Helical broach

Abstract

The shell of this helical broach (1) is formed by stacking a plurality of wafer shells (20.sub.W(1)-20.sub.W(N)) in the axial direction, and is obtained by forming on the wafer shells (20.sub.W(1)-20.sub.W(N)) finishing blades (30.sub.W(1)-30.sub.W(N)) corresponding to teeth grooves on a piece to be cut (W) and forming the finishing blades (30.sub.W(1)-30.sub.W(N)) such that the blade width gradually increases with each of the aforementioned wafer shells (20.sub.W(1)-20.sub.W(N)) from the leading end of the cutting direction toward the trailing end of the cutting direction.

Claims

1. A helical broach comprising: a cylindrical shell in which finishing blades having a predetermined tooth gear helix angle, with respect to an axial direction of the helical broach, corresponding to tooth grooves in a piece to be cut are formed on an outer peripheral side, wherein the cylindrical shell includes a plurality of wafer shells assembled coaxially and side by side along an axial direction of the helical broach, each wafer shell having finishing blades having the predetermined tooth gear helix angle with respect to the axial direction; and in the finishing blades in the plurality of wafer shells, a blade width gradually increases with each of the wafer shells from a leading end of a cutting direction toward a trailing end of the cutting direction.

2. The helical broach according to claim 1, wherein, in a case where the finishing blade provided in a wafer shell positioned at a rearmost end of the cutting direction is in a state of being unable to cut the piece to be cut to predetermined dimensions, a new wafer shell is inserted into the rearmost end of the cutting direction, and a wafer shell positioned at a foremost end of the cutting direction is removed and installation positions of the plurality of wafer shells assembled and arranged in the axial direction are moved toward the leading end of the cutting direction, thereby forming a new cylindrical shell.

3. The helical broach according to claim 1, wherein the finishing blades in the plurality of wafer shells cut both tooth surfaces along a tooth lead in the piece to be cut.

4. The helical broach according to claim 3, wherein all of tool angles at which the finishing blades in the plurality of wafer shells abut both of the tooth surfaces along the tooth lead in the piece to be cut are acute angles.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a side view illustrating a helical broach according to Embodiment 1.

(2) FIG. 2 is a side view and a longitudinal sectional view illustrating wafer shells of the helical broach of Embodiment 1.

(3) FIG. 3 is an explanatory view illustrating finishing by the helical broach according to Embodiment 1.

(4) FIG. 4 is a side view illustrating an example of an existing helical broach.

(5) FIG. 5 is an explanatory view illustrating an example of finishing by the existing helical broach.

DESCRIPTION OF EMBODIMENTS

(6) Hereinafter, an embodiment of a helical broach according to the present invention will be described in detail with reference to the accompanying drawings. As a matter of course, the present invention is not limited to the following embodiment, and it is natural that various modifications can be made without departing from the spirit of the present invention.

Embodiment 1

(7) First, the structure of the helical broach according to Embodiment 1 of the present invention will be described with reference to FIGS. 1 to 3.

(8) A helical broach 1 according to this embodiment is a cutting tool for forming a substantially cylindrical workpiece W which is a piece to be cut, to a helical internal gear having a tooth gear helix angle . As illustrated in FIG. 1, the helical broach 1 includes a shank section 2 to be installed in a broaching machine (not illustrated), a roughing section 3 for roughing of the workpiece W, and a finishing section 4 for finishing of the rough-worked workpiece W and is formed by assembling a plurality of (in this embodiment, N) wafer shells 20.sub.W(1), 20.sub.W(2), . . . , 20.sub.W(N) as a shell constituting the finishing section 4, to a broach body 10 having the shank section 2 and the roughing section 3.

(9) In the roughing section 3, roughing blades (not illustrated) having a tooth gear helix angle are formed integrally with the broach body 10 to protrude toward the outer peripheral side of the helical broach 1 in the radial direction and form a plurality of blade groups corresponding to each of the tooth grooves in the workpiece W. In addition, in order to form teeth having predetermined dimensions in the workpiece W, the roughing blades in the blade groups are arranged along the tooth gear helix angle such that the height of the blades gradually increases from the leading end of the cutting direction (the left side in FIG. 1) toward the trailing end of the cutting direction (the right side in FIG. 1).

(10) In the finishing section 4, the plurality of wafer shells 20.sub.W(1) to 20.sub.W(N) which are substantially cylindrical are arranged in the axial direction of the helical broach 1 and are assembled to the broach body 10. In the wafer shells 20.sub.W(1) to 20.sub.W(N), as illustrated in FIG. 2, finishing blades 30.sub.W(1) to 30.sub.W(N) having a tooth gear helix angle are formed to protrude toward the outer peripheral side of the wafer shells 20.sub.W(1) to 20.sub.W(N) in the radial direction, and since the wafer shells 20.sub.W(1) to 20.sub.W(N) are sequentially arranged along the axial direction of the helical broach 1, the finishing blades 30.sub.W(1) to 30.sub.W(N) form a plurality of blade groups which follow the tooth gear helix angle to correspond to each of the tooth grooves in the workpiece W.

(11) In order to form teeth having predetermined dimensions in the workpiece W, the finishing blades 30.sub.W(1) to 30.sub.W(N) in the blade groups are arranged such that the width of the blades gradually increases with each of the wafer shells 20.sub.W(1) to 20.sub.W(N) from the leading end of the cutting direction, (the left side in FIG. 2) toward the trailing end of the cutting direction (the right side in FIG. 2).

(12) That is, in the first wafer shell 20.sub.W(1) from the leading end of the cutting direction, the first smallest blades are respectively provided in the blade groups, and in the second wafer shell 20.sub.W(2) from the leading end of the cutting direction, the second smallest blades are respectively provided in the blade groups. In the same manner, in the third to N-th wafer shells 20.sub.W(3) to 20.sub.W(N) from the leading end of the cutting direction, the third to N-th smallest blades are respectively provided in the blade groups.

(13) As a matter of course, the number of each of finishing blades 30.sub.W(1) to 30.sub.W(N) respectively provided in the wafer shells 20.sub.W(1) to 20.sub.W(N) is not limited to one in each of the blade groups as in this embodiment and a plurality of blades may also be provided in each of the blades. Considering the degree of freedom of the shapes of the respective finishing blades 30.sub.W(1) to 30.sub.W(N) provided in the wafer shells 20.sub.W(1) to 20.sub.W(N), which will be described later, it is preferable that the number of blades in each of the blade groups is one.

(14) As illustrated in FIGS. 1 and 2, the wafer shells 20.sub.W(1) to 20.sub.W(N) are arranged in the axial direction of the helical broach 1, are engaged with a shell engagement portion 11 of the broach body 10, and are assembled such that the first wafer shell 20.sub.W(1) from the leading end of the cutting direction abuts a shell abutting surface 12 of the broach body 10, and the N-th wafer shell 20.sub.W(N) from the leading end of the cutting direction is pressed toward the leading end of the cutting direction by a fastener 40 together with the first to (N1)-th wafer shells 20.sub.W(1) to 20.sub.W(N1) from the leading end of the cutting direction (the left side in FIGS. 1 and 2) positioned closer to the leading end of the cutting direction than the N-th wafer shell 20.sub.W(N). In addition, the fastener 40 is fixed to the broach body 10 by a bolt (not illustrated) or the like.

(15) In order to relatively align the phases in the peripheral direction (around the axis of the helical broach 1) in the broach body 10 and the wafer shells 20.sub.W(1) to 20.sub.W(N), a positioning protrusion 13 is provided in the shell abutting surface 12, positioning grooves 21.sub.W(1) to 21.sub.W(N) are respectively provided in one end (the left end in FIG. 2) of the wafer shells 20.sub.W(1) to 20.sub.W(N), positioning protrusions 22.sub.W(1) to 22.sub.W(N) are provided in the other end (the right end in FIG. 2) of the wafer shells 20.sub.W(1) to 20.sub.W(N), and a positioning groove 41 is provided in one end (the left end in FIG. 2) of the fastener 40.

(16) The positioning protrusion 13 of the shell abutting surface 12 and the positioning groove 21.sub.W(1) of the first wafer shell 20.sub.W(1) from the leading end of the cutting direction are fitted to each other, and the positioning protrusion 22.sub.W(1) of the first wafer shell 20.sub.W(1) from the leading end of the cutting direction and the positioning groove 21.sub.W(2) of the second wafer shell 20.sub.W(2) from the leading end of the cutting direction are fitted to each other. In the same manner, in the second to N-th wafer shells 20.sub.W(2) to 20.sub.W(N) from the leading end of the cutting direction, the positioning protrusions 21.sub.W(2) to 21.sub.W(N) are respectively fitted to the corresponding positioning grooves 22.sub.W(2) to 22.sub.W(N) in the wafer shells 20.sub.W(2) to 20.sub.W(N) such that the wafer shells 20.sub.W(1) to 20.sub.W(N) are assembled to the broach body 10 in a state in which the positioning protrusion 22.sub.W(N) of the wafer shell 20.sub.W(N) positioned at the N-th position (at the rearmost end in the cutting direction) from the leading end of the cutting direction is fitted to the positioning groove 41 of the fastener 40, that is, in a state in which the phases are relatively aligned with each other.

(17) The positioning grooves 21.sub.W(1) to 21.sub.W(N) and the positioning protrusions 22.sub.W(1) to 22.sub.W(N) in the wafer shells 20.sub.W(1) to 20.sub.W(N), are provided at predetermined positions with respect to the finishing blades 30.sub.W(1) to 30.sub.W(N) in the wafer shells 20.sub.W(1) to 20.sub.W(N). That is, the installation positions of the positioning grooves 21.sub.W(1) to 21.sub.W(N) and the positioning protrusions 22.sub.W(1) to 22.sub.W(N) with respect to the finishing blades 30.sub.W(1) to 30.sub.W(N) are the same in any of the wafer shells 20.sub.W(1) to 20.sub.W(N).

(18) Therefore, when the wafer shells 20.sub.W(1) to 20.sub.W(N) are assembled to the broach body 10 in dimensional order such that the blade width gradually increases, as illustrated in FIG. 2, the finishing blades 30.sub.W(1) to 30.sub.W(N) are positioned to be arranged to follow the tooth gear helix angle , and the positioning grooves 21.sub.W(1) to 21.sub.W(N) and the positioning protrusions 22.sub.W(1) to 22.sub.W(N) are positioned to also be arranged to follow the tooth gear helix angle in the wafer shells 20.sub.W(1) to 20.sub.W(N).

(19) The finishing blades 30.sub.W(1) to 30.sub.W(N) are individually worked by a working machine (not illustrated) and a grinding wheel (not illustrated) to be formed in the wafer shells 20.sub.W(1) to 20.sub.W(N). The finishing blades 30.sub.W(1) to 30.sub.W(N) are provided such that the blade width gradually increases with each of the wafer shells 20.sub.W(1) to 20.sub.W(N). Therefore, in each of the wafer shells 20.sub.W(1) to 20.sub.W(N), the blades are not adjacent to each other along the tooth gear helix angle . Accordingly, when the finishing blades 30.sub.W(1) to 30.sub.W(N) are formed in the wafer shells 20.sub.W(1) to 20.sub.W(N), there is no concern of the grinding wheel interfering with the adjacent blades, and the finishing blades 30.sub.W(1) to 30.sub.W(N) can be formed in various shapes by setting the relief angle (second relief) of the blade to be large or the like.

(20) That is, compared to the finishing blades 130 (FIGS. 4 and 5) in the integration type shell 120 of the existing helical broach 101, in the wafer shells 20.sub.W(1) to 20.sub.W(N) which are division type shells of the helical broach 1 according to this embodiment, the degree of freedom of the shapes of the finishing blades 30.sub.W(1) to 30.sub.W(N) formed by the grinding wheel or the like is high.

(21) In this embodiment, the finishing blades 30.sub.W(1) to 30.sub.W(N) in the wafer shells 20.sub.W(1) to 20.sub.W(N) are set to cut both tooth surfaces 50 and 60 including the left tooth surface 50 (one tooth surface along the tooth lead of the workpiece W) and the right tooth surface 60 (the other tooth surface along the tooth lead of the workpiece W) in each workpiece W and cut each of both tooth surfaces 50 and 60 in the workpiece W with the same cutting amount d.sub.W(1) to d.sub.W(N).

(22) In addition, in this embodiment, the finishing blades 30.sub.W(1) to 30.sub.W(N) in the wafer shells 20.sub.W(1) to 20.sub.W(N) are set to cut both of the left tooth surface 50 and the right tooth surface 60 in the workpiece W with tool angles .sub.W(1) to .sub.W(N) and are set so as to allow the tool angles .sub.W(1) to .sub.W(N) to be appropriate acute angles for the cutting.

(23) In general, in the working blades of a cutting tool, a cutting portion having an acute angle has a higher cutting ability than that of those having an obtuse angle and enables cutting with good surface roughness for cut surfaces.

(24) Therefore, in this embodiment, the finishing blades 30.sub.W(1) to 30.sub.W(N) in the wafer shells 20.sub.W(1) to 20.sub.W(N) are set so as to allow the tool angles .sub.W(1) to .sub.W(N) with which both of the tooth surfaces 50 and 60 in the workpiece W are cut to be an acute angle.

(25) That is, in this embodiment, the wafer shells 20.sub.W(1) to 20.sub.W(N) are wafer shells for both tooth surfaces in which the finishing blades 30.sub.W(1) to 30.sub.W(N) that cut both of the left tooth surface 50 and the right tooth surface 60 in the workpiece W with the acute tool angles .sub.W(1) to .sub.W(N) are provided.

(26) As a matter of course, the helical broach according to the present invention is not limited to the finishing blades for both tooth surfaces as in this embodiment and may also be applied to finishing blades that cut one tooth surface at a time. In addition, the cutting amounts d.sub.W(1) to d.sub.W(N) and the tool angles .sub.W(1) to .sub.W(N) with which the left tooth surface 50 and the right tooth surface 60 in the workpiece W are cut may have numerical values that vary between the left tooth surface 50 and the right tooth surface 60.

(27) Subsequently, the finishing of the helical broach according to Embodiment 1 of the present invention will be described with reference to FIGS. 1 to 3.

(28) The helical broach 1 according to Embodiment 1 of the present invention and the workpiece W are installed in the broaching machine (not illustrated), and when the helical broach 1 is moved in the axial direction while being pulled with respect to the workpiece W, the workpiece W can be formed to the helical internal gear as described below.

(29) First, the roughing blades (not illustrated) in the roughing section 3 of the helical broach 1 come into contact with the inner peripheral surface of the substantially cylindrical workpiece W. By the roughing blades arranged such that the height of the blades gradually increases from the leading end of the cutting direction toward the trailing end of the cutting direction, teeth having predetermined dimensions are formed in the workpiece W.

(30) Next, as illustrated in FIG. 3, the finishing blade 30.sub.W(1) in the first wafer shell 20.sub.W(1) positioned at the first position from the leading end of the cutting direction in the finishing section 4 of the helical broach 1 comes into contact with the left tooth surface 50 and the right tooth surface 60 in the workpiece W subjected to roughing. The finishing blade 30.sub.W(1) abuts both of the tooth surfaces 50 and 60 in the workpiece W with the cutting amount d.sub.W(1). Therefore, both of the tooth surfaces 50 and 60 in the workpiece W subjected to roughing are cut with the cutting amount d.sub.W(1).

(31) Subsequently, the finishing blade 30.sub.W(2) in the second wafer shell 20.sub.W(2) positioned at the second position from the leading end of the cutting direction in the finishing section 4 of the helical broach 1 comes into contact with the left tooth surface 50 and the right tooth surface 60 cut by the finishing blade 30.sub.W(1) in the first wafer shell 20.sub.W(1). The finishing blade 30.sub.W(2) abuts both of the tooth surfaces 50 and 60 in the workpiece W with the cutting amount d.sub.W(2). Therefore, both of the tooth surfaces 50 and 60 in the workpiece W are further cut with the cutting amount d.sub.W(2).

(32) In the same manner, the finishing blades 30.sub.W(3) to 30.sub.W(N) in the third to N-th wafer shells 20.sub.W(3) to 20.sub.W(N) positioned at the third to N-th positions from the leading end of the cutting direction in the finishing section 4 of the helical broach 1 sequentially come into contact with the left tooth surface 50 and the right tooth surface 60 in the workpiece W. The finishing blades 30.sub.W(3) to 30.sub.W(N) sequentially abut both of the tooth surfaces 50 and 60 in the workpiece W with the cutting amounts d.sub.W(3) to d.sub.W(N). Therefore, both of the tooth surfaces 50 and 60 in the workpiece W are further cut sequentially with the cutting amounts d.sub.W(3) to d.sub.W(N).

(33) That is, by the finishing blades 30.sub.W(1) to 30.sub.W(N) that are arranged such that the blade width gradually increases from the leading end of the cutting direction toward the trailing end of the cutting direction, the left tooth surface 50 and the right tooth surface 60 in the workpiece W are cut sequentially with the cutting amounts d.sub.W(1) to d.sub.W(N) to predetermined finished dimensions.

(34) As described above, by broaching using the helical broach 1 according to Embodiment 1 of the present invention, the left tooth surface 50 and the right tooth surface 60 in the workpiece W are accurately cut to predetermined finished dimensions, thereby forming a helical internal gear having high accuracy.

(35) Next, a case where the finishing blades 30.sub.W(1) to 30.sub.W(N) of the finishing section 4 in the helical broach according to Embodiment 1 of the present invention are in a state of being unable to cut the workpiece W to predetermined dimensions due to wear and the like will be described with reference to FIGS. 1 to 3.

(36) The finishing blades 30.sub.W(1) to 30.sub.W(N) in the finishing section 4 wear due to the repeated broaching operations using the helical broach 1 according to Embodiment 1 of the present invention described above. During cutting by the worn finishing blades 30.sub.W(1) to 30.sub.W(N), the left tooth surface 50 and the right tooth surface 60 in the workpiece W cannot be cut with the predetermined cutting amounts d.sub.W(1) to d.sub.W(N). That is, the workpiece W cannot be formed to a helical internal gear having predetermined dimensions.

(37) First, each of the wafer shells 20.sub.W(1) to 20.sub.W(N) are removed from the broach body 10, and the finishing blades 30.sub.W(1) to 30.sub.W(N) in the wafer shells 20.sub.W(1) to 20.sub.W(N) are re-ground to predetermined dimensions so as to be able to form the workpiece W to a helical internal gear having predetermined dimensions.

(38) When wear and re-grinding of the finishing blades 30.sub.W(1) to 30.sub.W(N) in the wafer shells 20.sub.W(1) to 20.sub.W(N) are repeated, the ground surfaces of the finishing blades 30.sub.W(1) to 30.sub.W(N) in the finishing section 4 reach the second relief and thus the blade width of the finishing blades 30.sub.W(1) to 30.sub.W(N) becomes too small. That is, even when the finishing blades 30.sub.W(1) to 30.sub.W(N) are re-ground, the finishing blades 30.sub.W(1) to 30.sub.W(N) cannot have predetermined dimensions and cannot cut the left tooth surface 50 and the right tooth surface 60 in the workpiece W with the predetermined cutting amounts d.sub.W(1) to d.sub.W(N). Particularly, the finishing blade 30.sub.W(N) positioned at the rearmost end of the cutting direction determines the final dimensions of the workpiece W and is thus important. When the finishing blade 30.sub.W(N) is reduced in size, the workpiece W cannot be formed to a helical internal gear having predetermined dimensions.

(39) In this embodiment, in a case where the finishing blade 30.sub.W(N) positioned at the rearmost end of the cutting direction is reduced in size, a new wafer shell 20.sub.W(N+1) having a finishing blade 30.sub.W(N+1) having predetermined dimensions is inserted into the (N+1)-th position from the leading end of the cutting direction which is closer to the trailing end of the cutting direction than the wafer shell 20.sub.W(N) positioned at the N-th position from the leading end of the cutting direction, that is, into the rearmost end of the cutting direction. Accordingly, the workpiece W is finally cut by the new finishing blade 30.sub.W(N+1) positioned at the rearmost end of the cutting direction, and thus the workpiece W can be formed to a helical internal gear having predetermined dimensions.

(40) In addition, since the new wafer shell 20.sub.W(N+1) is inserted into the rearmost end of the cutting direction, the number of wafer shells 20.sub.W(1) to 20.sub.W(N+1) is increased and thus the finishing section 4 is elongated in the axial direction. Here, the wafer shell 20.sub.W(1) positioned at the foremost end of the cutting direction is removed.

(41) The finishing blade 30.sub.W(1) in the wafer shell 20.sub.W(1) positioned at the foremost end of the cutting direction is also reduced in size due to the wear and the repeated re-grinding operations and is thus in a state of being unable to sufficiently cut both of the tooth surfaces 50 and 60 in the workpiece W subjected to roughing. Therefore, it may be considered that the finishing blade 30.sub.W(1) needs to be discarded.

(42) In addition, initially, since the finishing blade 30.sub.W(2) in the second wafer shell 20.sub.W(2) positioned at the second position from the leading end of the cutting direction is also reduced in size due to the wear and the repeated re-grinding operations, by the re-grinding operations, the finishing blade 30.sub.W(2) is further re-ground to the dimensions corresponding to the finishing blade 30.sub.W(1) in the first wafer shell 20.sub.W(1) initially positioned at the first position from the leading end of the cutting direction. The second wafer shell 20.sub.W(2) positioned at the second position is moved toward the leading end of the cutting direction to serve as the first wafer shell 20.sub.W(1) positioned at the first position.

(43) In the same manner, the finishing blades 30.sub.W(3) to 30.sub.W(N) in the third to N-th wafer shells 20.sub.W(3) to 20.sub.W(N) initially positioned at the third to N-th positions are re-ground to the dimensions corresponding to the finishing blades 30.sub.W(2) to 30.sub.W(N1) in the second to (N1)-th wafer shells 20.sub.W(2) to 20.sub.W(N1) initially positioned at the second to (N1)-th positions from the leading end of the cutting direction, and the third to N-th wafer shells 20.sub.W(3) to 20.sub.W(N) initially positioned at the third to N-th positions are moved toward the leading end of the cutting direction to serve as the second to (N1)-th wafer shells 20.sub.W(2) to 20.sub.W(N1) positioned at the second to (N1)-th positions.

(44) In this embodiment, the positioning grooves 21.sub.W(1) to 21.sub.W(N) and the positioning protrusions 22.sub.W(1) to 22.sub.W(N) in the wafer shells 20.sub.W(1) to 20.sub.W(N) are provided at predetermined positions with respect to the finishing blades 30.sub.W(1) to 30.sub.W(N) in the wafer shells 20.sub.W(1) to 20.sub.W(N). Therefore, in a case where the installation positions of the second to N-th wafer shells 20.sub.W(2) to 20.sub.W(N) are moved toward the leading end of the cutting direction as described above, the second to N-th wafer shells 20.sub.W(2) to 20.sub.W(N) are assembled while being slightly changed in phases along the tooth gear helix angle .

(45) That is, the positions of the finishing blades 30.sub.W(2) to 30.sub.W(N) in the helical broach 1, that is, the positions of the finishing blades 30.sub.W(2) to 30.sub.W(N) in the finishing section 4 with respect to the roughing blades (not illustrated) in the roughing section 3 are not changed. Therefore, there is no influence on the cutting of the workpiece W during broaching.

(46) As described above, by inserting the new wafer shell 20.sub.W(N+1) into the rearmost end of the cutting direction and removing the wafer shell 20.sub.W(1) positioned at the foremost end of the cutting direction, a new shell can be formed.

(47) In the helical broach 1 according to this embodiment, as illustrated in FIG. 3, the left tooth surface 50 and the right tooth surface 60 of the workpiece W can be cut with the tool angles .sub.W(1) to .sub.W(N) of the finishing blades 30.sub.W(1) to 30.sub.W(N) in the wafer shells 20.sub.W(1) to 20.sub.W(N), and thus the surface roughness of the cut surfaces cut by the finishing blades 30.sub.W(1) to 30.sub.W(N) is good and the cutting amounts d.sub.W(1) to d.sub.W(N) can be set to sufficiently large amounts.

(48) Therefore, the number of finishing blades in the finishing section 4 along the tooth gear helix angle can be reduced, that is, the number of wafer shells 20.sub.W(1) to 20.sub.W(N) can be reduced. Accordingly, the axial length of the wafer shells 20.sub.W(1) to 20.sub.W(N) as the shell can be reduced to be smaller than the axial length of the existing shell 120.

(49) In addition, it is preferable that the cutting conditions such as the cutting amounts d.sub.W(1) to d.sub.W(N) and the tool angles .sub.W(1) to .sub.W(N) of the finishing blades 30.sub.W(1) to 30.sub.W(N) in the wafer shells 20.sub.W(1) to 20.sub.W(N) are equally set. By equally setting the cutting conditions of the finishing blades 30.sub.W(1) to 30.sub.W(N), the wear times and wear amounts of the finishing blades 30.sub.W(1) to 30.sub.W(N) are equal to each other. Therefore, the replacement times of the wafer shells 20.sub.W(1) to 20.sub.W(N) can be substantially adjusted.

(50) Accordingly, not all the wafer shells 20.sub.W(1) to 20.sub.W(N) corresponding to the shell are not discarded, and the wafer shells 20.sub.W(1) to 20.sub.W(N) positioned at the rearmost end of the cutting direction are sequentially replaced at the leading end of the cutting direction as the finishing blades 30.sub.W(1) to 30.sub.W(N) wear. Therefore, the wafer shells 20.sub.W(1) to 20.sub.W(N) as the shell can be used for a longer period than the typical shell.

REFERENCE SIGNS LIST

(51) 1 HELICAL BROACH 2 SHANK SECTION 3 ROUGHING SECTION 4 FINISHING SECTION 10 BROACH BODY 11 SHELL ENGAGEMENT PORTION OF BROACH BODY 12 SHELL ABUTTING SURFACE OF BROACH BODY 13 POSITIONING PROTRUSION OF BROACH BODY 20 WAFER SHELL 21 POSITIONING GROOVE OF WAFER SHELL 22 POSITIONING PROTRUSION OF WAFER SHELL 30 FINISHING BLADE 40 FASTENER 41 POSITIONING GROOVE OF FASTENER 50 LEFT TOOTH SURFACE IN WORKPIECE 60 RIGHT TOOTH SURFACE IN WORKPIECE