How to Make D-Bits
The turner, (lathe operator), made his own D-bits as follows:
Using a piece of silver steel he would turn and then polish the cutting diameter to the exact size of the desired hole in the drill bush. Then he would turn a shank some .015″ dia. less than that, so as to permit the application of the coolant, finally cutting it off further along the bar.
Thus there were 3 diameters concerned, the actual cutting diameter, the reduced shank and a length of untouched bar for holding in the drill chuck. The next step was to harden it, a med. bright cherry red and, after polishing, to temper to a light straw. Back to the lathe and the blank was polished until it was .001″ below the finished bush size.
To the tool grinder then and carefully, taking care not to draw the temper, grind away half of the cutting part to form the “D” cross section. This grinding was done on the periphery of the wheel so that the resulting “flat” was not actually flat, but slightly concave.
The dimension of the finished tool was determined by a mic. from the “flat” to the untouched side of the cutting dia. and was .001″ greater than half of that diameter. Anything less than that resulted in a bit which cut undersize. The front was backed off so that it cut on one side only and a slight chamfer added.
See also Lapping.
Author: Cyril Collins
