JP7762094B2 - Forming Tap - Google Patents

Description

The present invention relates to a forming tap.

Forming taps are known as tools for forming internal threads in workpieces (Patent Document 1). With forming taps, a pilot hole has been drilled into the workpiece using a drill or other tool. The threaded portion, which is a helical-shaped formed part, is rotated and gradually driven into the pilot hole, plastically deforming the pilot hole portion, thereby forming the internal thread.

Japanese Patent Application Laid-Open No. 2004-148430

Generally, when the threads of a threading tap begin to plastically deform the pilot hole in a workpiece, high pressure is applied to the outer diameter and flank surfaces, which are the crests of the threads of the threading tap, due to the resistance to deformation. As a result, heat is generated due to intense friction between the tool and the workpiece, and heat is generated due to the plastic deformation of the workpiece, causing the threading tap and the workpiece to reach high temperatures. Furthermore, the surface of the pilot hole in the workpiece is strongly pressed by the threads of the threading tap, stretching the metal structure. As a result, the metal structure on the surface peels off. The high pressure on the contact surface with the threading tap and the high temperatures described above cause the peeled metal structure to adhere to the outer diameter and flank surfaces of the thread as an adhered material. When the adhered material adheres to the surface of the thread, the surface roughness of the formed internal thread increases and the accuracy of the internal thread decreases, resulting in a weaker fastening force. Furthermore, when the adhered material falls off, it scratches the surface of the threading tap, shortening tool life.

The objective of the present invention is to provide a thread forming tap that can form better internal threads.

One aspect of the present invention provides a thread forming tap that plastically deforms a pilot hole in a workpiece to form a female thread. The thread forming tap has a threaded portion with multiple threads and multiple grooves arranged in the longitudinal direction so as to divide the threaded portion, and is configured so that the width of the outer diameter of the threads becomes narrower as it approaches the grooves.

The clearance angle of the outer diameter portion of the thread may be smaller than the clearance angle of the effective diameter portion. The clearance angle of the outer diameter portion may be in the range of 6° to 12° and the clearance angle of the effective diameter portion may be in the range of 9° to 16°, and within these ranges, the clearance angle of the outer diameter portion may be smaller than the clearance angle of the effective diameter portion by 1° to 5°.

The aspects of the present invention have the common effect of providing a thread forming tap that can form better internal threads.

FIG. 1 is a front view of a thread forming tap according to an embodiment of the present invention. FIG. 2 is a cross-sectional view taken along line AA in FIG. FIG. 3 is an enlarged view of the threaded portion. FIG. 4 is an enlarged view of the threads of the screw portion. FIG. 5 is a longitudinal cross-sectional view of the threads of the screw portion. FIG. 6 is a schematic diagram showing a method for manufacturing a thread forming tap. FIG. 7 is an enlarged photograph of the internal thread formed by the thread forming tap. FIG. 8 is a diagram showing the time-series change in tapping torque when forming an internal thread with a thread forming tap.

Embodiments of the present invention will be described in detail below with reference to the drawings. Corresponding components are designated by common reference symbols throughout the drawings.

Figure 1 is a front view of a thread forming tap 1 according to an embodiment of the present invention.

The thread forming tap 1 has a shank 2 and a processing portion 3. The thread forming tap 1 is attached to the spindle of a processing machine by gripping the shank 2 in a chuck. To reduce frictional resistance with the workpiece, the thread forming tap 1 is coated with a hard coating such as TiN, TiCN, AlCrN, or CrTiN, or is subjected to a surface treatment such as oxidation or nitriding. The processing portion 3 has a helically wound thread portion 4 and multiple grooves 5 arranged longitudinally to divide the thread portion 4. Cutting oil is supplied through the grooves 5. The multiple grooves 5 are, for example, five grooves 5, but may also be any other number such as two or three. The multiple grooves 5 are arranged at equal intervals along the circumferential direction.

A chamfer 4a is provided at the tip of the threaded portion 4. The chamfer 4a is tapered and serves as a guide for the forming tap 1 to enter a pilot hole in a workpiece that has been drilled or otherwise machined. In other words, the outer diameter of the thread at the tip of the chamfer 4a is smaller than the inner diameter of the pilot hole. The outer and effective diameters of the threads of the chamfer 4a gradually increase from the tip along the lead, and are formed to match the root diameter of the specified internal thread. The length of the chamfer 4a is 2 to 4 threads. With the forming tap 1, as the chamfer 4a is screwed into the pilot hole, the metal structure of the pilot hole is gradually crushed and plastically deformed, raising the inner surface of the pilot hole and forming the internal thread.

Figure 2 is a cross-sectional view taken along line A-A in Figure 1. Specifically, Figure 2 is a cross-sectional view of one lead along the helical valley 12 of the threaded portion 4, and is shown schematically as a continuous, integrated cross-sectional view. Five threads 10 separated by grooves 5 are shown in Figure 2. Figure 3 is an enlarged view of the threaded portion 4. Figure 3(a) is a circumferentially expanded view of the threaded portion 4 corresponding to portion M in Figure 1, and Figure 3(b) is a similarly circumferentially expanded view of the threaded portion 104 of a conventional thread threading tap as a reference example. Both Figures 3(a) and 3(b) show the threaded portion for one land. Figure 4 is an enlarged view of the threads 10 of the threaded portion 4. Figure 4(a) is an axially enlarged view of the threads 10 for one land, and Figure 4(b) is a view of the threads 10 of Figure 4(a) viewed perpendicular to the axial direction.

2, 3(a), and 4, the thread portion 4 of the thread forming tap 1 has threads 10, an outer diameter portion 11 corresponding to the crest portion (outer diameter) of the thread 10 along the circumferential direction, and valley portions 12 provided between adjacent threads 10. The outer diameter portion 11 has a protrusion portion 13, which is a margin portion and protrudes the furthest radially outward, a formation-side relief portion 14a located forward when the thread forming tap 1 rotates, and a relief-side relief portion 14b located rearward when the thread forming tap 1 rotates. In other words, the formation-side relief portion 14a is located near the end face on the formation side, and the relief-side relief portion 14b is located near the end face on the relief side. The surface of the protrusion portion 13 is formed in an arc shape when viewed axially, as shown in FIG. 4(a). FIG. 4(a) shows an effective diameter portion 20, which is a lobe-shaped portion of the effective diameter (flank surface) of the thread portion 4. The effective diameter portion 20 protrudes most radially outward at a position corresponding to the protruding portion 13 of the outer diameter portion 11.

Referring to Figure 3(b), the thread portion 104 of a conventional thread forming tap has threads 110, outer diameter portions 111 corresponding to the crests of the threads 110 along the circumferential direction, and valley portions 112 provided between adjacent threads 110. The outer diameter portion 111 has a marginal protrusion 113 that protrudes the furthest radially outward, a thread forming-side relief portion 114a that is located in the front when the thread forming tap rotates, and a relief-side relief portion 114b that is located in the rear when the thread forming tap rotates. In other words, the thread forming-side relief portion 114a is located near the end face on the thread forming side, and the relief-side relief portion 114b is located near the end face on the relief side. For comparison, the outer diameter portion 111 of a conventional thread forming tap is shown in Figure 4.

3(a) and 4(b), the outer diameter portion 11 of the thread forming tap 1 is formed so that its width narrows from the protruding portion 13 toward the forming-side relief portion 14a or the relief-side relief portion 14b. That is, if the width of the outer diameter portion 11 at the protruding portion 13 is width L1, the width of the outer diameter portion 11 at an intermediate position toward the relief-side relief portion 14b is width L2, which is narrower than width L1. In other words, the width of the outer diameter portion 11 of each of the threads 10 of the thread portion 4 is configured to narrow the closer it is to the groove portion 5. Furthermore, an imaginary circle circumscribing the outer diameter portion 11, i.e., the protruding portion 13, centered on the central axis of the thread forming tap 1, is defined as a first circle S1, and an imaginary circle circumscribing the effective diameter portion 20 is defined as a second circle S2. The clearance angle of the outer diameter portion 11 is defined as a first clearance angle α based on the first circle S1, and the clearance angle of the effective diameter portion 20 is defined as a second clearance angle β based on the second circle S2. The first clearance angle α is formed to be smaller than the second clearance angle β.

On the other hand, as shown in Figures 3(b) and 4(b), the outer diameter portion 111 of a conventional thread forming tap is formed with a constant width L3 from the protruding portion 113 toward the thread forming-side relief portion 114a or relief-side relief portion 114b. Also, as shown in Figure 4(a), assuming that the effective diameter portion 120 of the conventional thread forming tap is equal to the effective diameter portion 20 of the thread forming tap 1 according to the embodiment of the present invention, the outer diameter portion 111 is formed to be inclined radially inward more than the outer diameter portion 11 of the thread forming tap 1. That is, if the relief angle of the outer diameter portion 111 of the conventional thread forming tap is defined as a third relief angle γ based on the first circle S1, the first relief angle α of the outer diameter portion 11 of the thread forming tap 1 is formed to be smaller than the third relief angle γ of the outer diameter portion 111 of the conventional thread forming tap. Furthermore, the third relief angle γ of the outer diameter portion 111 of a conventional thread forming tap is generally formed to be equal to the second relief angle β of the effective diameter portion 20.

Figure 5 is a longitudinal cross-sectional view of the thread 10 of the threaded portion 4. Figure 5(a) is a cross-sectional view of the thread 10 at the protruding portion 13, taken along line B-B in Figure 4(a). Figure 5(b) is a cross-sectional view of the thread 10 at the line C-C in Figure 4(a). Figure 5(c) is a cross-sectional view of the thread 110 of a conventional thread forming tap, taken along line C-C in Figure 4(a), as a reference example. The angle between the thread 10 and the thread 110 is 60 degrees. The crest portion of the thread 10, i.e., the outer diameter portion 11, has a gentle R-shape in longitudinal cross-section, but may also be flat.

As shown in Figure 5, if the height of the thread 10 along line B-B is height H1 (Figure 5(a)), and the height of the thread 10 along line C-C is height H2 (Figure 5(b)), height H2 is formed to be lower than height H1. On the other hand, if the height of the thread 10 of a conventional thread threading tap along line C-C, which is approximately the same position, is height H3 (Figure 5(c)), height H3 is formed to be lower than height H2. Meanwhile, with regard to the width of the crest portion of the thread 10 or thread 110, i.e., the width of the outer diameter portion, as described above, width L2 of outer diameter portion 11 of thread 10 is narrower than width L3 of outer diameter portion 111 of thread 110.

Figure 6 is a schematic diagram showing a manufacturing method for thread forming tap 1. Figure 6(a) shows the process of forming thread 10 using circular grinding stone G1, and Figure 6(b) shows the process of forming outer diameter portion 11 on thread 10 using circular grinding stone G2. In each of Figures 6(a) and 6(b), the left image is a cross-sectional view similar to Figure 2, and the right image is a front view of the corresponding thread portion 4.

Referring to Figure 6(a), the grinding wheel G1 has an outer circumferential surface formed into a 60-degree triangular cross section. The thread 10 is formed by rotating the cylindrical tap blank 30, from which the thread thread 1 is made, around its central axis while rotating the grinding wheel G1 in the opposite direction and moving it axially. During this process, the grinding wheel G1 reciprocates radially outward or inward relative to the tap blank 30, depending on the area to be formed. Specifically, when forming the protruding portion 13 of the thread 10, the grinding wheel G1 is positioned at its radially outermost position. When forming the end faces of the thread thread 10's relief portions 14a and 14b, the grinding wheel G1 is positioned at its radially innermost position. The process shown in Figure 6(a) forms a thread 10 with a pointed crest. Next, proceed to the process shown in Figure 6(b).

Referring to Figure 6(b), the grinding wheel G2 has a flat or concave outer periphery so that the vertical cross-section of the crest portion of the thread 10 is flat or gently curved. The outer diameter portion 11 of the pointed thread 10 is formed by rotating the tap blank 30 around its central axis while rotating the grinding wheel G2 in the opposite direction and moving it axially. During this process, the grinding wheel G2 reciprocates radially outward or inward relative to the tap blank 30, depending on the area to be formed. Specifically, when forming the outer diameter portion 11 of the protruding portion 13 of the thread 10, the grinding wheel G2 is positioned at its radially outermost position. When forming the outer diameter portion 11 of the raised relief portion 14a and relief relief portion 14b of the thread 10, the grinding wheel G2 is positioned at its radially innermost position.

When the amount of movement D2 due to the reciprocating motion of grinding wheel G2 (Figure 6(b)) is equal to the amount of movement D1 due to the reciprocating motion of grinding wheel G1 (Figure 6(a)), an outer diameter portion 111 with a constant width L3 is formed on the thread 110, as with a conventional thread forming tap. On the other hand, when the amount of movement D2 due to the reciprocating motion of grinding wheel G2 is smaller than the amount of movement D1 due to the reciprocating motion of grinding wheel G1, an outer diameter portion 11 like that of a thread forming tap 1, as shown in Figure 5(b), is formed on the thread 10.

In general, with forming taps, the outer diameter portion, which corresponds to the crest of the thread in the threaded portion, bites into the pilot hole in the workpiece, spreading the workpiece material to the left and right. The flank surfaces of the thread in the threaded portion then further spread and raise the workpiece material, forming an internal thread. In conventional forming taps, the outer diameter portion is formed with a constant width around the circumference, as described above. In conventional forming taps, the clearance angle or clearance amount of the pitch diameter portion and outer diameter portion is constant, so the positional relationship between the pitch diameter portion and outer diameter portion is always constant around the circumference of the tool.

In the thread forming tap 1 according to an embodiment of the present invention, the clearance angle of the effective diameter portion 20 (second clearance angle β) is set to be the same as the clearance angle of the effective diameter portion 120 of a conventional thread forming tap so that the amount of material raised from the workpiece when forming the internal thread is the same as that of a conventional thread forming tap. Meanwhile, the first clearance angle α, which is the clearance angle of the outer diameter portion 11 of the thread 10, where high pressure and temperature are likely to cause adhesion when forming the internal thread, is set to be smaller than the third clearance angle γ of the outer diameter portion 111 of a conventional thread forming tap (Figure 4(a)). Therefore, the clearance amount of the outer diameter portion 11 of the thread 10 is relatively smaller than the clearance amount of the effective diameter portion 20. As a result, the outer diameter portion 11 of the thread forming tap 1 is formed so that its width narrows from the protruding portion 13 toward the thread-forming clearance portion 14a or the clearance-side clearance portion 14b.

In general, water-insoluble cutting fluids are used as lubricants to prevent adhesion in forming taps. However, due to growing environmental awareness in recent years, machining sites are increasingly switching from water-insoluble cutting fluids to water-soluble cutting fluids. Water-soluble cutting fluids have better cooling properties than water-insoluble cutting fluids, but their lubrication properties are significantly inferior. Furthermore, lubricants that do not contain extreme-pressure additives such as phosphorus and sulfur are becoming mainstream, and switching to water-soluble cutting fluids has become a major issue in plastic processing sites.

The thread forming tap 1 described above reduces damage to the thread portion 4, reduces the occurrence of adhesion, and reduces tapping torque, even when used in a water-soluble oil, compared to conventional thread forming taps. Furthermore, the thread forming tap 1 described above alleviates stress concentration in the outer diameter portion 11, which is the crest portion, and suppresses a sudden increase in surface pressure on the flank surface, thereby reducing adhesion of the workpiece. In particular, it is preferable that the first relief angle α of the outer diameter portion 11 be in the range of 6° to 12° and the second relief angle β of the effective diameter portion 20 be in the range of 9° to 16°, and that within these ranges, the first relief angle α of the outer diameter portion 11 be 1° to 5° smaller than the second relief angle β of the effective diameter portion 20.

The results of actually forming a female thread using the above-described thread forming tap 1 will be explained with reference to Figures 7 and 8 regarding the reduction in adhesion and tapping torque.

Figure 7 shows enlarged photographs of internal threads formed by a thread forming tap. Figure 7(a) is an enlarged photograph of internal thread 40 formed by thread forming tap 1, and Figure 7(b) is an enlarged photograph of internal thread 140 formed by a conventional thread forming tap as a reference example.

In both Figures 7(a) and 7(b), the same conditions and machine were used to form a female thread on SUS304 at a cutting speed of 15 m/min using a water-soluble cutting fluid diluted 20 times.

Referring to Figure 7(a), it can be seen that almost no irregularities or deposits are observed on the surface of the internal thread 40 formed by the thread forming tap 1, particularly on the flank surface 41, and that the flank surface 41 is formed very smoothly. On the other hand, referring to Figure 7(b), on the surface of the internal thread 140 formed by a conventional thread forming tap, particularly on the flank surface 141, irregularities 142 due to peeling of the metal structure of the workpiece, which would likely have occurred when the workpiece was machined with the thread forming tap, and deposits caused by the peeled metal structure, i.e., adhered material 143, are observed. In short, the thread forming tap 1 according to an embodiment of the present invention can reduce peeling of the metal structure of the workpiece compared to conventional thread forming taps, and can reduce irregularities and adhered material on the surface of the internal thread 40. Therefore, the thread forming tap 1 can form a better internal thread.

Figure 8 shows the time-series changes in tapping torque when forming an internal thread using a thread forming tap. Figure 8(a) is a graph when forming an internal thread using thread forming tap 1, and Figure 8(b) is a graph when forming an internal thread using a conventional thread forming tap as a reference example. In each graph in Figure 8, the horizontal axis represents time [s] and the vertical axis represents tapping torque [N·m].

Referring to Figure 8(a), at point 50, the threads 10 of the thread forming tap 1, particularly the threads of the chamfer 4a, first act on the workpiece as it rotates. Next, at point 51, the entire chamfer 4a has acted. Next, as the thread forming tap 1 rotates, the tapping torque increases approximately linearly, forming the internal thread. Next, at point 52, reverse rotation begins to remove the thread forming tap 1. Finally, at point 53, the internal thread is formed.

On the other hand, referring to Figure 8(b), as in Figure 8(a), at point 150, the threads of a conventional thread forming tap, particularly the threads of the chamfering portion, first act on the workpiece as it rotates. Next, at point 151, the entire chamfering portion has acted. Next, as the thread forming tap rotates, the tapping torque increases, forming an internal thread. However, unlike the trend in the graph of Figure 8(a), the increase in tapping torque is not approximately linear. In particular, there is a sudden increase in tapping torque at point E. This sudden increase in tapping torque is thought to be due to increased resistance resulting from peeling and even adhesion of the metal structure of the workpiece. In short, the thread forming tap 1 according to an embodiment of the present invention can form a good internal thread more easily than conventional thread forming taps.

In the thread forming tap 1, the width of the outer diameter portion 11 at the thread forming recess 14a and the width of the outer diameter portion 11 at the relief recess 14b are formed to be equal, but the width of the outer diameter portion 11 at the relief recess 14b may also be formed to be wider than the width of the outer diameter portion 11 at the thread forming recess 14a. The slope angle of the chamfer 4a may not be constant as shown in Figure 1, but may be a quadratic curve when viewed from the front so that the amount of threading gradually decreases. The thread forming tap 1 is preferably used to form internal threads with a reference outer diameter dimension of M3 to M16.

REFERENCE SIGNS LIST 1 thread tap 2 shank 3 machining portion 4 thread portion 5 groove portion 10 thread 11 outer diameter portion 12 root portion 13 protrusion portion 14a thread-side relief portion 14b relief-side relief portion 20 effective diameter portion

Claims (2)

A thread forming tap that forms a female screw by plastically deforming a pilot hole in a workpiece,
A screw portion having a plurality of threads and a plurality of grooves provided in the longitudinal direction so as to divide the screw portion,
A thread forming tap characterized in that the width of the outer diameter portion of the thread is configured to become narrower as it approaches the groove portion. The thread forming tap according to claim 1, wherein the clearance angle of the outer diameter portion of the thread is smaller than the clearance angle of the effective diameter portion.