US2556174A - Screw-thread swaging tool - Google Patents

Description

June 12, 1951 EVANS 2,556,174

SCREW THREAD SWAGING TOOL Original Filed Oct. 9, 1943 2 Sheets-Sheet .1

Edwin R. Evans June 12, 1951 Patented June 12, 1951 2,556,174. SCREW-THREAD SWAGINLG: T001 Edwin E. Evans, Orchard Lake Village, Mich, assignor to LockTh-read Corporation, a corporation of Delaware Original application October 9, 1943, Serial No. 504,981. Divided and this application August 13, 1945, Serial No. 610,619

5 Claims.

This invention relates to metallic screw threads and particularly to full or partial production of such threads by a cold working tool, as distinguished from cutting, grinding or molding. The present application is a division of my pending application Serial Number 504,981,, filed October 4, 1943, now abandoned.

Heretoiore there has been considerable use of. cold working in production of external metallic threads but the rolling methods employed have lacked practical application to production of in ternal threads except in the case of thin shells capable of being given a crude internal thread form by external pressure. Internal threads have heretofore been produced almost entirely by cutting processes such as tapping and chasing.

An object of the invention is to produce aninternal thread in metal by a novel cold, working tool securing a reinforcing effect and avoiding any production of scrap.

Another object is to provide for strengthening and accurately finishing an internal meta-llic thread by a cold working tool, resulting in: a desirable initial compression.

Another object is to form an internal screw thread in a metallic body by progression from one. end of such thread as is commonly done by taps, but employing a metal-displacing, tool rather than a cutting action, resulting in; a materially stronger thread than results from. cutting.

Another object is to provide an improved thread-forming tool suited to be progressivelyand rotatively' fed into an opening to form a cold: worked screw thread. or to. finish by cold working a thread previously formed in said opening.

These and various other objects are attained. by the construction hereinafter described and illustrated in the accompanying drawings,.

wherein;

Fig. 1 is a view in side elevation. of arr-internal thread forming tool constituting one: of various:

AmericanNational Dardelet threads.

provisions for practicing my improved method of cold working, the illustrated tool being particularly suited to form a thread of American National type.

Fig. 2 is an end View of said tool.

Fig; 3 is a relatively large view in side elevation of the tool for cold working threads, as: designed particularly to produce a thread? known as the American National Dardelet.

Fig. 4' is a cross sectional view taken on the line 44 of Fig. 3.

Fig. 5- is an enlarged axial sectional View of the tool shown in Figs. 3' and 4, showing how it dis"- places metal in finishing a previously tapped thread.

Fig. 6 is a view in side elevation and partial section of a bolt and nut, having interen aging American National Dardel'et threads", showing how these threads lend themselves to a predeterminedly tightv or looking fit of thenut on the bolt.

Fig. 8 is an. end View of a nut illustrating still. another provision for looking it on a bolt.

Considering the drawings now in greater detail,

I and describing first. the tool shown in. Figs. 1

and 2, the reference character I designates a substantially cylindrical body of tool steel having, a thread 2' formed thereon. by cutting, rolling or other suitable method. Such thread is the externalv complement of an A. N. internal thread which the tool is designed to form and the crest or major diameter of the thread 2 is reduced: in approaching the leading end of; the tool, as in-- dicated at 3, so as to facilitate. starting the tool in an opening to be either fully or finish threaded. Fig. 5 shows a member 4' formed" with such: an opening. Interrupting the thread 2' in all of: its convo-lutions are a plurality" of flutes 5', illustrated as: parallel to the tool axis, although such. paralleiismi is not essential. Thus said flutes divide the thread into sections and. the edges of the" end. faces of such. sections. are rounded so as to deprive them of any cutting effect. Furthermore, it ispreferred to quite acutely converge the endfaces of; the sections separatedby each flute, so: that tltese faces formv ramps graduating the pressure effect of the sections upon a wall to be threaded. The described tool is provided with a suitableshank 6 having a squared end 1' Or otherwise. adaptetd for'transmitting a rotary drive from a socket member (not shown) receiving such shank.

An A. N. D. thread, such as is indicated at 8 in. Fig: 5, has its flanks convergent at an angle of sixty degrees as is A. N. practice and: follows Dard'el'et practice in employing a wide root inclined at approximately six degreesto the thread axls.

Figs. 3' and. 4 illustrate a tool for producing A. N. D. threads by cold working, similar in prin ciple to that: shown in Figs. 1 and 2; Thus a cylindrical body 9 is formed. with a helical thread I18 interrupted and divided into sections by flutes H paralleling the axis of said body. The edgesof. the end. faces of said sections are rounded to deprive them. of any cutting effect and the root faces ['2 of the sections have asix degree inclina tion to the axis of. the tool; The thread sections are setback some. distance from the flutes, and from the ends of each section, shallow ramps illa lead to-the. flutes. Also the root faces 42 have ramp-forming: leading portions is extending to the flutes, between: thev ramps lilo. The ramps War and. i3 conform to the six'degree inclination ofv the. root faces and. further have a slight arcuate: convergency toward. the: tool. axis, in extending to the" flutes. This convergency is imparted preferably by letting the ramps form arcs of much less radius. 1', than. theminimum radius of the thread It]. Theramps I 3 act. upon any metal 3 of the work which has been unduly displaced toward the axis of the tool, ironing such material down to the proper crest dimension of the thread to be formed. As in the first described form of the tool, the A. N. D. thread forming tool has its leading end chamfered as regards the crest diam- 'eter of its thread Ill to facilitate starting the tool in an opening.

Fig. 6 discloses a cap screw I4 provided with an A. N. D. thread 15 having constant dimensions throughout the thread length. Engaging this screw is a nut 16, whereof the thread I! is constant as regards its major or root diameter, while its crest diameter is progressively reduced at a very gradual taper from the leading to the trailing end of the nut. Such a construction permits the convolution or convolutions of the nut thread nearest its trailing end to predeterminedly interfere with the bolt thread, resulting in a cold working of said convolution or convolutions when the nut is first applied. The efiect of such cold working may be so pronounced as to reduce the effort necessary to apply the nut by substantially one half, following an initial application and removal of the nut. The advantage of tapering the nut thread as described and thus obtaining a relatively tight interengagement of the two threads near the trailing end of the nut lies in the fact that working stresses acting transversely to the threads toward the trailing end of the nut, are thus initially primarily taken by convolutions adjacent said end and then progressively picked up by the remaining nut threads. This distribution of stresses materially increases the resistance of the bolt to fatigue failure. It is a matter of common engineering knowledge that the working stresses imposed on a common bolt engaged by a nut are very largely concentrated on the convolution or convolutions nearest the leading end of the nut, a condition quite undesirable from the standpoint of resistance to fatigue. Shearing strength of the nut is also increased by tapering its thread, as described.

To impart the described taper to the crest of the nut thread the opening of the nut may be reamed to such taper either prior or subsequent to tapping, this being obviously a quite simple operation. In present A. N. practice, any such tapering effect is obtainable only by employment of a tapered A. N. tap the production of which is quite complex and expensive, and which requires unusual skill in handling.

Fig. '7 illustrates another provision for securing the advantages described in connection with Fig. 6. The nut is of Fig. '7 is step reamed, either before or after tapping so that the crest diameter of its thread H9 in the portion X adjacent to the trailing end of the nut is slightly less than the crest diameter of its remaining portion Y. This achieves a predetermined interference adjacent to the trailing end only. Working stresses therefore initially take effect on the trailing end portion of the nut and such forces, as they increase, are resisted also by the remaining convolutions of the nut thread.

Fig. 8 illustrates a nut 26 threaded upon a bolt 2i, the opening of the nut being broached, either before or after an initial forming of its thread 22, to a slightly elliptical form. This has the eifect of producing two rows of maximum interference areas at opposite sides of the nut. Working stresses initially take effect at these maximum interference areas and then are distributed to the remaining portions of the thread, thus resisting fatigue, to a degree.

- will be imposed in use.

As regards each of the constructions illustrated in Figs. 6, 7, and 8, it is to be noted that the assembly of the parts serves to cold work the internal thread, compressing the metal of the latter particularly at the thread root, whereby the thread acquires an increased resistance to wear and resistance of the nut to splitting is increased. It is preferred, as regards these constructions, to cadmium plate and possibly also cyanide harden the bolt thread to prevent picking up or galling of the bolt metal where subject to the maximum interference. Such hardening, however, is usually superfluous in the case of cap screws, since these are generally threaded into cast iron or other relatively soft metal. Cap screws may be rendered self locking when desired by cold working the metal in a non-tapered opening, as by such tools as are illustrated in Figs. 1-5.

The A. N. D. thread is a somewhat recent development, being disclosed in my United States Patent 2,437,638, issued March 9, 1948. Primary advantages of this thread form include its reduction of thread depth as compared to prior types of threads, without sacrifice of strength, its adaptability to heavy interferences without resulting mutilation, its adaptability as an external thread to intermesh with the A. N. thread of an opening, and its special adaptability to be formed by rolling or other cold working methods.

In using the tool shown in Figs. 1 and 2 for finishing an A. N. tapped thread to burnish such thread and protect it against the tearing action of a companion bolt, cap screw, or stud, said tool applies such pressure to the crest and flanks of the thread as to induce a compression at the root adequate to fully resist the maximum force that The force referred to is a tearing effect deriving as a working strain from any two interengaged V-shaped threads such as A. N. threads. In using the tool of Figs.-l and 2 for reconditioning a badly worn A. N. thread, the

root diameter of the thread 2 is slightly increased as compared to such thread when used for finishing new threads, so that a somewhat greater pressure will be applied to the worn thread to induce compression at its root.

Finishing or reconditioning an A. N. tapped thread to receive an A. N. D. external thread is accomplished by the tool illustrated in Figs. 3, 4, and 5 and involves a materially greater flattening of the thread crest than is necessary when the tapped thread is intended to receive an A. N. external thread. Thus, as best appears in Fig. 5, the tapped thread must be reduced materially in depth so that its crest acquires the 6 degree taper characterizing an A. N. D. thread, acquiring also a width four or five times greater than its original width.

While tools have been exemplified for the cold working of A. N. and A. N. D. threads, it is to be understood that my improved method and tools can be suited to all thread forms, although most advantageous as applied to threads which by reason of a V shape exert a splitting effect on the internally threaded member, either under assembly or working stresses. In this connection it is to be noted, as an accepted engineering fact that metal cannot fail under tension when subjected to a compression involving a force exceeding the applied tension force. Since the splitting action exerted by an external thread on the com panion internal thread involves a tensional force,

and this force takes effect at the root of the internal thread, it follows that a sufiicient compres-.

sion of the metal at such root will eliminate the splitting action as a factor controlling the design of internally tapped members. In many instances such elimination will permit a material saving of metal and weight reduction in such design.

Since the feasible number of connections and separations of parts having screw threaded interengagement is far greater for cold worked threads than for tapped threads, it follows that the present improvement tends to greatly prolong the useful life of threadediy connected parts requiring occasional separation. There is a considerable and increasing demand for elimination of weight in modern machinery, particularly in the aircraft industry, and the present invention Will assist materially in meeting this demand by imposing a hardness on internally threaded parts permitting a dimensional reduction.

In using the tools exemplified in Figs. 1-5 for completely producing threads, it may be desirable in the range of larger threads to use a series of two or more such tools, progressively increased in size, to avoid imposing excessive work on a single tool.

It may be noted that in forming screw threads in holes accurately cored to a proper size, as is especially feasible in aluminum and other metals and alloys considerably softer than iron and steel, cold working of the threads by my improved tools may be accomplished without preliminary drilling or tapping.

A. N. D. threads lend themselves more advantageously to cold working at assembly than A. N. threads, since the metal is not readily forced to the root of an A. N. thread and even under conditions of heavy interference it is rarely found that root voids of A. N. threads are completely filled. Pressure applied to the relatively wide crest of an A. N. D. thread is better distributed than in case of an A. N. thread, with the result that the root voids may be readily completely filled, when desired.

It is evident that for the purpose of imparting a slight taper to an internal thread, such as that of the nut shown in Fig. 6, there may be used such tools as Figs. 1-5 illustrate modified by such taper as is desired. A cold working threadforming tool is better adapted than a cutting tool to produce a slightly tapered thread for reason that a cut taper thread is likely to have burrs therein which the burnishing action of a cold working tool does not permit.

It is essential to understand that all edges of the cold working thread-forming tools are to be rounded to an extent that will eliminate any cutting action. In Figs. 1-5 it has not been attempted to disclose all such edges so rounded, since this would prohibit any clear showing of such edges.

The feature exemplified in Fig. 6 of taper reaming an internally threaded member is of particular value in adapting the A. N. D. thread form to standard pipe thread practice. Pipe threads, as standardized, have a material taper inch to the foot) and fittings or the like for engagement by such threads have a corresponding taper. Ihus a joint established by standard pipe threads exhibits a substantially uniform tightness throughout the interengaged threads and frequently offers excessive resistance to unscrewing. Corrosion often adds considerably to origina1 tightness of such a joint. By providing a pipe with an A. N. D. thread, as per Fig. 6,

having either no taper other than that incident to this thread form or having a taper predeterminedly less than the standardized taper of the female thread, a fluid sealing interference of the two threads will be assured for the convolutions adjacent to the pipe end, and the tightness of the connection will be progressively reduced as the threads recede from such end. Such a connection will be liquid and gas sealed without need of a sealing compound but will be separable without danger of breakage. In installations subject to periodic vibration such a joint will be especially valuable, since fatigue is an important consideration in such installations. The improved joint will strongly resist fatigue, periodic stresses and strains taking initial effect at the interfering convolutions of the threads and being then progressively distributed to the remaining convolutions, so as to avoid concentration.

A further advantage of A. N. D. threads, as applied to pipe is that their weakening effect is materially less than that of A. N. threads, since their depth is materially less than that of A. N. threads.

What I claim is:

1. A tool for forming an internal screw thread comprising a substantially cylindrical body formed with a helical thread of a cross sectional contour determined by the thread to be formed, said body having at least one groove interrupting at least a majority of the convolutions of the thread and dividingthe thread into sections having the edges of their rotatively leading ends blunted to effect a cold working of metal to a desired thread form, ramps being formed by the leading portions of the root faces of the thread, and the radius of said ramps as measured from the axis of the tool being progressively reduced as they recede from such ends.

2. A tool as set forth in claim 1, said leading ends being set back from the groove and said ramps extending substantially to the groove from said ends.

3. A tool for forming an internal screw thread as set forth in claim 1, said ramps being inclined to the axis of the tool, in the axial direction.

4. A tool as set forth in claim 1, said leading ends being relieved, between said ramps, to accommodate metal displaced by the ramps.

5. A tool for forming an internal screw thread comprising a substantially cylindrical body formed with a helical thread of a cross sectional contour determined by the thread to be formed, said thread being interrupted at intervals and thus divided into sections having the edges of their rotatively leading ends blunted to effect a cold working of metal to a desired thread form, ramps being formed by the leading portions of the root faces of the thread, and the radius of said ramps as measured from the axis of the tool being progressively reduced as they recede from such ends.

EDWIN R. EVANS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,676,482 De Lapotterie July 10, 1928 1,912,517 Lapotterie June 6, 1933 2,029,514 Thomson Feb. 4, 1936

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