GB1568801A

GB1568801A - Thread-forming tap and its manufacture

Info

Publication number: GB1568801A
Application number: GB3175/78A
Authority: GB
Original assignee: SCHUMACHER KG W
Current assignee: SCHUMACHER KG W
Priority date: 1977-01-28
Filing date: 1978-01-26
Publication date: 1980-06-04
Also published as: NO774469L; ES238409U; ES238409Y; NL7800519A; DK148545B; FR2378973B1; AT362626B; SE7800519L; DK24678A; ATA59978A; DE2703433C2; FI68539C; FI780224A; FR2378973A1; SE434295B; NO146507C; IT1092132B; PT67580A; NL184127B; DK148545C

Abstract

A screw (2) which produces a thread which is to gauge when it is screwed into drilled holes is intended to be capable of being produced just as easily as normal screws. The shank in which the thread pitches (15) are rolled has a circular core cross-section over its entire length. Protuberances (16) which project radially out of the thread pitches (15), are formed out of the shank material. The protuberances (16) run steeply in a helical shape around the shank circumference and are superimposed on the thread pitches (15). Each protuberance (16) has a constant width over the thread edges and the thread tip of the respective thread pitch. The screws are produced on thread-rolling machines by means of grooved flat jaws which have a hardness between 2000 and 2600 N/mm<2>. In this case, screw blanks having a hardness of 400 to 500 N/mm<2> are used, into which the thread pitches (15) and the protuberances (16) are rolled at the same time. <IMAGE>

Description

(54) THREAD-FORMING TAP AND ITS MANUFACTURE (71) We, WILHEIM SCHUMACHER K.G., of Am Preist 5, 5912 Hilchenbach 1, Federal Republic of Germany, a Kommanditgesellschaft organised under the laws of the Federal Republic of Germany, 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 a thread-forming tap, in which the turns of the thread rolled into the stem have protruberances superimposed thereon. The invention is further concerned with a process for the manufacture of such thread-forming taps and also with rolling dies for carrying out this process.

Many different constructional forms of thread-forming taps are already known, as may be seen, for example, from page 358 of "Assembly Engineering Master Catalog" 1975/6. In this connection, a distinction is made between taps with which the screwthread is formed by a cutting operation, i.e.

with removal of swarf, and taps which form the turns of the thread by a grooving procedure, i.e. without removal of swarf.

Thread-forming taps which produce the screwthread in a previously tapped hole and in a swarfless manner by a grooving procedure are also known from German Offenle gungsschkifts 24 61 546 and 25 50 817.

Most of the known thread-forming taps, which with the turns of the screwthread are formed into the walls of previously drilled holes without removal of swarf by a grooving procedure, have the disadvantage that it is impossible with them to produce any gauging screwthread which would make it possible, after screwing out the threadforming tap, also to screw in normal or standard screws. An additional disadvantage of most of the known thread-forming taps consists in that these necessitate a relatively high insertion torque and that the threaded stems for producing the threadforming taps have to be provided with a specific profile of polygonal cross-section, which differs completely from normal screws.If in this connection the polygonal specific profile is extended over the entire length of the threaded stem, as is the case, for example, in connection with the screws illustrated in the "Assembly Engineering Master Catalog", then substantially smaller pulling-out values and thread-stripping torques are produced as compared with normal screws.

The invention seeks to minimize disadvantages connected with the previously described and known thread-forming taps.

Accordingly, the invention seeks to provide a thread-forming tap of the type as indicated above. which is capable of being made comparatively simply, but makes possible the provision of a gauging screw-thread in drilled holes, and in fact by producing low tapping moments with any desired length of screw stem. Furthermore, the invention seeks to provide a process for a simple manufacture of these thread-forming taps and also of rolling dies for carrying out this process of production.

According to this invention, we provide a thread forming tap having a stem, with a uniformly circular core cross-section over its entire length. into which has been rolled a helical screw thread, the tip and adjacent flanks of each turn of the screw thread having superimposed thereon at spaced locations on the circumference of the turn, radially projecting protuberances, the protuberances on adjacent turns being circumferentially offset so that lines of protuberances extend in helical form around the stem with a pitch greater than the pitch of the screw thread.

Even when the first turns of the thread are not fully developed on the free end of the stem, it is possible in accordance with the invention to produce a very soft screwing-in effect, when the turns of the threads are provided with a plurality of protuberances arranged distributed uniformly around the stem circumference. It has proved to be particularly expedient in this connection for the thread turns to be provided each with four protuberances, arranged distributed uniformly around the stem circumference.

In accordance with a further feature of the invention, it is also appropratie for the direction of the pitch of the protuberances to correspond to the pitch direction of the normal or standard threaded turns, i.e. to have a right-hand twist with a clock-wise screw thread and a left hand twist with an anti-clockwise screw thread.

With one particularly advantageous development of the thread-forming tap in accordance with the invention, the ratio of the nominal diameter of the thread turns relatively to the pitch of the protuberances is about 1:9, i.e. a complete turn of the helically extending threaded circumferential sections of greater profile height covers a length of the stem which is approximately equal to nine times the nominal diameter of the stem.

A process according to the invention. for the production of such a thread-forming tap comprises forming the turns of the thread by pressure exerted by means of grooved flat dies on a thread-rolling machine, wherein said thread turns and said protuberances which are superimposed on the said turns are simultaneously rolled into a tap blank having a smooth stem of circular crosssection and a hardness of 40 - 50 kp/mm2, flat dies with a hardness between 57 Rock- well and 60 Rockwell being used for this purpose.

Furthermore, preferably, provision is made for the fact that. during the rolling of the turns of the screw thread, the material displaced on the stem circumference is re-formed at least partially into the protuberances.

The rolling dies employed for carrying out this process, are provided with a grooved profile corresponding to the cross-section of the thread turns and troughs are formed into the flanks of the grooved profile along a plurality of strips arranged at an equal distance from one another and directed substantially transversely of the longitudinal direction of the grooves at an angle less than 90 , said troughs ending beneath the groove ridges, but passing through the groove bases.Furthermore, according to a preferred feature, the troughs are formed so as to be recessed, at least relatively to the groove base, by an amount of 0.025 mm to 0.075 mm, so that when the screw thread is rolled, protuberances are formed at least one the thread tips, in the region of which protuberances the screw thread diameter is larger by 0.05 mm to 0.15 mm than in those regions which lie between the protuberances.

Finally, as regards the rolling dies according to the invention preferably the troughs are impressed into the flanks of the groove profile.

The invention will now be described by way of example by reference to the accompanying drawings, wherein: Figure 1 is a perspective view in elevation on a greatly enlarged scale of a tap blank which has a smooth and substantially cylindrical shank or stem, which is however tapered frustoconically at its free end, Figure 2 is a section along the line II-II in Figure 1, Figure 3 is also an elevational view in perspective and on a greatly enlarged scale of a thread-forming tap according to the invention, which is produced by using the blank according to Figure 1, Figure 4 is a section along the line IV-IV in Figure 3, Figure 5 is a longitudinal section, again on an enlarged scale, through that region of the thread-forming screw which is designated by V in Figure 3.

Figure 6 is a diagrammatically simplified plan view of a rolling die. such as can be used for manufacturing a thread-forming tap according to Figure 3 from the tap blank according to Figure 1 in thread-rolling machines, Figure 7 is a view to a larger scale of that detail of the thread-rolling die which is designated in Figure 6 by VII.

Figure 8 is a section, again on an enlarged scale, along the line VIII-VIII through Figure 7, and Figure 9 is a section along the line IX-IX in Figure 7, through two rolling dies cooperating during the production of a threadforming tap.

Illustrated in Figure 1 of the drawing is a tap blank 1. from which is to be made a thread-forming tap 2, such as that shown in Figure 3. The tap blank 1 has a form such that it may also be used for manufacturing standard screws, i.e. it has for example a cylindrical head portion 3 with a transverse slot 4 and connected thereto a stem or, shank portion 5 of circular cross-section, such as may for example be seen from Figure 2. The major portion 6 of the length of this stem portion 5 is cylindrical, while the free end of the stem portion 5 terminates in a frustoconically tapered end portion 7.

The diameter 8 of the cvlindrical section 6 of the stem portion 5 corresponds here approximately to the flank diameter 9 of the screwthread 10 subsequently to be rolled on to the stem portion 5, as may be appreciated from comparing Figures 2 and 4. The diameter 8 of the smooth stem portion 5 is therefore smaller than the external diameter 11 of the finished screwthread 10, but is larger than the core diameter 12 of the stem portion which is provided with the screw thread 10.

In order that a thread-forming tap 2 according to Figure 3 is formed with the rolling of the thread 10 into the smooth stem portion 5, then simultaneously with the thread 10 being rolled in along linear or lamellar circumferential sections, beads or protuberances 16 formed from the material are superimposed on the flanks 13 and the tips 14 of each separate turn 15, in such a way that, in the region of the said beads or protuberances 16, the stem portion of the thread-forming screw 2 is given a diameter .17 which is larger, for example, by 0.05 mm, than that external diameter 11 of the screw thread 10 which forms the nominal dia meter.

It will be apparent from Figure 4 of the drawing that several, for example four, beads or protuberances are arranged, uni formly distributed, over the circumference of the stem of the thread-forming tap 2. in such a way that each thread turn 15 carries a corresponding number of beads or protu berances 16. The beads or protuberances 16 which here are disposed following one another on the individual turns 15 of the screwthread 10, are thus arranged so as to be circumferentially offset on adjacent turns so that the lines of protuberances extend in a steep helical form around the stem circum ference, this being clearly apparent from Figure 3. The direction of slope of the protuberances 16 and which extend along a steep helical path around the stem circum ference, advantageously corresponds to the direction of slope or pitch of the said turns 15.In other words, with a right-hand screw thread 10, the beads or protuberances are situated in lines which extend with a right handed spiral around the stem circumfer ence, while they are in lines provided with a left-handed spiral with a left-handed screw thread.

As a result of a thread-forming tap 2 according to Figure 3 being tapped into a previously drilled hole, then by means of the beads or protuberances 16 partially super imposed on the individual turns 15 of the thread, a gauging screwthread is formed in the walls of the drilled hole by means of the beads or protuberances 16 partially super imposed on the individual turns 15 by a so-called grooving, i.e. in a swarfiess man ner, in which gauging thread a normal screw may also be readily anchored after the thread-forming tap 2 has been screwed out.

Because of the helical arrangement of the beads or protuberances 16 which are superimposed on the turns 15. a very soft screwing effect and a small tapping moment are produced, although for example the first three turns 15, which are situated on the frusto-conical end portion 7. are not completely formed.

For producing high pull-out values and large thread-stripping moments. it is particularly advantageous for the screwthread 10 to extend in a substantially circular manner around the circumference of the stem, such as is also the case with a standard screw. In this way, the cross-section of the hole provided with the formed screwthread is completely filled by the introduced tap 2.

In addition, it is advantageous that the stem 5 of the thread-forming tap 2 may in theory be as long as desired. because the beads or protuberances 16 superimposed on the turns 15 at their flanks 13 and tips 14, because of their relatively small aggregations of material. can be directly formed by the thread-rolling operation. The forces of deformation applied in this case by the thread-rolling machine via the rolling dies always remain practicallv identical, because in fact the beads or protuberances 16 are not all formed simultaneously. but in succession during the operation of rolling the screw stem between the two rolling dies. This advantage is to be attributed to the steep helical position of the beads or protuberances 16 longitudinally of the screw stem.

Furthermore, it is sufficient with long taps for only the forward region to be provided with a thread-forming profile. The residual region may be formed with a normal screwthread.

A thread-rolling die 18. such as one which can be used for producing the threadforming tap 2 according to Figure 3, is shown diagrammatically in Figure 6. This rolling die 18 is provided throughout its length with a grooved profile 19 with a slight inclination relatively to its longitudinal axis, such profile form corresponding in the usual manner to the screw-thread 10 which is to be produced.

An enlarged detail of the thread-rolling die 18 is to be seen in Figure 7. It becomes apparent that the thread-rolling die 18 contains. as well as the grooved profile 19, also troughs 20 which are formed therein and which are formed into the flanks of the grooved profile 19, and in fact along a pluralitv of lines or strips 21 which are arranged with a uniform spacing from one another and substantially transversely of the length of the ribs. The lines or strips 21 which define the position of the troughs 20 in the rolling dies 18 are advantageously arranged with an inclination relatively to the longitudinal direction of the grooves of the grooved profile 19. and in fact at an angle 22. which is indicated in Figure 6.

It is made clear by Figure 8 of the drawing how the troughs 20 are formed into the flanks 23 of the grooved profile 19. The troughs 20 run out beneath the ridges of the grooves 24 while, in the region of the bases 25 of the grooves, they pass through two flanks 23 of the grooved profile 19 which are adjacent one another.

The troughs 20 can be incorporated into the flanks of the grooved profile 19 in various ways. For example, they may be milled into the flanks 23 of the grooved profile 19, in which case care is to be taken that the spacing 26 between the troughs 20 lying on two lines 21 adjacent to one another has to be kept. for example, at 0.01 mm, in order that the beads or protuberances 16 are satisfactorily formed on the flanks 13 of the turns 15 during the rolling of the tap stem between the two coacting rolling dies 18.

It has proved to be particularly advantageous for the troughs 20 to be impressed into the flanks 23 of the grooved profile 19.

by means of a suitable tool. However. the construction and the operation of the tool for the production of the rolling dies 18 does not form part of this invention.

Figure 9 of the drawing represents the operation as regards the production of a thread-forming tap 2 according to Figure 3.

In this connection, a tap blank 1 according to Figure 1 is introduced with its stem portion 5 between the two rolling dies 18 of a screwthread machine, whereafter the two rolling dies 18 are displaced relatively to one another, corresponding to the directions of the arrows as indicated, while they are operative through their grooved profile 19 under high pressure on the circumference of the stem portion 5. As a result, the tap blank carries out a rolling movement corresponding to the indicated direction of the arrow between the two rolling dies 18, in the course of which there is a displacement of material at the circumference of the smooth stem portion 5. which results in the formation of the screwthread 10 and also simultaneously the formation of the beads or protuberances 16 on the flanks 13 of the individual turns 15 of the thread.

For the manufacturing procedure in respect of the thread-forming tap 2 according to Figure 3, tap blanks 1 according to Figure 1 are employed, which have a hardness of about 40-50 kp/mm2. On the other hand, rolling dies 18 are used which have a hardness between 57 Rockwell and 60 Rockwell.

WHAT WE CLAIM IS: 1. A thread-forming tap having a stem, with a uniformly circular core cross-section over its entire length, into which has been rolled a helical screwthread, the tip and adjacent flanks of each turn of the screwthread having superimposed thereon at spaced locations on the circumference of the turn, radially projecting protuberances the protuberances on adjacent turns being circumferentially offset so that lines of protuberances extend in helical form around the stem with a pitch greater than the pitch of the screw thread.

2. A thread-forming tap according to claim 1, wherein the protuberances are arranged distributed uniformly around the stem circumference.

3. A thread-forming tap according to claim 2, wherein each thread turn is provided with four protuberances.

4. A thread-forming tap according to claim 1, 2, or 3, wherein the pitch direction of the protuberances corresponds to the pitch direction of said thread turns.

5. A thread-forming tap according to any one of claims 1 to 4, wherein the ratio between the nominal diameter of the thread turns and the pitch of the protuberances is about 1:9.

6. A thread-forming tap according to claim 1. substantially as described herein.

7. A thread-forming tap substantially as shown in Figures 3. 4 and 5 of the accompanying drawings and described herein with reference thereto.

8. A process for manufacturing threadforming taps according to any one of the preceding claims, comprising forming the turns of the thread by pressure exerted by means of grooved flat dies on a threadrolling machine, wherein said thread turns and said protuberances which are superimposed on the said turns are simultaneously rolled into a tap blank having a smooth stem of circular cross-section and a hardness of 40 - 50 kp/mm2, flat dies with a hardness between 57 Rockwell and 60 Rockwell being used for this purpose.

9. A process according to claim 8.

wherein the material displaced on the stem circumference during the rolling of the thread turns is re-formed at least partially into the protuberances.

10. A process according to claim 8 substantially as described herein.

11. A process for manufacturing threadforming taps substantially as described herein with reference to the accompanying drawings.

12. A pair of rolling dies for carrying out the process according to any one of claims 8 to 11. which dies are provided with a grooved profile corresponding to the crosssection of the thread turns and wherein troughs are formed into the flanks of said grooved profile along a plurality of strips arranged at a constant distance from one another and directed substantially trans

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. It is made clear by Figure 8 of the drawing how the troughs 20 are formed into the flanks 23 of the grooved profile 19. The troughs 20 run out beneath the ridges of the grooves 24 while, in the region of the bases 25 of the grooves, they pass through two flanks 23 of the grooved profile 19 which are adjacent one another. The troughs 20 can be incorporated into the flanks of the grooved profile 19 in various ways. For example, they may be milled into the flanks 23 of the grooved profile 19, in which case care is to be taken that the spacing 26 between the troughs 20 lying on two lines 21 adjacent to one another has to be kept. for example, at 0.01 mm, in order that the beads or protuberances 16 are satisfactorily formed on the flanks 13 of the turns 15 during the rolling of the tap stem between the two coacting rolling dies 18. It has proved to be particularly advantageous for the troughs 20 to be impressed into the flanks 23 of the grooved profile 19. by means of a suitable tool. However. the construction and the operation of the tool for the production of the rolling dies 18 does not form part of this invention. Figure 9 of the drawing represents the operation as regards the production of a thread-forming tap 2 according to Figure 3. In this connection, a tap blank 1 according to Figure 1 is introduced with its stem portion 5 between the two rolling dies 18 of a screwthread machine, whereafter the two rolling dies 18 are displaced relatively to one another, corresponding to the directions of the arrows as indicated, while they are operative through their grooved profile 19 under high pressure on the circumference of the stem portion 5. As a result, the tap blank carries out a rolling movement corresponding to the indicated direction of the arrow between the two rolling dies 18, in the course of which there is a displacement of material at the circumference of the smooth stem portion 5. which results in the formation of the screwthread 10 and also simultaneously the formation of the beads or protuberances 16 on the flanks 13 of the individual turns 15 of the thread. For the manufacturing procedure in respect of the thread-forming tap 2 according to Figure 3, tap blanks 1 according to Figure 1 are employed, which have a hardness of about 40-50 kp/mm2. On the other hand, rolling dies 18 are used which have a hardness between 57 Rockwell and 60 Rockwell. WHAT WE CLAIM IS:

1. A thread-forming tap having a stem, with a uniformly circular core cross-section over its entire length, into which has been rolled a helical screwthread, the tip and adjacent flanks of each turn of the screwthread having superimposed thereon at spaced locations on the circumference of the turn, radially projecting protuberances the protuberances on adjacent turns being circumferentially offset so that lines of protuberances extend in helical form around the stem with a pitch greater than the pitch of the screw thread.

9. A process according to claim 8.

10. A process according to claim 8 substantially as described herein.

versely of the longitudinal direction of the grooved profile at an angle less than 90 , which troughs terminate beneath the groove ridges, but pass through the bases of the grooves.

13. Rolling dies according to claim 12, wherein the troughs are formed so as to be recessed by an amount of 0.025 mm to 0.075 mm, at least relatively to the groove base.

14. Rolling dies according to claim 12 or 13 wherein the troughs are formed by pressure into the flanks of the grooved profile.

15. Rolling dies according to claim 12 substantially as described herein.

16. Rolling dies substantially as shwon in Figures 6 to 9 of the acompanying drawings and described herein with reference thereto.

17. An article having a screw thread formed using a tap according to any one of claims 1 to 7.