Patent Description:
<CIT> (PTL <NUM>) describes a drill having a first margin and a second margin.

A drill according to the present disclosure is a drill rotatable about an axial line, and includes a flank face, a thinning face, an outer peripheral surface, and a swarf discharging surface. The thinning face is contiguous to the flank face. The outer peripheral surface is contiguous to each of the flank face and the thinning face. The swarf discharging surface is contiguous to each of the flank face and the outer peripheral surface. A ridgeline between the flank face and the swarf discharging surface forms a cutting edge. The outer peripheral surface is provided with a first margin contiguous to each of the cutting edge and the flank face, and a second margin that is located on a rear side with respect to the first margin in a rotation direction and that is separated from each of the flank face and the thinning face. An outer peripheral portion of the first margin and an outer peripheral portion of the second margin have respective back tapers having the same angle. In a direction parallel to the axial line, a distance between a front end of the first margin and a front end of the second margin is more than or equal to <NUM> and less than or equal to <NUM>.

It is an object of the present disclosure to provide a drill to improve a roundness of a hole while maintaining rigidity to be high.

According to the present disclosure, there can be provided a drill to improve a roundness of a hole while maintaining rigidity to be high.

First, an overview of embodiments of the present disclosure will be described.

Hereinafter, the embodiments of the present disclosure (hereinafter, also referred to as "the present embodiment") will be described in detail with reference to figures. It should be noted that in the below-described figures, the same or corresponding portions are denoted by the same reference characters, and will not be described repeatedly.

First, a configuration of a drill <NUM> according to a first embodiment will be described. <FIG> is a schematic plan view showing the configuration of drill <NUM> according to the first embodiment. As shown in <FIG>, drill <NUM> according to the first embodiment mainly includes a front end <NUM>, a rear end <NUM>, flank faces <NUM>, thinning faces <NUM>, outer peripheral surfaces <NUM>, swarf discharging surfaces <NUM>, and a shank <NUM>. Drill <NUM> according to the first embodiment is a drill <NUM> for processing a metal. As shown in <FIG>, each of outer peripheral surfaces <NUM> is provided in the form of a helix around an axial line X. Outer peripheral surface <NUM> is contiguous to swarf discharging surface <NUM>. Swarf discharging surface <NUM> forms a flute. Swarf discharging surface <NUM> is provided in the form of a helix around axial line X. A cutting edge <NUM> is provided on the front end side of drill <NUM>.

Front end <NUM> of drill <NUM> is a portion to face a workpiece. Rear end <NUM> of drill <NUM> is a portion to face a tool main spindle that rotates drill <NUM>. Shank <NUM> is a portion to be attached to the tool main spindle. Axial line X extends through front end <NUM> and rear end <NUM>. A direction along axial line X is an axial direction. A direction perpendicular to the axial direction is a radial direction. In the present specification, a direction from front end <NUM> toward rear end <NUM> is referred to as "rear side in the axial direction". On the contrary, a direction from rear end <NUM> toward front end <NUM> is referred to as "front side in the axial direction". Drill <NUM> is rotatable about axial line X.

<FIG> is a schematic front view showing the configuration of drill <NUM> according to the first embodiment. As shown in <FIG>, drill <NUM> further includes flank faces <NUM> and thinning faces <NUM>. A ridgeline between each flank face <NUM> and each swarf discharging surface <NUM> forms cutting edge <NUM>. Swarf discharging surface <NUM> in the vicinity of cutting edge <NUM> functions as a rake face. Thinning face <NUM> is contiguous to flank face <NUM>. Thinning face <NUM> is located on the rear side with respect to flank face <NUM> in a rotation direction. Flank face <NUM> has a first region <NUM> and a second region <NUM>. First region <NUM> forms cutting edge <NUM>. Second region <NUM> is contiguous to first region <NUM>. Second region <NUM> is located on the rear side with respect to first region <NUM> in the rotation direction. Second region <NUM> is contiguous to thinning face <NUM>. Second region <NUM> is located between first region <NUM> and thinning face <NUM>.

As shown in <FIG>, coolant holes <NUM> may be provided in flank faces <NUM>. Each of coolant holes <NUM> may be provided in second region <NUM>, for example. As shown in <FIG>, when viewed along axial line X, a boundary between second region <NUM> and thinning face <NUM> is in the form of a curve (R thinning), for example. As another configuration, when viewed along axial line X, the boundary between second region <NUM> and thinning face <NUM> may be in the form of a straight line (X thinning), for example. Outer peripheral surface <NUM> is provided with a first margin <NUM> and a second margin <NUM>. Second margin <NUM> is provided on the rear side with respect to first margin <NUM> in the rotation direction. Second margin <NUM> is separated from first margin <NUM>. Outer peripheral surface <NUM> has an outer peripheral region <NUM>. Outer peripheral region <NUM> is located between first margin <NUM> and second region <NUM>. As shown in <FIG>, when viewed along axial line X, outer peripheral region <NUM> is in the form of an arc, for example.

<FIG> is a partial enlarged perspective view showing the configuration of drill <NUM> according to the first embodiment. As shown in <FIG>, outer peripheral surface <NUM> is contiguous to each of flank face <NUM> and thinning face <NUM>. Swarf discharging surface <NUM> is contiguous to each of flank face <NUM> and outer peripheral surface <NUM>. Thinning face <NUM> is contiguous to swarf discharging surface <NUM>. First margin <NUM> is contiguous to each of cutting edge <NUM> and flank face <NUM>. Specifically, first margin <NUM> is contiguous to a portion of first region <NUM> of flank face <NUM>. First margin <NUM> is contiguous to a boundary between swarf discharging surface <NUM> and outer peripheral surface <NUM>.

First margin <NUM> has a first front end <NUM>, a first outer peripheral portion <NUM>, and a first side surface portion <NUM>. First front end <NUM> is contiguous to cutting edge <NUM>. First outer peripheral portion <NUM> is contiguous to first front end <NUM>. First outer peripheral portion <NUM> is located on the rear end <NUM> side with respect to first front end <NUM> in the direction along axial line X. First side surface portion <NUM> is contiguous to first front end <NUM>. First side surface portion <NUM> is located on the rear end <NUM> side with respect to first front end <NUM> in the direction along axial line X. First side surface portion <NUM> is contiguous to each of first outer peripheral portion <NUM> and outer peripheral region <NUM>. First side surface portion <NUM> is located on the rear side with respect to first outer peripheral portion <NUM> in the rotation direction. From another viewpoint, it can be said that first outer peripheral portion <NUM> is located between swarf discharging surface <NUM> and first side surface portion <NUM> in the rotation direction.

Second margin <NUM> is separated from each of flank face <NUM> and thinning face <NUM>. Second margin <NUM> is contiguous to the boundary between swarf discharging surface <NUM> and outer peripheral surface <NUM>. Second margin <NUM> has a second front end <NUM>, a second outer peripheral portion <NUM>, and a second side surface portion <NUM>. Second front end <NUM> is contiguous to outer peripheral region <NUM>. Second outer peripheral portion <NUM> is contiguous to second front end <NUM>. Second outer peripheral portion <NUM> is located on the rear end <NUM> side with respect to second front end <NUM> in the direction along axial line X. Second side surface portion <NUM> is contiguous to second front end <NUM>. Second side surface portion <NUM> is located on the rear end <NUM> side with respect to second front end <NUM> in the direction along axial line X. Second side surface portion <NUM> is contiguous to each of second outer peripheral portion <NUM> and outer peripheral region <NUM>. Second side surface portion <NUM> is located on the front side with respect to second outer peripheral portion <NUM> in the rotation direction. From another viewpoint, it can be said that second outer peripheral portion <NUM> is located between swarf discharging surface <NUM> and second side surface portion <NUM> in the rotation direction.

<FIG> is an enlarged plan view showing a region IV of <FIG>. When a first distance A1 represents a distance between the front end (first front end <NUM>) of first margin <NUM> and the front end (second front end <NUM>) of second margin <NUM> in the direction parallel to axial line X, first distance A1 is more than or equal to <NUM> and less than or equal to <NUM>. The lower limit of first distance A1 may be more than or equal to <NUM> or more than or equal to <NUM>, for example. The upper limit of first distance A1 may be less than or equal to <NUM> or less than or equal to <NUM>, for example.

As shown in <FIG>, a third distance A3 represents a distance between front end <NUM> of drill <NUM> and first front end <NUM> of first margin <NUM> in the direction parallel to axial line X. As shown in <FIG>, third distance A3 may be shorter than first distance A1. As shown in <FIG>, when viewed in the direction perpendicular to axial line X, second front end <NUM> of second margin <NUM> may extend in the direction perpendicular to axial line X. From another viewpoint, it can be said that second front end <NUM> may be parallel to a plane perpendicular to axial line X. Second front end <NUM> extends along the rotation direction of the drill.

As shown in <FIG>, drill <NUM> according to the first embodiment has two cutting edges <NUM>. When viewed in the direction perpendicular to axial line X and perpendicular to a line segment that connects the outer peripheral end portion of first cutting edge <NUM> on one side to the outer peripheral end portion of second cutting edge <NUM> on the other side, a distance between the outer peripheral end portion of first cutting edge <NUM> on one side and the outer peripheral end portion of second cutting edge <NUM> on the other side is represented by diameter D of drill <NUM>. Diameter D of drill <NUM> is not particularly limited, but is, for example, <NUM>. As shown in <FIG>, a length L of swarf discharging surface <NUM> in the direction parallel to axial line X is, for example, <NUM> times or more and <NUM> times or less as large as diameter D of drill <NUM>. The lower limit of length L of swarf discharging surface <NUM> is not particularly limited, but may be, for example, <NUM> times or more or <NUM> times or more as large as diameter D of drill <NUM>. The upper limit of length L of swarf discharging surface <NUM> is not particularly limited, but may be, for example, <NUM> times or less or <NUM> times or less as large as diameter D of drill <NUM>.

<FIG> is a schematic cross sectional view taken along a line V-V of <FIG>. The cross section shown in <FIG> is a cross section that is perpendicular to axial line X and that intersects each of first margin <NUM> and second margin <NUM>. As shown in <FIG>, the width (second width C2) of second margin <NUM> may be larger than the width (first width C1) of first margin <NUM> in the rotation direction. The lower limit of second width C2 is not particularly limited, but may be <NUM> times or more or <NUM> times or more as large as first width C1, for example.

As shown in <FIG>, outer peripheral region <NUM> is contiguous to each of first side surface portion <NUM> and second side surface portion <NUM>. First side surface portion <NUM> is located on the front side with respect to outer peripheral region <NUM> in the rotation direction. Second side surface portion <NUM> is located on the rear side with respect to outer peripheral region <NUM> in the rotation direction. First outer peripheral portion <NUM> is located on the outer side with respect to outer peripheral region <NUM> in the radial direction. Similarly, second outer peripheral portion <NUM> is located on the outer side with respect to outer peripheral region <NUM> in the radial direction. In the radial direction, the position of first outer peripheral portion <NUM> is the same as the position of second outer peripheral portion <NUM>. From another viewpoint, it can be said that in the cross section perpendicular to axial line X, a distance between axial line X and first outer peripheral portion <NUM> is the same as a distance between axial line X and second outer peripheral portion <NUM>.

<FIG> is a schematic diagram showing a relation between a distance from axial line X and a position in the direction of axial line X. In <FIG>, the horizontal axis represents a position in the axial direction. The left side of <FIG> corresponds to the front end side of drill <NUM>. The right side of <FIG> corresponds to the rear end side of drill <NUM>. In <FIG>, the vertical axis represents a distance from axial line X. A first position P1 corresponds to the position of front end <NUM> of drill <NUM>. A second position P2 corresponds to the position of the front end (first front end <NUM>) of first margin <NUM>. In the direction parallel to axial line X, second position P2 is located between first position P1 and rear end <NUM> of drill <NUM>. A third position P3 corresponds to the position of the front end (second front end <NUM>) of second margin <NUM>. In the direction parallel to axial line X, third position P3 is located between second position P2 and rear end <NUM> of drill <NUM>. A fourth position P4 corresponds to the position of the rear end of first margin <NUM> and the position of the rear end of second margin <NUM>. In the direction parallel to axial line X, fourth position P4 is located between third position P3 and rear end <NUM> of drill <NUM>.

As shown in <FIG>, first margin <NUM> is provided in a region ranging from second position P2 to fourth position P4 in the direction parallel to axial line X. Second margin <NUM> is provided in a region ranging from third position P3 to fourth position P4 in the direction parallel to axial line X. A second distance A2 represents a distance from the front end (second front end <NUM>) of second margin <NUM> to the rear end of second margin <NUM> in the direction parallel to axial line X. Second distance A2 may be longer than first distance A1. Second distance A2 corresponds to the length of second margin <NUM> in the direction parallel to axial line X. The total of first distance A1 and second distance A2 corresponds to the length of first margin <NUM> in the direction parallel to axial line X. The length of first margin <NUM> may be longer than the length of second margin <NUM> in the direction parallel to axial line X.

Each of first outer peripheral portion <NUM> of first margin <NUM> and second outer peripheral portion <NUM> of second margin <NUM> has a back taper. The angle of the back taper of first outer peripheral portion <NUM> of first margin <NUM> is the same as the angle of the back taper of second outer peripheral portion <NUM> of second margin <NUM>. In other words, first outer peripheral portion <NUM> of first margin <NUM> and second outer peripheral portion <NUM> of second margin <NUM> have respective back tapers having the same angle. From another viewpoint, it can be said that first outer peripheral portion <NUM> and second outer peripheral portion <NUM> are located on the same conical surface.

As shown in <FIG>, a first length B1 represents a value obtained by subtracting a distance between axial line X and first outer peripheral portion <NUM> of first margin <NUM> at third position P3 from a distance between axial line X and first outer peripheral portion <NUM> of first margin <NUM> at second position P2. The tangent of the angle of the back taper of the outer peripheral portion of first margin <NUM> represents a value obtained by dividing first length B1 by first distance A1. Similarly, a second length B2 represents a value obtained by subtracting a distance between axial line X and second outer peripheral portion <NUM> of second margin <NUM> at fourth position P4 from a distance between axial line X and second outer peripheral portion <NUM> of second margin <NUM> at third position P3. The tangent of the angle of the back taper of the outer peripheral portion of second margin <NUM> represents a value obtained by dividing second length B2 by second distance A2. Each of the angle of the back taper of the outer peripheral portion of first margin <NUM> and the angle of the back taper of the outer peripheral portion of second margin <NUM> is, for example, arctan (<NUM>/<NUM>/<NUM>) (unit: rad). Each of the angle of the back taper of the outer peripheral portion of first margin <NUM> and the angle of the back taper of the outer peripheral portion of second margin <NUM> is, for example, <NUM> rad.

As shown in <FIG>, a distance between first outer peripheral portion <NUM> of first margin <NUM> and axial line X in the radial direction is monotonously decreased in the direction toward the rear end <NUM> side. Similarly, a distance between second outer peripheral portion <NUM> of second margin <NUM> and axial line X in the radial direction is monotonously decreased in the direction toward the rear end <NUM> side. A distance between axial line X and first outer peripheral portion <NUM> at second position P2 in the radial direction is larger than a distance between axial line X and second outer peripheral portion <NUM> at third position P3 in the radial direction. The distance between axial line X and first outer peripheral portion <NUM> at second position P2 in the radial direction is larger than a distance between axial line X and first outer peripheral portion <NUM> at third position P3 in the radial direction. The distance between axial line X and first outer peripheral portion <NUM> at third position P3 in the radial direction is the same as the distance between axial line X and second outer peripheral portion <NUM> at third position P3 in the radial direction.

Similarly, the distance between axial line X and first outer peripheral portion <NUM> at third position P3 in the radial direction is larger than a distance between axial line X and second outer peripheral portion <NUM> at fourth position P4 in the radial direction. The distance between axial line X and first outer peripheral portion <NUM> at third position P3 in the radial direction is larger than the distance between axial line X and first outer peripheral portion <NUM> at fourth position P4 in the radial direction. The distance between axial line X and first outer peripheral portion <NUM> at fourth position P4 in the radial direction is the same as the distance between axial line X and second outer peripheral portion <NUM> at fourth position P4 in the radial direction.

Next, a configuration of a drill <NUM> according to a second embodiment will be described. The configuration of drill <NUM> according to the second embodiment is different from the configuration of drill <NUM> according to the first embodiment in that the width of second margin <NUM> is large, and the other points are the same as those in the configuration of drill <NUM> according to the first embodiment. Hereinafter, the difference from the configuration of drill <NUM> according to the first embodiment will be mainly described.

<FIG> is an enlarged schematic plan view showing the configuration of drill <NUM> according to the second embodiment. <FIG> is a schematic cross sectional view taken along a line VIII-VIII of <FIG>. The cross section shown in <FIG> is a cross section that is perpendicular to axial line X and that intersects each of first margin <NUM> and second margin <NUM>. As shown in <FIG> and <FIG>, the width (second width C2) of second margin <NUM> of drill <NUM> according to the second embodiment is larger than the width (second width C2) of second margin <NUM> of drill <NUM> according to the first embodiment. As shown in <FIG>, the lower limit of second width C2 is not particularly limited, but may be, for example, <NUM> times or more or <NUM> times or more as large as first width C1. The upper limit of second width C2 is not particularly limited, but may be, for example, <NUM> times or less or <NUM> times or less as large as first width C1.

Next, a configuration of a drill <NUM> according to a third embodiment will be described. The configuration of drill <NUM> according to the third embodiment is different from the configuration of drill <NUM> according to the first embodiment in that second margin <NUM> is separated from swarf discharging surface <NUM>, and the other points are the same as those in the configuration of drill <NUM> according to the first embodiment. Hereinafter, the difference from the configuration of drill <NUM> according to the first embodiment will be mainly described.

<FIG> is an enlarged schematic plan view showing the configuration of drill <NUM> according to the third embodiment. <FIG> is a schematic cross sectional view taken along a line IX-IX of <FIG>. The cross section shown in <FIG> is a cross section that is perpendicular to axial line X and that intersects each of first margin <NUM> and second margin <NUM>. As shown in <FIG> and <FIG>, second margin <NUM> of drill <NUM> according to the third embodiment is separated from swarf discharging surface <NUM>. From another viewpoint, it can be said that second margin <NUM> is separated from a boundary line between swarf discharging surface <NUM> and outer peripheral surface <NUM>.

As shown in <FIG>, second margin <NUM> has a second outer peripheral portion <NUM>, a second side surface portion <NUM>, and a third side surface portion <NUM>. Second outer peripheral portion <NUM> is contiguous to each of second side surface portion <NUM> and third side surface portion <NUM>. Third side surface portion <NUM> is located on the rear side with respect to second side surface portion <NUM> in the rotation direction. Third side surface portion <NUM> is located opposite to second side surface portion <NUM>. In the rotation direction, second outer peripheral portion <NUM> is located between second side surface portion <NUM> and third side surface portion <NUM>. Second side surface portion <NUM> is contiguous to outer peripheral regions <NUM>. Third side surface portion <NUM> is contiguous to outer peripheral regions <NUM> on the rear side with respect to second outer peripheral portion <NUM> in the rotation direction. As shown in <FIG>, outer peripheral regions <NUM> are provided beside the both sides of second margin <NUM> in the rotation direction.

<FIG> is a schematic partial cross sectional view showing a step of performing an oblique through-hole forming process onto a workpiece using drill <NUM>. As shown in <FIG>, a through hole <NUM> is formed in a workpiece <NUM> using drill <NUM>. Workpiece <NUM> has an exit end surface <NUM>. Drill <NUM> is moved along a moving direction F parallel to the plane of sheet while rotating about axial line X. Moving direction F of drill <NUM> is inclined with respect to exit end surface <NUM>. Further, moving direction F of drill <NUM> is inclined with respect to a plane perpendicular to the plane of sheet and perpendicular to exit end surface <NUM>. Through hole <NUM> has a first exit end portion <NUM> and a second exit end portion <NUM>. When drill <NUM> is at a position of a first state S1, the outer peripheral end of cutting edge <NUM> is located at second exit end portion <NUM>. When drill <NUM> is at a position of a second state S2, the outer peripheral end of cutting edge <NUM> is located at first exit end portion <NUM>.

A roundness is defined by a magnitude of deviation of a round-shaped object from a geometrically correct circle as defined in JIS (Japanese Industrial Standards) B0621-<NUM>. The roundness is represented as a difference between radii of two concentric geometric circles in the case where a distance between the two circles is minimum when the round-shaped object is sandwiched between the two concentric geometric circles. The unit of the roundness is µm.

<FIG> is a schematic partial cross sectional view showing a method of measuring the roundness of through hole <NUM>. <FIG> is a schematic cross sectional view taken along a line XIB-XIB of <FIG>. The cross section shown in <FIG> is a view of a cross section that is perpendicular to axial line X and that intersects a plane H separated by a fifth distance G from second exit end portion <NUM> when viewed in a direction opposite to moving direction F. Fifth distance G is, for example, <NUM>. A first direction <NUM> represents a direction obtained by projecting, onto plane H, a direction that is perpendicular to axial line X and that extends from axial line X toward second exit end portion <NUM>. A second direction <NUM> represents a direction obtained by rotating first direction <NUM> counterclockwise by <NUM>° in plane H. In plane H, axial line X represents the origin, I represents a coordinate in first direction <NUM>, J represents a coordinate in second direction <NUM>, and a position in plane H is expressed as (I, J). An intersection line K represents an intersection line between workpiece <NUM> and through hole <NUM> in plane H. By measuring the positions of four points disposed on intersection line K at equal angles with respect to axial line X in plane H, the roundness is found.

Specifically, the roundness is found as follows. An intersection point Q1 represents an intersection point between intersection line K and a direction from axial line X toward first direction <NUM>. An intersection point Q2 represents an intersection point between intersection line K and a direction from axial line X toward second direction <NUM>. An intersection point Q3 represents an intersection point between intersection line K and a direction extending opposite to first direction <NUM> from axial line X. An intersection point Q4 represents an intersection point between intersection line K and a direction extending opposite to second direction <NUM> from axial line X. First, the coordinates of intersection Q1, intersection Q2, intersection Q3, and intersection Q4 are measured as (I1, <NUM>), (<NUM>, J2), (<NUM>, <NUM>), and (<NUM>, J4), respectively. Next, respective distances from axial line X serving as the origin to these four points are calculated as √(I1<NUM>), √(J2<NUM>), √(I3<NUM>) and √(J4<NUM>) by using the coordinates of intersection point Q1, intersection point Q2, intersection point Q3, and intersection point Q4. A difference between the maximum value and the minimum value of the calculated distances represents a provisional value of the roundness. In the measurement of roundness, the center (I0, J0) of intersection line K does not necessarily coincide with axial line X, i.e., the origin (<NUM>, <NUM>). Respective distances between the provisionally determined center of intersection line K and intersection points Q1, Q2, Q3, and Q4 are calculated, and the center (I0, J0) of intersection line K is determined to attain a minimum objective function with a difference between the maximum value and the minimum value of the calculated distances being regarded as the objective function. When the objective function is minimum, the minimum value of the objective function represents the roundness.

The roundness measured as described above is referred to as "minimum zone roundness". A more detailed method of measuring the minimum zone roundness is described in, for example, Non-Patent Literature (<NPL>).

Referring to <FIG>, in the case where a distance (fourth distance E) from the outer peripheral end of cutting edge <NUM> of drill <NUM> to second exit end portion <NUM> is shorter than first distance A1 (see <FIG>) when drill <NUM> is at the position of second state S2, second margin <NUM> does not reach second exit end portion <NUM>. Therefore, drill <NUM> is not guided by second margin <NUM> at second exit end portion <NUM>. As a result, drill <NUM> is moved to the lower right side of <FIG>. Thus, through hole <NUM> is expanded toward the lower right side, with the result that the roundness of the exit of through hole <NUM> is deteriorated. On the other hand, in the case where the distance (fourth distance E) from the outer peripheral end of cutting edge <NUM> of drill <NUM> to second exit end portion <NUM> is longer than first distance A1, second margin <NUM> is brought into contact with second exit end portion <NUM>. Therefore, drill <NUM> is guided by second margin <NUM> at second exit end portion <NUM>. As a result, drill <NUM> can be suppressed from being moved to the lower right side of <FIG>. Thus, the roundness of the exit of the hole can be suppressed from being deteriorated.

It should be noted that workpiece <NUM> is a metal such as steel, for example. Workpiece <NUM> may be, for example, 38MnS6, which is an inexpensive microalloyed steel. Workpiece <NUM> may be a carbon steel, an alloy steel, a difficult-to-cut material, or a stainless material (SUS). A processing method may be, for example, MQL (Minimum Quantity Lubrication) processing.

Next, functions and effects of drill <NUM> according to the present embodiment will be described.

When the distance between the front end of first margin <NUM> and the front end of second margin <NUM> is short, only first margin <NUM> is brought into contact with the inner wall surface of the hole at the start of drilling; however, immediately thereafter, both first margin <NUM> and second margin <NUM> are brought into contact with the inner wall surface of the hole. Therefore, during the drilling, excessive torque is applied to the inner wall surface of the hole. As a result, the roundness of the hole is deteriorated. On the other hand, when the distance between the front end of first margin <NUM> and the front end of second margin <NUM> is long, only first margin <NUM> continues to be in contact with the inner wall surface of the hole for a while after the start of drilling.

Thereafter, both first margin <NUM> and second margin <NUM> are brought into contact with the inner wall surface of the hole. Therefore, excessive torque can be suppressed from being applied to the inner wall surface of the hole during the drilling. As a result, the roundness of the hole is improved. Particularly, in the case of the MQL processing, the processing of a difficult-to-cut material, or the processing of SUS, the hole tends to be shrunk after the processing, with the result that the torque applied to the inner wall surface of the hole is likely to be excessive. Drill <NUM> according to the present embodiment particularly exhibits the effect in the case of the MQL processing, the processing of a difficult-to-cut material, or the processing of SUS.

Further, in the case where the distance between the front end of first margin <NUM> and the front end of second margin <NUM> is too long, drill <NUM> cannot be guided by second margin <NUM> when drill <NUM> is moved out of the oblique hole. Therefore, drill <NUM> interferes with the inner wall surface of the hole. As a result, the roundness of the hole cannot be improved.

According to drill <NUM> according to the invention, the distance between the front end of first margin <NUM> and the front end of second margin <NUM> in the direction parallel to axial line X is more than or equal to <NUM> and less than or equal to <NUM>. By setting the distance between the front end of first margin <NUM> and the front end of second margin <NUM> to be more than or equal to <NUM>, excessive torque can be suppressed from being applied to the inner wall surface of the hole. By setting the distance between the front end of first margin <NUM> and the front end of second margin <NUM> to be less than or equal to <NUM>, drill <NUM> can be suppressed from interfering with the inner wall surface of the hole when drill <NUM> is moved out of the oblique hole. As a result, the roundness of the hole formed in workpiece <NUM> can be improved.

As a method of attaining a long distance between the front end of first margin <NUM> and the front end of second margin <NUM>, it is conceivable to employ a method in which thinning face <NUM> is made large and second margin <NUM> is formed such that the front end of second margin <NUM> is contiguous to thinning face <NUM>. However, when thinning face <NUM> is made large, the web thickness of drill <NUM> becomes small. As a result, the rigidity of drill <NUM> is decreased.

According to drill <NUM> according to the invention, second margin <NUM> is separated from each of flank face <NUM> and thinning face <NUM>. Therefore, the distance between the front end of first margin <NUM> and the front end of second margin <NUM> can be made long without making thinning face <NUM> large. Thus, the roundness of the hole can be improved while maintaining the rigidity of drill <NUM> to be high.

According to drill <NUM> according to each of the above-described embodiments, the front end of second margin <NUM> may extend along the direction perpendicular to axial line X when viewed in the direction perpendicular to axial line X. When the front end of second margin <NUM> extends to be inclined with respect to the straight line perpendicular to axial line X, the width of second margin <NUM> in the rotation direction is smaller than that when the front end of second margin <NUM> extends along the straight line perpendicular to axial line X. Therefore, when the front end of second margin <NUM> extends to be inclined with respect to the straight line perpendicular to axial line X, the strength of second margin <NUM> becomes lower than that when the front end of second margin <NUM> extends along the straight line perpendicular to axial line X, with the result that second margin <NUM> is likely to be chipped. By extending the front end of second margin <NUM> along the direction perpendicular to axial line X when viewed in the direction perpendicular to axial line X, the strength of the front end of second margin <NUM> can be made high. As a result, the front end of second margin <NUM> can be suppressed from being chipped.

First, drills <NUM> of samples <NUM> and <NUM> were prepared. Drill <NUM> of sample <NUM> is a drill <NUM> according to a comparative example. Drill <NUM> of sample <NUM> is a drill <NUM> according to an example of the present disclosure. In drill <NUM> of sample <NUM>, second margin <NUM> was contiguous to flank face <NUM>. In the direction parallel to axial line X, the distance between the front end of first margin <NUM> and the front end of second margin <NUM> was <NUM>. In drill <NUM> of sample <NUM>, second margin <NUM> was separated from each of flank face <NUM> and thinning face <NUM>. In the direction parallel to axial line X, the distance (first distance A1) between the front end of first margin <NUM> and the front end of second margin <NUM> was <NUM>.

Next, oil holes were formed in a crankshaft using each of drills <NUM> of samples <NUM> and <NUM>. <FIG> is a schematic side view showing a configuration of the crankshaft. As shown in <FIG>, a crankshaft <NUM> mainly includes crank journals <NUM>, balance weights <NUM>, and crank pins <NUM>. As shown in <FIG>, oil holes <NUM> were formed in crankshaft <NUM> (workpiece) using each of drills <NUM> of samples <NUM> and <NUM>. Oil holes <NUM> are formed to extend from crank pins <NUM> to crank journals <NUM>. The workpiece was 38MnS6. The diameter of drill <NUM> was <NUM>. The diameter of each of the holes was <NUM> (+<NUM>/-<NUM>) mm. The depth of the hole was <NUM>. A cutting speed (peripheral speed) was <NUM>/min. A feed speed f was <NUM>/rev. As equipment, DH524 provided by NACHI-FUJIKOSHI CORP was used. The load current value of the main spindle of the processing equipment was measured while forming oil hole <NUM> in crankshaft <NUM> using each of drills <NUM> of samples <NUM> and <NUM>. Further, the roundness of the oil hole formed in crankshaft <NUM> was measured.

<FIG> is a diagram showing a relation between a current value and time. In <FIG>, the horizontal axis represents time (unit: millisecond). In <FIG>, the vertical axis represents a load current value (unit: amp) of the main spindle of the processing equipment. In <FIG>, pieces of data of five different types of lines are shown. The different types of lines represent respective pieces of data of different drills. In <FIG>, five drills (N = <NUM>) were used for each of the drills of samples <NUM> and <NUM>. The first count corresponds to data in the first round of drilling. The 40th count corresponds to data in the 40th round of drilling. The 80th count corresponds to data in the 80th round of drilling.

As indicated in variation in current value in an actual processing time of <FIG>, at the first count, the current value of the drill of sample <NUM> was more stable than the current value of the drill of sample <NUM>. Further, as the number of the rounds of processing is increased to the 40th count and the 80th count, the variation in current value of the drill of sample <NUM> during the actual processing time becomes large and unstable. On the other hand, the variation in current value of the drill of sample <NUM> during the actual processing time was small and stable even when the number of the rounds of processing was increased to the 40th count and the 80th count. As shown in <FIG>, at the 40th count, the current value of the drill of sample <NUM> was more stable than the current value of the drill of sample <NUM>. Similarly, at the 80th count, the current value of the drill of sample <NUM> was more stable than the current value of the drill of sample <NUM>.

Table <NUM> shows the roundness of the hole (oil hole <NUM>). The roundness of the hole was measured using a roundness measuring instrument (model number: Crysta-Apex C9166) provided by Mitsutoyo. For the measurement, a stylus (model number: MS2-3R27. <NUM> provided by Mitsutoyo) was used which had a shaft composed of cemented carbide with an effective length of <NUM> and had a ruby ball having a diameter of <NUM> at its tip. For each of first and second areas, the roundness of the hole was measured as follows. First, in a cross section that intersects plane H separated by a distance of <NUM> from second exit end portion <NUM>, an intersection line K was defined to represent an intersection line between workpiece <NUM> and through hole <NUM>. Next, in plane H, the positions of four points disposed on intersection line K at equal angles with respect to axial line X were measured. The roundness of the hole was found from these four positions as described above. The first area is oil hole <NUM> shown on the left side of <FIG> (oil hole <NUM> extending from crank pin <NUM> located the third from the left to crank journal <NUM> located the second from the left). The second area is oil hole <NUM> shown on the right side of <FIG> (oil hole <NUM> extending from crank pin <NUM> located the sixth from the left to crank journal <NUM> located the fourth from the left). As shown in Table <NUM>, the roundness of the oil hole formed in the first area using drill <NUM> of sample <NUM> was <NUM>. The roundness of the oil hole formed in the second area using drill <NUM> of sample <NUM> was <NUM>. On the other hand, the roundness of the oil hole formed in the first area using drill <NUM> of sample <NUM> was <NUM>. The roundness of the oil hole formed in the second area using drill <NUM> of sample <NUM> was <NUM>. In view of the above results, it was proved that the roundness of the oil hole formed by drill <NUM> of the example of the present disclosure was significantly improved as compared with the roundness of the oil hole formed by drill <NUM> of the comparative example.

The embodiments and examples disclosed herein are illustrative and non-restrictive in any respect. The scope of the present invention is defined by the terms of the claims, rather than the embodiments described above, and is intended to include any modifications within the scope of the claims.

Claim 1:
A drill (<NUM>) rotatable about an axial line (X), the drill comprising:
a flank face (<NUM>);
a thinning face (<NUM>) contiguous to the flank face;
an outer peripheral surface (<NUM>) contiguous to the flank face; and
a swarf discharging surface (<NUM>) contiguous to each of the flank face and the outer peripheral surface, wherein
a ridgeline between the flank face and the swarf discharging surface forms a cutting edge (<NUM>),
the outer peripheral surface is provided with a first margin (<NUM>) contiguous to each of the cutting edge and the flank face, and a second margin (<NUM>) that is located on a rear side with respect to the first margin in a rotation direction and that is separated from each of the flank face and the thinning face,
characterised in that
the outer peripheral surface is also contiguous to the thinning face,
an outer peripheral portion of the first margin and an outer peripheral portion of the second margin have respective back tapers having the same angle, and
in a direction parallel to the axial line, a distance (A1) between a front end (<NUM>) of the first margin and a front end (<NUM>) of the second margin is more than or equal to <NUM> and less than or equal to <NUM>.