GB2045666A - Coarse pitch screw thread manufacture - Google Patents

Abstract

A tap 13 for producing a coarse pitch screw thread with one flank steeper than the other has cutting teeth formed on a thread of the tap, the teeth reducing in axial length towards the lead-in tap end. This is achieved by applying a cutting tool to the teeth, when initially fully formed, the tool being of different pitch. The cuts are made successively to produce a progressive removal of tooth material at the steeper, e.g. radial flank, end. <IMAGE>

Description

SPECIFICATION Screw thread manufacture The invention is concerned with screw thread manufacture. More particularly the invention is concerned with the manufacture of female screw threads of a kind which have a coarse pitch as compared with their diameter. This normally implies a coarse pitch as compared with the root to crest dimension of the thread.

The most convenient way to produce an internal thread is normally by means of a tap.

A conventional tap is produced by making a complete screw thread of the required pitch and profile on a shank, tapering the threaded shank over a lead-in portion of the tap by removing part of the crest of the thread whilst retaining the normal thread root. The taper continues gradually until there is virtually zero thread at the lead-in end and the shank diameter is equal to or even slightly reduced from the root diameter. Longitudinal flutes are then ground through the screw threads to produce cutting edges.

To produce a female thread with a tap as described above, the tap is engaged in a hole to be threaded and is then simultaneously rotated and advanced through the hole. The cutting edges of the tap near the lead-in end remove from the interior of the hole a thin slice of material which has a dimension axially of the thread which is only slightly less than the pitch of the thread. As the tap advances, successive slices are removed, and with a conventional thread form these successive slices are progressively narrower as the newly produced thread becomes deeper. This procedure can be entirely satisfactory for conventional screw threads.

However, when the thread is of an unusually coarse pitch, compared particularly with its diameter but also in comparison with its root to crest dimension, the requirement to remove several wide slices of thread simultaneously by a succession of cutting edges of the tap produces an excessively high torque requirement. The comparatively small diameter of the shank in relation to the width of cut can make it impossible to provide a shank of sufficient strength to tap the thread without shearing the tap.

An object of the invention is to provide a thread manufacturing technique for overcoming this problem. While the invention is intended primarily for the production of coarse pitch threads, there is no specific lower limit of pitch at which the invention is applicable and the method of the invention could if desired be used with threads of conventional dimensions.

According to one aspect of the invention there is provided a tap for producing a screw thread in which one flank is steeper than the other flank, the tap including a shank and screw cutting teeth on the shank, the teeth reducing in size towards the lead-in end of the tap, characterised in that the size reduction of the teeth incorporates a reduction in the axial length of the teeth from the end of each tooth which has the steeper flank. In this way, the conventional requirement to remove wide slices of material when producing the thread no longer applies. Instead, the gradual removal of thread takes place in an axial direction.

According to a second aspect of the invention a method of making a tap according to the first aspect of the invention includes the steps of cutting a first thread of the desired profile on a workpiece and carrying out further thread cutting operations over the lead-in portion of the thread at a pitch differing slightly from that of the first thread, a series of such cuts being made over a series of phase relationships with the first thread at the top of the tap, each such cut being made between the tip and a position towards the root of the lead-in portion where the cut becomes in phase with the first mentioned thread.

Further details of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 shows a screw thread of the kind to which the invention is particularly applicable.

Figure 2 is a cross section of the thread of Fig. 1.

Figure 3 is a cross sectional view of a tap of the present invention; Figures 4 to 7 show progressive steps in the tapping of a thread in a nut; and Figures 8 to 10 illustrate a series of steps by which the thread of Figs. 1 and 2 can be modified to produce the desired taper for a tap.

The thread form shown in Figs. 1 and 2 is a buttress type thread with one shallow flank 11 and one steep flank 1 2 which is either radial or very near to radial. The pitch P of the thread is approximately equal to the mean diameter D of the thread which represents a very coarse pitch. Also the pitch P is very much greater than the root to crest dimension C of the thread; in this example P is approximately equal to 3C. In order to tap a corresponding female thread using a conventional tap, the width of cut for the initial cuts of the thread would be only slightly less than P.

With a conventional tap, several cuts are made at the same time and the torque required to make these cuts would be unacceptably high and would tend to break the tap.

A suitable tap in accordance with the invention for producing a female screw thread of the form corresponding to Figs. 1 and 2 is shown in Fig. 3. The tap itself is entirely conventional apart from the thread form, which is a matter of choice, and the nature of the taper of the screw thread on the lead-in portion L of the tap. Beyond the lead-in portion, the screw thread in the tap corresponds to the form of the thread as shown in Fig. 2.

In the root region 1 3 of the lead-in portion L the thread form has been slightly tapered by removal of a small area 14 which is shown in chain dotted outline to enable the actual shape of the tapered thread to be compared with the basic thread. This thread removal is primarily a reduction in the width of the thread but due to the buttress form of the thread there is also a corresponding reduction in the peak diameter of the thread. However remaining portions of the thread do retain their full local diameter. In other words, no material has actually been removed from the depth of the thread as such. This is in complete contrast with the conventional tapering of a tap and is a fundamental feature of the present invention.

As the thread progresses towards the tip 1 5 of the lead-in portion, there is a gradual and continuous further reduction in the width of the thread until at the lead-in tip, the width has been reduced to zero so that the thread has disappeared. The material removed, as shown in chain dotted lines, has of course increased correspondingly towards the tip 1 5.

The usual longitudinal grooves in the tap interrupt the continuous tapering screw thread and provide a series of radial cutting edges at the trailing side of each groove, each cutting edge having a length slightly greater than that of its predecessor (going around the thread form towards the tip) so that when used to cut a thread, successive cutting edges can remove a little more of the axial length of the required thread.

A tapping operation is illustrated in Figs. 4 to 7 each of which shows part of a nut diagrammatically in cross section, the tapping operation being further advanced in successive figures. In Figure 4, the final intended thread form is shown in chain dotted lines and the extent to which the thread has been produced by the tip portion of the tap is shown in full. As the tap progresses through the nut, the situation shown in Fig. 5 is reached, the previous cut of Fig. 4 being shown in chain dotted outline to give an indication of the additional cut which has been carried out to reach the stage of Fig. 5.

As the tap progresses through the nut, a series of additional cuts are made as shown by the series of lines in Fig. 6 and eventually the situation shown in Fig. 7 is reached in which the thread has been tapped completely.

During the thread cutting operation, it may not be possible to rely on the tap to feed in to the nut which it is cutting at the appropriate rate and this would lead to inaccuracies in the thread or even to effectively reaming out the nut to the full thread depth without leaving any thread. To guard against this possibility, the tapping operation should be carried out on a conventional controlled lead thread cutting machine to ensure that the various cuts are made in the correct axial positions.

In both Fig. 3 and in Figs. 4 to 7, the difference in size between adjacent cutting portions of the tap and the width of the corresponding cuts made by the tap have been exaggerated in order to illustrate the invention. In a typical example, the pitch P of the thread may be 4.23mm, the thread on the tap could be tapered over a distance of 84.6mm and the tap could have six flutes to give six longitudinal cutting edges around its circumference. In such a case, the thickness of material removed by each cut would be approximately 0.035mm.

One possible method of manufacture of the tapered screw thread on the tap will now be explained with reference to Figs. 8 to 10. Fig.

8 shows a cutter 1 6 in a position close to but spaced slightly from the tip of the lead-in portion of the tap. The cutting tool 1 6 is set up to move along the workpiece as the workpiece is rotated at a pitch which is slightly less than the pitch of the main screw thread. Due to this reduced pitch, the cutter gradually cuts into the steep flank of the screw thread and reduces the length of the thread by removing the areas marked in chain dotted lines. The position of the cutter after one complete revolution of the workpiece is also shown at 1 7.

As thus far described, the width of the teeth to the right of the starting position of the cutter 1 6 has been reduced slightly and progressively towards the tip. The next cutting operation is illustrated in Fig. 9. On this occasion, the cutter 1 6 is started in contact with the steep flank of the screw thread at a position a little further from the tip of the cutter and the cutting operation is repeated with the tool moving towards the tip with the same reduced pitch. The effect is for the cutter to cut progressively more deeply into the screw thread as it moves towards the tip as is illustrated in Fig. 9. However the depth of the cut does not become excessive because of the previous cut as described with reference to Fig. 8. The process is repeated for small increments of the starting position of the cutter away from the tip. Fig. 10 shows a series of cuts representing the whole of the material which has been cut away to leave a fully tapered thread as shown in Fig. 3. As with the cutting of the thread in a screw, the size of the individual increments has been exaggerated to facilitate illustration of the invention and in practice a large number of cuts at a corresponding number of starting positions would be employed.

In the above explanation, the direction of cut of the reduced pitch thread was towards the tip of the tap. In practice, it is simpler to carry out successive cuts from the tip in the opposite direction. This avoids the need to match the increment in the depth of cut accurately with the movement of the starting point along the tap. Instead each cut is continued until the cutter runs out into the root of the original thread form. The cutter must be withdrawn at the correct position to avoid cutting in to the back of the teeth.

A further feature of the screw thread as shown in Fig. 7 is that the steep flank in its final form has been produced by a single full width cutting face whereas the shallow flank has been produced in a series of steps. This could give a smoother finish on the steeply sloping or radial flank than on the shallow flank and this could be of advantage in certain applications of the thread. One primarily intended use of the thread is in a mechanism which requires a low friction high pitch thread in one direction to allow relative rotation of two threaded members when loaded axially against the radial or steep flank whilst preventing corresponding movement by a wedging action when the screw threads are loaded in the opposite sense and engaged together by the shallow flanks. A good finish on the radial or steep flank and a slightly irregular finish on the shallow flank could supplement the inherent difference in friction due to the angles of the shanks and thereby improve the operation of the threaded members.

Claims (3)

1. A tap for producing a screw thread in which one flank is steeper than the other flank, the tap including a shank and screw cutting teeth on the shank, the teeth reducing in size towards the lead-in end of the tap, characterised in that the size reduction of the teeth incorporates a reduction of the axial length of the teeth from the end of each tooth which has the steeper flank.

2. A tap as claimed in Claim 1 in which the thread form is such that the one flank is radial.

3. A method of making a tap according to Claim 1 including the steps of cutting a first thread of a desired profile on a workpiece, carrying out further thread cutting operations over the lead-in portion of the thread at a pitch differing slightly from that of the first thread, a series of such cuts being made over a series of phase relationships with the first thread at the tip of the tap, each such cut being made between the tip and a position towards the root of the lead-in portion where the cut becomes in phase with the first mentioned thread.