GB2063740A - Milling thread rolling dies - Google Patents

Abstract

A method of milling thread rolling grooves in a thread rolling die of the kind having comparatively wide angle thread forming ribs at the entry end of the dies to carry out the initial thread forming and having steeper angled wide ribs at the exit end of the die to produce a wide-root thread form with steeper flank angles in the resulting workpiece comprises using a milling cutter having a steep angle crest portion conforming to the thread form angle and a wider angle root portion corresponding to the angle of the thread forming ribs. The milling cutter cuts to full depth at the entry end of the dies to produce wide angle crests 41 which form initial wide angled grooves 43 in a workpiece. As milling progresses along the thread rolling die, the cutter operates at a reduced depth to produce grooves by which stage the resulting widened crests 48 in the die have spread the material of the workpiece to produce raised portions 51 and 52 which in the final workpiece become united as a single steep angle thread form. <IMAGE>

Description

SPECIFICATION Thread rolling The invention relates to a method of milling thread rolling dies, to a milling cutter for milling the dies and to a thread rolling die produced by the method.

The invention is concerned particularly with the rolling of threads of the kind often used as self tapping threads or woodscrew threads which have a wide flat root potion and steep angle flanks. The included angle between the two flanks may for example be 60 degrees and the length of the flat root may be about a third to a half of the pitch dimension of the thread. When rolling threads of this form, the resulting root diameter of the screw is only a little smaller than the diameter of the original screw blank because the crest portion which has to be formed by this displaced material is very much narrower than the root.Thus during the initial part of the thread rolling operation it is desirable to ensure that the material is being displaced laterally through a substantial distance rather than simply being displaced in an outward direction close to the initially narrow root. To achieve this type of displacement, it is desirable for the initial engagement of the die with the workpiece to be by a sharp pointed crest and for the width rather than the depth of this crest to increase continuously during the rolling operation. In the interests of resistance to wear of the crest and in the interests of achieving effective lateral displacement, it is desirable for this crest to have a wider angle than the required steep angle of the resulting thread form.

The formal way of achieving a wide angle at the start of the thread rolling die and a steeper angle at the end of the rolling die to correspond to the required thread form is to use two different milling cutters in the production of the die. This not only requires an additional operation but also involves the problem of setting up the two milling operations with respect to each other in such a way that the transition between the two flank angles occurs in a desired controllable manner.

An object of the present invention is to overcome this difficulty.

According to one aspect of the invention there is provided a method of milling a thread rolling die of the kind having comparatively wide angle thread forming ribs at the entry end of the dies to carry out the initial thread forming and having steeper angle wide ribs at the exit end of the die to produce a wide-root thread form in a workpiece, including the step of milling the surface of a die blank with a cutter having a steep angle crest portion conforming to the thread form angle and having a wider angle root portion corresponding to the profile of the initial thread forming ribs, the milling cutter cutting to full depth at the entry end of the dies to produce the wide angle crests (together with steep angle roots) and the milling cutter cutting to any part of the depth at the exit end to produce wide ribs with steep angle flanks.

The invention also extends to a milling cutter having a steep angle crest portion and a wider angle root portion for use in the above method. The invention further extends to a thread rolling die produced by the above method.

The die may additionally be equipped with additional features such as are employed in conventional dies including a chamfered leadin and trailing portion, a pointing and shearing section and a knurled portion to assist in rotating the workpiece.

An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of a thread rolling operation.

Figure 2 is a front elevation of a thread rolling die.

Figure 3 is an end elevation of the die of Fig. 2.

Figure 4 is a plan view of the die of Fig. 2.

Figure 5 diagrammatically represents the milling form of a milling cutter used in the production of the dies; and Figures 6a to 9b illustrate the form of the die and a threaded workpiece at various positions along the die and at corresponding stages of the production of a screw thread.

As shown in Fig. 1, a die set incorporates a fixed die 11 and a moveable die 1 2 which during a thread rolling operation is moved across the surface of the die 11 as indicated by arrow 1 3 and the chain dotted outlines of further positions of the die 1 2. The working faces 14 and 15 of the dies 11 and 12 incorporate alternate grooves and ribs which are engaged by a workpiece 1 6 as the workpiece is rolled between the dies in order to form a thread on the workpiece.

The die 1 2 also has a chamfered lead-in at 1 7 and a chamfered trailing edge at 1 8 to assist initial engagement and final release of the workpiece. In other respects the two dies are similar to each other, except that the fixed die 11 is a little longer than the moveable die 1 2. Further details of the two dies can be seen from the following description of the die 1 2 in connection with Figs. 2, 3 and 4.

Die 1 2 incorporates a main thread rolling region 24 incorporating alternate grooves and ribs, details of which will be described subsequently. At the lead-in edge of the die and over part of the length of the die, the ribs are serrated as illustrated at 1 9 in order to ensure that the workpiece does rotate between the dies rather than sliding along between them.

The die also incorporates a ridge 21 which starts a short distance in from the lead-in edge 22 and gradually increases in height. This ridge has a sloping flank towards the thread rolling region 24 of the die. The purpose of this ridge, in conjunction with a corresponding ridge on the other die 11, is to roll a point on to the workpiece. A knurled but ungrooved region 23 on the opposite side of the ridge 21 from the thread forming region 24 is intended to grip and rotate the end portion of the workpiece which is then sheared by the co-operating action of the ridges to leave a pointed screw.

As thus far described, the dies and the thread rolling operation are conventional. The invention is concerned solely with the details of the shape of the thread forming ribs and grooves and the way in which these ribs and grooves are produced.

Fig. 5 shows the cutting form of a milling cutter in accordance with the invention for use in producing the thread forming ribs and grooves in the rolling die. The milling cutter is shown as having three crests 31 enabling it to mill three adjacent grooves in one pass along the rolling die. The use of three crests is intended merely to facilitate understanding of the invention and in practice the milling cutter may have only one or two crests or any other number of crests. The included angle P between adjacent flanks 32 and 33 at each crest 31 is made to correspond to the included angle required in the final thread form. In this example, this angle is 60 degrees. The 60 degree flank angle continues for about 50% of the depth from crests 31 to roots 34. The included angle Q between the lower portions 35 and 36 of the flanks is greater than the 60 degree angle P.In practice a suitable angle is 75 degrees, but in the interests of illustrating the difference between the two angles more clearly in the drawings, the angle has been shown as 90 degrees.

At the entry end of the die, the milling cutter of Fig. 5 is arranged to mill the full depth of its profile in the face of the die as illustrated by the die form in Fig. 6a. This results in wide angled crests 41 in the die form together with narrow angle roots 42 between adjacent crests. Only a small region around the crests 41 engages with the workpiece so the steep angle roots are of no significance to the operation of the die. They are merely an incidental feature which comes about by virtue of the method of manufacture of the die. The thread form which is produced near to the entry end of the die is shown in Fig. 6b. The crests 41 have penetrated a small depth into the material of the workpiece to produce shallow grooves 43. The material which had occupied the grooves 43 in the original workpiece is displaced to both sides of the groove and is raised up slightly as shown at 44.

As the milling cutter of Fig. 5 progresses along the die, it is gradually raised out of the die so that the grooves in the die are less deep. Fig. 7a shows the situation where the grooves are slightly less deep than in Fig. 6.

One effect of this less deep milling is that the roots 34 of the milling cutter remain proud of the original surface of the die so that small flats 45 occur at the crests of the ribs on the due. During the rolling operation, the dies are set so that the flattened crests 45 penetrate into the workpiece to approximately the same distance as the penetration of the pointed crest 41 of Fig. 6. However the width of the crest has increased and this results in an increase of the lateral displacement of the material from the groove in the workpiece.

The resulting widened groove 46 is shown in Fig. 7b. The additional material which has been displaced from the groove results in a raised portion 47 which is higher and wider than the raised portion 44 in Fig. 6b.

Fig. 8 shows the situation still further along the die from the entry end. The flat crest portion 48 is wider than in Fig. 7 with the result that the groove 49 in the recess is still wider than that of Fig. 7. Also, the ridges of displaced material from two adjacent grooves 49 are beginning to join up in Fig. 8 to form a single raised portion although the amount of raising at the centre of this portion is less than at the edges. Also, these raised portions 51 have by the stage of Fig. 8 reached into the shallow flank angle portion 42 so that the beginnings of a shallow flank angle are becoming evident on the raised portions at 52.

The transition evident from Figs. 6, 7 and 8 continues with further distance along the die until the depth of cut of the milling cutter is equal only to the depth of the steep angled flanks 32 and 33. This results in a final die form as shown in Fig. 9a which corresponds to the required final thread form shown in Fig.

9b. The grooves in the die continue at this depth for some further distance along the die in order to provide the resulting workpiece with a regular well finished thread form to the shape shown in Fig. 9b.

The initial wide angle of the effective part of the thread rolling die as shown in Fig. 6 results in a greater area of contact between die and screyv blank than would be the case with the steeper thread angle, thereby reducing the tendency of slippage. Other advantages are stronger crests on the ribs of the die and better distribution of work along the face of the die both of which tend to increase the life of the die. This wide initial angle is achieved in conjunction with a shallow angle thread form without having to resort to two different milling cutters with different angles.

Instead of having a single wider angle Q for the lower part of the milling cutter and thus for the crest part of the die form shown in Fig.

6, a stepped transition from the 60 angle at the crest of the cutter could be employed.

Thus instead of an angle Q of 75 , the profile could be stepped with portions at angles of say 70 , 80 and 90 . This would give a very wide initial angle for the die, giving good wear characteristic, with a stepped transition to the final 60 angle. Instead of a stepped transition, the milling cutter profile could be arcuate, giving a gradual transition.

The invention can be used in connection with dies for wood screws or for self tapping screws or for any other kind of screw which requires a thread form with a wide root and steep flank angles. The detailed shape of the milling cutter and of the grooves in the thread rolling die will of course depend on details of the thread form required. For example with a required thread angle P of 40 degrees, the angle Q could be 55 degrees. In more general terms the ratio between the angles should preferably be between 1.2:1 and 1.4:1.

Claims (5)

1. A method of milling a thread rolling die of the kind having comparatively wide angle thread forming ribs at the entry end of the dies to carry out the initial thread forming and having steeper angle wide ribs at the exit end of the die to produce a wide-root thread form in a workpiece including the step of milling the surface of the blank with a cutter having a steep angle crest portion conforming to the thread form angle and a wider angle root portion corresponding to the profile of the initial thread forming ribs, the milling cutter cutting to full depth at the entry end of the dies to produce the wide angle crests (together with steep angle roots) and the milling cutter cutting to only part of the depth at the exit end to provide wide ribs with steep angle flanks.

2. A method as claimed in Claim 1 wherein the ratio of the wide angle of the thread forming ribs to the angle of the steep angle thread form is between 1.2:1 and 1.4:1.

3. A method as claimed in Claim 2 wherein the wide angle thread forming ribs have an included angle of approximately 75 degrees and wherein the steep angle thread form has an included angle of approximately 60 degrees.

4. A milling cutter for use in the method of Claim 1 having a steep angle crest portion conforming to a desired thread form angle and a wider angle root portion corresponding to the required angle for initial thread forming ribs in a thread rolling die.

5. A thread rolling die manufactured by a method as claimed in Claim 1 or Claim 2 or Claim 3.