least 0 02 in thick the improved cutting discs of this invention have a radius r 1 of 0 262 in for the bottom half thereof the rear half when considered in relation to the cutting function which is the size of the larger commercially available cutting discs the cutting edge 126 has a radius corresponding to the radius of one of the small cutting discs which have been used for drilling hard formations viz a radius r 2 of 0 165 in this disc therefore has a lower half or main body portion of large radius 0 262 in tapering along arcs 127 and tangential lines to a somewhat pointed end having a cutting edge 126 of small radius 0 165 in the various radii given are based on the sizes of cutting discs which are commercially available at the present time obviously other sizes can be used as materials become available for constructing them when the drill bit is operated under a normal load the depth of penetration per revolution of the bit is usually no more than 0 0625 in under these conditions only the cutting edge 126 of small radius 0 165 in penetrated the formation the cutting disc has an area of penetration into the formation at a rate of 0 0625 in per revolution of 0 00491 in 2 which is 63 of the penetration area for the larger 0 262 in radius discs these discs while having a tapered and rounded cutting edge 126 for cutting hard formations have essentially the full surface area of the larger 0 262 in radius cutting discs for bonding to the supporting studs 19 as a result of this construction the.