Tapping Tool

Abstract

To prevent deformation such as bending or falling of a tapping tool when the cutting edge of a chamfer at the tip of the tapping tool cuts into a workpiece. [Solution] A tapping tool 10 has a shank portion 12 having a central axis Om, and a cutter body 14 connected to one end of the shank portion. The cutter body has a threaded portion 16 that engages with a pilot hole H formed in a workpiece W, and a non-engaging portion 18 that does not engage with the pilot hole. The threaded portion has a completely threaded portion 22 on the base end side and an incomplete threaded portion 20 formed continuously with the tip side of the completely threaded portion. The incomplete threaded portion has a cutting edge 16a that machines the pilot hole, and a guide pad portion 16b located rearward in the rotational direction of the cutting edge, and the outer peripheral surface of the guide pad portion is formed from a cylindrical surface having the radius of the cutting edge relative to the central axis.

Description

[Technical field]

[0001] The present invention relates to a tapping tool for machining a pilot hole provided in a workpiece into a screw hole. [Background technology]

[0002] When tapping is performed using a machining center and a tapping tool attached to it, so-called synchronous tapping is performed in which the rotation and linear feed are synchronized. In synchronous tapping, the rotation and feed must also be synchronized when the tapping tool is withdrawn from the tapped hole. Therefore, the time required to withdraw the tapping tool is approximately equal to the time required to advance the tapping tool to form the thread groove.

[0003] Patent Document 1 describes a tapping tool that includes a threaded portion having a blade portion for machining a thread groove in a screw hole, a pad portion that engages with the thread groove machined by the blade portion, and a non-engaging portion that forms a space between the screw hole and the threaded portion when viewed in cross section with the rotation axis and the central axis of the screw hole aligned. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2020-168698 A Summary of the Invention [Problem to be solved by the invention]

[0005] In the tapping tool of Patent Document 1, a space is formed between the non-engaging portion and the threaded hole. Therefore, after thread machining is completed, the tapping tool can be shifted in a direction perpendicular to the rotation axis within the threaded hole from a state in which the rotation axis and the central axis of the threaded hole are aligned, thereby simultaneously releasing the engagement between the threaded portion and the thread groove and the engagement between the pad portion and the thread groove, and the tapping tool can be pulled out axially from the threaded hole.

[0006] In the tapping tool of Patent Document 1, when the cutting edge of the chamfer at the tip of the tapping tool first cuts in, the chamfer cannot be sufficiently supported at the entrance of the pilot hole, causing deformation of the tapping tool such as bending or tilting. Furthermore, the tapping tool of Patent Document 1 has a problem in that deformation such as bending or tilting is likely to occur especially when the tapping tool first cuts in, because the cutting edge is provided with a relief angle.

[0007] The present invention is an improved invention of Prior Art Document 1, which is the applicant's earlier application, and aims to solve the problems of the conventional technology. That is, the object of the present invention is to provide a tapping tool that can be shifted in a direction perpendicular to the rotation axis within a screw hole after machining is completed, and then pulled out of the screw hole in the axial direction, and that does not cause deformation such as bending or tilting when the cutting edge of the leading edge of the tapping tool cuts into a workpiece. [Means for solving the problem]

[0008] In order to achieve the above object, according to the present invention, a tapping tool for machining a pilot hole formed in a workpiece into a screw hole includes a shank portion having a central axis and a cutter body connected to one end of the shank portion, the cutter body having a threaded portion that engages with the pilot hole formed in the workpiece and a threaded portion that does not engage with the pilot hole. Asymmetrically formed with respect to the central axis a non-engagement portion, the threaded portion having a complete thread portion on a base end side and an incomplete thread portion formed continuously with the complete thread portion on a tip end side, the incomplete thread portion having a cutting edge that processes the pilot hole and a guide pad portion located behind the cutting edge in a rotational direction, the outer peripheral surface of the guide pad portion being a cylindrical surface parallel to the central axis and having a radius equal to the distance from the central axis to the cutting edge Part of Formed from The cutting edge has no relief angle. A tapping tool is provided. Effect of the Invention

[0009] According to the present invention, the outer peripheral surface of the guide pad portion of the incomplete thread portion is formed from a cylindrical surface that is parallel to the central axis and has a radius equal to the distance from the central axis to the cutting edge. Therefore, during machining, particularly when the incomplete thread portion of the tapping tool begins to engage with the pilot hole in the workpiece (when the cutting edge cuts in), the outer peripheral surface of the guide pad supports the cutting resistance, thereby preventing the tapping tool from bending. [Brief description of the drawings]

[0010] [Figure 1] 1 is a schematic diagram showing an example of a machine tool for performing tapping using the tapping tool of the present invention. [Diagram 2] FIG. 1 is a front view of a tapping tool according to an embodiment of the present invention. [Diagram 3] FIG. 3 is a side view of the tapping tool of FIG. 2. [Figure 4] FIG. 3 is an end view of the tip side of the tapping tool of FIG. 2. [Diagram 5] FIG. 3 is a perspective view of the tapping tool of FIG. 2. [Figure 6] FIG. 3 is a partially enlarged cross-sectional view showing an incomplete thread portion of the tapping tool of FIG. 2. [Figure 7] FIG. 1 is an enlarged partial cross-sectional view showing an incomplete thread portion of a prior art tapping tool. [Figure 8] FIG. 2 is a cross-sectional view of a tapping tool and the threaded hole it machines. [Figure 9] FIG. 9 is a cross-sectional view similar to FIG. 8, showing cutting resistance and its principal force and thrust force. [Figure 10] FIG. 9 is a cross-sectional view of the tapping tool and the screw hole, showing a state in which the tapping tool has been shifted from the state in FIG. 8; [Figure 11] FIG. 2 is a front view of a tapping tool having grooves. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. Referring to FIG. 1, a machine tool 100 to which the tapping tool and tapping method of the present invention are applied is illustrated. In FIG. 1, the machine tool 100 is equipped with a bed 102 as a base fixed to the floor of a factory. On the upper surface of the bed 102, a Y-axis guide rail 118 is provided extending in the horizontal front-rear direction or the Y-axis direction (left-right direction in FIG. 1), and a table 104 is provided so as to be capable of reciprocating along the Y-axis guide rail 118. A workpiece W is clamped and fixed to the table 104. The machine tool 100 is equipped with a Y-axis servo motor (not shown) as a Y-axis drive device that drives the table 104 to reciprocate in the Y-axis direction. The machine tool 100 is also equipped with a Y-axis digital scale (not shown) that detects the Y-axis coordinate of the table 104.

[0012] A column 108 is provided upright on the upper surface of the rear end side of bed 102. An X-axis guide rail 120 is provided on the front surface of column 108, extending in the horizontal left-right direction or in the X-axis direction (a direction perpendicular to the paper surface in FIG. 1), and an X-axis slider 110 is provided so as to be able to reciprocate along X-axis guide rail 120. Machine tool 100 is provided with an X-axis servo motor (not shown) as an X-axis drive device that drives X-axis slider 110 to reciprocate in the X-axis direction. Machine tool 100 also is provided with an X-axis digital scale (not shown) that detects the X-axis coordinate of X-axis slider 110.

[0013] A Z-axis guide rail 122 is provided on the front surface of X-axis slider 110, extending in the vertical or Z-axis direction, and headstock 112 is provided so as to be able to reciprocate along Z-axis guide rail 122. Machine tool 100 is equipped with a Z-axis servo motor (not shown) as a Z-axis drive device that drives headstock 112 to reciprocate in the Z-axis direction. Machine tool 100 also is equipped with a Z-axis digital scale (not shown) that detects the Z-axis coordinate of headstock 112.

[0014] A spindle head 116 is attached to the headstock 112, which supports the spindle 114 rotatably about a vertical rotation axis Om. A rotary tool T such as an end mill, a face mill, or a drill, in particular a tapping tool 10 described below, can be attached to the tip of the spindle 114. The spindle head 116 is equipped with a spindle servo motor 124 that rotates the spindle 114 about the rotation axis Om. The machine tool 100 is equipped with a rotation position detection device that detects the rotation position of the spindle 114 about the rotation axis Om. The rotation position detection device can be, for example, a rotary encoder 126 attached to the spindle servo motor 124.

[0015] Machine tool 100 can further be configured as a machining center equipped with a tool magazine (not shown) that stores a plurality of tools used for machining, an automatic tool changer (not shown) that changes tools between the tool magazine and spindle 114, and a coolant supply device (not shown) that supplies coolant to the machining area of ​​machine tool 100, and these peripheral devices are housed within a cover (not shown) together with machine tool 100.

[0016] The machine tool 100 further includes a control device 130 that controls the machine tool 100. The control device 130 can be composed of a computer and associated software including a CPU (Central Processing Unit), a memory device such as a RAM (Random Access Memory) or a ROM (Read Only Memory), a storage device such as a HDD (Hard Disk Drive) or an SSD (Solid State Drive), an input / output port, and a bidirectional bus that interconnects these. The control device 130 can be formed by an NC control device that controls, in particular, the X-axis servo motor, the Y-axis servo motor, the Z-axis servo motor, and the spindle servo motor 124.

[0017] The machine tool 100 controls the X-axis servo motor, Y-axis servo motor, Z-axis servo motor, and spindle servo motor 124 in accordance with a machining program supplied to the control device 130, to move the rotating tool T attached to the tip of the spindle relative to the workpiece W fixed to the table 104 in three orthogonal axis directions, X, Y, and Z, to machine the workpiece W.

[0018] Machine tool 100 performs tapping by synchronizing the rotation of spindle 114 with the feed in the Z-axis direction. In this specification, the linear movement in the Z-axis direction by tapping tool 10 to perform tapping is referred to as forward movement, and the linear movement in the Z-axis direction by which tapping tool 10 is withdrawn from threaded hole 6 is referred to as backward movement.

[0019] The tapping tool 10 performs tapping, i.e., machining the pilot hole H into a threaded hole 6, with its central axis Ot coinciding with the central axis Oh of the pilot hole H formed in the workpiece W. In Figs. 8 to 10, reference numeral 8 indicates the inner circumference of the threaded hole 6, which coincides with the pilot hole H. Reference numeral 6 indicates the root of the threaded hole 6 that corresponds to the nominal diameter of the tapping tool 10. The tapping tool 10 rotates in the direction of arrow A.

[0020] The tapping tool 10 has a rod-shaped shank portion 12 extending along a central axis Ot, and a cutter body 14 coupled to the tip end of the shank portion 12. The shank portion 12 is formed so as to be attached to the tip end of a spindle 114. The shank portion 12 can be formed so as to be directly attached to a mounting hole (not shown) formed in the tip end of the spindle 114. Alternatively, the tapping tool 10 can be attached to the tip end of the spindle 114 via a tool holder (not shown) or a tool chuck (not shown) by mounting the shank portion 12 so as to be attachable to the tool holder or the tool chuck.

[0021] The cutter body 14 is formed asymmetrically with respect to the central axis Ot, and has a male-threaded threaded portion 16 that engages with the inner peripheral surface of a pilot hole H pre-formed in the workpiece W to machine a screw groove, and a non-engagement portion 18 that does not engage with the inner peripheral surface of the pilot hole H. The threaded portion 16 has a plurality of threads 24, 26, 28, 30 (see Figs. 6 and 7) that are spaced apart at a predetermined thread pitch in the longitudinal direction. In this embodiment, the tapping tool 10 forms a triangular screw groove.

[0022] 8 and 9, each of the threads of the screw portion 16 has a cutting edge 16a formed at the tip in the rotation direction of the tapping tool 10, and a guide pad portion 16b which is a portion that spreads behind the cutting edge 16a in the rotation direction. The guide pad portion 16b is a portion from the cutting edge 16a to the rear end 16d in Figs. 8 to 10, and has an outer circumferential surface 16c. In this embodiment, the outer circumferential surface 16c is formed from a part of a cylindrical surface having a radius (the distance between the central axis Ot and the cutting edge 16a) equal to that of the cutting edge 16a. That is, in this embodiment, the cutting edge 16a of the screw portion 16 is not provided with a relief angle, and in this embodiment, the tangent of the cutting edge 16a and the tangent of the cutting edge 16a to the circumference described by the cutting edge 16a coincide with each other.

[0023] The threaded portion 16 has an incomplete threaded portion 20 forming a lead portion formed at the tip portion, and a complete threaded portion 22 on the base end side. In this embodiment, the incomplete threaded portion 20 has three threads 24, 26, 28. The complete threaded portion 22 has a plurality of threads 30. Referring to FIG. 6, the three threads 24, 26, 28 of the incomplete threaded portion 20 are spaced apart at a predetermined thread pitch in the axial direction of the tapping tool 10, and have outer peripheral surfaces 24a, 26a, 28a and flank surfaces 24b, 26b, 28b. The flank surfaces 24b, 26b, 28b are arranged approximately symmetrically in the axial direction on both sides of the outer peripheral surfaces 24a, 26a, 28a.

[0024] In this embodiment, the outer peripheral surfaces 24a, 26a, 28a are each formed from a part of a cylindrical surface, and the outer peripheral surfaces 24a, 26a, 28a are parallel to the central axis Ot. The outer peripheral surface 28a at the tip end of the tapping tool 10 has the smallest radius, and is therefore formed to be the widest in the axial direction. In contrast, the outer peripheral surface 24a at the base end has the largest radius, and is therefore formed to be the narrowest in the axial direction. Thus, when viewed in a longitudinal cross section or when the incomplete thread portion 20 is projected onto a plane, the outer peripheral surfaces 24a, 26a, 28a are formed so as to have a stepped shape from the outer peripheral surface 24a at the tip end to the outer peripheral surface 28a at the base end.

[0025] In contrast, in the conventional tapping tool, as shown in Fig. 7, the incomplete thread portion 50 forming the cutting edge has three threads 52, 54, 56 spaced apart at a predetermined thread pitch in the axial direction, and has outer peripheral surfaces 52a, 54a, 56a and flank surfaces 52b, 52c; 54b, 54c; 56b, 56c. The outer peripheral surfaces 52a, 54a, 56a are arranged in a common conical surface, and therefore the incomplete thread portion 50 is formed in a tapered shape tapering toward the tip. Therefore, in each of the threads 52, 54, 56, the flank surfaces 52b, 54b, 56b on the base end side are wider than the flank surfaces 52c, 54c, 56c on the tip end side, and during machining, a larger machining load acts on the flank surfaces 52b, 54b, 56b on the base end side than on the flank surfaces 52c, 54c, 56c on the tip end side.

[0026] In conventional tapping tools, the difference in size between the base end and tip end flank surfaces of the threads of the incomplete thread portion that forms the biting portion causes a problem in that the tapping tool falls and deforms so as to bend during machining. In contrast, in this embodiment, the flank surfaces 24b, 26b, and 28b are arranged approximately symmetrically in the axial direction on both sides of the outer circumferential surfaces 24a, 26a, and 28a, so that the tapping tool 10 is prevented from falling and bending during machining, particularly when the incomplete thread portion 20 of the tapping tool 10 begins to engage with the pilot hole H of the workpiece W.

[0027] The fully threaded portion 22 has a plurality of threads 30, seven in this embodiment. The threads 30 have flank surfaces 30b and an outer peripheral surface 30a, and the flank surfaces 30b are arranged axially symmetrically on both sides of the outer peripheral surface 30a. The fully threaded portion 22 has a shape that matches the root shape of the internal thread to be machined.

[0028] The non-engagement portion 18 has a smooth curved surface, and can be formed, for example, by a part of a cylindrical surface having a smaller radius than the shank portion 12. A space 4 is formed between the non-engagement portion 18 and the inner peripheral surface 8 of the screw hole 6 machined in the workpiece W. The space 4 has a size such that when the tapping tool 10 moves (shifts) from a position coinciding with the central axis Oh of the screw hole 6 in the workpiece W in a direction perpendicular to the rotation axis Om as shown by the arrow S in FIG. 10 from a position coincident with the central axis Oh of the screw hole 6 in the workpiece W as shown in FIGS. 8 and 9, the engagement between the screw portion 16 and the female thread machined in the workpiece W is released, that is, the threads 24, 26, 28, and 30 of the screw portion 16 can be completely separated from the inner peripheral surface 8 of the female thread of the workpiece W. In FIG. 10, the arrow S is a vector indicating the movement direction and the movement amount of the spindle 114.

[0029] It is desirable to form the size of the space 4 slightly larger than the space 16a to provide a safety margin. During cutting, the space 4 serves as a path for supplying cutting fluid to the cutting edge 16a and a path for discharging chips generated by cutting, improving the quality of the machined surface and extending the tool life.

[0030] 10, when the threaded portion 16 is disengaged from the thread groove of the workpiece W by shifting the spindle 114, the tapping tool 10 can be pulled out of the threaded hole 6 by moving backward along the rotation axis Om (moving upward along the Z-axis in FIG. 1). This pulling out of the tapping tool 10 can be performed at high speed because it does not involve a rotational motion as in the conventional method.

[0031] In the following description, the 3 o'clock direction in Fig. 8 is set as the origin of the rotational position around the rotation axis Om of the spindle 114 (the position where the rotational angle θ = 0°). In Fig. 8, the rotational angle (hereinafter referred to as "phase angle") of the tapping tool 10 is illustrated as 180 degrees as the phase angle of the cutting edge 16a. The phase angle of the tapping tool 10 that has stopped after machining differs for each screw hole 6 because the number of rotations of the tapping tool 10 required for machining differs depending on the depth of the screw hole 6. Therefore, the direction of the shift S of the tapping tool 10 also differs for each screw hole 6.

[0032] In order to determine the direction in which the spindle 114 is shifted, in this embodiment, when the tap tool 10 is positioned at a processing start point (not shown), the spindle 114 is set to a predetermined phase angle. Then, when the tap tool 10 stops after processing, the control device 130 calculates the phase angle of the tap tool 10 based on information from the rotary encoder 126 of the spindle servo motor 124. Based on the calculated phase angle of the tap tool 10, the control device 130 calculates the shift direction of the spindle 114 (angle θ in FIG. 0). On the other hand, the required shift amount is determined by the cross-sectional shape of the tap tool 10, and can be stored in advance in the control device 130.

[0033] An example of a tapping operation performed using a tapping tool 10 according to an embodiment of the present invention and a vertical machine tool 100 as shown in FIG. 1 will now be described. First, the tap tool 10 is attached to the spindle 114 and then positioned at the machining start point. That is, the rotation axis Om of the spindle 114 of the machine tool to which the tap tool 10 is attached is aligned with the central axis Oh of a pilot hole H provided in a workpiece, and the tip of the tap tool 10 is positioned at the machining start point at a predetermined height in the Z-axis direction. At this time, the spindle 114 is rotated and fed at a predetermined phase angle so that the cutting edge 16a is positioned at a predetermined rotational position.

[0034] Next, the rotation speed of the spindle 114 and the feed speed in the Z-axis direction are synchronized according to the pitch of the thread to perform synchronous tapping. When the tapping tool 10 moves a predetermined distance in the Z-axis direction and reaches the commanded thread depth, the rotation of the spindle 114 and the feed in the Z-axis direction are stopped. This causes the pilot hole H in the workpiece to be machined into a threaded hole 6.

[0035] Next, the tapping tool 10 is shifted in a direction perpendicular to the rotation axis Om. The shift direction of the tapping tool 10 (the direction indicated by the arrow S in FIG. 10) is calculated based on the phase angle of the tapping tool 10 (the rotational position of the cutting edge 16a) calculated based on information from the rotary encoder 126 of the spindle servo motor 124. A value previously registered in the control device 130 is used as the shift amount. This process completely releases the engagement between the threaded portion 16 of the tapping tool 10 and the thread groove of the screw hole 6.

[0036] Next, the tapping tool 10 is retracted upward to the processing start point without rotating. The feed speed of the spindle 114 in the Z-axis direction at this time can be set to be more than twice as fast as the feed speed during processing, i.e., during forward movement. In this manner, the tapping of one screw hole 6 is completed.

[0037] According to this embodiment, a space 4 is formed between the non-engagement portion 18 of the tapping tool 10 and the inner diameter of the screw hole 6, and the space 4 has a size that disengages the threaded portion 16 from the thread groove when the tapping tool 10 moves in a direction perpendicular to the rotation axis Om. Therefore, after machining, the tapping tool 10 that has been disengaged from the thread groove of the screw hole 6 can be pulled out of the screw hole 6 in the rotation axis direction at high speed without rotating.

[0038] 9, the cutting resistance Rc acting on the cutting edge 16a of the tapping tool 10 as a reaction force against the cutting force while the tapping tool 10 rotates to perform tapping, and the thrust force Rb and principal force Rp are shown as vectors. Since the principal force Rp is larger than the thrust force Rb, the resultant cutting resistance Rc acts toward the guide pad portion 16b behind the cutting edge 16a in the rotation direction. The outer peripheral surface of each thread of the guide pad portion 16b is a cylindrical surface parallel to the center line Ot and has a radius equal to the distance from the center line Ot to the cutting edge 16a. Therefore, the cutting resistance Rc is supported by the contact of the outer cylindrical surface of the guide pad portion 16b with the workpiece W. Thus, according to this embodiment, the outer peripheral surface 16c of the guide pad portion 16b is formed from a part of a cylindrical surface with a radius equal to the radius of the cutting edge 16a. Therefore, the cutting resistance Rc is supported by the screw hole 6, particularly the surface of the screw groove (valley) formed in the workpiece W, via the outer peripheral surface 16c of the guide pad portion 16b. As a result, the tap tool 10 does not tip or bend due to the cutting resistance Rc. As a result, the machining accuracy is equivalent to that when a conventional normal tap tool is used.

[0039] Although it has been described that the radius of the outer peripheral surface 16c of the guide pad portion 16b is equal to the radius of the cutting edge 16a, "equal" does not mean that they are completely the same in the strict sense. In the present invention, "equal radii" includes the same radius or a radius several μm to several tens of μm smaller. In other words, "equal radii" includes "approximately equal radii." For example, "equal radii" may include machining errors within a reasonable range.

[0040] Furthermore, in order to lubricate the gap between the outer peripheral surface 16c of the guide pad portion 16b and the surface of the thread groove (valley) formed in the workpiece W with a coolant, or to prevent the cutting resistance Rc from increasing excessively, the radius of the outer peripheral surface 16c of the guide pad portion 16b may be made smaller than the radius of the cutting edge 16a within a range in which the guide pad portion 16b can support the cutting resistance Rc and prevent the tap tool 10 from bending, thereby forming a gap between the outer peripheral surface 16c of the guide pad portion 16b and the workpiece W. This gap can be, for example, 100 μm or less, preferably several tens of μm or less, and more preferably 50 μm or less.

[0041] Alternatively, the radius of the outer peripheral surface 16c of the guide pad portion 16b may be gradually reduced in the circumferential direction from the cutting edge 16b toward the rear end 16d, so that a gap is formed between the outer peripheral surface 16c and the workpiece W. This gap can be, for example, 100 μm or less, preferably several tens of μm or less, and more preferably 50 μm or less at the rear end 16d of the guide pad portion 16b.

[0042] If the radius of the outer peripheral surface 16c of the guide pad portion 16b is smaller than the radius of the cutting edge 16a, a gap will be formed between the outer peripheral cylindrical surface 16c of the guide pad portion 16b and the workpiece W, which may cause the tapping tool 10 to bend. However, the flank surfaces 24b, 26b, 28b of the threads 24, 26, 28 come into contact with the threads of the female thread formed in the workpiece W, thereby supporting the cutting resistance Rc, and if the difference in radii is in the range of a few μm to a few tens of μm, bending of the tapping tool 10 can be prevented.

[0043] The widths GW1, GW2, and GW3 of the outer peripheral surface 16c of the guide pad portion 16b can be made constant in the circumferential direction. As a result, the cutting resistance Rc is supported not only by the outer peripheral surface 16c but also by the flank surfaces 24b, 26b, and 28b of the threads 24, 26, and 28 of the incomplete thread portion 20, so that the effect of preventing the tap tool 10 from being bent is further enhanced.

[0044] As a preferred embodiment of the present invention, an example has been described in which the radius of the outer peripheral surface 16c of the guide pad portion 16b is equal to the radius of the cutting edge 16a. As another example, the widths GW1, GW2, and GW3 of the outer peripheral surface 16c of the guide pad portion 16b may increase slightly in the circumferential direction, i.e., the cutting edge 16a may have a slight clearance angle.

[0045] The threaded portion 16 may have one or more grooves extending in the axial direction. Referring to FIG. 11, the tapping tool 60, like the tapping tool 10, has a shank portion 62 and a cutter body 64 coupled to the tip end of the shank portion 62. The cutter body 64 is formed asymmetrically with respect to the central axis Ot and has a male-threaded threaded portion 66 that engages with the inner circumferential surface of a pilot hole H pre-formed in the workpiece W to machine a thread groove, and a non-engagement portion (not shown) that does not engage with the inner circumferential surface of the pilot hole H. The threaded portion 66 has an incomplete threaded portion 68 that forms a biting portion formed at the tip portion, and a complete threaded portion 70 on the base end side. In this embodiment, the tapping tool 10 has two grooves 72 extending in the axial direction formed in the threaded portion 66. By providing the grooves 72, lubrication by the coolant and discharge of chips are further improved. [Explanation of symbols]

[0046] 6 Screw holes 8 Inner surface 10 Tapping Tools 12 Shank 14 Cutter body 16 Threaded part 16a cutting edge 16b Guide pad part 16c Outer surface 18 Non-engaging part 20 Incomplete thread 22 Fully threaded 24, 26, 28, 30 threads 24a, 26a, 28a, 30a outer surface 24b, 26b, 28b, 30b flank surface

Claims

1. In tapping tools that turn pilot holes formed in workpieces into screw holes, a shank portion having a central axis; a cutter body coupled to one end of the shank portion, The cutter body has a threaded portion that engages with a pilot hole formed in a workpiece, and a non-engagement portion that is formed asymmetrically with respect to the central axis and does not engage with the pilot hole, The threaded portion includes a complete threaded portion on a base end side and an incomplete threaded portion formed continuously with the complete threaded portion on a tip end side, The incomplete thread portion includes a cutting edge that processes the pilot hole and a guide pad portion located behind the cutting edge in a rotational direction, The outer peripheral surface of the guide pad portion is formed of a part of a cylindrical surface that is parallel to the central axis and has a radius equal to the distance from the central axis to the cutting edge, A tapping tool characterized in that the cutting edge has no relief angle.

2. A tapping tool as described in claim 1, wherein the outer peripheral surfaces of the multiple threads of the incomplete thread portion are formed in a stepped shape with a gradually increasing radius from the threads at the tip end to the threads at the base end.

3. The tapping tool according to claim 1 , wherein the axial width of the outer circumferential surface of the guide pad portion is constant in the circumferential direction.

4. The tapping tool according to claim 1 , wherein the threaded portion has an axially extending groove.

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