GB1562385A

GB1562385A - Thread forming members

Info

Publication number: GB1562385A
Application number: GB2963875A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1976-10-11
Filing date: 1976-10-11
Publication date: 1980-03-12

Description

(54) THREAD FORMING MEMBERS (71) We, WALTER THURSTON and WIL LIAM KERRIGAN, both British Subjects, of 28 Atwood Avenue, Kew Gardens, Surrey and 22 Feltham Hill Road, Ashford, Middlesex, respectively, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to screw thread forming male members, for convenience herein referred to as screws although members such as bolts, studs and taps are intended to be included.

Some tapping screws cut a thread in a plain hole producing swarf or chips, others form or swage an internal thread without the production of swarf, and this invention is concerned with the type of screw which forms a thread.

Screw thread-forming is effected by this type of screw by circumferentially spaced lobes and the screw has relief sectors between adjacent lobes for reducing friction and possible binding during thread forming. The circumferential extent of the relief portions is in practice important.

The most efficient screw thread-forming cross-sectional shape for the screw perpendicular to its longitudinal axis is such that the pitch surface of the thread on the screw has a lobular cross section, the pitch surface being an imaginary envelope surface, co-axial with the thread, which intersects the surface of a thread in such a manner that the axial distance between the points where a generator of the pitch surface meets the opposite flanks of the thread groove is equal to half the basic pitch of the thread. The screw should have a continuous thread.

This configuration offers the following advantages.

1. The relief sectors offer proper relief from contact between the thread of the screw and the internal thread in the region between the lobes.

2. The continuous thread ensures that the screw does not cut the material in which the internal thread is being formed. Certain thread forming devices have gaps in the thread for relief sectors and the leading faces of the interrupted thread cut into softer materials, producing chips which jam the screw movement.

A requirement of a good screw-thread forming member for use as a fastener is that the lobular cross-sectional shape is restricted to the leading end of the screw and that the main body of the screw has a circular cross section holding portion. One known screwthread forming member commercially available has a lobular cross section throughout the length of the shank so that, when tightened, contact between the flanks of the thread of the screw and the flanks of the generated internal thread which has a circular cross section is intermittent and the stripping strength of the union is low.

Thus a screw having a lobular cross section towards the free end and a circular cross section holding portion offers the best possibility of an efficiently generated internal thread and a strong union between this thread and that of the screw.

There is described in Patent Specification No. 1,231,805 a screw which has a lobular cross section at the forming end and holding portion with a circular cross section. The lobular form at the end of the shank is obtained by deforming a screw blank of circular cross section throughout its length which has been previously threaded in a conventional manner. The deformation is achieved by pressure pads being - applied to the forming end of the screw and being pressed radially inwards against the screw. The pads are threaded internally to mate with the threads on the screw to ensure that these do not become damaged. This radially inward deformation forms the relief sectors of the lobular shape and is carried out so that there is an outward flow of metal between the pads, this outwardly flowing metal forming the lobes.The radial distances from the axis of the screw to the crest and the root of the thread on the lobes are therefore greater than the corresponding distances in the circular cross section holding portion. The design of the screw of Patent Specification No. 1231805 is such that the lobes form an internal thread whose major, minor and pitch circle diameters are slightly greater than the corresponding diameters of the holding portion and this reduces or completely eliminates an unacceptably high build up of required torque..

However where conditions of vibration are present in the member or members to be fastened, and the member to be internally threaded is not thick in relation to the diameter of the fastening screw so that no great depth of penetration of the screw is required, then a screw with a different arrangement of thread forming and holding portion dimensions to that just described would be advantageous.

Such a screw is described in Patent Specification No. 1,022,355. The design is such that the radially outward extent of the various thread dimensions on the lobes of the lobular cross section screw thread forming end are never greater than the outward extent of the corresponding thread dimensions on the circular cross section holding portion. However this screw has serious disadvantages. The lobular pitch surface cross section has a radius of curvature of the lobes substantially less than one half of the width of the cross section, and a radius of curvature of the relief sectors greater than one half of the width and is designed to be produced by the thread rolling process described therein which requires expensive dies which are difficult to set up. Moreover, a different die is required for every different length of screw as well as for every different diameter and thread pitch.

In this design of screw the flattened arcuate sides forming the relief sectors between the lobes disappear gradually as the lobular thread blends into the circular holding portion. This gradual change is a feature of the screw necessary for it to be produced.

However it is a feature detrimental to the screws performance as a screw thread forming member since it is advantgeous to keep the relief sectors as large as possible. This is particularly so on the turn of thread of the screw which completes the formation of the internal thread. The metals into which these screws are driven are strain hardening and as the metal of the internal thread undergoes successive stages of thread forming, during each lobe pass an increased torque is required to induce the same deformation in the hardened metal. Moreover as the internal thread nears its full form it offers a greater area of friction between itself and the screw thread. Thus on a turn of thread where the extent of the relief sectors is of greatest importance, this earlier screw design has relief sectors whose extent is progressively reduced.

The object of one aspect of the invention is to provide a screw-thread forming male member which is particularly but not exclusively, useful for fastening together members subject to vibration.

According to this aspect of the invention we provide a screw-thread forming male member comprising a shank having; a first portion of substantially constant and generally circular cross-section; a second portion extending to or adjacent to the free end of the shank and tapering towards said free end, the second portion being of lobular cross-section and presenting circumferentially a plurality of lobes extending axially for the whole of the axial length of the second portion, the lobes being separated circumferentially by axiallyextending relief sectors; and a screw thread extending continuously from the first portion into and along said second portion and of constant form (as herein defined) along the first portion and the lobes of the second portion; the crests of the threads on the lobes of the second portion being coincident for substantially the whole of their circumferential lengths with an imaginary generally conical surface (as herein defined) whose axis of revolution is coincident with the longitudinal axis of the shank and of which the radial distance between said axis of revolution and the second point is equal to the radial distance of the thread crests on the first portion from said longitudinal axis, the pitch surface (as herein defined) and the root surface (as herein defined) of the thread of the second portion having lobular cross sections for the whole of the axial length of the second portion.

By generally conical surface we mean a surface of revolution formed by rotating a generating line about an axis of revolution, the generating line being either straight or smothly curved away from said axis, as shown in the accompanying Figure 13, and extending from a first point on the axis to a second point spaced radially from said axis, said surface of revolution lying wholly within a cylindrical surface of revolution about said axis and of a radius equal to the distance between the axis and the second point.

Where we say that a thread is of "constant form" and refer to a full thread and a truncated thread, the parts of the thread which exist are of the same form as the corresponding parts of the full thread, i.e. the grooves of the truncated thread parts are of the same shape as the grooves of the full thread part.

The term "thread form" is used in the specification to mean the shape of one complete profile of the thread between corresponding points, at the bottom of the adjacent grooves, as shown in an axial plane section.

The term "root surface", as used herein, is to be understood to refer to an imaginary surface of tubular or envelope form which just touches the roots of a thread.

The term "crest surface", as used herein, is to be understood to refer to an imaginary surface of tubular or envelope form which just touches the crests of a thread.

The second portion of the screw which is small enough at its smaller end to enter the plain hole in which the thread is to be formed may be produced by having the thread progressively truncated towards the smaller end.

Alternatively the thread may be fully formed throughout and the pitch surface conical towards the free end of the screw, with the crest diameter of the thread progressively decreasing towards the free end so that the screw will enter the plain hole in which the thread is to be formed.

The generally conical surface may be of straight line generation or convex. A convex surface assists in progressively reducing the metal required to be manipulated with each successive lobe pass.

Adjoining the second portion there may be a conical circular cross section portion at the free end having a thread on all or part of its axial length, this thread being a continuation of the one on the second portion.

Although described as a screw, the thread forming member may be in the form of a bolt, stud or tap, and may have conventional driving means, and be of conventional design in parts other than the thread forming portions.

If desired, one or more lobes and relief sectors may be provided on a part of the cylindrical portion of the blank in addition to the generally conical portion. This produces an intermediate third portion of the screw of constant lobular cross section which slightly reduces the amount of torque required to drive and tighten the screw in conditions where this is thought desirable.

In those embodiments having an intermediate or third portion, this portion may have an axial length of less than one turn of thread, i.e. less than one pitch of the thread, and to establish the cross sectional shape of the pitch surface of the thread and the surface defined by the crest of the thread it is neces sary to project an imaginary full turn of thread.

Screws embodying the invention have the following features.

At the position axially of the screw where the second or third portions of lobular cross section change to the circular cross section of the first portion, the pitch surface of the thread on the lobes of the lobular portion has the same centre and radius of curvature as the pitch surface of the thread on the cylindrical portion. This enables the screws to be made cheaply as described below.

This feature also ensures that the generated internal thread has the same size pitch circle diameter as the thread on the holding first portion of the screw, and because the metal of the internal thread endeavours to spring back after deformation, inter-thread pressure is exerted between the generated internal thread and the thread on the holding portion of the screw producing a tight union. Unlike inter-surface pressure in a normal threaded union that is produced by tensioning the screw shank when tightening, the interthread pressure produced by the screw of the invention is not affected to the same extent by vibration. Thus the screw of the invention is particularly valuable as a fastener under these conditions.

The change of pitch surface of the thread of the screw from a lobular to a circular cross section may be immediate. Preferably there are full relief sectors on the thread forming portion even on the turn of thread adjacent to the holding portion and so each turn of thread on the thread forming portion is fully effective. In those embodiments where the pitch surface of the thread on a lobular portion is generally conical, the cross section will change size but the shape and thread forming efficiency will remain substantially constant because the generally conical surfaces governing the change of size of the crest pitch and root surface cross sections are geometrically similar. This feature is particularly important in embodiments of the invention where there is no lobular third portion.

Demarcation between the thread forming and holding portions is preferably sharp and all turns of thread are clearly either threadforming or holding. This means that generally a greater proportion of the length of the screw is available as a holding portion than with known screws which are completely formed by a thread-rolling process.

An object of another aspect of the invention is to provide a method of making a screwthread forming male member which is simple and inexpensive.

According to this aspect of the invention we provide a method of making a screwthread forming male member from a blank comprising a shank having: a first portion of substantially constant and generally circular cross-section; a second portion extending to or adjacent to the free end of the shank and tapering towards said free end; and a screwthread of constant form throughout (as herein defined) extending continuously from the first portion onto and along said second portion; the method comprising engaging the second portion of the blank with a plurality of internally threaded pads spaced apart circumferentially about the longitudinal axis of the shank, at least the crests of the internal thread on the pads mating with the roots of the external thread on the shank, and moving said pads inwardly towards said axis to deform said second portion into a lobular cross-section which presents circumferentially a plurality of lobes extending axially for the whole of the axial length of the second portion, the lobes being separated circumferentially by axially extending relief sectors; said relief sectors being formed by the parts of the second portion engaged by the pads and the deformation being such that the parts of the second portion between the pads and which form the lobes do not move radially as a result of said deformation.

The stages of the method of manufacture may be as follows: 1. A unthreaded blank is formed in a conventional manner with a portion of cylindrical shape and an adjoining portion of generally conical circular cross section. The generally conical surface may be of straight line generation or convex. The conical portion will have the same centre and diameter as the cylindrical portion where they adjoin, and will taper inwardly away from the cylindrical portion towards the free end of the shank.

2. All or part of the cylindrical and conical portions are then rolled between conventional thread rolling dies to produce on them a continuous thread of constant form.

3. The threaded blank is then fed into a power forming machine which, whilst supporting the threaded blank, forces pressure pads radially inward on a selected axial length of the blank which consists of or includes at least part of the conical portion, each pad re-forming an arc of the circular cross section of the blank over the selected axial length. The pads are preferably concave and have a number of thread projections which mate with at least the roots of the thread on the screw, although the form of the thread projections on the pad may allow clearance on the crest and flanks of the thread on the screw.

The radially inward movement of the metal of the screw under pressure from the pads is arranged to be small enough not to cause any radially outward movement of the metal of the screw between the pads. However the magnitude of the arc and inward movement produced by the pads is arranged to be sufficient to form a series of effective relief sectors.

The inward movement of the metal of the screw produces an inward movement of the root, flanks and crest of the thread on the relief sectors of the screw. The areas of the screw surface that lie between the pads retain their shape and position and become the lobes. Thus the surfaces of the thread on the lobes where they axially adjoin the surfaces of the thread on a portion of the screw not acted upon by the pads will have the same centre and radius of curvature as the surfaces of the thread on this axially adjoining portion.

Preferably the pads are sections of an internally threaded body, the internal thread being parallel and/or tapered.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figures 1 and 2 are elevations of two conventionally manufactured screw blanks from which screws embodying the invention may be made.

Figure 3 is an elevation of the thread forming end of a screw constituting one embodiment of the invention; Figure 4 is a diagrammatic cross section of the surfaces defined by thread features of the screw of Figure 3; Figure 5 is an elevation of the thread forming end of a screw constituting a second embodiment of the invention; Figure 6 is a diagrammatic cross section of the surfaces defined by thread features of the screw of Figure 5; Figure 7 is an elevation of the thread forming end of a screw constituting a third embodiment of the invention; Figure 8 is a diagrammatic cross section of the surfaces defined by thread features of the screw of Figure 7; Figure 9 is an elevation of the thread forming end of a screw constituting a fourth embodiment of the invention; Figure 10 is a diagrammatic cross section of the surfaces defined by thread features of the screw of Figure 9;; Figure 11 is a fragmentary cross section on the line 11-11 of Figure 12 of deforming tools used to make a screw embodying the invention with a screw in position prior to deformation; Figure 12 is a diagrammatic end view of the tools and screw of Figure 11; and Figure 13 is a diagram to illustrate what we mean by "a generally conical surface".

Referring to Figure 1 a conventionally manufactured screw used as a blank for the making of a screw of the invention is indicated generally at 10 with a shank 11 and a head 12 at one end. The shank has a continuous thread of constant form with a cylindrical portion 13 and adjoining it a generally conical portion 14. The crests of the thread on the portion 14 are coincident with a convex generally conical surface as herein defined.

The taper of the portion 14 is formed by the thread thereon becoming increasingly truncated towards the free end as at 15. The pitch surface of the thread on the blank is cylindrical throughout and is indicated by the dotted line 16.

Figure 2 shows another conventionally manufactured screw used as a blank which differs from the screw of Figure 1 in the shape and thread of the tapering portion 14. The thread on the portion of this screw is fully formed with sharp crests as indicated at 17, and the crests of the thread are coincident with a right conical surface 18. The thread on the portion 14 has a conical pitch surface generated by a straight line shown by the dotted line 19.

Figure 13 illustrates what we mean by a "generally conical surface" as defined above, such a surface is a surface of revolution formed by rotating a generating line, 100 in Figure 13 about an axis 101 of revolution.

The generating line 100 extends from a first point 102 on the axis of revolution to a second point 103 on a line 104 which is perpendicular to the axis 101, the second point 103 being spaced therefrom by a distance r. The generating line can either be straight as indicated at 100 in which case the surface of revolution is a right circular cone or the generating line may be smoothly curved away from the axis 101 as indicated by the dotted line 105 in which case the surface of revolution is generally conical but convex. If the generating line 105 is convex it must not be so curved as to cut the line 106 which is parallel to the axis 101 and passes through the second point 103.In other words, the surface of revolution must lie wholly within a cylindrical surface of revolution formed by rotating the line 106 about the axis 101, the line 106 being spaced from the axis 101 by the distance r.

Relating this to the screw blanks of Fig ures 1 and 2 it will be seen that the crests of the thread on the portion 14 of the blank of Figure 1 lie on a generally conical surface as defined in which the generating line is smoothly curved away from the axis of revolution 101 so as to produce a convex conical surface. The axis of revolution of this surface is coincident with the longitudinal axis of the screw blank.

Turning now to the screw blank of Figure 2, the crests of the thread on the portion 14 lie on a right circular cone formed by rotating a straight line such as 100 about the axis of revolution 101. The pitch surface 19 is also a right circular cone about the longitudinal axis of the screw blank of Figure 2.

Referring now to Figures 3 and 4, these illustrate a first embodiment of the screw. The screw has a first portion 20 of substantially constant and generally circular cross section, a second portion 21 which tapers towards the free end 22 of the screw and is of lobular cross section as will be described, and a third portion 23 between the portions 20 and 21.

The crest and root surfaces of the thread on the third portion 23 are of constant lobular cross section. Referring to Figure 4 particularly, the line 24 represents the crests of the thread on the first portion 20. The lobular cross section of each of the portions 21 and 23 has three lobes and, between adjacent lobes, a relief sector. Figure 4 includes a number of lines indicated at 25 radiating from the longitudinal axis 26 of the screw and these lines divide the cross section into those parts which provide lobes and those parts which provide relief sectors. The parts are labelled Ll, L2, and L3 for the parts which provide lobes and RS1, RS2, and RS3 are the parts which provide relief sectors.

The crests of the thread on the lobes of the third portion 23 are represented by the parts of the line 24 in the parts L1, L2 and L3. The crests of the thread in the relief sectors of the portion 23 are represented by the lines 27 in the relief sectors RS1, RS2 and RS3. It will be seen that the thread crests on the lobes of the portion 23 do not extend radially out wardly beyond the thread crests on the first or holding portion 20.

Turning now to the tapering second por tion 21, the crest of the threads on the lobes of this portion are coincident for substantially the whole of their circumferential lengths with a generally conical surface as defined above. As will be described below, when the screw is formed from the blank of Figure 1, the parts of the screw which form the lobes in the portions 21 and 23 are not moved so that since the thread crests in the portion 14 of the screw blanks of Figure 1 were coincident with a generally conical surface as defined above and which was convex, the thread crests on the lobes of the second portion 21 in the finished screw will also lie on this same generally conical surface. This means that as one moves towards the free end 22 the radius of curvature of the thread crests on the lobes will decrease.The thread crests on the lobes for one complete turn of thread are indicated in Figure 4 by the part 28 of the line 24 in the part L1 and the parts 29 and 30 of the spiral line 31 in the parts L2 and L3. The thread crests in the relief sectors of the second part 21 are represented by the line 27 in the part RS1 and by the parts 32 and 33 of the spiral line 31 in the parts RS2 and RS3.

The pitch surface of the thread on the first part 20 is indicated by the dotted lines 34 in Figures 3 and 4 and as will be seen from Figure 4 is circular in cross sectional shape. The pitch surface of the lobular portions 21 and 23 is indicated in Figures 3 and 4 by the dotted line 35 which is coincident with the dotted line 34 in the parts L1, L2 and L3 of Figure 4.

That is to say the pitch surface of the thread on the lobes in the portions 21 and 32 is an axial continuation of the pitch surface 34 of the thread on the holding or first portion 20.

The parts of the dotted line 35 in the parts RS1, RS2 and RS3 represents the pitch surface of the thread on the relief sectors of the lobular portions 21 and 23. It will be seen, therefore, that the cross sectional shape of the pitch surface 35 is lobular as shown by the line 35, the pitch surface 35 and the root surface of the thread of the second portion 21 having lobular cross sections for the whole of the axial length of the second portion 21. The pitch surface of the thread on the relief sectors is tangent to a cylinder indicated at 36 in Figure 4. The pitch surface of the relief sectors for the whole of the axial length of the portions 21 and 23 is tangent to this cylinder 36.

This means that there is an immediate or sharp change of pitch surface from lobular to cylindrical. Thus, the pitch surface changes at the line 37 from the pitch surface 35 which is lobular to the pitch surface 34 which is cylindrical. The flank 38 of the turn of thread 39 on the first portion 20 which is adjacent to the third portion 23 has a circular pitch line and will have 360C contact with the internal thread formed by the screw and therefore will be fully able to withstand any loading required on it.

It will be seen that in this embodiment of Figures 3 and 4 the taper of the second portion 21 is provided by progressive truncation of the thread but that the pitch and root diameters of the thread on the lobes are uniform and are substantially identical to the pitch and root diameters respectively of the thread on the first portion 20. This is clear from the fact that the parts of the dotted line 35 in Figure 4 in the parts L1, L2 and L3 are coincident with the pitch surface line 34 of the holding portion 20.

Because of this feature, when the metal of the internally formed thread springs back, there will be a tight fit between the formed thread and the thread on the holding or first portion 20 because the two threads will have the same pitch circle diameter.

Referring now to Figures 5 and 6, the screw shown in these Figures is similar to the screw shown in Figures 3 and 4 except that the intermediate or third portion 23 is omitted.

Thus, there is a first or holding portion 40 of circular cross section and a second portion 41 which tapers towards the free end 42 of the screw. The taper is produced by truncation of the thread. The shape of the portion 41 is identical to the shape of portion 21 as will be seen from Figure 6. It will be noted that in this Figure, which has been divided as in Figure 4 into parts, L1, L2, L3 and RS1, RS2 and RS3, lines corresponding to the lines 27 in Figure 4 in the parts RS2 and RS3 are not present. The crests of the thread on the lobes and relief portions are indicated by a spiral line indicated generally at 43. In the part L1, this line is coincident with the line 44 which indicates the thread crests on the holding or first portion 40.The crests of the thread on the other lobes are indicated by the parts 45 and 46 of the line 43 and as before, these crests lie on the generally conical surface described above. The crests of the thread on the relief sectors are represented by the parts 47, 48 and 49 of the line 43. As before, the pitch surface of the thread on the portion 41 is lobular and is indicated by the dotted line 50. This line 50 is coincident, in the parts L1, L2 and L3 with the pitch surface of the holding portion 40 which is indicated by the dotted lines 51 in Figures 5 and 6.

As in the embodiment of Figures 3 and 4 there is an abrupt or immediate change from the lobular pitch surface 50 to the cylindrical pitch surface 51 at the line 52 between the two portions 40 and 41. There are thus no wasted turns in which there is a gradual transition of the relief sectors where the lobular portion joins the cylindrical holding portion.

A disadvantage of such a gradual transition is described above. A further feature of this screw is that as the turn of thread on the portion 41 adjacent to the portion 40 carries the heaviest thread-forming workload due to strain hardening of the metal of the internal thread being formed after a series of previous lobe passes, and the area of interthread friction progressively increases with the development of the internal thread, it is important in this last turn of thread on the portion 41 that the circumferential extent of the relief sectors is not reduced, and thus the sudden or immediate change from lobular to cylindrical pitch surface described above is highly desirable.

Referring now to Figures 7 and 8, these show a screw in which there is a first, cylindrical holding portion 53, a tapering second portion 54 and an intermediate third portion 55. The screw of Figures 7 and 8 is made from the screw threaded blank of Figure 2 in which, it will be recalled, the tapering portion has a thread of full depth with a generally conical pitch surface. As in the embodiments of Figures 3 to 6, the portions 54 and 55 have a lobular cross section, the portion 55 being of constant lobular cross section and the portion 54 being of generally conical lobular cross section.

In Figure 8, the line 56 represents the crest line of the threads on the first portion 53. As before, the cross section of Figure 8 is divided into lobe parts L1, L2 and L3, and relief sector parts RS1, RS2 and RS3. The crests of the thread on the lobes of the third portion 55 are coincident with the crest line 56 in Figure 8. The crest of the threads of the portion 55 are indicated in Figure 8 by the lines 57 in the relief sector parts RS1 RS2 and RS3. As before there is an immediate change from a lobular pitch surface indicated by the line 58 in Figure 7 to a circular pitch surface indicated by the line 59 in Figures 7 and 8, this change taking place at the line 60 in Figure 7.

The lobular pitch surface 58 is identical to the lobular pitch surface of the said part 23 in the embodiment of Figures 3 and 4.

The crests of the thread on the lobes of the tapering second portion 54 lie on the surface of a right circular cone. As has been described above, and as will be described in more detail below, when the screw is formed from the blank, the lobes in the second portion 54 are not moved. The crests on the lobes and relief portions of the portion 54 lie on a spiral line which is substantially coincident with the line 56 in the part L1 and with the part 57 in the part RSl and then has the parts 61 and 62 in the parts L2 and L3 and has the parts 63 and 64 in the parts RS2 and RS3. The parts 61 and 62 lie on the cone designated by the line 18 in Figure 2 and the lines 63 and 64 touch the surface of a cone which is similar to the cone designated by the line 18.Similarly, the pitch surface of the thread in the portion 54 is of lobular cross section and tapering and is indicated by a dotted line which is coincident with the line 59 in the part L1 and the line 69 in the part RSI and then has parts 65, 66, 67 and 68 in the parts L2, RS2, L3 and RS3 respectively. The line 59 represents the pitch cylinder of the first portion 53.

Figures 9 and 10 illustrate a screw similar to the screw of Figures 7 and 8 except that the third portion 55 is omitted. There is thus a first or holding portion 70 of cylindrical shape and a tapering second portion 71 of lobular shape. The pitch surface of the first or holding portion is cylindrical and is indicated at 72 and the pitch surface of the portion 71 is indicated at 73 and is tapering and of lobular cross section. There is a sudden or immediate change between the lobular and cylindrical pitch surfaces at 74 in the manner described above. It is believed that further description of this screw is unnecessary in view of the foregoing description.

Referring now to Figures 11 and 12, these illustrate the method of making a screw of the invention from the blank of Figure 1. The free end of the blank is engaged by a spring loaded stop 75 and the shank 11 is received in a bush 76. The tapering portion 14 of the screw blank and the immediately adjacent part of the portion 13 are surrounded by three pressure pads 77 which are equiangularly spaced about the longitudinal axis of the blank. The pressure pads are provided with internal thread formations 78. The crests 79 of the thread formations 78 are arranged to mate with the roots of the thread on the screw blank. The flanks 80 and roots 81 may be arranged to give clearance to the flanks and crests of the thread on the screw blank.The minor diameter of the thread on the pads may be approximately 12 times the minor diameter of the thread on the shank of the screw blank.

The pads 77 are moved inwardly towards the longitudinal axis of the screw blank by means not shown. They press inwardly the metal of the screw blank to form the relief sectors in the second and third portions 21 and 23 of the finished screw of Figure 3. The amount of inward deformation is controlled, however, by the relationship between the minor diameters of the thread on the pads and the thread on the screw so that the portions of the screw blank between the pads are not affected and move neither inwardly nor outwardly. The thread on the lobes, therefore, remains in the same position it occupied in the screw blank so that the thread on the lobes has the same pitch circle diameter as the thread on the holding portion as described above and with the advantages described above.Moreover, the manufacture is extremly simple requiring only the application of the pressure pads to the screw blank.

The arrangement shown in Figures 11 and 12 produces the screw of Figures 3 and 4 from the screw blank of Figure 1. If one wanted to produce the screw of Figures 5 and 6 then the pads 77 would be of less axial extent or the relative positions of the screw blank in the pads would be changed so that the pads would operate only on the tapering portion 14 of the screw blank.

To manufacture screws of the embodiment of Figures 7 and 8 the threads on the pads 77 would need to have a cylindrical portion and a tapering portion and to form a screw of the enbodiment of Figures 9 and 10 it would be necessary to have pads with tapering threads to operate only on the tapering portion of the screw blank.

It will be seen that the invention provides a screw forming male member which is particularly useful as a fastener for two members where vibration is present. However, as mentioned above, the screw forming member can be in the form of a tap, stud or bolt.

Moreover the screw forming members can be simply formed from a threaded blank by means of a simple and inexpensive method.

Although the screws of the invention have been shown as having three lobes, any convenient number may be provided.

WHAT WE CLAIM IS 1. A screw-thread forming male member comprising a shank having; a first portion of substantially constant and generally circular cross-section; a second portion extending to or adjacent to the free end of the shank and tapering towards said free end, the second portion being of lobular cross-section and presenting circumferentially a plurality of lobes extending axially for the whole of the axial length of the second portion, the lobes being separated circumferentially by axiallyextending relief sectors; and a screw thread extending continuously from the first portion onto and along said second portion and of constant form (as herein defined) along the first portion and the lobes of the second portion; the crests of the threads on the lobes of the second portion being coincident for substantially the whole of their circumferential lengths with an imaginary generally conical surface (as herein defined) whose axis of revolution is coincident with the longitudinal axis of the shank and of which the radial distance between said axis of revolution

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. portions of the portion 54 lie on a spiral line which is substantially coincident with the line 56 in the part L1 and with the part 57 in the part RSl and then has the parts 61 and 62 in the parts L2 and L3 and has the parts 63 and 64 in the parts RS2 and RS3. The parts 61 and 62 lie on the cone designated by the line 18 in Figure 2 and the lines 63 and 64 touch the surface of a cone which is similar to the cone designated by the line 18.Similarly, the pitch surface of the thread in the portion 54 is of lobular cross section and tapering and is indicated by a dotted line which is coincident with the line 59 in the part L1 and the line 69 in the part RSI and then has parts 65, 66, 67 and 68 in the parts L2, RS2, L3 and RS3 respectively. The line 59 represents the pitch cylinder of the first portion 53. Figures 9 and 10 illustrate a screw similar to the screw of Figures 7 and 8 except that the third portion 55 is omitted. There is thus a first or holding portion 70 of cylindrical shape and a tapering second portion 71 of lobular shape. The pitch surface of the first or holding portion is cylindrical and is indicated at 72 and the pitch surface of the portion 71 is indicated at 73 and is tapering and of lobular cross section. There is a sudden or immediate change between the lobular and cylindrical pitch surfaces at 74 in the manner described above. It is believed that further description of this screw is unnecessary in view of the foregoing description. Referring now to Figures 11 and 12, these illustrate the method of making a screw of the invention from the blank of Figure 1. The free end of the blank is engaged by a spring loaded stop 75 and the shank 11 is received in a bush 76. The tapering portion 14 of the screw blank and the immediately adjacent part of the portion 13 are surrounded by three pressure pads 77 which are equiangularly spaced about the longitudinal axis of the blank. The pressure pads are provided with internal thread formations 78. The crests 79 of the thread formations 78 are arranged to mate with the roots of the thread on the screw blank. The flanks 80 and roots 81 may be arranged to give clearance to the flanks and crests of the thread on the screw blank.The minor diameter of the thread on the pads may be approximately 12 times the minor diameter of the thread on the shank of the screw blank. The pads 77 are moved inwardly towards the longitudinal axis of the screw blank by means not shown. They press inwardly the metal of the screw blank to form the relief sectors in the second and third portions 21 and 23 of the finished screw of Figure 3. The amount of inward deformation is controlled, however, by the relationship between the minor diameters of the thread on the pads and the thread on the screw so that the portions of the screw blank between the pads are not affected and move neither inwardly nor outwardly. The thread on the lobes, therefore, remains in the same position it occupied in the screw blank so that the thread on the lobes has the same pitch circle diameter as the thread on the holding portion as described above and with the advantages described above.Moreover, the manufacture is extremly simple requiring only the application of the pressure pads to the screw blank. The arrangement shown in Figures 11 and 12 produces the screw of Figures 3 and 4 from the screw blank of Figure 1. If one wanted to produce the screw of Figures 5 and 6 then the pads 77 would be of less axial extent or the relative positions of the screw blank in the pads would be changed so that the pads would operate only on the tapering portion 14 of the screw blank. To manufacture screws of the embodiment of Figures 7 and 8 the threads on the pads 77 would need to have a cylindrical portion and a tapering portion and to form a screw of the enbodiment of Figures 9 and 10 it would be necessary to have pads with tapering threads to operate only on the tapering portion of the screw blank. It will be seen that the invention provides a screw forming male member which is particularly useful as a fastener for two members where vibration is present. However, as mentioned above, the screw forming member can be in the form of a tap, stud or bolt. Moreover the screw forming members can be simply formed from a threaded blank by means of a simple and inexpensive method. Although the screws of the invention have been shown as having three lobes, any convenient number may be provided. WHAT WE CLAIM IS

1. A screw-thread forming male member comprising a shank having; a first portion of substantially constant and generally circular cross-section; a second portion extending to or adjacent to the free end of the shank and tapering towards said free end, the second portion being of lobular cross-section and presenting circumferentially a plurality of lobes extending axially for the whole of the axial length of the second portion, the lobes being separated circumferentially by axiallyextending relief sectors; and a screw thread extending continuously from the first portion onto and along said second portion and of constant form (as herein defined) along the first portion and the lobes of the second portion; the crests of the threads on the lobes of the second portion being coincident for substantially the whole of their circumferential lengths with an imaginary generally conical surface (as herein defined) whose axis of revolution is coincident with the longitudinal axis of the shank and of which the radial distance between said axis of revolution

and the second point is equal to the radial distance of the thread crests on the first portion from said longitudinal axis, the pitch surface (as herein defined) and the root surface (as herein defined) of the thread of the second portion having lobular cross sections for the whole of the axial length of the second portion.

2. A member according to claim 1 wherein the taper of the second portion is provided by progressive truncation of the thread form (as herein defined) and the pitch and root diameters of the thread on the lobes are uniform and are substantially identical to the pitch and root diameters respectively of the thread on the first portion.

3. A member according to claim 1 wherein the thread is of constant depth throughout.

4. A member according to any one of claims 1 to 3 wherein the shank includes a third portion between the first and second portions, the crest and root surfaces (as herein defined) of the thread on the third portion being of constant lobular cross-section and presenting at least one lobe and at least one relief sector, the or each lobe and relief sector of the third portion being axially aligned wits a lobe and relief sector respectively of the second portion, and wherein the crest, pitch and root diameters of the thread on the or each lobe of the third portion are identical to the crest, pitch and root diameters of the thread on the first portion.

5. A member according to claim 2 or claims 2 and 4 wherein the pitch surface and root surface (as herein defined) of the thread on the relief sectors of the second portion are tangent to imaginary cylinders whose longitudinal axes are coincident with the longitudinal axis of the shank.

6. A member according to claim 3 or claims 3 and 4 wherein the pitch surface and root surface (as herein defined) of the thread on the relief sectors of the second portion are tangent to imaginary, generally conical surfaces as herein defined whose axes of revolution are coincident with the longitudinal axis of the shank and which are geometrically similar to said first-mentioned conical surface.

7. A method of making a screw-thread forming male member from a blank comprising a shank having; a first portion of substantially constant and generally circular cross-section; a second portion extending to or adjacent to the free end of the shank and tapering towards said free end; and a screwthread of constant form (as herein defined) extending continuously from the first portion onto and along said second portion; the method comprising engaging the second portion of the blank with a plurality of internally threaded pads spaced apart circumferentially about the longitudinal axis of the shank, the crests of the internal thread on the pads mating with the roots of the external thread on the shank, and moving said pads inwardly towards said axis to deform said second portion into a lobular cross-section which presents circumferentially a plurality of lobes extending axially for the whole of the axial length of the second portion, the lobes being separated circumferentially by axially-extending relief sectors; said relief sectors being formed by the parts of the second portion engaged by the pads and the deformation being such that the parts of the second portion between the pads and which form the lobes do not move radially as a result of said deformation.

8. A method according to claim 7 wherein the pads are curved in cross-sections perpendicular to said longitudinal axis so that the minor diameter of the thread on the pads is approximately 1-2 times the minor diameter of the thread on the shank.

9. A method according to claim 7 or claim 8 wherein the taper of said second portion is provided by progressive truncation of the thread form and the pads have a cylindrical thread thereon.

10. A method according to claim 7 or claim 8 wherein the second portion has a completely formed taper thread thereon and the pads have a taper thread thereon.

11. A method according to any one of claims 7 to 10 wherein the pads are also arranged to deform the blank to provide, in the finished member, a - third portion between said first and second portions, the third portion being deformed so that the crest pitch and root surfaces (as herein defined) of the thread thereon have a constant lobular crosssection which presents at least one lobe and at least one relief sector the or each lobe and relief sector of the third portion being axially aligned with a lobe and relief sector respectively of the second portion, the or each axial relief sector of the third portion being formed by a part thereof which is engaged by a pad and the deformation being such that the parts of the third portion between the pads and which form the lobes do not move radially as a result of said deformation.

12. A screw-thread forming male member substantially as hereinbefore described with reference to and as shown in Figures 3 and 4 of the accompanying drawings.

13. A screw-thread-forming male member substantially as hereinbefore described with reference to and as shown in Figures 5 and 6 of the accompanying drawings.

14. A screw-thread forming male member substantially as hereinbefore described with reference to and as shown in Figures 7 and 8 of the accompanying drawings.

15. A screw-thread forming male member substantially as hereinbefore described with reference to and as shown in Figures 9 and 10 of the accompanying drawings.

16. A method of making a screw-threadforming male member substantially as hereinbefore described with reference to Figures 1, 2, 11 and 12.