Thread forming tap

Abstract: A reamer edge 16 is provided at a chamfer part 13 of a thread forming tap. When the tap is pulled out after the internal thread 22 is formed, a seam portion 23 present at the crest of the internal thread 22 is cut off whereby, even in a blind hole, formation of internal thread 22 and cutting of seam portion 23 are performed in a single process. The formation of the internal thread 22 and processing of inside diameter portion are thus achieved in a single process so that concentricity in ensured. Further, even if there is a deviation in the diameter of the hole 21 before threading, the inside of the internal thread 22 is processed to a constant value.

Description: FIELD OF THE INVENTION This invention relates to a thread forming tap for forming an internal thread by plastically deforming a hole before threading in a material to be processed, which intends to improve the accuracy of inside diameter of the internal thread. DESCRIPTION OF THE PRIOR ART A thread forming tap is one of tools for forming an internal thread, which is provided on the outer periphery of the main body with a thread-formed radial portion forming an external thread corresponding to the internal thread to be formed. The material to be processed is provided with a hole before threading having a diameter nearly equal to the pitch diameter of the external thread, which is screwed into the hole before threading in the material to be processed to plastically deform the inner peripheral surface of the hole to form an internal thread. When an internal thread is formed using a thread forming tap, generation of swarf is small as compared with grinding and, further, the over size of the internal thread is small thus providing a stable processing accuracy. The thread forming tap, which forms an internal thread by building up the thread on the inner peripheral surface of the material to be processed, has tended to generate a seam portion 2 at the crest of an internal thread 1 as shown in Fig.14(a). The seam portion 2 has a danger to cause burrs, defective products, and adverse effects to the product by falling down when screwed with the external screw. Further, when the threaded part is plated, the plating solution remains at the seam portion 2 which leads to rust generation. Therefore, the total numbers of the processed material have to be inspected to remove the seam portion 2 generated at the crest of the internal thread 1. Removal of the seam portion 2 is achieved by means of a reamer after the internal thread is formed using the thread forming tap. However, in this case, formation of internal thread requires two steps resulting in a substantial deterioration of the work efficiency. US 3067509 shows a combination of a reamer and threader type tool according to the preambule of claim 1. In light of the above circumstances, a primary object of the present invention is to provide a thread forming tap which can remove the seam portion generated at the crest of the internal thread under the same process as that for forming the internal thread without dividing the process, not extending the length of the tool. SUMMARY OF THE INVENTION In accordance with the present invention, which attains the above object, there is provided a thread forming tap comprising a plurality of radial portions disposed on the outer periphery of a main body to form a thread part, the main body being turned in one direction to plastically deform an inner peripheral surface of a material by the radial portions disposed on the thread part to form an internal thread, wherein a cutting part for cutting the crest of the internal thread is formed at least one position between the radial portions in the peripheral direction of the thread part, the crest of the internal thread is cut off by the cutting part formed between the radial portions after the internal thread is formed by the radial portions. The thread part is provided with a chamfer part, which is formed so as to decrease in diameter towards the tip portion, and the cutting part has an outside diameter almost equal to the inside diameter of the internal thread, which is provided so that the crest of the internal thread is cut off when the main body is turned in the other direction. Further, a plurality of grooves extending in the axial direction of the main body are formed in the peripheral direction of the main body, and the cutting part is provided at a position of the radial portion between the grooves adjacent to the chamfer part. In the process in which the main body is turned in one direction to form an internal thread by the radial portions, the internal thread is formed to an effective depth by the radial portions without contacting the cutting part of the chamfer part against the internal thread and, when the main body is turned in the other direction, the crest of the internal thread is cut off by the cutting part and the main body is removed from the internal thread. When the cutting part is formed at the root such that the root of the chamfer part is formed smaller in depth than the root of the parallel thread portion, the crest of the internal thread can be cut off by the angle-shaped edge without generating burrs, which is preferable. Still further, the thread part is provided at the rear end portion with a run off which gradually decreases in diameter towards the rear end, the cutting part has an outside diameter almost equal to the inside diameter of the internal thread, and the run off is provided so that when the main body is turned in one direction, the crest of the internal thread is cut off after the internal thread is formed. In the process in which the main body is turned in one direction to form the internal thread, by the radial portions, the internal thread is formed to an effective depth by the radial portions without contacting the cutting part of the run off against the internal thread and, when the main body is turned further in one direction, the crest of the internal thread is cut off by the cutting part and the main body is removed from the internal thread. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view showing the entire construction of an embodiment of the thread forming tap according to the present invention; Fig.2 is a schematic enlarged view showing part of Fig. 1; Fig.3 is a schematic cross sectional view taken along line III-III in Fig. 2; Fig.4 is a schematic cross sectional view taken along line IV-IV in Fig. 2; Fig.5 is a schematic cross sectional view when the internal thread is being formed; Fig.6 is a schematic cross sectional view when the thread forming tap is being removed; Fig.7 is a schematic view for explaining dimensions of the individual parts of the thread forming tap; Fig.8 is a schematic enlarged view showing part of the thread forming tap provided with a run off 24; Fig.9 is a schematic cross sectional view taken along line IX-IX in Fig. 8; Fig.10 is a schematic cross sectional view taken along line X-X in Fig. 8; Fig.11 is a schematic enlarged view showing part of another embodiment of the thread forming tap according to the present invention; Fig.12 is a schematic cross sectional view taken along line XII-XII in Fig. 11; Fig.13 is a schematic view showing the entire construction of a further embodiment of the thread forming tap according to the present invention; Fig.14 is a schematic view explaining the shape of the internal thread. Description of symbols: 11. 31: main body 12. 32: thread part 13, 33: chamfer part 14. 34: parallel thread portion 15, 36: oil 1 groove 16, 37: reamer edge 17, 38: radial 1 portion 18: run off 21: hole before threading 22: internal thread 23: seam portion 35: run off DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig.1 is a schematic view showing the entire construction of an embodiment of the thread forming tap according to the present invention, Fig.2 is a schematic enlarged view of part of Fig.1, Fig.3 is a schematic cross sectional view taken along line II-II in Fig.2, and Fig.4 is a schematic cross sectional view taken along line IV-IV in Fig.2. As shown in Fig.1, a thread part 12 is formed on the outer periphery of a main body 11, the thread part 12 is provided at the tip portion (right side in the Figure) with a chamfer part 13 which gradually decreases in diameter towards the tip, and a parallel thread portion 14 continues from the chamfer part 13. Further, a plurality of oil grooves 15 extending in the axial direction of the main body 11 are formed in the radial direction of the main body 11, and a reamer edge 16 as a cutting part is formed at a position between the oil grooves 15 adjacent to the chamfer part 13. The thread forming tap shown is one of metric fine pitch thread M10, 1 mm pitch, and 2 mm length of chamfer part 13. As shown in Figs.2 to 4, the thread part 12 is provided with a thread-formed radial portion 17, and the reamer edge 16 is formed at the position where the radial portion 17 between the oil grooves 15 adjacent to the chamber part 13 is located. The thread-formed radial portion 17 is disposed on the parallel thread portion 14 between the oil grooves 15 corresponding to the position where the reamer edge 16 is formed. The reamer edge 16 is provided with a run off 18 at the front side of the turning direction of the main body 11, and a cutting edge is formed at the rear side of the turning direction. That is, when the internal thread is formed by the thread-formed radial portion 17 by turning the main body 11 in one direction, the edge part of the reamer edge 16 does no affect the internal thread. Further, the reamer edge 16 is formed at the position corresponding to the last radial portion of the chamfer part 13. Outside diameter of the reamer edge 16 is set to a thread engagement of 85% (nearly equal to the inside diameter of the internal thread) to satisfy the thread engagement of 100% (8.917 mm) to 78.3% (9.158 mm) of JIS Class 6H internal thread. Functions when an internal thread is formed in a blind hole by the thread forming tap of the above arrangement will be described with reference to Figs.5 to 7. In the present embodiment, an internal thread 22 is formed using a material to be processed, a cast aluminum alloy, which is provided with a hole before threading 21 so that the thread engagement is 95% to 100%. Fig.5 shows a cross sectional view when the internal thread is being formed, Fig.6 shows a cross sectional view when the thread forming tap is being removed, and Fig.7 shows description of dimensions of the individual parts of the thread forming tap. The thread forming tap is mounted to a tapping machine or the like (not shown), and the thread forming tap is turned in one direction (direction of arrow A in Fig.5) to drive the tap into the hole 21 before threading from the chamfer part 13 (direction of arrow X in Fig.5). The material to be processed is deformed due to plastic flow by the thread-formed radial portion 17 to start formation of the thread. As shown in Fig.7, since inside diameter of deformation by the chamfer part 13, even when diameter of the hole before threading is set to a thread engagement of 100%, both in the pitch diameter and outside diameter, is decreased according to the taper angle of the chamfer part 13, the thread is formed up to only 80% by the chamfer part 13. Therefore, there is a clearance between the reamer edge 16 and the inside diameter of the internal thread 22 in the course of formation and the reamer edge 16 and the internal thread 22 will never contact with each other. Next, when the parallel thread portion 14 advances, the thread in the material to be processed is further formed and, since the reamer edge 16 is not present in the parallel thread portion 14, the internal thread 22 is formed without removing the crest (condition as shown in Fig.5). At this moment, a seam portion 23 of about 0.15 to 0.2 mm deep exists at the crest of the internal thread 22. When the tip of the thread forming tap reaches the bottom of the hole before threading 21, the thread forming tap is turned reverse in the other direction (direction of arrow B in Fig.6) to begin to pull out the thread forming tap from the hole before threading 21 (direction of arrow Y in Fig.6). As the thread forming tap is pulled out, the seam portion 23 present at the crest of the internal thread 22 is cut off by the reamer edge 16 having a cutting part formed at the rear side of the turning direction, and the thread forming tap is removed from the material to be processed, thus completing the formation of the internal thread 22. Since the diameter of the reamer edge 16 has a thread engagement of 85% (9.078 mm), the inside diameter of the internal thread after processing is cut within a predetermined range from 9.08 to 9.10 mm. Further, as shown in Fig. 14(b), the seam portion 23 present at the crest of the internal thread 22 is almost cut off. With the above-described thread forming tap, which is provided with the reamer edge 16 on the chamfer part 13, by turning the thread forming tap in reverse after the internal thread 22 is formed, the seam portion 23 present at the crest of the internal thread 22 can be cut off. As a result, even in a blind hole, formation of the internal thread 22 and cutting off the seam portion 23 are achieved in a single process, and since the formation of the internal thread 22 and processing of the inside diameter of the internal thread 22 is performed in a single process, concentricity of the internal thread 22 is in line with that of the inside diameter. Further, even there is some dispersion in the diameter of the hole before threading 21, the inside diameter of the internal thread 22 can be processed to a constant value, and the length of the thread forming tap will never be increased. The above-described embodiment corresponds to Claim 3. In the above embodiment, the reamer edge 16 is formed at the position where the radial portion 17 between the adjacent oil grooves 15 is located. However, alternatively, the reamer edge 16 may be formed in the oil groove 15 between the radial portions 17 (Claim 2). Further, an example is shown in which the thread-formed radial portion 17 is disposed on the parallel thread portion 14 between oil grooves 15 corresponding to the position where the reamer edge 16 is formed. However, as shown in Figs.8 to 10, it is also possible that the radial portion 17 is not provided on the parallel thread portion 14 between the oil grooves 15 corresponding to the position where the reamer edge 16 is formed, but the run off groove 24 is provided thereon. Fig.8 is a schematic enlarged view showing part of the thread forming tap provided with the run off groove 24, Fig.9 is a schematic cross sectional view taken along line IX-IX in Fig.8, and Fig.10 is a schematic cross sectional view taken along line X-X in Fig.8. In this case, discharge of cutting debris is superior due to the run off groove 24 when the seam portion is cut off. Further, in the above-described embodiment, the reamer edge 16 is provided at one position in the peripheral direction of the main body 11, however, the reamer edge 16 may be provided at two or more positions (at least one position). Still further, while the oil groove 15 is formed along the axial direction, it is also possible that the oil groove 15 is spirally formed and the radial portion 17 is disposed according to the spiral oil groove 15. Yet further, in some cases, it is possible that the oil groove 15 is not provided, but only debris discharge means. Figs.11 and 12 show an embodiment in which the root of the chamfer part 13 is formed smaller in depth than the root of the parallel thread portion 14 and the reamer edge 16 is formed in the former root (Claim 4). Fig.11 is a schematic enlarged view showing part of the thread forming tap, and Fig.12 is a schematic cross sectional view taken along line XII-XII in Fig. 11. In the present embodiment, the root of the chamfer part 13 of the thread portion 12 is formed smaller in depth (wider) than the root of the parallel thread portion 14, and the radial portion 17 is not formed only at one position in the peripheral direction where the radial portion 17 between the oil grooves 15 adjacent to the chamfer part 13, and that root is formed as the reamer edge 16. The thread-formed radial portion 17 is disposed on the parallel thread portion 14 between the oil grooves 15 corresponding to the position where the reamer edge 16 is formed. The reamer edge 16 is provided with a run off 18 at the front side in the turning direction of the main body, and an angle-formed cutting part 16a is formed at the rear side in the turning direction. That is, by turning the main body in one direction, the thread-formed radial portion 17 engages with the hole before threading 21 of the material to be processed and, when the internal thread 22 is formed, the cutting part 16a of the reamer edge 16 will never affect the internal thread 22. Further, the reamer edge 16 is formed at the last root of the chamfer part 13 to have an outside diameter almost equal to the inside diameter of the internal thread 22. Therefore, after the thread forming tap is turned in one direction to drive the tap into the hole before threading 21 from the chamfer part 13, and the internal thread 22 is formed by plastic flow due to the radial portion 17 from the chamfer part 13 to the parallel thread portion 14, when the thread forming tap is removed from the hole before threading 21 by turning the thread forming tap in reverse, as in the previous embodiments, the seam portion 23 present at the crest of the internal thread 22 is cut off by the reamer edge 16 provided with the cutting part 16a formed at the rear side in the turning direction. In the present embodiment, since, at the time of the cutting, cutting is made while the crest of the internal thread 22 is bound (closely contacted) with the angle-formed cutting part 16a, generation of burrs or the like is suppressed. Further, since the reamer edge 16 is formed only at the last root of the chamfer part 13, cutting torque is small as compared with the case where the reamer edges are provided at a plurality of roots. In addition, in the thread forming tap of the present invention, since the root of the chamfer part 13 is formed small in depth, in other words, the root is widely formed, the strength of the chamfer part can be increased and, when tapping, grind workability can be improved with decreased grinding man-hours and reduced generation of defective products. In the above-described thread forming tap, a reamer edge 16 having a cutting part formed in reverse is provided on the chamfer part so that formation of the internal thread 22 and cutting of seam portion 23 can be made in a single process even in a blind hole. On the other hand, Fig.13 shows an embodiment in which formation of the internal thread 22 and cutting of seam portion 23 are made without reverse turning the thread forming tap in a through hole, such as nut or the like (Claim 5). As shown in Fig.13, a thread part 32 having a thread-formed radial portion 38 is formed on the outer periphery of a main body 31, the thread part 32 has a tip side (right side in the Figure) as a chamfer part 33 which gradually decreases in diameter towards the tip, and a parallel thread portion 34 continues from the chamfer part 33. Further, at the rear end side (left side in the Figure) the thread part 32 has a run off 35 continuing from the parallel thread portion 34 and decreasing in diameter towards the rear end. On the other hand, a plurality of oil grooves 36 extending in the axial direction of the main body 31 are formed in the peripheral direction of the main body 31, and a reamer edge 37 as a cutting part is formed at a position between the oil grooves 36 adjacent to the run off 35. The reamer edge 37 has a cutting part formed at the front side in the turning direction, whereby the crest of the internal thread is cut off by turning the main body 31 in one direction. It is also possible that the reamer edge 37 is provided at the rear end side of the parallel thread portion 34, rather than the run off 35. With the above-described thread forming tap, the thread forming tap is turned in one direction to drive the chamfer part 33 into the hole before threading to begin thread forming, then the parallel thread portion 34 is advanced to further form threads in the material to be processed thereby forming the internal thread. The thread forming tap is further continued to be turned, the seam portion present at the crest of the internal thread is cut off by the reamer edge 37 at the position of the run off 35, and the thread forming tap passes through the internal thread. Using the above procedure, as with the above-described thread forming tap, formation of the internal thread and cutting of the seam portion can be performed in a single process and, since formation of the internal thread and processing of the inside diameter portion of the internal thread are performed in a single process, concentricity of the internal thread is in line with that of the inside diameter. Further, in the above embodiment, the reamer edge is formed as a cutting part to cut the crest of the internal thread. However, alternatively, it is needless to say that an end mill edge may be formed to enable high-speed processing. In the thread forming tap of the present invention, the cutting part to cut the crest of the internal thread is formed between the radial portions in the peripheral direction of the thread portion, after the internal thread is formed by the radial portion, formation of the internal thread and processing of the inside diameter portion of the internal thread can be performed in a single process without increasing the length of the tool, and the inside diameter of the internal thread can be processed to a constant value even if the hole before threading has a deviation in diameter. As a result, cutting of the crest of the internal thread is efficiently achieved, with improved accuracy of the internal thread. Further, since the cutting part is provided on the chamfer part so that the crest of the internal thread is cut off when the main body is turned in the other direction, in the process in which the main body is turned in one direction to form the internal thread by the radial portion, the internal thread can be formed by the radial portion to the effective depth without contacting the cutting part of the chamfer part against the internal thread, and when the main body is turned in the other direction, the crest of the internal thread is cut off by the cutting part and the main body is pull out from the internal thread. As a result, even in a blind hole, formation of the internal thread and cutting of the crest of the internal thread can be performed in a single process. Further, since the root of chamfer part is formed smaller in depth than the root of the parallel thread portion and the cutting part is formed at the root, the crest of the internal thread can be cut off by the angle-formed cutting part without generating burrs, and grindability and durability of the chamfer part is improved.

Claims: A thread forming tap comprising a plurality of radial portions (17, 38) disposed on the outer periphery of a main body (11, 31) to form a thread part (12, 32), said main body (11, 31) being turned in one direction to plastically deform an inner peripheral surface of a material by the radial portions (17, 38) disposed on the thread part (12, 32) to form an internal thread (22), and comprising a cutting part (16, 37) for cutting off the crest of the internal thread (22) characterised in that the cutting part (16, 37) is formed at at least one position between the radial portions (17, 38) in the peripheral direction of the thread part (12, 32). The thread forming tap of Claim 1, wherein said thread part (12, 32) is provided at a tip portion with a chamfer part (13, 33) which gradually decreases in diameter towards the tip, and said cutting part (16, 37) has an outer diameter substantially equal to the inner diameter of the internal thread (22), so that the crest of the internal thread (22) is cut off when said main body (11, 31) is turned in the other direction. The thread forming tap of Claim 2, wherein a plurality of grooves (15, 36) extending in the axial direction of said main body (11, 31) are formed in the peripheral direction of said main body (11, 31), and said cutting part (16, 37) is provided at a position of said radial portion (17, 38) between said grooves adjacent to said chamfer part (13, 33). The thread forming tap of Claim 2 or 3, wherein the root of said chamfer part (13, 33) is smaller in depth than the root of said parallel thread part (12, 32), and said cutting part (16, 37) is formed at said root. The thread forming tap of Claim 1, wherein said thread part (12, 32) is provided at the rear end portion with a run off which gradually decreases in diameter towards the rear end, said cutting part (16, 37) has an outer diameter substantially equal to the inner diameter of the internal thread (22), and said run off is provided so that, when said main body (11, 31) is turned in one direction, the crest of the internal thread (22) is cut off after the internal thread (22) is formed.

Inventors: SATO KAZUTOSHI, SAWABE TERUO, YOKOYAMA ISAO, SATO, KAZUTOSHI, SAWABE, TERUO, YOKOYAMA, ISAO, Sato, Kazutoshi, c/o Miyagitanoi Co., Ltd., Sawabe, Teruo, c/o Tanoi Mfg. Co., Ltd., Yokoyama, Isao, c/o Tanoi Mfg. Co., Ltd.
Applicants: TANOI MFG CO LTD, TANOI MFG. CO., LTD.
Application year: 1996
Main classification: B23G7
IPC codes: B23G7, B23D77