GB1571786A - Method of producting an internal screw thread and a tap for use in the method - Google Patents

Description

(54) METHOD OF PRODUCING AN INTERNAL SCREW THREAD AND A TAP FOR USE IN THE METHOD (71) We, GKN BOLTS AND NUTS LIMITED, a British Company of P.O. Box 12, Kings Hill, Wednesbury, West Midlands, 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: This invention relates to a method of producing an internal parallel screw-thread and to a tap for use in the method.

In the production of nuts, especially rela timely low priced nuts produced in large quantiuies, an apertured blank and a tap are subjected to a relative rotation and to relative displacement along an axis which is at least approximately common to the tap and blank so that a tapered leading end portion of the tap enters the aperture in the blank. This displacement is promoted in various ways, for example by means of a spring, by gravity and by means of a cam, and does not normally occur at a speed which is exactly related to the speed of relative rotation by the pitch of the thread which is to be formed.As the displacement continues, the tap engages the blank in such a manner as to commence to produce the thread in the blank and eventually a stage is reached at which the thread in the blank is sufficient to co-operate with the thread of the tap to control the relative displacement in accordance with the pitch of the thread and the speed of rotation. In so far as the thread of the nut is produced after this stage, the thread of the nut will have the correct pitch, but the form of that part of the thread which was produced prior to this stage (i.e.

when the speed of relative displacement is not controlled by co-operating threads on the tap and the nut) is commonly incorrect.

A further cause of a part of the thread produced in an apertured blank having an incorrect form is a change in the angular relation between the respective axes of the tap and blank after production of the thread in the blank has commenced. Normally, a smaller diameter portion of the tapered leading end portion of a tap has radial clearance with respect to the blank when it enters the aperture thereof. Accordingly, when the tap first enters the aperture of the blank, co-operation, if any, between the tap and blank does not contribute to the establishment of a truly coaxial relation between the tap and blank.

Reliance is placed entirely upon respective supports for the blank and tap to control the respective positions and attitudes of the tap and blank. In consequence of wear of the part of the machine which supports the tap and blank, and variations in the dimensions of blanks within permitted tolerance ranges, the accuracy with which the blank and tap are positioned is not generally very high and varies during use of a particular machine.

Accordingly, when the tap first begins to produce the thread in the blank, the respective axes of tap and blank may not be properly aligned. As the tap penetrates further through the blank, it will exercise a greater degree of control over the position of the blank so that, when production of the internal thread is completed, the axis of this thread will coincide exactly with the axis of the tap. Thus, it is common for some change in the angular or positional relation of the respective axes of tap and blank to take place during the production of the thread in the blank. That part of the thread which is produced before this change is completed is commonly not of the correct form.

The production of malformed threads in nuts can be reduced by the use of more elaborate, and therefore expensive, machinery.

For example, nuts which are required to have threads of high accuracy are produced on machines which incorporate a lead screw for controlling relative displacement of the nut and tap exactly in accordance with the speed of relative rotation and the pitch of the thread. Substantial coincidence of the respect tive axes of tap and blank can be ensured by the provision of elaborate support means for supporting the tap and blank. These solutions cannot be used in the production of relaitvely low priced nuts, since they involve the use of very expensive machinery and generally achieve only a relatively low production rate.

Relatively low priced nuts are commonly produced by a method in which the nuts pass along the threaded portion of the tap and then along a shank of the tap to be discharged from an end of the tap opposite to the tapered leading end portion. With this procedure, the tap rotates continuously in one direction and the thread is produced on one blank whilst a previously threaded nut is being discharged from the tap. With this method, the tap is located by a row of nuts which embrace the shank of the tap and these nuts are in turn confined in a tube of the machine which carries out the operation.

Variations in the dimensions of the blanks, within permitted tolerance ranges, prevent accurate control of the position of the row of nuts by means of the tube and of the tap by means of the row of nuts. Wear of the machine during its working life further contributes to the difficulty of producing nuts with properly formed threads.

It is an object of the present invention to provide a method and a tap for use in the method which reduces or overcomes the foregoing problems without necessarily resorting to the use of elaborate machines which control the positions and relative motion of the tap and blank with a high degree of accuracy.

According to a first aspect of the invention there is provided a method of producing an internal parallel screw thread in an apertured blank wherein by subjecting the apertured blank to a thread forming tap a thread is produced initially in the apertured blank having a greater separation, as hereinafter defined, between its flanks than is required in the finished thread and is of a form such that its flanks are connected by a crest which is narrow, as compared with the distance be- tween successive crests, rhe thread form subsequently being thinned progressively to give the required separation between its flanks.

With this method, a small misalignment between the respective axes of the blank and tap during an initial part of the threading operation will not cause material to be removed from a position in the blank at which material is required to be present in the finished nut. Similarly, relative displacement of the tap and blank at a speed which differs a little from the speed which corresponds exactly to the pitch of the thread and the speed of relative rotation will not cause material to be removed from a part of the blank at which material is required to be present in the finished nut, provided that the speed of relative displacement is corrected during an initial part of the threading operation.If the stage at which co-operation between the tap and blank is sufflcient to bring their respective axes into coincidence and to ensure that the speed of relative displacement is exactly correct, is reached before the thinning of any part of the thread is completed, the thread of the completed nut will be satisfactory.

By the separation between the flanks of the thread form, we mean the distance measured along a line, which line is parallel to the axis of the thread form and is spaced in each case by the same distance from the axis, between successive points where the line intersects the surface of the thread form, the space between said successive points being occupied by the solid material in which the thread is formed.

Preferably, in the form of the thread which is produced initially, each flank is spaced by substantially the same amount from the position of the corresponding flank in the final thread form. In other words, the finished thread is produced by removing substantially the same thickness of material from all of the flanks of the initially produced form.

The form of the thread at any selected position along the blank may be shallow initially and progressively deepened. The thinning of the thread form at any such position may be completed before it acquires its final depth.

According to a second aspect of the invention there is provided a tap having a leading portion and a following portion, the leading portion being the portion which determines the initial form of the thread cut by the tap in use, wherein the separation, as hereinbefore defined, between the flanks of the thread form of the tap is greater in the following portion than it is in the leading portion and increases along the leading portion in a direction from a leading end of the tap towards the following portion, and wherein the thread form od the leading portion has more than two cutting edges, the pitch of the thread form being substantially the same throughout the leading and following portions.

When a tap of this form is used to produce an internal thread in an apertured blank, the leading portion preceding the following portion into the aperture, each turn of the thread produced in the blank is produced initiallv with a greater separation between its flanks and is subsequently thinned to give a separation corresponding to that between the flanks of the following portion of the tap.

A portion of the tap which includes the leading portion is preferably tapered so that the height of the thread increases progre sively in a direction from a leading end of the tap towards the following portion.

The entire leading portion of the tap may be tapered. Furthermore, the tapered portion may be longer than the leading portion and may extend beyond both ends of the leading portion.

The invention will now be described by way of example with reference to the accompanying drawings, wherein: FIGURE 1 shows a longitudinal cross section of a threaded part of a tap in accordance with the invention, and FIGURE 2 is a similar section showing the tap, two nuts which have been threaded by the tap and a blank which is in course of being threaded by the tap.

The tap illustrated in Figure 1 is a fluted bent shank tap. Bent shank taps are well known in the art for use in processes in which the tap is rotated continuously in one direction, blanks are fed onto an end of the tap, are threaded thereby and pass along the tap to be discharged from an opposite end thereof substantially continuously. Commonly, such taps are provided with four longitudinallyextending flutes which may be parallel to a longitudinal axis 10 of the tap, or of helical form. Whilst the invention has been developed primarily for this particular type of tap, it can be applied to other types of tap, for example straight shank taps and taps which are stationary in use, the blanks being rotated.

The tap is formed with a thread, the turns of which are indicated by the reference numbers 11 to 24, beginning at a leading end of the tap. An end portion 25 of the tap is tapered so that the turns 11 to 22 of that portion are truncated so as to provide the thread with three cutting edges. The first two turns, 11 and 12, are truncated so severely that they can be inserted into the aperture of a blank in which an internal thread is to be cut by the tap, without removing any material from the blank. The four turns 13 to 16 of the thread occupy a portion of the tap referred to herein as the leading portion 26.

This portion is shorter than the tapered portion 25 and lies within that tapered portion.

The threaded portion of the tap which lies outside the tapered portion 25 is referred to herein as the following portion and is indicated by the reference number 27 in Figure 1.

Although, for convenience of illustration, only two turns, turns 23 and 24, of the following portion are shown, it will be appreciated that this portion would normally comprise a considerably larger number of turns of the thread.

For convenience of illustration, the leading portion 26 is shown in Figure 1 as comprising only 4 turns of the thread. Normally, the leading portion would comprise a larger number of turns, typically from 10 to 14 turns..

The turns of the thread which lie in the following portion 27 of the tap are each identical with one another and correspond in form to the thread which is required to be produced in each blank. The respective forms of the cross sections of the turns 13 to 16 of the leading portion of the tap shown in Figure 1 differ from the form of the thread in the following portion in two respects.

Firstly, as previously mentioned, the turns 13 to 16 are truncated. Secondly, each of these turns is thinner than the turns of the following portion. As the flanks of the thread diverge towards the axis 10 of the tap, the thicknesses of respective forms of the thread at different positions along the tap can be compared only by reference to the separation between the flanks of such forms at a given distance from the axis 10. The separation of the flanks of the cross sections of the turns 14 to 17 and 23 of the thread at a distance X from the axis 10 are indicated by the reference numbers 28 to 32 respectively. The dimensions 31 and 32 are identical. The dimension 30 is slightly less, for example two or three thousandths of an inch less, than the dimension 31.The dimension 29 is smaller by a similar amount than the dimension 30, the dimension 28 is smaller by a corresponding amount than the dimension 29 and the turn 13 is similarly slightly thinner than the turn 14 of the thread.

Preferably, the angle of inclination of the flanks of all of the thread to. the axis 10 is the same.

The pitch of the thread of the tap is also uniform along its entire length. Adjacent to the turns 13, 14, 15 and 16in Figure 1 there is shown in broken lines a thread form identical to. that of the turns 23 and 24 of the following portion. It will be seen that the leading flank of each of the turns 13 to 16 is displaced rearwardly with respect to the thread form shown in broken lines by the same distance as the following flank is displaced forwardly with respect to the thread form shown in broken lines.

The tap shown in Figure 1 may be used to. produce an internal thread in apertured blanks by rotating the tap continuously in one direction and in a fixed position and feeding blariks, which are constrained against rotation, onto the tapered portion 25 of the tap. Such feeding may be affected by means of springs or of a cam. In Figure 2, there is shown a blank 33 which has been fed sufficiently far onto the tap for the cutting of the thread in the blank to have commenced.

Since the end portion of the tap which comprises the turns 11 and 12 of the thread can be received within the aperture 34 of the blank with a slight clearance, this end portion cannot exercise accurate control over the relative positions of the blank and tap. When contact is first established between the tap and the blank, the axis 35 of the blank is unlikely to coincide exactly with the axis 10 of the tap, unless the machine which supports the tap and the blank is sufficiently elaborate and unworn to control the positions of both the blank and the tap with a high degree of accuracy. As shown in the drawing in an exaggerated manner, for purposes of illustration, the axis 35 of the blank may be inclined to the axis of the tap.The axis 35 rnay also be offset from the axis 10 so that these axes do not intersect or do not intersect at a position within the aperture 34.

When the blank 33 has advanced to the position shown in Figure 2, in which it is en gaged with some of the turns of the leading portion 26 of the tap, this leading portion will exercise some control over the portion of the adjacent end portion of the blank rela tive to the axis 10. If the blank is sufflciently free from constraint, at least this end portion of the blank is likely to be brought into a position in which it is accurately centered on the axis 10. However, at this stage, the tap cannot accurately control the position of the opposite end of the blank which may therefore remain out of alignment with the tap. As will be seen from Figure 2, when the blank has reached the position illustrated therein, cutting of the thread in the blank has commenced.If the tap was of conven tional form, i.e. with the spacing of the flanks of the thread being uniform throughout the thread, misalignment between the axes 35 and 10 may result in removal from the blank of material which is required to be present in the finished nut The amount of thread which has been cut in the blank 33 at the stage of formation of the thread illustrated in Figure 2 is not suffi cient to co-operate with the thread of the tap and reliably control movement of the blank along the axis 10 in accordance with the speed of rotation of the tap and the pitch of the thread. Prior to and at the stage illu strated in Figure 2, the speed of advancement of the blank along the axis 10 will depend upon the spring or other means used to feed the blank onto the tap and upon the resistance to such movement resulting from engagement between the tap and blank.Since this latter resistance will vary as the blank moves onto the tap, the speed of advancement is also likely to vary. If the speed of advancement of the blank differs from that speed which corresponds exactly to the pitch of the thread of the tap and the speed of rotation of the tap, the pitch of the thread cut in the blank will differ from the pitch of the thread of the tap. It will be appreciated that if the separation between the flanks of the turns 13 to 16 of the tap was the same as the separa tion of the flanks of the turns 23 and 24, then it is likely that there would be removed from the blank some material which is required to be present in the finished nut.

As the blank advances further along the tap and contact between the nut and tap becomes more extensive, the tap exerts an increasing control over the position of the blank relative to the axis 10 and also over the speed of advancement of the blank along this axis. The leading portion 26 of the tap is made sufficiently long to ensure that before any part of the blank moves cn from the leading portion towards the following portion, the axis 35 of the blank coincides with the axis 10 and the speed of advancement is exactly in accordance with the speed of rotation of the tap and the pitch of the thread thereof.The turns of the thread of the leading portion of the tap are sufficiently thin, as compared with the turns of the following portion, to ensure that, prior to this stage at which the tap exercises full control over the blank, no material which is required to be present in the finished nut is removed from the blank. It will be appreciated that, as the turns 11 and 12 of the thread of the tap do not remove material from the blank, it is not necessary for these turns to be thinner than the turns 23 and 24.

The thread at any position along the aperture 34 of the blank is formed progressively by the turns 13 to 23 of the thread of the tap. The turn 13 cuts material away from the flanks of the thread of the blank to a shallow depth only forming a turn having a form with a crest which is narrow relative to the distance at the minor diameter between that turn and the next turn formed. Furthermore, this turn leaves the thread of the blank wider than is required in the finished nut. The turns 14, 15 and 16 of the thread of the tap progressively deepen and reduce the width of the thread formed in the blank until the required separation between the flanks is achieved. The turns 17 to 23 progressively deepen the thread of the blank without altering the separation between the flanks.

The blank travels further along the tap until the thread in the blank is completed and the blank becomes a nut. The nuts continue to travel along the tap and are discharged from an end thereof remote from the tapered end. The tap may be formed with an unthreaded shank which is a sliding fit within the threaded apertures of the nuts so that the tap can be supported by means of a row of nuts 36 which embrace the shank.

Whilst the method of the invention is ap plicable with especial advantage to the production of nuts through which the tap can pass completely, it may also be applied to the production of internal threads in blind bores, particularly in cases where the thread is not required to extend completely up to the closed end of the bore, and generally to the production of threads by relative reciprocating motion of the tap and nut.

The Manufacture of taps of known form includes the step of relieving each cutting tooth by removing material from the back of the tooth to give a greater radial dimension at the cutting edge of the tooth than at other parts of the tooth. It will be understood that the front and back of the tooth are spaced apart around the circumference of the tap, each tooth forming a part of one turn of the thread form of the tap. This step of relieving each cutting tooth is carried out by means of a cutting wheel having a form complementary to the form od the thread of the tap. The tap is turned about its own axis and the cutting wheel is fed in towards the axis of the tap as the cutting wheel traverses along the length of each tooth in a row of cutting teeth.To produce a tap as shown in Figure 1, the form of the cutting wheel in a region corresponding to the leading portion 26 of the tap is modified so that the cutting wheel removes material from the flanks of the teeth in the leading portion. The relieving operation is carried out with this modified cutting wheel in the same manner as with the known cutting wheel.

WHAT WE CLAIM IS: 1. A method of producing an internal parallel screw thread in an apertured blank by subjecting the apertured blank to a thread forming tap, wherein a thread is produced initially in rhe apertured blank having a greater separation, as hereinbefore defined, between its flanks than is required in the finished thread and is od a form such that its flanks are connected by a crest which is narrow, as compared with the distance between successive crests, the thread form subsequently being thinned progressively to give the required separation between its flanks.

2. A method according to Claim 1 wherein, in the thread form produced initially, each flank is spaced from the position of the corresponding flank of the final thread form by substantially the same distance.

3. A method according to Claim 1 or Claim 2 wherein the respective angles, at which the flanks of both the thread form produced initially and the final thread form are inclined to the axis of the thread are the same.

4. A method according to any one of Claims 1 to 3 wherein the form of the thread at any selected position along the blank is shallow initially and is progressively deepened.

5. A method according to Claim 4 wherein the thinning of the thread form at any selected position is completed before the thread form acquires its final depth.

6. A tapl having a leading portion and a following portion, the leading portion being the portion which determines the initial form of the thread cut by the tap in use, wherein the separation, as hereinbefore defined, between the flanks of the thread form of the tap is greater in the following portion than it is in the leading portion and increases along the leading portion in a direction from a leading end of the tap towards the following portion, and wherein the thread form of the leading portion has more than two cutting edges, the pitch of the thread form being substantially the same throughout the leading and following portions.

7. A tap according to Claim 6 wherein each flank of the thread form of the tap is inclined to the axis of the tap at the same angle.

8. A tap according to Claim 6 or Claim 7 wherein a portion of the tap is tapered and the tapered portion is longer than the leading portion and extends beyond both ends of the leading portion.

9. A tap according to any one of Claims 6 to 8 wherein a portion of the tap which includes the leading portion is tapered so that the height of the thread form increases pro; gressively in a direction form a leading end of the tap towards the following portion.

10. A method of producing an internal parallel screw thread substantially as herein descrl.

11. A tap substantially as herein described with reference to and as shown in the accompanying drawings.

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

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. and the cutting wheel is fed in towards the axis of the tap as the cutting wheel traverses along the length of each tooth in a row of cutting teeth. To produce a tap as shown in Figure 1, the form of the cutting wheel in a region corresponding to the leading portion 26 of the tap is modified so that the cutting wheel removes material from the flanks of the teeth in the leading portion. The relieving operation is carried out with this modified cutting wheel in the same manner as with the known cutting wheel. WHAT WE CLAIM IS:

1. A method of producing an internal parallel screw thread in an apertured blank by subjecting the apertured blank to a thread forming tap, wherein a thread is produced initially in rhe apertured blank having a greater separation, as hereinbefore defined, between its flanks than is required in the finished thread and is od a form such that its flanks are connected by a crest which is narrow, as compared with the distance between successive crests, the thread form subsequently being thinned progressively to give the required separation between its flanks.

2. A method according to Claim 1 wherein, in the thread form produced initially, each flank is spaced from the position of the corresponding flank of the final thread form by substantially the same distance.

3. A method according to Claim 1 or Claim 2 wherein the respective angles, at which the flanks of both the thread form produced initially and the final thread form are inclined to the axis of the thread are the same.

4. A method according to any one of Claims 1 to 3 wherein the form of the thread at any selected position along the blank is shallow initially and is progressively deepened.

5. A method according to Claim 4 wherein the thinning of the thread form at any selected position is completed before the thread form acquires its final depth.

6. A tapl having a leading portion and a following portion, the leading portion being the portion which determines the initial form of the thread cut by the tap in use, wherein the separation, as hereinbefore defined, between the flanks of the thread form of the tap is greater in the following portion than it is in the leading portion and increases along the leading portion in a direction from a leading end of the tap towards the following portion, and wherein the thread form of the leading portion has more than two cutting edges, the pitch of the thread form being substantially the same throughout the leading and following portions.

7. A tap according to Claim 6 wherein each flank of the thread form of the tap is inclined to the axis of the tap at the same angle.

8. A tap according to Claim 6 or Claim 7 wherein a portion of the tap is tapered and the tapered portion is longer than the leading portion and extends beyond both ends of the leading portion.

9. A tap according to any one of Claims 6 to 8 wherein a portion of the tap which includes the leading portion is tapered so that the height of the thread form increases pro; gressively in a direction form a leading end of the tap towards the following portion.

10. A method of producing an internal parallel screw thread substantially as herein descrl.

11. A tap substantially as herein described with reference to and as shown in the accompanying drawings.