IMPROVEMENTS IN OR RELATING TO SCREWING DIES

Abstract: A blank for a screwing die on which there is to be formed a bore (4) having a screw thread (5) on its surface, includes one or more shaped cavities (2; 32; 42) such that openings (6) are formed between the bore and the remainder of the cavities, the openings being flanked by land portions (7). For each of a variety of sizes of the bore a cutting edge (10) having a positive rake angle of between about 5° and 15° is formed on the leading edge of each land portion, and the land portions (7) form in total between about 30% and 40% of the bore circumference. The cavities may have a leading edge (8; 48; 49) in the form of an Archimedes spiral or an edge (38; 39) made up of a plurality of linear portions. The cavities (2; 32; 42) may have two such edges so that the die is reversible.

Description: This @@vention relates to screwing dies, and to novel form Of @@ank from which auch dies may by produced. A known form of screwing die comprises a cylinder having an axial threaded bore therethrough upon which bore the cutting threads are formed. In this form of die, a plurality of similar circular holes are drilled parallel to the threaded bore, these holes being of such a diameter, and so positioned with their centres equidistant from the threaded bore, that these bores "overlap" (ie form openings into) the threaded bore. At an edge of each opening so formed1 there is thus created a cutting edge whose rake angle is determined by the diameter and positioning of the holes relative to the threaded bore. Between these openings there remain lands upon which the cutting threads are disposed, the cutting edge being the leading edge of each land. For efficient cutting, the rake angle should normally have the same value irrespentive of the diameter of thread to be cut. ideally this will be in the approximate range 10 to 11 , at least at the leading end of the die where cutting commences, although values as low as 5 or as great as 150 can provide acceptable results. The choice of land dimensions will depend upon the diameter of the thread to be cut, and ideally will strike the optimum compromise between the ability of the die to resist breakage during use, and the provision of openings of adequate size to permit all waste material produced during cutting to be cleared through the openings without fouling the newly cut thread. The ideal relationship between thread diameter and land dimension is determined eRpiricnlly, and data providing details of st,ch relationships for a variety of thread diameters will be familiar to those skilled in the art. Normally the land area will account; for about 35-45% of the circumference of the threaded bore, although values down to 30% can give acceptable results. A disadvantage of the known form of die described above is that for each thread diameter there is only one predetermined size and configuration of axial holes which is suitable. The present invention seeks to make possible the provision of a single die blank from which dies for cutting various thread diameters and forms can be made simply by the formation therein of an appropriate threaded bor According to the present invention, a blank for a screwing die has a longitudinal axis on which is to be formed a bore having a screw-cutting thread on its surface, and one or lore shaped cavities extending though the blank in a generally longitudinal sense, the shap and disposition of the shaped cavities being such that for each of a variety of sizes of the bore, (i) one or more openings are formed through which the bore cornnnmicates with the remainder of the said one or more shaped cavities, said remainder being flanked by one or more land portions of the blank which carry the screw-cutting threads, (ii) in at least one plane transverse to the longitudinal axis, a cutting edge having a positive rake angle of between about 5 and 15 is formed on the leading edge of each land portion, and (iii) in the said at least one plane the land portions form in total between about 30 and 45% of the bore circumference with the said openings forming the remainder of the circum reference. Normally the said at least one plane will be that of an end face of the die blank. The said positive rake angle shouldpreferably be between about and 110. Ent @@@@ portiens Ehould preferably form bout 35 and 45% of the bore circumference. Preferably there will be a plurality of mutually separate shaped cavities. A plurality of shaped cavities conceivably might be linked in a zone around the longitudinal axis to form a single cavity. The latter arrangement might, however, lead to difficulty in forming a bore by drilling or a similar method. Preferably each of the shaped cavities is of similar form. Preferably the shaped cavities are symmetrically disposed with reference to the longitudinal axis. In one advantageous arrangement, each shaped cavity has an edge in the said at least one plane in the form of an Archimedes viral Iavig its origin cn the longitudinal axis and being of a pitch such that a cutting edge having substantially the same positive rake angle is formed for each of the variety of size of the bore. Th another advantageous arrangement, each shaped cavity has an edge in the said at least one plane made up of a plurality of linear portions such that a cutting edge having a positive rake angle between about and 150, preferably between about 100 and 1l0 is formed for each of the variety of sizes of the bore. Advantageously. in the case of this latter arrangement, the shaped cavity has two edges in the said at least one plane each made up of a plurality of linear portions, such that a cutting edge having a positive rake angle between about 50 and 150, preferably between about 100 and 110 is formed for each of the variety of sizes of the bore and for both possible directions of rotation of the die blank about its longitudinal axis. In some cases, each shaped cavity can conveniently have the same transverse cross-section throughout the length of the blank. Alternatively, where each shaped cavity is;ot symmetrical about a plane radial with respect to the longitudinal axis of the die blank, the shape and configuration of each shaped cavity can be such that as viewed from one end of the blank in the longitudinal direction the shape of the shaped cavity at the said at least one plane is the same as the shape of the sbaped cavity at a second transverse plane as viewed from the other end of the blank in the longitudinal direction. The cross-sect of each shaped cavity can be varied smoothly from the said at least one t the second transverse pit ;e - eg by linking corresponding points at each respective section by straight lines. Alternatively, the change of section might be achieved by having two zones of constant transverse cross-section with a rapid or abrupt changes of section between these zones, preferably mid-way between the said at least one and second transverse planes. Preferably the said at least one and the second transverse planes are at opposite longitudinal end faces of thc dic blank. One very convenient method of making a screwing die blank in accordance with the invention is investment casting. The invention also resides in a screwing die comprising a blank in accordance with the invention having a threaded bore formed on the said longitudinal axis and extending through the blank. The invention will now be described by way of example only with reference to the accompanying drawings, of which Figure I shows a screwing die blank in accordance with the invention; Figure 2 shows a screwing die made from the blank shown in Figure 1; Figure 3 shows a different form of screwing die blank in accordance with the invention; and Figure 4 shows a still further form of screwing die blank in accordance with the invention. As showy in Figure 1, a screwing die bl=ik 1 is of cylindrical Thrum, and is provided with four shaped cavities 2, which extend through the blank from end to end, parallel with, and symsletrically disposed around its longitudinal axis. It is intended that the blank should be used to form a die 3 (Figure 2) having a bore 4 centred on its longitudinal axis, the bore 4 having a screw thread 5 formed on its surface. As will be apparent from Figure 2, the minimum sized threaded bore 4 for which the blank 1 is designed overlaps the shaped cavities 2, and hence communicates therewith through openings 6 which extend the full axial length of the bore 4. Between the openings 6 there remain lands 7 upon which the thread 5 is formed. The die 3 is intended to cut a screw thread on a male screw blank inserted from the upper end when the die is rotated in an anti-clockwise direction as viewed in the Figures. The. shaped 'cavities 2 are each of similar constant cross-section, when viewed in section transverse to the longitudinal axis of the blank Each shaped cavity 2 has a curved rearward side 8 which forms, with the surface of the bore 4, a cutting edge 10 having an acute rake angle of about 10 to 110 for all bore diameters within the intended range. Such curve is in fact an Archimedes spiral having its origin at the cylinder axis and is so referred to herein. Each shaped bore 2 has also, when se viewed,a curved forward side 9, which is shaped so as to provide for all diameters of threaded bore 5 which the die blank 1 can accommodate, the best compromise between the dimension of the lands 7 and the openings 6. This compromise has been determined empirically for various screw threac diameters to be cut. and will generally be such as to provide a land are of about 35 to 45% of the threaded bore circumercrce for all bore sized which can be accommodated. The forward sides 9 in this instance are thu also in the form of an Archimedes spiral of the same form as the sides 8 but displaced circumferentially therefrom sufficiently to provide a constant percentage land area. The outer side Il and the inner side 12 of each of the shaped cavities 2 is in the form of an arc centred on the longitudinal axis of the blank, but the shape of the outer and inner sides is not critical. ! A small slit 13 is formed along the axial length of the die extending from one of the shaped bores 2 to the exterior of the die bland This imparts some degree of resilience to the finished die 3 in the circa ential direction, which is a desirable characteristic in use, as is knowr The die blank 1 is also provided with a pair of notches 14 in its exterior surface, by means of which the finished die may be held for ;.t n > conventional die holder. Xn or to form a finished die such as 3 from the die blank 1, it necessary only to form a bore 4 of appropriate diameter on the -=n;,itudinal axis thereof, and to provide a screw thread 5 of desired form on the land portions 7. The blank 1 is intended to accommodate a range of sizes of threaded bore 4, the limits of the range being fixed by the diameter of the inner surface 12, and by the need to leave a sufficient clearance between the workpiece and the outer surface 11 for the disposal of metal cut away when forming a thread. The blank itself can conveniently be formed by investment casting. In most instances the die blank material will be hard in order to produce a durable cutting edge 10. It will be apparent that the invention permits the production of a die from the blank material with a minimum of machining, thus effecting an appreciable saving both in terms of machining effort and material. The accuracy of the rake angle at the cutting edge 10 is an important matter for the efficient cutting of screw threads by means of a die, and the present invention enables the provision of a die in which the rake angle can be accurately controlled. The value of the optimum rake angle does not normally vary with screw thread diameter, hence the curved rearward side 10 in the form of an Archimedes spiral can provide the optimum rake angle for all intended thread diameters. In the known design of die described above the accurate control of rake angle tends to be critically dependent upon accurate positioning of the plurality of circular holes trilled parallel to the threaded bore and is thus more open to inaccuracy than with a die formed from a cast blank in accordance with the invent ion. The embodiment described with reference to Figures 1 and 2 is suitable for use only one way up, ie if a right hand thread is formed on the bore 4, the male blank on which a thread is to be formed must enter from the upper face of the die as viewed in Figures 1 and 2. If the blank enters the die from the other (lower) face, clockwise rotation of the die necessary to "screws' the die along the workpiece will move the cutting edges2Ointhewrongsense for the removal of metal, ie the cutting edges will form the trailing, not the leading sides of the lands 7. There is a requirement for a die which is reversible, ie is suitable for use from both end faces, and the die blanks shorn in Figures 3 and 4 are intended to meet this requirement whilst still being suitable for forming a range of die sizes. In Figures 3 and 4, those parts which are similar to parts showy in Figure 1 are given like reference numbers, and repetition of description is avoided. Refrain to Figure 3 the die blank 31 showy therein also has four mutually similar shared cavities 32 extending longitudinally therethrough parallel to the longitudinal axis and symmetrically disposed with respect thereto. The shaped cavities 32 of the Figure 3 embodiment are, however, of a different shape from the shaped cavities 2 of the Figure 1 embodiment. Specifically, the cavities 32 are of stepped form, having stepped sides 38 and 39 corresponding to the curved sides 8 and 9 of the Figure 1 embodiment, while the inner and outer bounding sides 12 and 11 are of arcuate form as in Figure 1. Each of the stepped sides 38, 39 is made up of a number (in this case four) of planar portions or "steps", each inclined at an angle of about 10-11 to at least one, and preferably all radial planes which intersect that portion. The cavities 32 are of constant cross-section transverse to the longitudinal axis. It is intended that as with the Figure 1 embodiment a die tor cutting a thread on a male screw blank should be formed from the die blank 31 by @imply forming a threaded bore (not shown) on the longitudinal axis of the hlank. The discontinuities between the steps are so dimensioned as to ensure that at each intended size of threaded bore the openings formed between each shaped cavity 32 and the bore account for between 5552 and 65% of the bore circumference, and the lands which separate the openings thus account for the remaining 35 to 45. It will be seen that from a single blank of this form it is possible to produce dies for a ranee of thread diameters. Furthermore this blank has the advantage over that shown in Figure 1 that it is suitable for the formation of a die for cutting a right or a left-hand thread, and that the male screw blank may be introduced to the die from either end, ie the die is reversible. This arises because of the stepped design of the forward and rearward sides 38 and 39. For example, if the blank shown in Figure 3 is made into a die for forming a right-hand thread, then if the male screw blank is introduced into the lower side of the die as viewed in Figure 3, the die needs to be turned clockwise to form the thread, and the cutting edges are those formed where the sides 38 meet the threaded bore (not shown). If the screw blank is introduced from the upper side (Figure j), then the die needs to be turned counter-clockwise to cut the threads, and the cutting edges are those formed where the sides 39 meet the threaded bore. In either event, the cutting edges have the correct positive rake angle of about 10 to 11 for a variety of intended bore sizes, because of the slope of the stepped planar portions of the sides 38, 39. The stepped arrangement permits a design in which the correct land proportions are also maintained through the intended range of bore sizes. With the Figure 3 arrangements, two die blanks are needed to provide for the whole of a range of thread diameters, because it is necessary to avoid bores just smaller than the discontinuities between the s'eps. where only a range of standard, discrete sizes is required, it may be possible to provide a single blank suitable for all such sizes1 but where a capability for all sizes throughout a particular range is required, the need for two different blanks can be a disadvantage. It may also be noted that for only one single thread diameter within each step is it possible to provide a combination of optimum rake angle and land area - although a close to optimum combination can be achieved throughout each step. The cylindrical die blank 41 shown in Figure 4 embodies a still further configuration of shaped cavities, which again is designed to permit production of a range of reversible dies for cutting a range of thread diameters. The embodiment shown in Figure 4 is for the formation of dies for cutting left-hand screw threads only, but it will be apparent from the following description that a similar embodiment can be made for right-hand threads. The die blank 41 has four shaped cavities 42 extending therethrough from top to bottom in the direction of its longitudinal axis and disposes' symmetricallY with respect thereto. Each cavity 42 has an outer bounding surface 11 and an inner bounding surface 12, both being in the form of an arc centred on the longitudinal axis of the blank. Each shaped cavity is also bounded by sides having a complex shape, and extending in a generally radial direction. In order to appreciate the reasons for the shaping of these sides, it is necessary to recall that the blank is intended to receive a threaded bore centred on its longitudinal axis, and cutting edges for cutting a thread on a male screw blank are formed where the shaped cavities meet this bore. As shown in Figure 4 the die thus formed needs to be rotated anticlockwise to cut a rightQhsnd) acrn thread on a male screw blank introduced fTr3E the top, and clockwise if introduced from the bottom. At the top surface of the die blank 41, each shaped cavity 42 has curved edges 48 and 49 and the shaping of these edges corresponds exactly to that of the respective sides 8, 9 as viewed in plan from above in Figure 1, At the lower surface of the die blank 41, each shaped cavity has curved edges 481, and 491 whose shape again corresponds exactly to th respective sides 8, 9 but as viewed in plan from below. Thus as shown in Figure 4, the cross-section of the cavities 42 at the upper surface of th die blank 41 is the mirror image of that at the lower surface - when both cross-sections are viewed from above as in Figure 4. It will thus be appreciated that when the blank 41 is converted into a die by the formation of an axial bore there through in the manner alread described, the die will have a cutting edge forked with the correct rake angle at its upper face where the edge 48 meets the central bore, for cutting a screw thread on a blank which enters from that face, and at its lower face where the edge 491 meets the bore for cutting a screw thread on a blank entering from the lower face. The correct proportion of land to opening will be maintained as noted above with reference to Figures 1 and 2. Between the upper and lower faces of the die blank, each shaped cavity can be formed by joining the upper and lower faces through straight lines linking points on a common radius (ie points on edge 48 to points on edge 48 , points on edge 49 to points on edge 491) The inner and outer surfaces 12, 11 will of course conveniently be partcylindrical as with the other described embodiments. Alternatively, the shaped cavity can have some other form which smoothly merges the upper cross-section into the lower. @@ @@@@@ @@@@@@@ alternative, the shaped cavities. 42 @ight each @e between sides 11 and 12, the cross-section defined by edges 48 and @@ for the urper half of their depth, and that defined by edges 481 @1 for the lower half, with a rapid or abrupt transition from one cross - tion to the other at some point in between, eg at the mid-plane.

Claims: Claims 1. A blank for a screwing die having a longitudinal axis on which is to bs formed a bore (4), having a screw-cutting thread (5) on its surface, and one or more shaped cavities (2 ; 32; 42) extending through the blank in a generally longitudinal sense, characterised in that the shape and disposition of the shaped cavities (2; 32; 42) is such that for each of a variety of sizes of the bore, i one or more openings (6) are formed through which the bore communicates with the remainder of the said one or more shaped cavities said remainder being flanked by one or more land portions (7) of the blank which carry the screw-cutting threads (5), ii in at least one plane transverse to the longitudinal axis, a cutting edge (leo) having a positive rake angle of between about 5 and l' formed cn the leading edge of each land portion, and iii in the said at least one plane the land portions () form in total between about 30 and 45% of the bore circumference with the said openings (6) forming the remainder of the circumference. 2. A blank for a screwing die according to claim l characterised in tat the said at least one plane is that of an end face of the die blank. 3. A blank for a screwing die according to claim 1 or claim 2 characteri± in that the said positive rake angle is between about 100 and 11 . 4. A blank for a screwing die according to any one of claims 1 to 3 characterised in that the said at least one plane the land portions form in total between about 35 and 45% of the bore circumference. 5. A blank for a screwing die according to any one of the claims 1 to 4 characterised by a plurality of the said shaped cavities (2; 32; 42) which are mutually separate. @@@@@@@@@@@@@ for a @@@ewing die according to claim 5 characterised in that @@@ said shaped cavities (2: 32: 42) are of similar form. A blank for a screwing die according to claim 5 or claim 6 characterised in that the said shaped cavities (2; 32; 42) are symmetrical @lsposed with reference to the longitudinal axis. 8. A blank for a screwing die according to any one preceding claim tharacterised in that each shaped cavity (2 42) has an edge (8; 48; 49) in the said at least one plane in the form of an Archimedes spiral having origin on the longitudinal axis and being of a pitch such that a cutting edge (10) having substantially the same positive rake angle is formed fo each of the variety of sizes of the bore (4). 9. h blank for a screwing die according to any one of claims 1 to 7 characterised in that each shaped cavity (32) has an edge (38; 39) in the said at least one plane made up of a plurality of linear portions, such th; a cutting edge (10) having the said positive rake angle is formed for each of the variety of sizes of the bore. 10. A blank for a screwing die according to any one of claims 1 to 7 characterised in that each shaped cavity (32) has two edges (38,39) in the said at least one plane each made up of a plurality of linear portions, su@ that a cutting edge (10) having the said positive rake angle is formed for each of the variety of sizes of the bore and for both possible directions of rotation of the die blank about its longitudinal axis. 11. A blank for a screwing die according to any one preceding claim characterised in that each shaped cavity (2; 32) is of the same transverse cross-section throughout the length of the blank. 12. A blank for a screwing die according to any one @@ @@@@ characterised in that the shape of each cavit (4?) is such that as viewed from one end of the blank in the longitudinal direction the shape of the shaped cavity at the said at least one plane is the same as the shape of the shaped cavity at a second transverse plane as viewed from the other end of the blank in the longitudinal direction. 13. A blank for a screwing die according to claim 12 characterised in that the cross-section of each shaped cavity (42) is varied smoothly from the said at least one to the second transverse plane. 14. A blank for a screwing die according to claim 12 characterised in that there are two zones of constant transverse cross-section with an abrupt change of section between the said zones. 15. A blank for screwing die according to claim 14 characterised in that the abrupt change of section is mid way between the said at least one and second transverse planes. 16. A blank for a screwing die according to any one of claims 12-15 characterised in that the said at least one and second transverse planes are at opposite longitudinal end faces of the die blank. 17. A blank for a screwing die according to any one preceding claim characterised in that it is formed by investment casting. 18. A screwing die characterised in that it is formed fron a blank according to any one preceding claim having a threaded bore formed on the said longitudinal axis and extending throngh the blank.

Inventors: GREEVES, COLIN STUART
Applicants: THE SECRETARY OF STATE FOR INDUSTRY IN HER BRITANNIC MAJESTY'S GOVERNMENT OF THE UNITED KINGDOM OF GREAT BRITAIN AND
Application year: 1980
Main classification: B23G5
IPC codes: B23G5