The disclosure of Japanese Patent Application No. 2008-333380 filed on Dec. 26, 2008 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a drill including a cutting edge a point angle of which varies depending on a position.
2. Description of the Related Art
Examples of the drill including a cutting edge a point angle of which varies depending on a position may be double angle drills disclosed in Japanese Unexamined Patent Application Publication No. 1988-306812 (hereinafter, referred to as document '812) and in Japanese Unexamined Utility Model Application Publication No. 1994-75612 (hereinafter, referred to as document '612).
Such a double angle drill includes a primary cutting edge and a secondary cutting edge. The tip section of the primary cutting edge has a ridgeline shape of a drill for metal. The secondary cutting edge continuously extending from the primary cutting edge is flat and has a smaller point angle than the primary cutting edge.
Any of documents '812 and '612 teaches that the double angle drill is suitable for drilling a hole in a fiber reinforced plastic composite material and a metal material at a time.
With the double angle drill, the primary cutting edge drills a primary hole with a relatively small diameter, and then the secondary cutting edge cuts the outer periphery of the primary hole to drill a secondary hole with a target diameter. During drilling, transient delamination may appear at the periphery of the primary hole of the composite material. The delamination is removed by cutting the primary hole by the secondary cutting edge.
Unfortunately, as mentioned in document '612, the double angle drill typically has low wear resistance. Document '612 discloses a configuration to increase the wear resistance of the cutting edge by covering the cutting edge with a diamond film with a suitable film thickness.
However, the double angle drill has corners at the boundary between the primary and secondary cutting edges and at the outermost periphery of the secondary cutting edge. A stress is likely concentrated at the corners, and hence chipping likely appears at the corners. Thus, such a shape of the drill decreases the wear resistance of the drill.
In order to increase the wear resistance of the cutting edge, the applicant of this application has filed an application for a drill in Japanese Unexamined Patent Application Publication No. 2008-36759 (hereinafter, referred to as document '759). The drill includes a cutting edge. The cutting edge has a point angle which is continuously decreased from a center-position point angle A (herein, 0°<A<180°) at a center position to a maximum-diameter-position point angle of 0° at a maximum diameter position. The cutting edge has a relief angle which is continuously decreased from the center position to the maximum diameter position.
Document '759 describes the point angle of the cutting edge as follows. “The point angle of the cutting edge is decreased from the center-position point angle A to the maximum-diameter-position point angle of 0° in a continuously changing manner. Accordingly, the ridgeline of the cutting edge defines a smooth curve without a corner (point of discontinuity). Also, it is assumed that the curve defined by the ridgeline of the cutting edge is a curve protruding to the outside without a point of inflection. For example, in the ridgeline of the cutting edge, the curve from the center-position point angle A to the maximum-diameter-position point angle of 0° is defined as a part of a parabola. The ridgeline of the cutting edge may partly have a straight line. In this case, the point angle of the cutting edge is continuously changed even at a transition portion between the straight line and the curve (not having a straight line), thereby eliminating a corner.” Also, document '759 describes that the ridgeline of the cutting edge may be an arcuate ridgeline formed of a single arc, instead of the parabolic ridgeline.
With the drill disclosed in document '759, the point angle of the cutting edge is continuously decreased from the center position to the maximum diameter position. Thus, the ridgeline of the cutting edge has no corner, which is likely chipped, and the wear resistance of the cutting edge is increased.
The drill disclosed in document '759 has a cutting edge section mainly for drilling, which is located close to the tip end among the entire cutting edge, and a cutting edge section mainly for reaming, which continuously extends from the former cutting edge and is located close to the rear end. The cutting edge mainly for drilling gradually shifts to the cutting edge mainly for reaming.
The present invention is made as a result of advanced studies on the drill of such a type disclosed in document '759 by the inventors of this application, for a fiber reinforced plastic composite material. The present invention provides a drill that is ideally suited for drilling a hole in the fiber reinforced plastic composite material.
A first object of the studies by the inventors of this application has been to increase reaming performance by a cutting edge section located close to the rear end.
A second object of the studies by the inventors of this application has been to increase cutting performance by a cutting edge section located close to the tip end.
In light of this, an object of the present invention is to allow a drill including a cutting edge a point angle of which varies depending on a position to have better reaming performance by a cutting edge section located close to the rear end among the entire cutting edge and better cutting performance by a cutting edge section located close to the tip end.
A drill according to a first aspect of the present invention includes a cutting edge having a ridgeline. The ridgeline is entirely or partly formed along a part of a reference ellipse.
A drill according to a second aspect of the present invention includes a cutting edge having a ridgeline. A portion of the ridgeline continuously extending to a maximum diameter position from a predetermined point located closer to a tip end of the drill than the maximum diameter position is formed along a part of a reference ellipse.
In the above-described configuration, a portion of the ridgeline continuously extending from the predetermined point toward the tip end of the drill may be formed straight along a tangent to the reference ellipse at the predetermined point.
A drill according to a third aspect of the present invention includes a cutting edge having a ridgeline. The ridgeline is entirely or partly formed along a reference curve that is defined by connecting end points of a plurality of arcs. Each of the arcs has end points on a reference ellipse. Also, a tangent to the arc is common to a tangent to the reference ellipse, or the arc has a common point with the reference ellipse at a position other than the end points.
A drill according to a fourth aspect of the present invention includes a cutting edge having a ridgeline. A portion of the ridgeline continuously extending to a maximum diameter position from a predetermined point located closer to a tip end of the drill than the maximum diameter position is formed along a reference curve that is defined by connecting end points of a plurality of arcs. Each of the arcs has end points on a reference ellipse. Also, a tangent to the arc is common to a tangent to the reference ellipse, or the arc has a common point with the reference ellipse at a position other than the end points.
In the above-described configuration, a portion of the ridgeline continuously extending from the predetermined point toward the tip end of the drill may be formed straight along a tangent to the reference curve at the predetermined point.
In the above-described configuration, the reference ellipse may be an ellipse having a minor axis located at the maximum diameter position.
In the above-described configuration, the cutting edge may have a constant relief angle in a predetermined area extending from a position closest to the tip end of the drill.
In the above-described configuration, the cutting edge may have a constant relief angle in an area closer to the tip end of the drill than the predetermined point.
In the above-described configuration, the cutting edge may have a relief angle δ1 at the position closest to the tip end of the drill and a relief angle δ2 at the maximum diameter position. The relief angles δ1 and δ2 may have a relationship δ1>δ2. The relief angle δ1 may be gradually decreased to be the relief angle δ2 in at least a part of an area from the position closest to the tip end of the drill to the maximum diameter position.
In the above-described configuration, the cutting edge may have a relief angle δ1 at a position closest to the tip end of the drill and a relief angle δ2 at a maximum diameter position. The relief angles δ1 and δ2 may have a relationship δ1>δ2. The relief angle δ1 may be gradually decreased to be the relief angle δ2 in an area from the predetermined point to the maximum diameter position.
In the above-described configuration, the drill may have a margin continuously extending from a relief face of the cutting edge toward a rear end of the drill. The margin may have the relief angle δ2.
With the drill according to the aspects of the present invention, the ridgeline of the cutting edge is formed along the ellipse. That is, with the aspects of the present invention, since the curve defined by expanding the ellipse is used, the point angle can be decreased toward the rear end of the drill, at which a cutting edge section mainly for reaming is provided. Also, with the aspects of the present invention, since the point angle is decreased toward the rear end, reaming can be sufficiently performed while cutting load is decreased at a position near the maximum diameter position close to the rear end. Reaming performance of the cutting edge section close to the rear end among the entire cutting edge can be increased. Thus, a fine cut surface can be obtained.
The ridgeline of the cutting edge may have the straight line portion near the tip end of the drill. In the straight line portion, the ridgeline is formed straight. When the straight line portion has a properly selected point angle, at the start of drilling, catching performance of the tip section of the drill to bite into a workpiece, centrality, holding stability, etc., are improved. The cutting performance of the tip section mainly for drilling is increased. A single drill can provide both good cutting performance by the cutting edge section located close to the tip end of the drill, and provide good reaming performance by the cutting edge section located close to the rear end.
Also, when the relief face is kept constant at the straight line portion, the holding stability is further increased.
Further, with the drill according to the third and fourth aspects of the present invention, the ridgeline of the cutting edge is formed without reference to the reference ellipse, but with reference to the plurality of connected arcs substantially extending along the reference ellipse. Thus, by shaping the drill by successively following the arcs, the drill, in which the ridgeline of the cutting edges is formed substantially along the ellipse, can be obtained. Therefore productivity can be increased.
Hereinafter, embodiments of the present invention are described with reference to the attached drawings. These embodiments are merely examples of the present invention, and hence the present invention should not be limited to these embodiments.
First, two examples of drills will be described, to which the shape of a drill of the present invention may be applied. One of the two drills is a drill having a helical flute (see
Referring to
The cutter edge section 1 includes a pair of cutting edges 6 arranged symmetrically about an axis 9. The cutting edges 6 each have a rake face 7 and a relief face 8.
The cutter edge section 1 is finished by X thinning (cross thinning), and rake faces 7 are formed at positions where portions of the cutter edge section 1 have been removed by thinning. The positions where the portions have been removed by thinning are continuous to the helical flutes 3. The helical flutes 3 are two threads which are twisted at a predetermined helix angle. Lands 4 are formed between the helical flutes 3. Margins 5 are formed at both edges of the lands 4 along the helical flutes 3. The margins 5 contact the inner surface of a hole being processed and supports the drill 21.
Referring to
The point angle of the cutting edge 6 is decreased from the center-position point angle θ to the maximum-diameter-position point angle of 0° in a continuously changing manner. Accordingly, the ridgeline of the cutting edge 6 defines a smooth curve without a corner (point of discontinuity).
Referring to
Referring to
The shape of a drill according to an embodiment of the present invention will be described below.
Here is described the shape of the drill. Since the size of the drill is not so important, the size of the drill is assumed to φD=1 as shown in
Referring to
A straight line f2 is a tangent to the ellipse f1 at the separation point H. It is assumed that a1 is an area from the tip end of the drill to x0, a2 is an area from x0 to the origin, and a3 is a negative area, along the x-axis. The ridgeline of the cutting edge is formed straight along the tangent f2 in the area a1, and is formed along the reference ellipse f1 in the area a2. That is, the ridgeline of the cutting edge overlaps with the tangent f2 in the area a1 and overlaps with the reference ellipse f1 in the area a2. A margin is formed in the area a3 continuously from the relief face of the cutting edge.
A relational expression of θ, φA, and L may be derived below as Expression 1 under the conditions described above. The shape of the drill can be designed by using Expression 1.
Next, cutting performance of the ellipse portion of the cutting edge will be more specifically described with reference to
When the cutting edge has a constant point angle at any position, the cutting volume per unit length of the cutting edge is increased toward the rear end where the radius is large. In contrast, in the case of the drill shown in
Referring to
Further, when the ellipse f1 serves as the reference, the life of the drill can become long because the point angle is zero at the maximum diameter position (x=0) and hence no corner, which is likely subjected to wear, is produced.
Next, a method of determining a reference curve will be described. The reference curve is defined by connecting end points of a plurality of arcs.
An arc having an end point at the maximum diameter position is determined as follows.
Referring to
A desirable arc, an end point of which is not at the point A or the separation point H, but at a position between the point A and the separation point H, is determined as follows.
Referring to
The arc, an end point of which is not at the point A or the separation point H, but at a position between the point A and the separation point H, has been provided in the above description. However, such an arc may not be provided. If the arc is provided, a desirable number of arcs may be provided.
An arc having an end point at the separation point H is determined as follows.
Referring to
The ridgeline of the cutting edge is formed along the reference curve, which has been determined as described above. That is, the ridgeline of the cutting edge overlaps with the reference curve. Referring to
Next, a relief angle of a cutting edge and a margin will be described.
In the drill shown in
Next, a relief angle of a cutting edge and a margin will be described according to another embodiment.
In the drill shown in
Referring to
When the drill (former drill) shown in
(1) Regarding the cutting edge section mainly for drilling located closer to the tip end of the drill than the separation point H, the relatively large relief angle δ1 of the former drill has a small area, and hence the cutting resistance is large. In contrast, the relief angle δ1 of the latter drill has a large area, and hence the cutting resistance is small. (2) The change in relief angle of the latter drill from δ1 to δ2 is a slight change as compared with that of the former drill. Hence, the cutting edge is highly resistant to uneven wear, the hole quality, i.e., the quality of a produced hole is hard to be degraded, and the life of the drill tends to be long. (3) Regarding the cutting edge section mainly for reaming located closer to the rear end than the separation point H, the former drill has the relatively small relief angle δ2 in the substantially entire area, and hence the cutting resistance is large. In contrast, the latter drill has the relief angle in the transition area from δ1 to δ2. The average relief angle of the latter drill is large, and hence the cutting resistance is small.
Using the above-mentioned differences in the characteristics as the guidelines, the distribution of the relief angles can be designed in accordance with the desired characteristics.
Next, another embodiment of the point angle will be described.
In the drill shown in
As described above, when the tip section of the cutting edge is formed along the reference ellipse f4, the major axis b of the ellipse f4 is shifted beyond the axis of the drill (x-axis) as shown in
The point at which the minor axis of the reference ellipse f4 is coincided with the y-axis is the same as that of the drill shown in
For the ridgeline of the cutting edge of the drill which is entirely defined by a part of the reference ellipse f4, a method of determining a reference curve that is defined by connecting end points of a plurality of arcs will be described.
An arc having an end point at the maximum diameter position is determined in a similar manner to the method described above with reference to
An arc having an end point at the drill tip K is determined as follows.
Referring to
As a result, the ridgeline of the cutting edge is formed along the reference curve, which has been determined as described above. The ridgeline of the cutting edge overlaps with the reference curve.
In any of the drill shown in
Next, a method of determined φD, L, θ, and φA, which are suitable for drilling a hole in a fiber reinforced plastic composite material, will be described. Here, carbon fiber reinforced plastics (CFRP) will be used as an example of the fiber reinforced plastic composite material. Table 1 shows changes in characteristics in accordance with the value of L relative to the value of φD, i.e., L/φD.
When L/φD is increased as shown in Table 1, the length of the cutting edge is increased, and the distance of a stroke required for drilling is increased. Also, since drilling reaction force and cutting resistance are proportional to the length of the cutting edge, the drilling reaction force and cutting resistance are increased. Further, when the length of the cutting edge is increased, the cutting volume per unit length of the cutting edge is decreased and hence the cutting edge is hard to wear. Thus, hole quality, i.e., the quality of a produced hole (the level of delamination, inner surface roughness of the hole) is improved, and the life of the drill becomes long. The most right column of Table 1 shows the comprehensive evaluations of the characteristics for the CFRP when the value of L/φD ranges from 1.0 to 2.0. As shown in the comprehensive evaluations, good results can be obtained when the value of L/φD is in a range from 1.4 to 1.6, particularly around 1.5.
Regarding the results, the value is determined as L/φD=1.5. Table 2 shows values of φA/φD and changes in characteristics in accordance with the value of θ when L/φD=1.5.
As shown in Table 2, the values of θ are in a range from 60° to 180°. The values of φA/φD respectively corresponding to the values of θ are obtained from Expression 1 and the expression L/φD=1.5, as shown in Table 2. When the value of θ is decreased from 180° to 60°, centrality (deflection state of the tip end, holdability) is good. This is because the tip end of the drill becomes sharper and deflection of the tip end less occurs as the value of θ is decreased. Also, the drilling reaction force and cutting resistance are decreased. This is because the cutting edge bites into a member to be cut by a larger distance as the radio of φA to φD is increased. Also, since the length of the cutting edge section mainly for reaming located closer to the rear end than the separation point H is increased as the value of φA/φD is decreased, the hole quality, i.e., the quality of a produced hole (the generated state of delamination, inner surface roughness of the hole) are improved. Also, since the tip end of the drill becomes sharper and hence the tip end is more likely to be chipped as the value of θ is decreased, the durability of the tip section of the drill against chipping is decreased.
The most right column of Table 2 shows the comprehensive evaluations of the characteristics for the CFRP when the value of θ ranges from 60° to 180°. As shown in the comprehensive evaluations, good results can be obtained when the value of θ is in a range from 100° to 140°.
With the results, a drill for drilling a hole in the CFRP preferably has a value of L/φD in the range from 1.4 to 1.6, and a value of θ in the range from 100° to 140°.
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