1. A three-flute drill comprising: three chip discharge flutes axially disposed to open in a tip portion; and
three cutting edges each formed at an intersecting portion between an inner wall surface of each of the chip discharge flutes facing in a drill rotation direction during hole drilling and a tip flank formed on the tip portion,
the cutting edge having a concavely-curved cutting edge portion in a concavely-curved shape formed on an inner circumferential side and a convexly-curved cutting edge portion in a convexly-curved shape formed on an outer circumferential side,
an axially perpendicular cross-sectional view orthogonal to an axial center O having a first convex curve corresponding to the convexly-curved cutting edge portion and a first concave curve corresponding to the concavely-curved cutting edge portion intersecting with each other at an intersection A,
in the axially perpendicular cross-sectional view, a concave amount LF of the first concave curve being within a range of 0.01 D to 0.05 D in terms of a drill cutting diameter D relative to a reference line K connecting an outer circumferential point B, at which a drill outer circumferential portion intersects with the first convex curve, and the axial center O, and
in the axially perpendicular cross-sectional view, a rake chamfer width LW being a distance from an intersection E between a straight line passing through the intersection A and orthogonal to the reference line K and the reference line K, to the outer circumferential point B, and being within a range of 0.005 D to 0.06 D in terms of the drill cutting diameter D.
2. (canceled)
3. (canceled)
4. The three-flute drill of claim 1, wherein
in the axially perpendicular cross-sectional view, a rake angle \u03b8 is an angle between the reference line K and the first convex curve at the outer circumferential point B and is negative.
5. The three-flute drill of claim 1, wherein
the three-flute drill has a web thickness CD within a range of 0.15 D to 0.50 D in terms of the drill cutting diameter D.
6. The three-flute drill of claim 1, wherein
in the axially perpendicular cross-sectional view, an inner wall surface of the chip discharge flute facing opposite to the drill rotation direction during hole drilling is made up of a second concave curve formed on the inner circumferential side and a second convex curve formed continuously from the second concave curve on the outer circumferential side, and wherein the second convex curve reaches a heel.
7. The three-flute drill of claim 6, wherein
when the three-flute drill has the first convex curve with a curvature radius of R1, the first concave curve with a curvature radius of R2, the second concave curve with a curvature radius of R3, and the second convex curve with a curvature radius of R4 in the axially perpendicular cross-sectional view, the curvature radii R1 to R4 are within ranges of the following equations (1) to (4), respectively, in terms of the drill cutting diameter D:
R1: 0.02 D to 0.4 D \u2003\u2003(1);
R2: 0.10 D to 0.45 D \u2003\u2003(2);
R3: 0.10 D to 0.45 D \u2003\u2003(3); and
R4: 0.3 D to 1.2 D \u2003\u2003(4).
8. The three-flute drill of claim 7, wherein
a relationship between the curvature radius R2 of the first concave curve and the curvature radius R3 of the second concave curve satisfies the following equation (5):
0.5\u2266R3R2\u22661.1 \u2003\u2003(5).
The claims below are in addition to those above.
All refrences to claim(s) which appear below refer to the numbering after this setence.
1. An electronic power steering apparatus comprising:
a steering force assist device having a motor as a drive source, wherein the steering force assist device applies an assist force to a steering system of a vehicle;
a torque sensor for outputting sensor signals generated by a plurality of systems based on torsion of a torsion bar provided in a steering shaft;
a vehicle speed sensor for detecting a vehicle speed;
a torque calculation section for detecting a steering torque based on the sensor signals outputted from the torque sensor; and
a control section for controlling the steering force assist device, for calculating an assist control variable based on the steering torque and the vehicle speed, and for executing an assist control for applying an assist force based on the assist control variable to the steering system; wherein
upon detection that only one of the sensor signals generated by the plurality of systems is normal, the control section executes an assist restriction process for setting an upper limit value and a lower limit value of the assist control variable based on a map according to at least any one of the lateral acceleration, steering angle, steering speed and yaw rate,
the control section has a first map selected when the vehicle speed is lower than or equal to a first speed and a second selected when the vehicle speed is a second speed being higher than the speed, and
the control section executes, when the vehicle speed is higher than the first speed but lower than the second speed, the assist restriction process based on an upper limit value and a lower limit value of the assist control variable obtained by interpolation of the first map and the second map.
2. The electronic power steering apparatus according to claim 1, wherein
the control section executes the assist restriction process when the vehicle speed is higher than or equal to a predetermined speed.
3. The electronic power steering apparatus according to claim 1, wherein:
the control section further has a third map selected when the vehicle speed is higher than or equal to a third speed being higher than the second speed; and
the control section executes, when the vehicle speed is higher than the second speed but lower than the third speed, the assist restriction process based on an upper limit value and a lower limit value of the assist control variable obtained by interpolation of the second map and the third map.
4. The electronic power steering apparatus according to claim 1, wherein
upon detection of no normal sensor signal in the sensor signals generated by the plurality of systems, the control section suspends the assist control by gradually decreasing the assist force.