Coated twist drill

Abstract

The present invention concerns a twist drill comprising a shank and a cutting portion which is defined by the axial length of the chip flutes and which is at least partially coated. To provide a twist drill having the aforementioned features and a process for the production thereof, which on the one hand have very good properties in terms of service life and chip formation or chip transport, but which on the other hand are also highly suited for reconditioning including coating, in accordance with the invention it is proposed that the cutting portion has substantially over its useable length a first coating, wherein there is provided a second coating which is different from the first coating only in the region of the tip of the drill and which extends over an axial length which corresponds at a maximum to twice the nominal diameter of the drill.

1 . A twist drill comprising a shank and a cutting portion which is defined by the axial length of the chip flutes and which is at least partially coated, wherein the cutting portion has substantially over its useable length a first coating, wherein in addition a second is applied only in a region at the tip of the drill, which extends over an axial length which corresponds at a maximum to twice the nominal diameter of the drill. 
     
     
         2 . A twist drill as set forth in  claim 1  wherein the cutting portion is of an axial length which is at least four times the nominal diameter. 
     
     
         3 . A twist drill as set forth in  claim 2  wherein the axial length of the cutting portion is at least six times the nominal diameter. 
     
     
         4 . A twist drill as set forth in  claim 1  wherein the second coating is restricted to a region of the tip of the drill, which is between 0.3 times and 1.5 times, preferably between 0.3 times and 1.2 times, the nominal diameter. 
     
     
         5 . A twist drill as set forth in  claim 1  wherein the first layer is applied over the entire useable length of the cutting portion with the exception of the end clearance surfaces, wherein the end clearance surfaces are provided only with the second coating. 
     
     
         6 . A twist drill as set forth in  claim 1  wherein at least the cutting part of the drill and preferably the entire drill comprise solid hard metal. 
     
     
         7 . A twist drill as set forth in  claim 1  wherein the two layers differ at least by their color. 
     
     
         8 . A twist drill as set forth in  claim 1  wherein at least the color of the first layer differs from the base color of the drill. 
     
     
         9 . A twist drill as set forth in  claim 1  wherein the first layer is smoothed in the chip flutes at least outside the region of the second layer. 
     
     
         10 . A twist drill as set forth in  claim 9  wherein in addition the peripheral surfaces of the drill lands and/or the round lands of the cutting part are also smoothed. 
     
     
         11 . A process for the production of a twist drill comprising the following steps:
 a) providing a drill blank comprising a shank and a cutting portion having possibly pre-shaped chip flutes as well as a drill tip to be provided with main cutting edges,   b) grinding the chip flutes, the round lands and the peripheral surfaces adjoining the round lands,   c) completely coating at least the chip flutes of the round lands and the peripheral surfaces forming the periphery of the drill lands with a first layer,   d) grinding the drill tip including removing any first layer also applied to the end clearance surfaces, and   e) coating the drill tip with a second layer different from the first layer, wherein the second layer, measured from the tip of the drill, extends axially over a region which at a maximum corresponds to twice the nominal diameter of the drill, wherein the sequence of steps c) and d) is reversible.   
     
     
         12 . A process as set forth in  claim 11  wherein the end clearance surfaces of the drill prior to step are not coated with the first layer and then only with the second layer. 
     
     
         13 . A process as set forth in  claim 11  wherein the first layer is smoothed at least in the region of the chip flutes outside the tip portion provided with the second layer. 
     
     
         14 . A process as set forth in  claim 13  wherein the first layer is additionally smoothed in the region of the round lands and the relieved peripheral surfaces. 
     
     
         15 . A process as set forth in  claim 11  wherein the first and second layers comprise the same material.