CA1054315A - Die for making thread-forming fastener - Google Patents

Abstract

DIE FOR MAKING THREAD-FORMING FASTENER

Abstract of the Disclosure Thread-rolling dies for forming a lobular fastener have die sections with grooves of equal depth to roll a pilot thread on the fastener. The die sections have grooves of equal depth and with the groove depth being less than the depth of the grooves in the adjacent die section whereat a tapered thread-swaging thread is formed on the fastener.

Description

1~)59~3~5 Back~round of ~he Invention " _ This invention relates to improvements in thread-rolling dies usecl i~or making thread-Eorming fasteners.
One type of thread-forming screw which has been commercially successful in many parts of the world is that of the type shown in the United Sta-tes patent to Phipard ;
; 3,195~196 that issued July 20, 1965. The screw shown in that patent is of a type having a lobular pitch surface cross section in the form of a generally arcuate polygon with an odd number of arcuate sides merging gradually with inter-mediate thread-swaging lobes. The work-entering end of the screw is tapered toward the work-entering point of the screw .. . . .
for insertion into the workpiece pilot hole. Preferably the ~;
screw has three lobes. A screw of the foregoing type has a ;
generally uniform lobulation throughout its threaded length. ~
Since the work-entering end forms the thread and the holding -or shank section subsequently mates with the thread in the workpiece, the thread must have a lobula~ion that provides a low driving torque for thread formation and yet a high locking ~ 20 ability for engagement with the workpiece. Consequently, the amount of lobulation in the screw is necessarily a compromise to effect the best possible combination of low-driving torque , and good locking ability. Generally, the screws are designed , to favor thread-forming.
It has also been proposed to produce thread-forming fasteners with a lobular work-entering portion and a circular

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holding section, Typical of such screws are those shown in United States patents to Phipard 3,246,556 that issued April19, 1966 and to Muenchinger 3,681,963 that issued August 8, 1972, ~lowever, where the holding section is of circular cross section there is a reduction of stripping torque and a reduction in the effective locking action with the workpiece as compared to the lobular form, Because the holding section is of circular cross section there are no lobes or like areas that tend to bite into the workpiece to ~ ;
I 10 effect a locking action and an increase in both strip torque ¦ and locking action, ¦ Another problem in self-tapping fasteners resides ¦ in the difficulty in aligning the fastener with the hole in I the workpiece, particularly in workpieces of substantial I thickness, When a self-tapping fastener is introduced into -¦ the pilot hole of a workpiece, the fastener tends to have its central axis lie at an angle to the central axis of the hole of the workpiece. The reason for this lies in the fac~ that ~:
the tapering lead thread of the screw tends to trace a spiral path with the result that in any cross section there is sub-stantially no more than one point from the ~hread whose ;;-~
` distance from the axis is one half the pilot hole diameter.
Thus, in theory the screw axis at the ~tarting may have an angularity as much as the angle o~ the taper of the thread.
Such angularity or "cocking" of the screw is objectionable in that there is difficulty in starting the screw in a threading operation without undue end pressure. Also the screw may end up "cocked" when Eully threaded into the workpiece,

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, ". ~ , . .,. , , ~L0543~5 Ob~ects and Summary of the Inventlon An object of this invention is to provide dies for makLng a thread-forming fastening device in which the tapered work-entering section is of a substantial lob-ulation to insure ease oE thread formation whereas the : `
holding section of the screw is of a considerably lesser lobulation, namely only a very moderate out of rbund to the extent necessary to insure an adequate prevailing torque ~: :
and locking ability of the screw. This is in contrast to ~.
10 present day lobular thread-forming screws wherein the ~ ~
lobulation tends to favor thread-formation rather than ~ :
holding power. - :
A further object of this invention is to provide dies ' .! .
for making a thread-forming fastener with dual lobulation . -of the type and for the purpose stated in which the relative ~ :~
~ . .
.' lobulation of the work-entering section and the work hold- .` -. , .
ing section is defined within certain empirical limits which will result in a screw that is more effective for,.~ . .
both threading and holding than has been possible with~ ~:
-` 20 screws of the types referred to previously herein.
Another object o t'nis invention is to prov-ide dies for making a self-threading fastener of the type s~ated ~ .
in which there is a pilot thread of about one turn which is used to prevent cocking of the fastener as it is in-troduced into the pilot hole of the workpiece and also to :~
reduce the amount of end pressure needed to start the screw. :

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This pilot thread has a crest cross section of subs~an-tially uniform width throughont 36Q~ and has a maximum crest diameter that is less than the maximum crest diameter of thread in the work-holding section. The uniform width oE the pilot thread is slightly less than the diameter of the pilot hole in the workpiece.
A still further object of this invention is to provide a die pair in which each die of the pair having thread-forming ridges for rolling a thread on the holding section and the tapered thread formation on the work-entering section~ and a further section having ridges and grooves of equal depth to form the pilot thread.
In accordance with the foregoing objects there ~;~
- : - .
is provided a thread-r`olling die for fo~ming a self-thread-ing fastener having a holding section and a work-entering ` `~ ~`
section, said work-entering section having a tapered thread- - ~
swaging thread formation and a non-tapered pilot thread ~ `` `
formation between the larger end of said thread-swaging for-mation and an adjacent tip of the fastener, said pilot ~hread formation being of uniform width throughout 360 and extending ; -axially a~ least approximately one thread turn; said die com-prising first and second adjacent longitudinally extending sections of thread'forming ridges for rolling a thread on said holding section and for rolling the thread on the work~
entering section, said second section having truncated ridges and grooves of varying depth to form the taper on said thread- ~ `
swaging formation, and said die having a third section with -~
grooves of equal depth to form said pilot thread, the depth ~ID5fl~3~S
of said last-mentioned grooves keing less than the depth of the rldjacent grooves in said second section, said third section including at least one groove of a depth less than said grooves of ec~ual clepth.
The die may also include a ramp extendiny over at least a part of one of said sections to fonm a thread with ; a decreasing cross-sectional width of root.
The die further may include a fourth section having ,. a flat surface, the fla~ surfa oe being in a plane that coincides with the truncations on the ridges in said second and third sections.
For further clarity, the screw that is made by the dies is described in detail hereirl.
E~ief Descri~tion of the Figures ` Figure 1 is a side elevational view of a screw blarik used with the dies of the invention;
Figure 2 is a front elevational view of the blank of Figure l; ;~
Figure 3 is a sectional view taken along line 3-3 of Figure l;
~ Figure 4 is an enl æ ged fragmentary side elevational : view of a screw that is roll-threr~ded on the dies of Figures `~
., .
9-10, utilizing the blank of Fig~res 1-3, and showing the screw inserted into a w~rkpiece (shown in section) preparatory to the threading operation:

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~ 6 `; ~ ' ~0543~5 Figures 5, 6 and 7 are fragmentary sectional views .
: taken approxlmately along lines 5-5, 6-6 and 7-7 of Figure 4;
Figure 8 is a graph indicating the ranges of lobu-lati.on for the work-entering section and the holding section of the screw of this invention; ~ :
Figure 8a is a diagrammatic view of a lobular thread cross section, to be used with Figure 8 to determine out of roundness;
Figure 8b is a diagram relating to the analysis of . minimum out of round of the holding section;
` Figure 9 is a fragmentary perspective view, partially in section, and showing a pair of dies rolling a blank there-~ between to form the screw of this invention;
Figure lO is a transverse sectional view through the :
dies of Figures 9 and 10 but with the screw being shown in ~`~ elevation; `~
. Figure 11 is a sectional view similar to Figure 10 `~ and showing a modified form of die construction; and Figure 12 is a fragmentary side elevational view -~
showing an enlarged scale the screw of Figure 11 inserted into `~
a workpiece preparatory to the threading operation. ~ -. Detailed Description -:
:- ~
Referring now in more detail to the drawings there ^ ~ -. is shown in Figures 1 - 3 a screw blank 2 having at one end :~ thereof a hexagonal driving head 4 and adjacent flange washer 5. .

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~ 0S 43 The blank also comprises a shank sect;on 8 oE lobular, arcuate triangular cross section and an intermediate tapered transi-tional portion 10 also of lobular, arcuate triangular cross section ~lowever, the extent of lobulation, namely the amount oE out of round, of the tapered transitional portion 10 is greater than the extent of lobulation in the section 8, as may best be seen from Figure 2 Forwardly of the tapered transi- :
tional portion the blank has a lead section 12 also of lobular arcuate triangular cross section and preferably the same as that in the portion 10. As will be seen hereafter, when the blank 2 is passed between the thread rolling dies o Figures 9 and 10, the thread formation on the holding section B (Figure 4) will be rolled out of the metal of the section 8 whereas the thread formation on the work-entering section A (Figure 4) : ~ .
will be rolled on the metal on the tapered transitional portion . ~
10 and lead section 12. ~ ~;

-. The lobular cross section of the blank section 8 is defined by the lobes 14,14,14 which merge with arcuate sides .

16,16,16 having longer radii of curvature than that of the ` 20 lobes 14,14,14. Likewise, the transitional portion 10 has - lobes 18,18,18 and intermediate sides 20,20,20, the latter `' having longer radii of curvature than do the arcuate sides 16.

The respective arcuate sides and lobes are all symmetrically -: arranged about the longitudinal axis 22 of the blank 2. The lobes 18 form respPctive axial continuativn~ of the lobes 14, ~ although the lobes 18 are more sharply defined than are the lobes 14. The transverse width of the blank taken through ~, ,.
~ ~8~

~ 43~

the axis 22 is uniform throughout 360. An important aspect of this invention9 however, lies in the fact that the cross-; sectional configuration of the section 8 is not circular but ~ ;
is lobular, but with a lesser amount of lobulation than that which is in the transitional portion 10 or the lead sections 12, The preerr~d relationship between the lobulation in the section 8 and that in the portion 10 and section 12 will depend upon the desired relative lobulation in the finished screw between the work-entering section and the holding section.
These preferred relative amounts of lobulation have been analyt-ically determined, and will be hereinafter more fully described. ~
Figures 4 - 7 show the screw 13 that is formed by ~ ~ -rolling the blank of Figures 1 - 3 in the dies of Figures 9 -10. More particularly, the screw 13 comprises a work-holding zone or section B and a work-entering zone or section A as .. ,~ . ~
shown in Figure 4. It will be seen that the section A runs from thè beginning of the thread, namely at or near the work-- ., :- ..
~ entering tip 25 to the maximum diameter part 15 of the tapered : .
thread formed in the work-entering section A. The work-holding :..
section B extends from the maximum diameter part 15 of the work-entering section A for a predetermined length, usually the balance of the thread. This generally runs substantially ~, to the washer 6. The juncture of the zones A and B is not -abrupt but preEerably the zones A and B gradually and smoothly merge with one another.
The thread formation in the work-entering section A

is of the usual profile having a root 28, a pitch 30 and a .
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~ 3 ~ 5 crest 32. The pitch line is indicated by bro~en line 30.
Furthermore, the root, pitch, and crest cross sections are esch in the form of a generally arcuate polygon with an odd number of arcuate sides merging gradually with intermediate arcuate lobes. More specifically, and as is preferred, the arcuate polygon is a triangular one. Fur~hermore, the taper-ing of the thread formation on the work-entering section A
results from the fact that the crest cross section progres- ;~
sively diminishes in width, the taper being toward the tip 26. :~
Consequently, and as best seen in Figure 6, the work-entering section A may be said to have thread-swaging lobes 34,34,34 joined by arcuate sides 36,36,36 gradually merging with the lobes 34,34,34. The crest cross-section, the root cross section, or the pitch cross section, as the case may be, will ~: :
each be a lobular, arcuate, triangular shape similar to that shown in Figure 6. ~
The thread formation in the work-entering section A ~ ~.
continues into the work-holding section B wherein the pitch 30 ~
of the thread as well as the root 28 are the same as in the `~ ; -section A. The crest 38 is not tapered but is of uniform .: ~;
width throughout 360. By uniform width it is meant that the distance 4etween any two parallel planes tangent to the crest will be` uniorm or constant regardless of the orientation of ~ :~
, .
those planes. Preferably also, this uniform width throughout ~ ~ :

1 360 is also true of the pitch and root. In any event and as ~ ~

¦ best sèen in Figures 6 and 7, the holding sec~ion B has a crest ~ ~-! cross section with lobes 40,40,40 that merge with intermediate .¦ sides 42,42,42.

, ~ .

.,. . ~ . - .. ~ ., . . , -1~5~3~5 As will best be seen by a comparison of Figures 6 and 7, the amount or extent of lobulation of the holding section B is considerably less than that of the work-entering section A. The moderate amount of lobulation in the holding sectlon L3 will result in some elastic deflection of the lobular portions 40 due to stress concentrations therein. Consequently, an improved locking action of the holding section B thread with the thread formed in the workpiece P will be provided. ;
Also on the work-entering section A intermediate -~
the tapered thread formation and the tip 26 is a pilot thread 44 which may be a thread formation of one or more turns, as -desired. The pilot thread 44 merges with the tapered thread formation of the zone A; however, ~he pilot thread 44 is not tapered but is of uni~orm width throughout 360. This is best shown in Figure 5, which illustrates the pilot thread as having a crest dlameter approximately the same as the diameter ;
of the hole 46 in the workpiece. The one or two turns of pilot thread 44 result in the alignment of the screw aXis 22 approximately with the longitudinal axis or center iine of 20 the workpiece hole 46, thereby to prevent cocking of the screw ~ ;
during beginning of the threading operation. The pilot thread ~ 44 also reduces the amount of end pressure required to start ;;~ the threading operation. The crest diameter of the pilot thread 44 is preferably the same as or slightly less than the pitch diameter 30 for most general applications. -Figures 8 and 8a show the manner in which the pre~
- ferred amounts of lobulation in the sections A and B can be ~;
~. ' ' ' , " `';`:
-11- ,.

1~35~S
I determined. Fig. 8a shows a ~alue K, which is a measure of out of roundness or lobulation. Thus, the value of K or lobulation may be defined as the maxlmum distance from the I crests of the thread at ~n arcuate side 36 or 42, as the case I may be, to the clrcle 48 that inscribes the crest oE the thread ¦ as shown in Figure 8a. This value of K is plotted as a function of the diameter of the stress area, abbreviated as DSA. For a screw of circular cross section (cres~ 9 root and pitch) the stress area is the area of the circle whose diameter is the arithmetic mean of the pitch diameter and the root diameter ¦ of the thread. For a thread of lobular cross section the definition is the same except that the root diameter is measured at the point of maximum root diameter at a lobe in the holding section B.
The straight or substantially straight lines 50 and ` 52 of Figure 8 (not to scale) were determined by computation of the stress area (and hence its diameter) for various known ' lobular arcuate triangular screws, such as United States -; National Fine and National Coarse screw series in addition to 20 various standard metric sizes. A~scattering of points (not ~ -~
shown) based on out of round produced the lines 50, 52 which were approximately at the boundary of the spread of polnts ~ ~
plotted. In each instance the lobulation was uniform through- ~ ;
out. Consequently, for each DSA ~alue the ordinate extending ` between plots 50 and 52 represents the range of maximum and minimum lobulation for the work-entering section A. That ~` same ordinate line extended downardly to cut across the plots ':

~5 ~3 ~5 S4, 56 will produce the maximum and minimum lobulation for the holding section B for that particular screw, Like lines -50,52, it is assumed that plots 54 and 56 are also substan-tially straight ].ines.
It has been found that the slope o the line 50 is substantially 0,042 and the llne 50 intersects the zero line for DSA at a point 50a which is 0,5 millimeters, The line 52 intersects the zero point for DSA at 52a~ which is 0,04 milli-I meters and the slope of the line 52 is approximately 0,020.
Line 54 representing the maximum holding section lobulation has a slope of substantially 0,0176 and intersects the 0 line for DSA at 54a which is also 0,04 millimeters, Finally~ line 56 ha's a slope of 0,0025 and intersects the origin 56a of the graph of Figure 8, Thus, the work-entering section, the max;mum and minimum lobulations K texpressed in millimeters) are as ~ollows:
Maximum K _ 0.042 DSA ~ 0,05 Minimum K = 0,020 DSA f 0,04 The maximum and minimum lobulations K (expressed in millimeters~ for the holding section are as follows:
-~ ~Maximum K = 0.0176 DSA ~ 0.04 ~ Minimum K = 0,0025 DSA -~
`l The range of lobulation in the work-entering section may vary depending upon the material of the workpiece, In any ' event, the lobulation can be designed to favor low torque threading, On the other hand, the lobulation in the holding , ., - .

'.' '' ~:' ~ ~ .' ~ L0543~S
section need only be sufficient such that the lobes therein are defleeted so as to provide a locking action with the workpiece. For thin metals only a slight amount of lobulation may be neecled.
With urther regard to the minimum lobulation oE
the holding section B, the value K 0.0025 DSA is a practlcal minimum lobulation that can be fabricated and measured. The suitability of this K value may be verified by analogy with the contact stress between cylinders of different diameter, one rolling within the other, and having a load applied radi-ally outwardly through the center of the smaller circle. By this analogy it is believed that the radial force on the rol-ling cylinders is analogous to the radial component of force of the thread engaging its mating thread where elastic materials are involved. Referring to Figure 8b, the larger and smaller circles Cl, C2 have radii R and r respectively. The value of K represents the out of round of the lobular form and the load through the centers of the circles represented by P.
' The elastic deflection of the cylinders represented by circles ~ ;
`~ 20 Cl and C2 is assumed as the value K. If K = 0.0025 DSA, then K ~ .005R. The relationship of r and R is known for a given ;
K. Thus, for K = .005R, the value r = .98134R. Also it may ~` be readily derived from the known equation a2 _ K (2r -K) ; -that the contact zone a = .099R where a is the half chord of -the circle C2 shown in Figure 8b.
The average ~mit compressive stress, denoted as T, : may b~ derived by using the so-called Her~z equations and -14~

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3~0543~5 related material which are known and may, for example, be Eound in M. F. Spotts, Mechanical Desi~ Analysis, pp. 166-171, published in 1964 by Prentice Hall, Englewood, New Jersey, ll.S.A., especially the Eormulae at Figure 9.6 on p. 171.
Assume that the cylinclers Cl, C2 are of steel (modulus of elasticity El = E2 = 3 x 107 psi) and that the load P = T(2a). Using the known Hertz formula:

~ - 1.076 ~

It can be determined that T = 8936 psi or 61614 Kilo Pascals.
For a determination of maximunl compressive stress the known Hertz equation below may be used:

~ Max. Stress = 0.591 ~ (El ~ E2) (r ~ R) ~ ;

`; ;
Using the same values as above, it can be derived that the maximum stress is about 13,276 psi (91538 Kilo Pascals). In a thread the stress would actually be distributed over a smaller area per uni~ length of the screw which would tend to raise the ;
maximum stress. Nevertheless, even doubling this maximum `l stress is well within the elastic limits of the materials.
¦ The following represent test data (Metric and English) on the screw of the present invention compared with (1) stan-dard tri-lobular screws having uniform lobulation in the holding and thread-forming sections (designated Type I), and (2) screws having a lobular thread-forming section and circular holding sections (designated Type II). Specimen screws were ! tested in weld nuts of Rockwell B hardness of 82-84 having a ~S ~3 ~ ~

0,280 inch (7.11 mm) pilot hole diameter. Screws were nominally size 5/16 - 18 (M7.9 x 1.41). The value X in each case represents the arithmetic average of the data. For the Type I, screw K ~ 0.30 mm. For the type II screw, K - 0.46 mm.
maximum Ln the lead section. For the screw of the invention, K ~ 0.152 mm in the holding section B and K = 0.356 mm in the thread-forming section A. The DSA in each instance is approx-imately 6.56 mm.

DATA I
. _ 10 STA~llNr l!ND~ F~ESSURE:
INVENTION TYPE II
Pounds Newtons Pounds Newtons 7 31.1 7 31.1 6~ 28.9 12 53.4 ; 6 26.7 9 3/4` 43.4 8 35.6 7 31.1 ;
5~ 24.5 11~ 51.2 5~ 24.5 10 44.5 ~''' ' ' ` ; ~ '~.. '' X : 6.42 X = ~8.5 X = 9.54 X , 4~.5 .~ , " ~," '.

.
: ~ . . -:

10~4~5 DATA II

Maximum angularity (cocking) at start of driving operation:

INVRNTION TYPE II
2 140' 1~ 310' 1 2 5O5~ 3 3/4 2 215' 2~ 150' 1~ 2~ 2~ 3~

DATA III
~:
Maximum drive torque to fully thread test nut: -. . ..
INVENTION TYPE I TYPE II
LB-INS. NEWTON- LB-INS. NEWTON- LB-INS. NEWTON-~ METRES MF.TRES METRES
`~ 74 8.36 80 9.04 78 8.8 ~ `~
86 9.72 80 9.04 81 9.15 8.47 70 7.91 ` 76 8.6 -9.04 95 10.73 68 7.68 9.04 85 9~60 86 9.72 85 9.60 809.04 73 8.75 ~ ;~
91 10.78 809.04 79 8.93 83 9.38 809.04 79 8.93 ;`~
; 83 9.38 809.04 75 8.47 82 9.26 809.04 80 9.04 ` X _ 81.9 X = 9.25X _ 81 X = 9.15 X = 77.5 X = 8.75 , , ' ' `:,' ~""
,' ', : ' ' -17~

DATA IV
Prevaillng (locking) torque:
TNVENTION TYPE I TYPE II
LB-INS, NEWTON- LB INS. NEWTON- LB-INS. NEWTON-M~TRES METRES _ METRES
~2 4,75 20 2,26 5.08 30 3.39 Ten (10)specimens 47 5.31 20 2.26 showed zero pre-62 7,00 20 2.26 vailing torque after 5.65 20 2.26 forming internal 6.78 20 2.26 thread in test nut -5.65 15 1.69 virtually finger free ;~ 60 6.78 25 2.82 engagement.
6.21 30 3.39 ;~
5.65 35 3 95 ~ `
'~ X = 52.1 X = 5.89 X = 21.50 X = 2.65 ;
` , ' '.; . ~- `''' DATA V
~ Fail (Strip) torque: ;~
`, 380 42.93 320 36.16 342 38.64 `
365 41.24 - 300 33.90 305 34.46 380 42.93 295 33.33 346 39.09 -; 380 42.93 3~0 33.90 315 35.59 `
375 42.37 320 36.16 303 34.23 ~, .
:~` 365 41.24 325 ` 36.72 322 36.38 `~
. 370 41.80 315 35.6 297 33.56 "~` 375 42.37 275 31.07 341 38,53 380 42.93 310 35.02 332 37.51 ` 355 40,11 319 36.04 - 363 41.01 `~ -... , . ;.
X - 372,5 X ~ 42.08 X = 307.9 X _ 34.79 X ~ 326.6 X - 36.90 , . .

3~054;3~
DATA VI
Axial tensile pull-out load rom test nut aEter tightening to 275-285 pound-inches (31-32 Newton-Metres) LNVENTION TYPE I TYPE II
LBS, KN LBS~ KN l.BS. KN
7375 32.80 7000 31.14 7500 33.36 8125 36.14 7000 31.14 7585 33.74 7500 33.36 6500 28.91 7750 34.47 7187 31.97 6750 30.02 8000 35.58 10 7625 33.92 6375 28.36 78J5 34.81 7125 31,69 6687 29.74 7915 35.21 7750 34.47 7250 32.25 7500 33.36 7937 35,30 6625 29.47 8000 35.58 ~ ' 7625 33.92 6625 29.47 8165 36.32 `
7500 33,36 6750 30.02 8165 36.32 X = 7574.9 X _ 33.69 X _ 6756X = 30.0S X = 7840 X = 34.87 ~ ~ ~
, ' ` .
Turning now to Figures 9 and 10, the pair of dies !
shown comprises a stationary die 51 and a movable die 53, the -;~
. ~ . .
; latter movable in the direction indicated by the arrow 53a.
` 20 The dies are of similar construction so far as their-thread- `~rolling construction is concerned; therefore, like numbers will ~: ~
l indicate parts in the two dies 51, 53. The dies 51, 53 at -~` least in the direction extending along the length of the blank~
(i.e. transversely along the dies) has a generally flat profile.
' Thus, the die 51 has a flat section 55 along one side thereof.
The die 51 also has alternating ridges and grooves 56, 58 ., ~:. .
. . . : . ... . . .. . .. ., , - : .

~1~543~1LS
for rolling the thread onto ~lanks More particularly the die 51 comprises a first longitudinal section 60 with thread-forming ridges and grooves for forming the thread in the ,hold-lng section B. AdJacent the first section 60 is a second longitudinal section 62 for forming the thread-swaging thread on the work-entering section A. Additionally, there is a third die section 64 for rolling the pilot thread 44. The `~ second section 62 has truncated ridges 66 plus grooves 68 of varying depth to form a taper on the thread in the section A.
10 In the section 64 the ridges are likewise truncated but the ~ ;
- grooves 70 are of equal depth so as to form the pilot thread ~;
44 of equal width throughout 360.
The flat end portions 55 of each die lie in respective planes substantially coincident with the plane of the ridge 66 associated therewith. Such ridge 66 also coincides with those truncated die ridges that are between the ridge 66 and the flat surface 55.
` The dies 51a, 55a of Figure 11 are similar to the ~;
dies 51, 55 except that each includes a ridge and groove ~; 20 design resulting in a screw in which the width of the root~ `
; namely its transverse dimension~ decreases toward the tip 26.
Thus, the ridges 66, 67, 69, etc. are progressively further away ~ `
from the plane of the flat end portion 53. The result is that `
in the fastener of Figure 12 the roots 29, 31, 33, etcO are ` progressively wider in the direction toward the tip 26.
.. ~ ;
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As further shown by reference to Figs. 10 and ~`~
11~ the two grooves at the righthand end of each die are of a depth that i5 less than the adjacent grooves of equal depth. These two grooves thus form the first two thread turns o~ the screw as clearly seen in Figs. 10~ 11 and 12.

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Claims (3)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A thread-rolling die for forming a self-threading fastener having a holding section and a work-enter-ing section, said work-entering section having a tapered thread-swaging thread formation and a non-tapered pilot thread formation between the larger end of said thread-swaging for-mation and an adjacent tip of the fastener, said pilot thread formation being of uniform width throughout 360° and extending axially at least approximately one thread turn; said die com-prising first and second adjacent longitudinally extending sections of thread-forming ridges for rolling a thread on said holding section and for rolling the thread on the work-entering section, said second section having truncated ridges and grooves of varying depth to form the taper on said thread-swaging formation, and said die having a third section with grooves of equal depth to form said pilot thread, the depth of said last-mentioned grooves being less than the depth of the adjacent grooves in said second section, said third section including at least one groove of a depth less than said grooves of equal depth.

2. A die according to claim 1 including a ramp extending over at least a part of one of said sections to form a thread with a decreasing cross-sectional width of root.

3. A die according to claim 1 or claim 2 including a fourth section having a flat surface, the flat surface being in a plane that coincides with truncations on the ridges in said second and third sections.