Workpieces with an inclined end surface were drilled by a gun drill and a long drill. The inclination angles are 0°, 15°, 30° and 45°. The tool diameter is 18 mm. Besides this, another investigation with a parallel hole next to a probored hole was made. The wall thickness between the two holes, and the diameter of the prebored hole and that of the parallel hole were varied. As a result, the following points were clarified. The direction of the cutting force during the cutting of an inclined end surface influences the hole deviation. In the case of the gun drill, the hole deviates in the direction of the inclination of the end surface because the hole diameter becomes smaller than that of the tool, while in the case of the long drill, the hole deviates in the opposite direction of the inclination because the hole diameter becomes larger than that of the tool. Parallel hole deviation increases with the increase of prebored hole diameter. Hole deviation increases suddenly when the wall thickness between two holes decreases below a certain value.
