capability. Elimination of these steps, as compared to known processes, allows the use of substantially cheaper metals in making the screws. By omitting the case-hardening and induction heating steps, further, the desired ductility of the screw is kept, because the entire core, reaching the roots and the major portions oE the Elanks of the thread, reaches, i.e.
extends into, the thread itselE. As clearly illustrated in Figure 2, only the hardened areas 26 do not pertain to what the industry calls the core. The remaining, major portion of the screw forms part of the core. The result is a screw made from relatively inexpensive material and having a ductility corres-ponding to a hardness of approximately Rockwell C = 30, or less, while having sel~-tapping pilot end thread crests with a hard-ness of approximately Rockwell C = 70, or more.
Experiments with aluminum screws thus described indicate that such a limited hardening deposit will allow, as presently believed, for the first time, aluminum and other non-ferrous metals and low-carbon steels to be used in the pro~uction of self-tapping screws of any variety.
Having described the self-tapping screws of the invention in connection with Figures 1 and 2, methods for producing the screw, as imp].emented by equipment operable in accordance with the methods, will now be discussed.
~ 96S DA-ll The method of producing self-tapping screws of the type disclosed herein is sub~tantially less difficult and complex than the method of preparing case-hardened screws requi.ring induction heating treatments in the presently known methods.
The tungsten or titanium carbide, constituting a particularly suited hardening substance, can be deposited upon the crest o the thread, or threads in case of parallel threads, in any of currently known methods, It is preferred, however, that the presently known electric arc, vibrating electrode-deposition .10 technology be used. This procedure is described in detail in United States Patents Nos. 3,415,970 and 3,524,956, The electrode-deposition equipment is suitably positioned such that it operates upon each screw immediately at the end of the thread-roll forming step performed by a thread-rolling machine, or equipment, as described in more detail further below, One of the many advantages gained by combining thread-rolling techno-logy with electrode-deposition technology resides in the fact that a thoroughly cleaned thread is not required for the carbide deposition. Thus, there is no need to remove the lubricating oil used, nor to remove the grit of manufacture produced in the thread-rolling process. The high voltage applied to, and the impact force of, the electrode, or electrodes, during operation, are sufficient to deposit electrode material upon a screw through the oils and grit, i.e, in spite of thei.r presence.
During this treatment, the temperature of the core of the screws, thus produced, remains substantially the same, and therefore the ductility is not changed, The screws may then be heat-treated ~or neutral hardness, thereby to increase their strength. Such a heat treatment up to approximately ]i350 F will not affect the carbide deposi-t, see Tun~Lsten, 3rd Edition (1955), ~3 DA-ll by Li and Wang (A~.erican Chemical Society Monograph No. ~4), on page 39~. It has been found that, under use of electrode-deposition technology, also descriptively referred to as elec-tric arc impregnation, some of the thread crest is removed, but that the deposited carbide substantially compensates therefor.
The carbide so deposited impregnates, and thus anchors within, the thread crest or other selected parts oE a screw to a depth of approximately 0.~01 of an inch. Moreover, it forms an addi-tional buildup of approximately from 0.001 of an inch to 0.0015 of an inch beyond the original surface. The deposit is solidly implanted upon, and penetrates into, the screw. This explains why it is highly resistant to impact and breaking, as has been found.
It is not necessary to deposit the carbide smoothly or evenly upon the thread crest. Indeed, it is preferred that the deposition occurs unevenly, resulting in a rough surface condi-tion which, along the crest line, is comparable to a sawtooth-type configuration upon the crest 24. Such an irregular appli-cation is illustrated in Figures 1 and 2 where the hardening deposit 26 at one location is shown to be somewhat thicker than at another location. Generally speaking, but particularly when a screw is intended to remain permanently, once it has been insexted into another material, it was sufficient to apply the deposit at only a selected number of discrete, mutually spaced locations along at least one turn, preferably at the pilot end of the thread.
It can be seen that a treated body, such as a scre~7 of the self tapping class, provides a harder and sharper thread crest for functional forming and cutting its mating thread, yet the core, ~7hich constitutes the major portion of the whole body, DA-ll ~(~9~65 is not brittle and consequently useful where known products failed. Moreover, it is not susceptible to chipping in ship-ping and use.
It is contemplated that the deposition of carbide or a similar substance can be made upon the crest of the screw ~hread during a single pass by producing the deposit immediately subse-quent to the thread-rolling step in a slightly modified, otherwise conventional, thread-rolling machine. In the schematic illus-tration of Figure 3 is shown a die block 40 and die block 42, constituting elements of such conventional machine. Die block 40 represents the stationary die, while die block 42 is the movable die. In the conventlonal and well-known method of manu-facturing threaded bodies, a cylindrical blank 44 is rolled between the dies 40, 42 as the die 42 is forcefully moved in the direction of the arrow. Threads are formed on the blank 44 by the ridges between mating grooves 46 and ~8 on the respective working surfaces of the die blocks 40 and 42. The ~acing grooves 46, 48 on these working surfaces are so matched that, when the blank 44 is rolled between the two dies 40, 42, a continuous thread, or a set of parallel continuous threads, is, or are, fo~med on the shank of the blank.
A slightly different thread-rolling arrangement is illus-trated in Figure 4. As in Figure 3, a stationary die block 50 is shown having grooves 52 on its working surface, and a die block 54 is movably positioned with its working face opposite that of die block 50. Die block 54 has grooves 56 so that when a blank 58 is rolled between the dies 50, 54 when die block 54 is moved in the direction of the arrow as shown, a conkinuous thread or a set of parallel continuous threads, is, or are, formed on the blank 58.
DA-ll The die blocks constituting dies 50, 54 differ from the dies 40, 42 in that the dies of Fi~ure 4 have grooved ridges 5~, 56 which approach each other in such a way as to form, in accordance with known techniques, a threaded point which will be the pilot end of the resulting screw. The thread-rolling machine schematically illustrated in Figure 3 does not form suGh a pilot end.
The pilot end of a threaded body, such as a screw, generally is the end of the screw shank opposite the head of the screw. This end normally has a gradually increasing outside thread diameter beginning from the shank's end. The inside, or root, diameter of the thread may or may not be tapered in corres-pondence with the -thread's outside diameter in order to have a useful pilot erld. The tapered length may have no thread at all at the pilot end, although, normally, the thread will extend over such tapering. The screw produced by the dies of Fig~re 3, for example, will not have a pilot end.
With a modification of conventional thread-rolling machines, the deposit causing a qurface-hardening ef~ect only in limited surface areas, such as a deposit of carbide or metal, can be made automatically and effectlvely upon the threads of each screw as it is ejected from the space between the two dies of the machine. In each of the machines illustrated in Figures 3 and 4, an extension 60 of the stationary die block 40 or 50~
respectively, is shown. This extension 60 is a support for at least one electrode 62, suitabl~ three such electrodes, of the type described in United States Patents Nos. 3,524,956 and 3,415,970, mentioned above. Such an electrode 62 of tungsten carbide or other similar material, during operation, causes a deposit of which approximately 0.001 of an inch penetrates into, DA-ll and thus impregnates, a metal sur~ace, and additionally forms a layer of approximately 0.001 of an inch, for example, oE the carbide, protru~ing from the original surface.
The extension, i.e. support, 6~ is constructed and posi-tioned such that it makes electrical contact with the newly formed threads of the screw, thus constituting a guide element for each screw and, at the same time, closing the electrical circuit by which power is applied between each screw, as it is guided by support 60, and one after the other of the electrodes.
The illustrated energizing circuits are believed to be self-explanatory.
A plurality of electrodes 62 may be placed in the support 60, as is illustrated in Figures 3 and 4. As explained in the United States patents mentioned above, the electrodes 62 are caused to vibrate by a mechanism suitable for this purpose and not shown in the drawings, as corresponding explanations can be found in the pertinent literature, such as these patents. By virtue of these vibrations, the electrodes 62 rapidly move into and out of contact with the newly formed thread crests. The electrodes 62 are preferably positioned so that only the end, such as the pilot end 14, will receive deposits 26. Alterna-tively, the electrodes 62 can be positioned so that the entire axial length of the thread along the shank will be impregnated as the result of the electrode-deposition treatment. Also, one continuous electrode 64 could be used, and positioned as illus-trated in Figure 5. The continuous electrode 64 would ef~ectively cause a deposit along the entire circumference oE the thread cxest, or crests, at the pilot end, during a combined thread-rolling and electrode-deposition treatment in which the second immediately follows the first.
DA-ll A useful thread-rolling equipment could be constructed wherein the moving die 42 or S4 could cause energization of the electrodes 62 at a predetermined moment, namely when the blank 44 or 58 reaches the support 60. A pressure-actuated switch 70, as schematically shown in Figure 4, or alternatively a lever, a radiation sensor or any other well-known electric circuit-actuating device could be employed to caus~e the electrodes 62 to become energi2ed at the desired moment and remain energized for the desired time period.
It has been found that producing discrete areas of the hardening deposit 26 upon the tapping portion oE the shan~ of the screw, which includes the Pilot end approximately every 120, provides satisfactory sel~-tappinq capabilities in most instances of later use of the screw. Thus, the three electrocles 62 shown in Figures 3 and 4 are carefully positioned so that the screws rolled from blanks 44 or 58 will be caused to perform their rolling motion past the electrodes such that discrete deposits are made at precise circumferential distances along the circum-ference of the thread on which it is desired to produce such ~0 intermittent deposition. By way of illustration, the distance between the electrodes 62 in Figure 3 will be exactly equal to that between points which are 120 apart along the clrcumference of the thread on the screw formed from blank 44.
The electrodes 62 may be triggered into energization by the moving die 42 or 54 so that they will operate only when the screw, as it has been roll-formed from a blank 44 or 58, approaches the guide element, i.e. support, 60, a~d the grooves 48 or 56 of the moving die have moved beyond the screw. It was found advan-tageous to provide the moving die 42 or 54 with an extension 68 shown only in Figure 4. Such an extension 68 will not have DA-ll ~O~ 5 grooved ridges. Rather, it will serve as a second guide element, as it assists in the continuation oE the rolling motion of the screw past the electrodes 62 or 64 after the conventional thread-rolling operation has been completed, If there is a pilot end on the screw, such as screw 58 shown in phantom in Figure 6, the electrodles 62 preferably have a canted electrode tip. Such a canted elec-trode tip may be at an appropriate angle to fit complementally against the taper o the diminishing outside diameter of the thread on the pilot end of the screw 58. Such a complemen-tally cantecl fit is illus-trated in Figure 6. This type of specially formed electrode tip also has particular advantages when used to transfer hardening deposits onto the thread-cutting edges o~ an ax.ially grooved thread of this type of self-tapping screws. The thread-cutting edge (not illustrated) will thus be completely treated along its axial length.
If a self-tapping, thread-cutting type pilot end is desired, it may prove to be advantageous to cause the deposi~ion upon the self-tapping, thread-cutting type pilot end of the shank from the lower side of the thread-rolling mach.ine, when viewed as illustrated, as opposed to deposit.ion from the side~
as shown in Figures 3 and 4. In Fi.gures 7 and 8, for example, a basic scheme for electrode-deposi.tion upon a threaded body on its tip end from the bottom is shown A stationary thread-rolling.
die 80 is shown having an extension 82. The extension 82 is a guide element, as the support 60, ~igures 3 and 4, but does not carry an electrode. It extends only so far down from the neck of the screw 84, illustrated as it has just been formed from a blank, so as to retain the screw 8~ in its -travel for a short distance beyond the thread-roll.ing dies.
3.9-DA-ll A die 86 movable in relation to d.ie 80 is shown having an extension, namely guide element 88, compa.rable in shape and purpose to extension 82. The die 86 and guide element 88 move in the direction of the arrows shown in Figure 7 when the screw 84 is being thread-rolled and the threads thereon are being formed.
An electrode 90 oE the description given above is posi-tioned in axial parallelism with the screw 84 by a suitable support 92. The electrode 90 is electrically connected to the power source, not shown, by a connecting cable 94.
Electrode 90 is shown having a slanted tip 96 which, during the travel of the screw 84, will come into cngagement with the selE-tapping thread-forming type pilot end 98 of the screw 84, when the screw comes into axial alignment with -the electrode. If the pilot end 9B had axial grooves across the thread, not shown in Figures 7 and 8, the cutting edges and the crests of the tip end 98 of the screw 84 will receive, in these limited surface areas, a deposit of a hard substance, such as tungsten carbide or titanium ca.rbide, transferred immediately after the thread-rolling process in a single pass, and thus in a manner which is econom.ical and efEicient, because the process is conducted at very high speeds. When the screw 84 is rollecl in the direction of the arrows in Figure 7, the extensions 82, 88 operate as guide elements, as they cause the roll-formed screw 84 to continue its rolling motion for a short distance beyond the thread-rolling dies 80, 86. The self-tapping pilot end 9~ of the screw 84 will then be brought into close proximity to the slanted tip 96 of the carbide electrode 90. The electrode 90 may be triggered into energization by an actuating device 100, and l~e caused to vibrate, thereby to come into rapicl and repeated con~
tact with the pilot end 98. Actuating device 100 may be a switch DA-ll or a sensor, operated by the screw as it leaves the space between the dies, and electrically connected to cable 94 so as to cause energization of the electrode 90 for selected, predetermlnecl periods sufficient to ensure that electrodle 90 will be operating when the screw 8~ is rolling through successive positions of axial alignment with the electrode 90.
It is of interest to note that vertical electrodes, as shown in Figures 7 and 8, and horizontal or angled electrodes, as shown in ~igures 3, 4, 5 and 6, may be combined in the same equipment composed of both a thread-rolling machine and an electrode-deposition arrangement, so that the threads at the pilot end, as well as threads in other or all areas of the shank, oE the screw will be subjected to the treatment described. Such dual deposition treatment could be caused to occur simultaneously or seriatim.
Self~tapping screws of the thread-forming, thread-cutting or self-drilling type, when produced as described herein, are particularly useful when made of aluminum, other non-ferrous metals or their alloys. The carbide deposit on the crests of the thread, or threads, of the selE-tapping screws, particularly a-t the pilot end, allows an aluminum or other non-ferrous snetal screw to be used in the assembly of parts made Erom light-gauge steel and aluminum sheets.
It will have become clear that, in accordance with the combined technology described herein, the hardening deposit 26 is ~ade only in those surface areas of a self tapping screw which will displace material into which the screw will be caused to tap itself. The result of this combination of technologies is a screw having a relatively ductile core and relatively hard surface areas of the crest, or crests, of its thread, with the specific 1091965 DA-ll feature that the relatively ductile core extends -to, and is defined by, surfaces pertalning to grooves constituting the root of the thread between relatively hard crest areas, Reviewing the present invention, it may be noted that a new technique for economically producing high strength self-tapping screws has been developed~ The body of such screws may be made. of tough, inexpensive low-carbon steels or of non-ferrous metals, and are not subject to the embrit-tlement or other adverse effects of conventional heat treatments for surface hardening. The hardening deposit, or deposi-ts, may be placed on two, three or more turns on the lead, i,e, forward, threads of the self-tapping screws, thus providing self-tapping capabilities without impai.ring the basic toughness of the body of the screw, Also, the efficiency of self-locking features provided by 5 resiliently bendable ribs of the threads of screws is preserved, This application is a divisional of Canadian Patent Application 196,237 filed March 28, 1974.
a~
This invention relates to self-tap,ping screws, as well as methods and equipment for producing ~uch screws, The fastener manufacturing industry has had the problem of making threaded metal fasteners possessing at once charac-S teristics of ductility and toughness, yet having relative threadhardness, The problem is particularly acute in connection with self-tapping screws, though it may be encountered in different environments, too, By being ductile or tough, a metal i9 considered herein as being pliant yet strong, as having the ability oE being bent and/or twisted without tearing or br0aking. Antonymously, the term "hardness" is u~ed herein to connote pronounced suscepti-bility to breaking, i.e, brittlene~, in addition to resistance to penetration, In the pertinent industry, it is customary to use the Rockwell testing devices for measuring hardness. Using such devices, a metal is penetrated by a probe of known hardness, such as a diamond, with a predetermined force. The depth of penetration gives an indication of hardness. One o~ a series of nur~bers is assigned directly to every penetration depth. The results of such tests are ordinarily expressed on the scales designated Rockwell A, Rockwell B and Rockwell C. The higher the number given as a result of such a test, the harder the metal, Thus, a metal having a hardness corresponding to a reading of Rockwell C = 30 or less would be considered ductile and is used in the manufacture of relatively tough, ductile fasteners, A screw made from such tough, ductile material generally fails as a self-ta~ping ~crew. A self-tapping screw is one which forrns its own mating thr0ad, or threads (if there are two parallel threads), in a material into which the screw taps DA-ll 109~l965 itself, when rotated under the application of an axially oriented ~orce, In one form of self-tapping screw, a previously drilled hole initially receives the more or less pointed, so-called piIot end of the screw, and then the thread on the self-tapping screw forms a mating thread by partially displacing material around ~he hole, substantially without removing any such material. This type of screw is frequently referred to as a thread-forming screw, as di~tinguished from the thread-cutting screws which, mostly by virtue of axially oriented cutting edges acro8s the thread, particularly in the pilot end, actually cut their mating thread by removing material in which the initial hole has been drilled. As a specific form of thread-forming screws, the so-called self-drilling variety should also be men-tioned, This type of self-tapping screw has a pointed pllot end whose thread drills its own hole, thus avoiding the need for a previously drilled hole in the material into which the screw taps itself, Any one of these classes of self-tapping screws i9 fre-guently subjec~ to fai~ure, inasmuch as such failure occurs when the ductile or tough thread on the fastener collapses within the unthreaded hole.
In general, the hole for thread-forming self-tapping screws should be sized to produce approximately 8~ thread engagement, This value can vary, however, with the flexibility of the materials used, Thread-cutting self-tapping screws, on the other hand, are manufactured to function as a tap to cut threads into the material being joined, Usually, as a thread-cutting screw cuts into the mate~ial, chips are generated and pushed ahead of the screw, For further explanations and illus-trations, see, for example, "Solutions to Plastic Fastening Prob-lems" in Assembly Engineerin~, September, 1971.
DA-ll ~9~ti5 Various methods are presently used to make the fastener harder only on it~ surface while maintaining toughness by virtue of the unchanged ductility of the internal part of the body of the screw, descriptively referred to in pertinent literature as a core, The most common, presently used process is a three-step process, First, the fastener is heat-treated to approximately 1700 F or(more, The fastener is then, in a second ~tep, sub-jected to a treatment commonly and descriptively designated ~Ica~e-hardening~, inasmuch as a hardened shell, i.e~ case or casing, is produced around the core, which is the interior o the body of the screw which retains its ductility. In this second step, the astener's surface i9 thoroughly cleaned, heated to approximately 1250 F to 1300 F and placed into a carbon-rich atmosphere, i,e, an atmosphere containing a large proportion of hydrocarbons. In such an environment, the metaI has a tendency to attract the carbon. The usual result is a carbon layer of approximately 0.004 of an inch to 0,006 of an inch in thickness deposited upon the fastener' 5 surface, The third procedural step is an induction heating treatment, In this step, the pilot end of the threaded fastener is placed in a rapidly changing electro-magnetic field, Upon cooling, it is found that the molecular structure in a surface layer of at least approximately 0,06 of an inch in thickness of the pilot end has been changed. The result of such treatment, unfortunately, hardens the fastener and makes it more brittle, In fact, such an induction heating pro-ce~ure with the carbon layer, known as "carburization", has made test fasteners possess a hardness in the order of Rockwell C = 45 and higher.
To manufacture these relatively hard fasteners in -the currently known manner, a premium alloy steel containirlg an 10~1965 DA-ll additional metal component is required, Such additional me-tal could be manganese, chromium, nickel and the li~se, which makes these alloys expensive, Furthermore, the three-step procedure outlined above requires continuous attention and careful handling.
Prior to case-hardening, for example, the fasteners, and parti-cularly the thread root areas between the threads, must be thoroughly cleaned to remove the lubricating oils and other grit of manufacture, so that the carbon will become evenly and finely deposited upon the surface of the fastener~ The composition of the carbon-rich atmosphere must be precisely controlled and require~ constant attention, Additionally, the induction heating step requires meti-culous positioning of the fasteners with only the pilot end exposed to the field, in order to prevent that the entire fastener be 90 treated. Moreover, the equipment for the induc-tion heating~step is complex and very costly.
The result of all of these manipulations and attendant expenses is a Eastener which has substantially lost the desired ductility and toughness, and in some instances has not even achieved the desired hardness. For example, a fastener having a Rockwell C = 45 measure of hardness would be too hard and brittle in certain applications where predictable shocks require the fastener to have a certain resiliency, ductility and toughness.
Yet, the thread~ of such a fastener, even after case-hardening and induction treating, have collapsed when attempts were made to produce and then use such fasteners in the form of self-tapping screws.
A self-tapping screw or other threaded fastener made of comparatively tough, resilient material possessing hardness characteristics necessa~y for successful use as self-tapping screws has been long sought but was heretofore unavailable~
~091965 This invention is based on the recognition that this long-felt need could be filled, specificallv with respect to self-tapping screws, by producing them from comparatively tough, ductile material, and providing a com-paratively hard substance deposited at, and bonded to, points of maximum wear, ~.e. on the thread crests exclusively.
To achieve this resuIt, t~e threads ma~ be subjected to an electrode-depOsitiOn treatment ~h;~ch preferahl~r advantageousl~
immediately folloT.rs, in one s~ngle pass, the thread-rolling step wherein the threads are rolled onto cylindrical blanks.
S~rews with the resulting thread structure are particularl~
useful in the case of seIf-tapping screws having self-locking features, as described below, and for high strength fasteners, norm~ formed o mater~als wh~ch are adversely affected bv conventional hardening methods, such as case-hardening.
~ ne aspect of a specific method resulting in a self-tapp~ng screw of the invention involves the eIectrode-deposition, which is assumed to be associated with impregnation, upon t~e screw threads or other metal parts as a production phase o~ the machin~ng, i.e. thread-rolling, process. Unlike other coating processes which reauire a high degree of clean-liness, the car~ide deposition, i.e. reinforcing by coating, process su~gested here~n can tolerate the precence of coolant, lubricating fluid and machining grit, and thus can be incor-porated into manufacturing metal-working processes with little additional expense.
As used herein,~the phrases "eIectrode-deposition"
and "electrode-deposition technology", described and illu.s-trated further beIow in more detail, are intended to denote a class of methods, and the relevant technology, wherein the suhstance of an eIectrode is deposited from the electrode upon a workpiece under the application of electric power between the electrode and the workpiece. T~e depositi~n mb/ )~7`~ 6 -.. . . .
19~S
effect is. achieved under the formation of an electricalarc, resulting in a strongly adhering deposit constituting a coating on the workpiece, so th.at re.ference to impreg-nation is justified, inasmuch as penetration of the elect-rode material into -the material of the workpiece, as it is deposited, may safely be assumed to occur. The strong bo~d, thus achieved, may be explained by the molecular forces between the mutually penetrati.n~ materials of deposit and workpiece along the interface.
~hus in accordance with a broad aspect of the invention, there is provided a se.lf-tapping screw having a relatively ductile core and carb.on-containing material substantially harder than the relatively ductile core deposited on to the crest, or crests, of its thread in at least the area of the pilot end of the screw, wherein the relatively ductile core extends to, and is defined by, surfaces pertaining to grooves consti.tuting the root of the thread ~etween relatively hard crest areas.
In accordance with specific features of an embodiment of the invention, a self-tapping screw has relatively hard sur~ace areas of the crest, or crests, of the thread only in the area of the pilot end of the screw.
There may be discrete, relatively hard sur~ace areas of the crest, or crests, disposed along the circumference o a turn, or turns, of the thread. Alternati~ely, a relative-ly hard surface area of the crest, or crests, along the entire circumference of a turn, or turns, of the thread - may be provided. In the case of self-tapping screws of the thread-cutting variety, the. relatively hard surface areas of the crest, or crests, suitably include cutting edges o-f axial notches in the thread. ~s to useful mater-ials, generally any metal or alloy having.the des:;re~ ~uctility and toughness may be selec-ted and gocd results have been.a~e~ed when the core was made oE lo~-cæbon or a.luminum. With ~ .
-- pg/~ - 7 -s DA-ll respect to the relatively hard surface area, or areas, of the crest, or crests, a deposit of titanium carbide or tungsten carbide has been found particularly suitable, as such deposits lead to successfully tested reinforcement effects.
The invention will become better understood~from the following detailed description of several emhodiments thereof, when taken in conjunction with the drawings, wherein:
Figure 1 illustrates, in a side view, one embodiment of a self-tapping screw in accordance with the invention' Figure 2 is a partial, relatively enlarged cross-sectional view taken al.ong an axial plane through the pilot end of the screw of Figure l, Figure 3 schematically illustrates, in.a perspective view, one emhodiment of a combination of a thread-rolling machine with electrode-deposition equipment in accordance with the invention, Figure 4 is a schematic top view of an al-ternative embodiment of a combination of a thread-rolling machine with electrode-deposition equipment, Figure 5 is a schematic, partial side view of a different embodiment of thè electrode-deposition equipment, Figure 6 is a cross-sectional view, taken along line 6-6 of Figure 4, Figure 7 is a fragmentary perspective view of yet another alternative embodiment of the combination of thread-rolling and electrode-deposition equip-ment, and Figure 8 is a cross-sectional view taken along line 8-8 of Figure 7, 1 ~9 ~ ~ ~ S DA-ll To satisfy the aforementioned, seemingly contradictory needs in a single threaded fastener, particularly a self-tapping screw, a new construction for such screw has been developed which is shown in Figures 1 and 2 of the drawings. As a representative example of possible applications, a self-tapping screw of the thread-forming variety having a head 12 and the conventional pilot end 14, described in more detail below, is shown. The fastener has a shank 10 with a helical screw thread 22 thereon, The shank 10, head 12, pilot end 14 and substantially the entire thread 22 are made from a relatively ductile, possible resilient, metallic material which is ductile and tough enough to stand up to the stresses under which the fastener could be -foreseeably placed. The pilot end 14 of the screw of Figure 1 is shown in enlarged cross section and more detail in Figure 2, The thread 22 of the pilot end has applied to its outside edge, conventionally referred to as the crest 24, a small layer 26 of metal, or other relatively hard substance such as, in some successfully tested examples, tungsten carbide or titanium carbide~ A~ such substance is harder than the ductile material used when initially making the body of the screw, such as the shank 10 and other parts, the illustrated screw has a relatively ductile core and relatively hard surface areas of the crest, or crests, of its thread, and the relatively ductile core extends to, and is defined by, surfaces pertaining to grooves constitut-ing the root of the thread between relatively hard crest areas 26A preferred construction i3 ko produce the deposit only upon the crest 24 of the forward turns of the thread 22, i,e, at the pilot end 14 of the screw. As shown in Figure 1 and producecl suitably as described further below, there may be discrete, relatively hard surface areas 26 of the crest disposed along the circumference ,. .
,. ~.
19~5 of a turn, or turns, of the thread. When the screw is forcibly introduced into a non-threaded or poorly grooved hole, the reinforced hardened crest 24 at the pilot end 14 will form a correctly shaped, mating thread therein. The following non-treated thread 22 of the shank portion closer to the head 12 will then fit within the mating thread formed by the self-tapping pilot end 14 so treatedO No undue axial stress will be placed on the base,'i.e.- ~he root, of the thread 22 where it joins the shank 10 in such a construction.
Such an impregnated construction of a screw, wherein the word "impregnated" connotes the penetration of mater-ials when the layer 26 is deposited upon the crest 24 at the pilot end 14, is especially useful when employed in combination with the recently developed self-locking, ribbed screw thread 23 described in United States Patent No. 3,517,717 and in Canadian Patent Application Serial No. 157,345 filed November 21, 1972. The thread in such a screw, known under the trademark ORLO ~, has a resiliently bendable rib which protrudes outside the boundary of the flank of a normal thread. The thread 23 in Figure l r'epre-sents such thread shaped in accordance with this patent, or patents. The rib of the thread 23 is forced against the remaining thread when the screw is inserted into a conventional mating internal thread in a nut or a tapped hole. A ~ery superior grip is maintained by the outward resilient tension exerted by the rib of the thread 23. It can be seen that a rela'_ively ductile, tough and non-brittle material is preferable for the success of such a self-locking screw thread. Moreover, a precise, correctly formed mating thread in the nut or hole is highly desirable, in order to supply the surface against which the resiliently bendable rib will be forced.
)~
.
~V~ 5 DA-11 A self-tappincJ screw having its thread 22 reinforced by a hardening deposit ~6 at the pilot end 14 of the shank 10, as described herein, will self-tap a precisely mating thread, which then will constitute a nut or a tapped hole. The portion with thread 23 formed beyond the pilot end 14 and thus beyond the hardened crest portion will accurately fit into the thus sel~-tapped nut or hole and the resiliently bendable rib will provide the desired self-locking ef~ect.
Self-tapping screws o~ the thread-cutting variety, not shown, frequently have their pilot ends notched, i.e. grooved, in the axial direction. The axial groove has an axial edge which, when the screw is forcefully rotated in a hole, cuts its mating thread into the material around the hole. In the past, such cutting edge necessarily had to be of material which was harder than the material around the hole which was being cut. It has now been found that a hardening, thus reinforcing, deposit upon the edges provides sufficient hardness to the cutting edges to permit them to cut the hole itself and to cut the matiny threads in the material around the hole. Such a satisfactor~ thread is cut despite the fact that the main body, and therewith the core, of the screw is made oE a tough, ductile material which may even be softer than the material surrounding the hole and to be cut.
Other resilient, self-locking screw thread arrangements have also been proposed in the fastener industry. For example, one such arrangement involves the displacement, at regular inter-vals, of a portion of the entire crest along several turns of the thread of a fastener. The so-called "impregnation", i.e. appli-cation, of hardening material on these modifications of screw threads avoids the need ~or sur~ace-hardening by heat treatment or case-hardeningj thus preventing the otherwise resultant DA-ll degradation and embrittlement of these types of self locking structures when used on self-tapping screws.
It has been found that the resultant screws possess a hardness at the hardened crest 24 which is comparable to a metal having a hardness Rockwell C = 70, or more A screw having its thread so treated, it has been found, does not require case-hardening nor induction heating treatment to possess the required self-tapping capability. Elimination of these steps, as compared to known processes, allows the use of substantially cheaper metals in making the screws. By omitting the case-hardening and induction heating steps, further, the desired ductility of the screw is kept, because the entire core, reaching the roots and the major portions oE the Elanks of the thread, reaches, i.e.
extends into, the thread itselE. As clearly illustrated in Figure 2, only the hardened areas 26 do not pertain to what the industry calls the core. The remaining, major portion of the screw forms part of the core. The result is a screw made from relatively inexpensive material and having a ductility corres-ponding to a hardness of approximately Rockwell C = 30, or less, while having sel~-tapping pilot end thread crests with a hard-ness of approximately Rockwell C = 70, or more.
Experiments with aluminum screws thus described indicate that such a limited hardening deposit will allow, as presently believed, for the first time, aluminum and other non-ferrous metals and low-carbon steels to be used in the pro~uction of self-tapping screws of any variety.
Having described the self-tapping screws of the invention in connection with Figures 1 and 2, methods for producing the screw, as imp].emented by equipment operable in accordance with the methods, will now be discussed.
~ 96S DA-ll The method of producing self-tapping screws of the type disclosed herein is sub~tantially less difficult and complex than the method of preparing case-hardened screws requi.ring induction heating treatments in the presently known methods.
The tungsten or titanium carbide, constituting a particularly suited hardening substance, can be deposited upon the crest o the thread, or threads in case of parallel threads, in any of currently known methods, It is preferred, however, that the presently known electric arc, vibrating electrode-deposition .10 technology be used. This procedure is described in detail in United States Patents Nos. 3,415,970 and 3,524,956, The electrode-deposition equipment is suitably positioned such that it operates upon each screw immediately at the end of the thread-roll forming step performed by a thread-rolling machine, or equipment, as described in more detail further below, One of the many advantages gained by combining thread-rolling techno-logy with electrode-deposition technology resides in the fact that a thoroughly cleaned thread is not required for the carbide deposition. Thus, there is no need to remove the lubricating oil used, nor to remove the grit of manufacture produced in the thread-rolling process. The high voltage applied to, and the impact force of, the electrode, or electrodes, during operation, are sufficient to deposit electrode material upon a screw through the oils and grit, i.e, in spite of thei.r presence.
During this treatment, the temperature of the core of the screws, thus produced, remains substantially the same, and therefore the ductility is not changed, The screws may then be heat-treated ~or neutral hardness, thereby to increase their strength. Such a heat treatment up to approximately ]i350 F will not affect the carbide deposi-t, see Tun~Lsten, 3rd Edition (1955), ~3 DA-ll by Li and Wang (A~.erican Chemical Society Monograph No. ~4), on page 39~. It has been found that, under use of electrode-deposition technology, also descriptively referred to as elec-tric arc impregnation, some of the thread crest is removed, but that the deposited carbide substantially compensates therefor.
The carbide so deposited impregnates, and thus anchors within, the thread crest or other selected parts oE a screw to a depth of approximately 0.~01 of an inch. Moreover, it forms an addi-tional buildup of approximately from 0.001 of an inch to 0.0015 of an inch beyond the original surface. The deposit is solidly implanted upon, and penetrates into, the screw. This explains why it is highly resistant to impact and breaking, as has been found.
It is not necessary to deposit the carbide smoothly or evenly upon the thread crest. Indeed, it is preferred that the deposition occurs unevenly, resulting in a rough surface condi-tion which, along the crest line, is comparable to a sawtooth-type configuration upon the crest 24. Such an irregular appli-cation is illustrated in Figures 1 and 2 where the hardening deposit 26 at one location is shown to be somewhat thicker than at another location. Generally speaking, but particularly when a screw is intended to remain permanently, once it has been insexted into another material, it was sufficient to apply the deposit at only a selected number of discrete, mutually spaced locations along at least one turn, preferably at the pilot end of the thread.
It can be seen that a treated body, such as a scre~7 of the self tapping class, provides a harder and sharper thread crest for functional forming and cutting its mating thread, yet the core, ~7hich constitutes the major portion of the whole body, DA-ll ~(~9~65 is not brittle and consequently useful where known products failed. Moreover, it is not susceptible to chipping in ship-ping and use.
It is contemplated that the deposition of carbide or a similar substance can be made upon the crest of the screw ~hread during a single pass by producing the deposit immediately subse-quent to the thread-rolling step in a slightly modified, otherwise conventional, thread-rolling machine. In the schematic illus-tration of Figure 3 is shown a die block 40 and die block 42, constituting elements of such conventional machine. Die block 40 represents the stationary die, while die block 42 is the movable die. In the conventlonal and well-known method of manu-facturing threaded bodies, a cylindrical blank 44 is rolled between the dies 40, 42 as the die 42 is forcefully moved in the direction of the arrow. Threads are formed on the blank 44 by the ridges between mating grooves 46 and ~8 on the respective working surfaces of the die blocks 40 and 42. The ~acing grooves 46, 48 on these working surfaces are so matched that, when the blank 44 is rolled between the two dies 40, 42, a continuous thread, or a set of parallel continuous threads, is, or are, fo~med on the shank of the blank.
A slightly different thread-rolling arrangement is illus-trated in Figure 4. As in Figure 3, a stationary die block 50 is shown having grooves 52 on its working surface, and a die block 54 is movably positioned with its working face opposite that of die block 50. Die block 54 has grooves 56 so that when a blank 58 is rolled between the dies 50, 54 when die block 54 is moved in the direction of the arrow as shown, a conkinuous thread or a set of parallel continuous threads, is, or are, formed on the blank 58.
DA-ll The die blocks constituting dies 50, 54 differ from the dies 40, 42 in that the dies of Fi~ure 4 have grooved ridges 5~, 56 which approach each other in such a way as to form, in accordance with known techniques, a threaded point which will be the pilot end of the resulting screw. The thread-rolling machine schematically illustrated in Figure 3 does not form suGh a pilot end.
The pilot end of a threaded body, such as a screw, generally is the end of the screw shank opposite the head of the screw. This end normally has a gradually increasing outside thread diameter beginning from the shank's end. The inside, or root, diameter of the thread may or may not be tapered in corres-pondence with the -thread's outside diameter in order to have a useful pilot erld. The tapered length may have no thread at all at the pilot end, although, normally, the thread will extend over such tapering. The screw produced by the dies of Fig~re 3, for example, will not have a pilot end.
With a modification of conventional thread-rolling machines, the deposit causing a qurface-hardening ef~ect only in limited surface areas, such as a deposit of carbide or metal, can be made automatically and effectlvely upon the threads of each screw as it is ejected from the space between the two dies of the machine. In each of the machines illustrated in Figures 3 and 4, an extension 60 of the stationary die block 40 or 50~
respectively, is shown. This extension 60 is a support for at least one electrode 62, suitabl~ three such electrodes, of the type described in United States Patents Nos. 3,524,956 and 3,415,970, mentioned above. Such an electrode 62 of tungsten carbide or other similar material, during operation, causes a deposit of which approximately 0.001 of an inch penetrates into, DA-ll and thus impregnates, a metal sur~ace, and additionally forms a layer of approximately 0.001 of an inch, for example, oE the carbide, protru~ing from the original surface.
The extension, i.e. support, 6~ is constructed and posi-tioned such that it makes electrical contact with the newly formed threads of the screw, thus constituting a guide element for each screw and, at the same time, closing the electrical circuit by which power is applied between each screw, as it is guided by support 60, and one after the other of the electrodes.
The illustrated energizing circuits are believed to be self-explanatory.
A plurality of electrodes 62 may be placed in the support 60, as is illustrated in Figures 3 and 4. As explained in the United States patents mentioned above, the electrodes 62 are caused to vibrate by a mechanism suitable for this purpose and not shown in the drawings, as corresponding explanations can be found in the pertinent literature, such as these patents. By virtue of these vibrations, the electrodes 62 rapidly move into and out of contact with the newly formed thread crests. The electrodes 62 are preferably positioned so that only the end, such as the pilot end 14, will receive deposits 26. Alterna-tively, the electrodes 62 can be positioned so that the entire axial length of the thread along the shank will be impregnated as the result of the electrode-deposition treatment. Also, one continuous electrode 64 could be used, and positioned as illus-trated in Figure 5. The continuous electrode 64 would ef~ectively cause a deposit along the entire circumference oE the thread cxest, or crests, at the pilot end, during a combined thread-rolling and electrode-deposition treatment in which the second immediately follows the first.
DA-ll A useful thread-rolling equipment could be constructed wherein the moving die 42 or S4 could cause energization of the electrodes 62 at a predetermined moment, namely when the blank 44 or 58 reaches the support 60. A pressure-actuated switch 70, as schematically shown in Figure 4, or alternatively a lever, a radiation sensor or any other well-known electric circuit-actuating device could be employed to caus~e the electrodes 62 to become energi2ed at the desired moment and remain energized for the desired time period.
It has been found that producing discrete areas of the hardening deposit 26 upon the tapping portion oE the shan~ of the screw, which includes the Pilot end approximately every 120, provides satisfactory sel~-tappinq capabilities in most instances of later use of the screw. Thus, the three electrocles 62 shown in Figures 3 and 4 are carefully positioned so that the screws rolled from blanks 44 or 58 will be caused to perform their rolling motion past the electrodes such that discrete deposits are made at precise circumferential distances along the circum-ference of the thread on which it is desired to produce such ~0 intermittent deposition. By way of illustration, the distance between the electrodes 62 in Figure 3 will be exactly equal to that between points which are 120 apart along the clrcumference of the thread on the screw formed from blank 44.
The electrodes 62 may be triggered into energization by the moving die 42 or 54 so that they will operate only when the screw, as it has been roll-formed from a blank 44 or 58, approaches the guide element, i.e. support, 60, a~d the grooves 48 or 56 of the moving die have moved beyond the screw. It was found advan-tageous to provide the moving die 42 or 54 with an extension 68 shown only in Figure 4. Such an extension 68 will not have DA-ll ~O~ 5 grooved ridges. Rather, it will serve as a second guide element, as it assists in the continuation oE the rolling motion of the screw past the electrodes 62 or 64 after the conventional thread-rolling operation has been completed, If there is a pilot end on the screw, such as screw 58 shown in phantom in Figure 6, the electrodles 62 preferably have a canted electrode tip. Such a canted elec-trode tip may be at an appropriate angle to fit complementally against the taper o the diminishing outside diameter of the thread on the pilot end of the screw 58. Such a complemen-tally cantecl fit is illus-trated in Figure 6. This type of specially formed electrode tip also has particular advantages when used to transfer hardening deposits onto the thread-cutting edges o~ an ax.ially grooved thread of this type of self-tapping screws. The thread-cutting edge (not illustrated) will thus be completely treated along its axial length.
If a self-tapping, thread-cutting type pilot end is desired, it may prove to be advantageous to cause the deposi~ion upon the self-tapping, thread-cutting type pilot end of the shank from the lower side of the thread-rolling mach.ine, when viewed as illustrated, as opposed to deposit.ion from the side~
as shown in Figures 3 and 4. In Fi.gures 7 and 8, for example, a basic scheme for electrode-deposi.tion upon a threaded body on its tip end from the bottom is shown A stationary thread-rolling.
die 80 is shown having an extension 82. The extension 82 is a guide element, as the support 60, ~igures 3 and 4, but does not carry an electrode. It extends only so far down from the neck of the screw 84, illustrated as it has just been formed from a blank, so as to retain the screw 8~ in its -travel for a short distance beyond the thread-roll.ing dies.
3.9-DA-ll A die 86 movable in relation to d.ie 80 is shown having an extension, namely guide element 88, compa.rable in shape and purpose to extension 82. The die 86 and guide element 88 move in the direction of the arrows shown in Figure 7 when the screw 84 is being thread-rolled and the threads thereon are being formed.
An electrode 90 oE the description given above is posi-tioned in axial parallelism with the screw 84 by a suitable support 92. The electrode 90 is electrically connected to the power source, not shown, by a connecting cable 94.
Electrode 90 is shown having a slanted tip 96 which, during the travel of the screw 84, will come into cngagement with the selE-tapping thread-forming type pilot end 98 of the screw 84, when the screw comes into axial alignment with -the electrode. If the pilot end 9B had axial grooves across the thread, not shown in Figures 7 and 8, the cutting edges and the crests of the tip end 98 of the screw 84 will receive, in these limited surface areas, a deposit of a hard substance, such as tungsten carbide or titanium ca.rbide, transferred immediately after the thread-rolling process in a single pass, and thus in a manner which is econom.ical and efEicient, because the process is conducted at very high speeds. When the screw 84 is rollecl in the direction of the arrows in Figure 7, the extensions 82, 88 operate as guide elements, as they cause the roll-formed screw 84 to continue its rolling motion for a short distance beyond the thread-rolling dies 80, 86. The self-tapping pilot end 9~ of the screw 84 will then be brought into close proximity to the slanted tip 96 of the carbide electrode 90. The electrode 90 may be triggered into energization by an actuating device 100, and l~e caused to vibrate, thereby to come into rapicl and repeated con~
tact with the pilot end 98. Actuating device 100 may be a switch DA-ll or a sensor, operated by the screw as it leaves the space between the dies, and electrically connected to cable 94 so as to cause energization of the electrode 90 for selected, predetermlnecl periods sufficient to ensure that electrodle 90 will be operating when the screw 8~ is rolling through successive positions of axial alignment with the electrode 90.
It is of interest to note that vertical electrodes, as shown in Figures 7 and 8, and horizontal or angled electrodes, as shown in ~igures 3, 4, 5 and 6, may be combined in the same equipment composed of both a thread-rolling machine and an electrode-deposition arrangement, so that the threads at the pilot end, as well as threads in other or all areas of the shank, oE the screw will be subjected to the treatment described. Such dual deposition treatment could be caused to occur simultaneously or seriatim.
Self~tapping screws of the thread-forming, thread-cutting or self-drilling type, when produced as described herein, are particularly useful when made of aluminum, other non-ferrous metals or their alloys. The carbide deposit on the crests of the thread, or threads, of the selE-tapping screws, particularly a-t the pilot end, allows an aluminum or other non-ferrous snetal screw to be used in the assembly of parts made Erom light-gauge steel and aluminum sheets.
It will have become clear that, in accordance with the combined technology described herein, the hardening deposit 26 is ~ade only in those surface areas of a self tapping screw which will displace material into which the screw will be caused to tap itself. The result of this combination of technologies is a screw having a relatively ductile core and relatively hard surface areas of the crest, or crests, of its thread, with the specific 1091965 DA-ll feature that the relatively ductile core extends -to, and is defined by, surfaces pertalning to grooves constituting the root of the thread between relatively hard crest areas, Reviewing the present invention, it may be noted that a new technique for economically producing high strength self-tapping screws has been developed~ The body of such screws may be made. of tough, inexpensive low-carbon steels or of non-ferrous metals, and are not subject to the embrit-tlement or other adverse effects of conventional heat treatments for surface hardening. The hardening deposit, or deposi-ts, may be placed on two, three or more turns on the lead, i,e, forward, threads of the self-tapping screws, thus providing self-tapping capabilities without impai.ring the basic toughness of the body of the screw, Also, the efficiency of self-locking features provided by 5 resiliently bendable ribs of the threads of screws is preserved, This application is a divisional of Canadian Patent Application 196,237 filed March 28, 1974.
a~
Claims (18)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A self-tapping screw having a relatively ductile core and carbon-containing material substantially harder than said relatively ductile core deposited on to the crest, or crests, of its thread in at least the area of the pilot end of the screws, the relatively ductile core extending to and being defined by surfaces pertain-ing to grooves constituting the root of the thread be-tween carbon-containing relatively hard crest areas.
2. Self-tapping screw according to claim 1 wherein said deposited material includes discrete, carbon-contain-ing surface areas of the crest, or crests, disposed along the circumference of a turn, or turns, of the thread.
3. Self-tapping screw according to-claim 1, wherein said deposited material includes a carbon-containing sur-face area of the crest, or crests, along the entire cir-cumference of a turn, or turns, of the thread.
4. Self-tapping screw according to claim 1 or claim 2 wherein said deposited material is provided on cutting edges of axial notches in the thread.
5. Self-tapping screw according to claim 1, claim 2 or claim 3, wherein the core is made of low-carbon steel or aluminum.
6. Self-tapping screw according to claim 1, claim 2 or claim 3, wherein said deposited material is formed by a deposit of titanium carbide or tungsten carbide.
7. A self-tapping threaded fastener comprising:
(a) a shank including driving means at a first end thereof, and self-tapping means at a second end there-of, said shank being formed of a relatively ductile material;
(b) said shank including at least one helically wound thread each having a plurality of helical turns formed about said shank for at least a portion of the length of said shank, said self-tapping means including a continuation of said threads, each said thread including at least one crest; and (c) a substance substantially harder than said rela-tively ductile material, deposited on and into selected portions of only said thread crest which defines at least a portion of said self-tapping means, said selected portions located along substantially at least one full helical turn length said substance impregnating the material of said fastener.
(a) a shank including driving means at a first end thereof, and self-tapping means at a second end there-of, said shank being formed of a relatively ductile material;
(b) said shank including at least one helically wound thread each having a plurality of helical turns formed about said shank for at least a portion of the length of said shank, said self-tapping means including a continuation of said threads, each said thread including at least one crest; and (c) a substance substantially harder than said rela-tively ductile material, deposited on and into selected portions of only said thread crest which defines at least a portion of said self-tapping means, said selected portions located along substantially at least one full helical turn length said substance impregnating the material of said fastener.
8. A fastener as claimed in Claim 7 wherein said self-tapping means includes cutting edges comprising said impregnat-ing substance.
9. A fastener as claimed in Claim 7 wherein said im-pregnating substance is taken from the group consisting of titanium carbide and tungsten carbide.
10. A fastener as claimed in Claim 7 wherein said im-pregnating substance is impregnated intermittently along the length of said crest beginning at said second end of said fastener for at least three helical turns of said thread.
11. A fastener as claimed in Claim 7 wherein said im-pregnating substance is impregnated on said crest continuously beginning at said second end for at least three helical turns of said thread.
12. A fastener as claimed in Claim 7 wherein said im-pregnating substance is impregnated on said crest at a maximum outside diameter of said thread.
13. A fastener as claimed in Claim 7 wherein at least a portion of said thread not at said second end comprises deform-able locking means.
14. A fastener as claimed in Claim 13 wherein said lock-ing means are resilient.
15. A fastener as claimed in Claim 14 wherein said resil-ient locking means further comprises dual crests on said at least one thread.
16. A fastener as defined in Claim 7 wherein the body of said fastener is made of a relatively tough, soft material.
17. A fastener as defined in Claim 16 wherein the body of said fastener is made of low-carbon steel.
18. A fastener as defined in Claim 16 wherein the body of said fastener is made of nonferrous metal alloys.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA280,378A CA1091965A (en) | 1973-04-23 | 1977-06-13 | Self-tapping screws |
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US353449A US3894570A (en) | 1973-04-23 | 1973-04-23 | Self-tapping fastener |
| CA196,237A CA1022703A (en) | 1973-04-23 | 1974-03-28 | Self-tapping screws |
| CA280,378A CA1091965A (en) | 1973-04-23 | 1977-06-13 | Self-tapping screws |
| US353,449 | 1989-05-18 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1091965A true CA1091965A (en) | 1980-12-23 |
Family
ID=27163398
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA280,378A Expired CA1091965A (en) | 1973-04-23 | 1977-06-13 | Self-tapping screws |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1091965A (en) |
-
1977
- 1977-06-13 CA CA280,378A patent/CA1091965A/en not_active Expired
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