GB2325288A

GB2325288A - Threaded fastener, and method and rolling dies for making it

Info

Publication number: GB2325288A
Application number: GB9709669A
Authority: GB
Inventors: Ali Akdar Pasalar; Melvyn Douglas Jackson
Original assignee: EUROP IND SERVICES Ltd
Current assignee: EUROP IND SERVICES Ltd
Priority date: 1997-05-14
Filing date: 1997-05-14
Publication date: 1998-11-18
Also published as: WO1998051933A1; EP0981692A1; GB9709669D0; AU7438898A

Abstract

A fastener (20) has a substantially parallel portion (12) with a head (13) at one end and a tapered portion (14) leading to a point (15) at the other end, and a helical formation (21) providing an edge (23) along its length and extending outwardly from the body, wherein the helical formation (21) starts substantially at the point (15). The fastener (20) is made by means of a pair of rolling dies, and the helical formation 21a is of different cross-section to that of helical formation 21 b. The fastener may comprise a two start thread.

Description

Title: Threaded fastener, method of making a threaded fastener and rolling dies for making a threaded fastener.

Description of Invention The invention relates to a threaded fastener of the kind commonly called a screw, to the method of making the threaded fastener and to the rolling dies for making the threaded fastener.

Many forms of threaded fastener of the kind generally called screws are known. These generally comprise a substantially cylindrical shank with a head provided with a formation for engagement of a driving tool, such as a screwdriver, at one end and a tapered portion leading to a point at the other end. They further comprise at least one helical formation, or thread, of substantially triangular cross section providing an edge along it's length and extending outwardly from the body. Screws having a single thread are known as single start and those with two threads are known as twin start. The two threads on known twin start screws start and end on opposite sides of the body at the same distance from the point and head respectively.

In general the threads, on both single and twin start screws do not start at the point. This provides a significant disadvantage, when the screw is to be used, in getting the thread to "bite" into the material into which it is being inserted. Essentially such screws have to act as a drill at first to create a hole of sufficient depth that the thread is then in contact with the material. This requires either rotation of the screw over a period of time, for example thirty seconds, with a light load applied as sufficient to drive the screw in to a hard material once the thread has "bitten", or the application of significant extra force until the thread "bites".

Threaded fasteners, such as screws, are manufactured by rolling a blank comprising a body having a cylindrical shank with a head at one end, between a pair of rolling dies. The rolling forms a substantially parallel portion adjacent the head leading to a tapered portion which in turn leads to a point at the other end of the shank and least one helical formation, or thread, of substantially triangular cross section providing an edge along it's length and extending outwardly from the substantially parallel and tapered portions.

The dies are blocks of metal machined on one face with a pattern of straight diagonal indentations whose cross section alters along their length t;ich form the thread or threads during rolling. In use one die is held stationary whilst the other is moved past it, with the blank being rolled in between. A raised portion of the face of each die, towards the finish end, forms the tapered portion and point.

However, in the prior art, as this occurs the blank is elongated at this end region which puts the partially formed thread(s) on the tapered portion out of pitch with the diagonal indentations on the dies. Hence the partially formed threads rather than being refined as the rolling continues tend to be flattened, resulting in a lack of thread(s) on the tapered portion adjacent to the point. This problem is exacerbated in the manufacture of twin start screws when compared with single start screws due to the higher pitch angle of the threads.

It is first object of the invention to provide a fastener which overcomes the above identified problem.

According to a first aspect of the invention there is provided a fastener having: (a) a body comprising a substantially parallel portion with a head at one end and a tapered portion leading to a point at the other end, and (b) a helical formation providing an edge along it's length and extending outwardly from the body, wherein the helical formation starts substantially at the point.

The invention provides the advantage that, as the helical formation, or thread, starts at the point of the fastener it is much simpler in use to insert the fastener into a hard material, as it is simpler to get the thread to "bite".

The helical formation may be of a first cross section adjacent the head and a second cross section adjacent the point.

Preferably the helical formation is of the first cross section along the parallel portion and of the second cross section along the tapered portion.

The first cross section may be a symmetrical V-shape about a line perpendicular to the parallel portion passing through the edge, and the second cross section may be non-symmetrical about that line. The second cross section may have a longer leading face than trailing face. The second cross section may have a greater cross sectional area than the first cross section and may extend further outwardly from the body than the first cross section.

The fastener may further comprise a second helical formation of like kind. In the alternative the fastener may further comprise a second helical formation having a single cross section and extending only along the parallel portion or along the parallel portion and partially along the tapered portion.

It is a second object of this invention to provide rolling dies which alleviate the above described problem.

According to a second aspect of the invention there is provided a pair of rolling dies for the manufacture of a fastener according to the first aspect of the invention.

According to the second aspect of the invention there is provided a pair of rolling dies for the manufacture of a threaded fastener, each die having a machined face bearing: (a) a plurality of elongate indentations for the formation of a thread, (b) a ramp portion for the formation of a tapered portion, and (c) a cut-off portion for the formation of a point, wherein the elongate formations extend to substantially an interface between the ramp and cut-off portions and are of different form on the ramp portion.

Preferably the difference of the form of the elongations on the ramp portion comprises a scollop on the leading edge thereof.

The machined faces may further have a second plurality of elongate indentations, alternating with the first plurality, for the formation of a second thread. The second plurality of elongate indentations may be of the same kind as the first plurality of elongate indentations. Alternatively, the second plurality of elongate indentations may extend only partially across the ramp portion.

It is a third object of the invention to provide a method of making fasteners which overcomes the problems described above.

According to the third aspect of the invention there is provided a method of making fasteners according to the first aspect of the invention comprising rolling a blank using a pair of rolling dies according to the second aspect of the invention.

It is a fourth object of the invention to provide a method of making rolling dies according to the second aspect of the invention.

According to a fourth aspect of the invention there is provided a method of making a pair of rolling dies for the manufacture of threaded fasteners comprising the use of three different cutting tools in succession.

Embodiments of a fastener, a method of making a fastener, a pair of dies, and a method of making a pair of dies according to the invention will now be described, by way of example only, with reference to the accompanying drawings in which: FIGURE 1 illustrates a prior art twin start screw fastener; FIGURE 2 illustrates a twin start screw fastener according to the present invention; FIGURE 3 is a sketch cross section through the prior art fastener of Figure 1; FIGURE 4 is a sketch cross section through the fastener of Figure 2; FIGURE 5 is a detail sketch of the pointed end of the prior art fastener of Figure 1; FIGURE 6 is a detail sketch of the pointed end of the fastener of Figure 2; FIGURE 7 is a cross section through a prior art thread; FIGURE 8 is a cross section through a thread on the tapered portion of a fastener according to the invention; FIGURE 9 is a schematic illustration of the manufacture of threaded fasteners by rolling; FIGURE 10 is a perspective view of a rolling die according to the invention; FIGURE 11 is a portion of the die of Figure 10 in more detail, and FIGURES 12 & 13 illustrate the cross sections of elongate indentations on a ramp portion of the die of Figure 10.

Figures 1, 3 and 5 illustrate a prior art twin start screw fastener 10. It comprises a body 11, having a parallel portion or shank 12 with a head 13 at one end and a tapered portion 14 leading to a point 15 at the other end. It further comprises two helical formations 16, commonly called threads, extending outwardly from the body 11. The threads 16 start on the tapered portion 14 a short distance from the point 15 and extend along the shank 12 to a distance adjacent the head 13. The threads 16 are substantially identical but start and end on opposite sides of the tapered portion 14 and shank 12 respectively.

Figures 2, 4 and 6 illustrate a twin start screw fastener 20 according to the invention. Those parts which are common with Figure 1 are like referenced. Thus, it also comprises a body 11, having a parallel portion or shank 12 with a head 13 at one end and a tapered portion 14 leading to a point 15 at the other end. It further comprises two threads 16' and 21 extending outwardly from the body, which provide edges 22 and 23 respectively.

Thread 16' is substantially identical with threads 16 of Figures 1, 3 and 5 and starts on the tapered portion 14 some distance from the point 15. It is of substantially the same cross section throughout it's length, as shown in Figure 7.

That is the cross section is a V-shape which is symmetrical about a line perpendicular with the shank 12 and passing through the edge 22.

Thread 21 differs in two respects. It starts substantially at the point 15, and it's cross section is not constant throughout it's length. The portion 21b of the thread 21 on the shank 12 is of a first cross section which is of the same form as that illustrated for thread 16 in Figure 7. The portion 21a of thread 21 on the tapered portion 14 is of a second cross section illustrated in Figure 8. It is asymmetric and has a longer leading edge 24 than trailing edge 25. In addition the cross sectional area is substantially greater than that for the first cross section, as best seen in figures 4 and 6. This results both from the asymmetry and the portion 21a extending further outwardly from the body than the portion 21b.

The screw 20 is much easier to insert into a hard material than the screw 10 of the prior art. It is not necessary effectively to drill a hole before one or more threads come into contact with the material and "bite" thus pulling the screw 20 into the material with each subsequent rotation. In addition, due to the larger cross sectional area of the thread portion 21a the screw 20 has greater resistance to stripping.

The thread 21 may be used as the only thread on a single start screw.

In addition two threads 21 may be employed on a twin start screw.

The screw 20 is described with the change in cross section of the thread 21 occurring around the junction of the shank 12 and the tapered portion 14. However, the change of cross section is of course gradual. Also, it may be located elsewhere such that the cross section 21a extends onto the shank 12 and the change occurs part way along the shank 12.

The manufacture of screws according to the prior art and according to the invention will now be described briefly, with reference first to Figure 9. A pair of identical rolling dies 30 and 31 are arranged with machined faces 32, 33 facing each other, with a blank 34 therebetween, as shown in Figure 9a. Rolling die 30 is held stationary whilst rolling die 31 is moved in the direction of arrow A past die 30, squeezing the blank 34 between the two dies 30, 31. Half way through the process the relative positions of the dies 30,31 and the blank 34, now a partially formed screw, are as shown in Figure 9b. The rolling continues until the blank 34 has moved to the other end of the die 30 as shown in Figure 9c, and is now a fully formed screw ready for finishing treatments, such heat treatment and plating.

Referring now to Figure 10 where a rolling die 40 according to the invention is illustrated, with start and finish ends 40a and 40b respectively, and a machined face 41 uppermost, and to Figure 11 where a portion of the die 41 towards the finish end is shown in more detail. The machined face 41 bears a plurality of straight diagonal indentations 42 which in the rolling process described above cause the formation of the threads 16' and 21 on the blank 34. Alternate indentations 42, designated 42a and 42b in Figure 11, are for the formation of threads 16' and thread 21 respectively. The machined face 41 also bears an elongate ramp portion 43 and cut-off portion 44 towards it's finish end 40b. The ramp portion 43, in the rolling process described above, causes the formation of the tapered portion 14. Likewise the cut-off portion 44 causes the formation of the point 15 on the blank 34.

The majority of the length of each indentation 42 is of known form of cross section for the formation of a twin start screw, the only differences over the prior art being their form on the ramp portion 43 of the die 40. In addition the overall form of the ramp portion 43 and cut-off portion 44 are of known kind.

The differences in the forms of the indentations 42a and 42b over the prior art are as follows.

The indentations 42a which are responsible for the formation of the thread 16', are towards the finish end 40b of the die of V-shaped cross section as is usual for the final definition of the thread form. However, they become narrower and shallower at the start of the ramp portion 43, and disappear altogether part way across the ramp portion 43. Hence, in contrast to the prior art, there is no attempt to form the thread 16' from the point 15 of the screw 20, rather it is deliberately started part way along the tapered portion 14 formed by the ramp portion 43.

The indentations 42b, which are responsible for the formation of thread 21, extend substantially to the interface between the ramp portion 43 and cut-off portion 44. In addition their form is altered on the ramp portion 43 by the addition to their leading edge of a scollop 45, which alters the cross section from a symmetric V-shape as shown Figure 12 to the form shown in Figure 13. The scallops 45 ensure that even when the partially formed threads 21 get out of pitch with the dies 30 and 31, due to elongation of the blank 34 as the tapered portion 14 is formed, the they are not flattened on the next revolution of the blank 34 but are further formed.

Dies comprising indentations of the form of indentations 42b may also be used the manufacture of single start screw fasteners. In addition dies comprising two sets of indentations 42b may be used for the manufacture of twin start screw fasteners with two threads 21.

The machined faces of rolling dies for the manufacture of screw fasteners are generally machined using two different cutting tools. The first tool is used to machine from the start end of a die to approximately two thirds of the way along the machined face. That is the part of the die over which the indentations are relatively wide and flat bottomed. The second tool is used to machine the remaining surface of the machined face to the finish end. That is the part of the die over which the indentations are relatively narrow and have pointed bottoms. Clearly, there is also an overlap part of the face which is partially cut by the first tool and finished by the second tool.

The machined faces 41 of the rolling dies 40 according to the invention require the use of a third tool. The majority of each machined face 41 is cut as described above for the prior art. A third tool is then used to cut the scallops 45.

Although the dies according to the invention are described as having a particular form of indentation for the manufacture of screw fasteners according to the invention, other forms of indentation may also be employed within the scope of the invention to ensure that a thread starts at the point of a screw.

The features disclosed in the foregoing description the following claims or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, or a class or group of substances or compositions, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

1. A fastener having: (a) a body comprising a substantially parallel portion with a head at one end and a tapered portion leading to a point at the other end, and (b) a helical formation providing an edge along it's length and extending outwardly from the body, wherein the helical formation starts substantially at the point.

i

2. A fastener according to claim 1 wherein the helical formation is of a first cross section adjacent the head and of a second cross section adjacent the point.

3. A fastener according to claim 2 wherein the helical formation is of the first cross section along the parallel portion and of the second cross section along the tapered portion.

4. A fastener according to claim 2 or 3 wherein the first cross section is a symmetrical V-shape about a line perpendicular to the parallel portion passing through the edge, and the second cross section is non-symmetrical about that line.

5. A fastener according to claim 4 wherein the second cross section has a longer leading face than trailing face.

6. A fastener according to claim 4 or 5 wherein the second cross section has a greater cross sectional area than the first cross section.

7. A fastener according to anyone of claims 4 to 6 wherein the second cross section extends further outwardly from the body than the first cross section.

8. A fastener according to any one of the preceding claims wherein it further comprises a second helical formation of like kind.

9. A fastener according to any one of claims 1 to 7 wherein it further comprises a second helical formation having a single cross section and extending only along the parallel portion or along the parallel portion and partially along the tapered portion.

10. A fastener substantially as hereinbefore described with referenc-.. to Figures 1 to 4 of the accompanying drawings.

11. A pair of rolling dies for the manufacture of a fastener according to any one of the preceding claims.

12. A pair of rolling dies for the manufacture of a threaded fastener, each die having a machined face bearing: (a) a plurality of elongate indentations for the formation of a thread, (b) a ramp portion for the formation of a tapered portion, and (c) a cut-off portion for the formation of a point, wherein the elongate formations extend to substantially an interface between the ramp and cut-off portions and are of different form on the ramp portion.

13 .A pair of rolling dies according to claim 12 wherein the difference of the form of the elongations on the ramp portion comprises a scollop on the leading edge thereof.

14. A pair of rolling dies according to claim 12 or 13 wherein the machined faces further have a second plurality of elongate indentations, alternating with the first plurality, for the formation of a second thread.

15. A pair of rolling dies according to claim 14 wherein the second plurality of elongate indentations are of the same kind as the first plurality of elongate indentations.

16. A pair of rolling dies according to claim 14 wherein the second plurality of elongate indentations extend partially across the ramp portion.

17. A pair of rolling dies substantially as hereinbefore described with reference to Figures 6 to 9 of the accompanying drawings.

18. A method of making a fastener according to any one of claims 1 to 8 comprising rolling a blank using a pair of rolling dies according to any one of claims 9 to 17.

19. A method of a making a pair of rolling dies for the manufacture of a threaded fastener comprising the use of more than two cutting tools in succession.

20. Any novel feature or novel combination of features as herein defined and/or shown in the accompanying drawings.