1. Field of the Invention
This invention relates to apparatus for grinding twist drills each having two clearance surfaces and a helical flute associated with each clearance surface, which apparatus comprises a rotatable drive shaft, a grinding wheel having a grinding surface and axially movably mounted on and coupled to said drive shaft and spring biased toward a drill-guiding structure, which faces said grinding surface and has guiding passages differing in diameter and adapted to guide respective ones of said twist drills differing in diameter toward said grinding surface until the latter engages such twist drill at one clearance surface thereof, also comprising in alignment with each of said guiding passages at least one guide projection for extending into a flute of a twist drill extending through said guiding passage, and drill stop means for engaging a twist drill extending through said guiding passage at the other clearance surface thereof so as to limit the advance of said twist drill toward the resiliently yielding grinding surface.
2. Description of the Prior Art
Such apparatus in which twist drills can be ground without being chucked are known from U.S. Pat. No. 3,753,320 and Published German Application No. 16 52 239.
In such known apparatus an adjustable abutment acting between a member rigidly connected to the mounting for the grinding wheel and the drill-guiding structure is arranged to limit the spring-urged movement of the grinding surface toward the drill-guiding structure in an end position in which said grinding surface is closer to the guide projections than the drill stop means.
For a satisfactory result of the grinding operation, a proper position of the clearance surface at which the drill is being ground relative to the grinding surface is of crucial significance. That position will depend on the radial position of the guide projection relative to the guiding passage and, owing to the helical flutes of the twist drill, on the distance from the guide projection to the drill stop means. That distance must be short because otherwise the fact that the flutes of twist drills from different manufacturers have different helix angles will result in an excessive deviation from the proper position of the clearance surface of some of such twist drills relative to the grinding surface.
One disadvantage of said known apparatus resides in that the axial distance from the grinding surface to the guide projections increases with the wear of said grinding surface and eventually the drills may no longer contact the grinding wheel at all. The proper distance must then be reestablished by means of the adjustable abutment means. Owing to the short axial distance between the guide projections and the drill stop means, a wrong adjustment of said drill abutment means may have the result that the grinding wheel grinds on the drill-guiding structure and damages the guide projections.
Another disadvantage of such known apparatus will arise during a prolonged grinding of twist drills which are so small in diameter that they will not contact the entire grinding surface during each revolution of the grinding wheel and resides in that a groove will then be formed in the grinding surface. When a twist drill larger in a diameter is subsequently ground, that groove will be reflected by the clearance surface that has been ground, i.e. by the cutting edge of such larger drill, and the grinding wheel cannot be used any longer when such groove has reached a certain size.