Tool rooms and shops in Illinois, Indiana, Michigan, Wisconsin, Iowa or anywhere, have your drills sharpened by the master tool makers at General Cutting Tools in Chicago. Twist drills are cutting tools designed to perform concurrently several functions, such as penetrating directly into solid material, ejecting the removed chips outside the cutting area, maintaining the essentially straight direction of the advance movement and controlling the size of the drilled hole. The geometry needed for these multiple functions is incorporated into the design of the twist drill in such a manner that it can be retained even after repeated sharpening operations. Twist drills are resharpened many times during their service life, with the practically complete restitution of the original operational characteristics. However, in order to assure all the benefits which the design of the twist drill is capable of providing, the surfaces generated in the sharpening process must agree with the original form of the tool’s operating surfaces, unless a change of shape is required for use on a different work material.
The principal elements of the tool geometry which are essential for the adequate cutting performance of twist drills are:
- POINT ANGLE: Commonly 118 degrees, except for high strength steels, 118 degrees to 135 degrees; aluminum alloys, 90 degrees to 140 degrees; and magnesium alloys, 70 degrees to 118 degrees.
- HELIX ANGLE: Commonly 24 degrees to 32 degrees, except for magnesium and copper alloys, 10 degrees to 30 degrees.
- LIP RELIEF ANGLE: Commonly 10 degrees to 15 degrees, except for high strength or tough steels, 7 degrees to 12 degrees. The lower values of these angle ranges are used for drills of larger diameter, the higher values for the smaller diameters. For drills of diameters less than 1/4 inches, the lip relief angles are increased beyond the listed maximum values up to 24 degrees. For soft and free machining materials, 12 degrees to 18 degrees except for diameters less than 1/4 inch, 20 degrees to 26 degrees.