By Bob Daley
Here’s an old trick often employed by toolmakers to improve the size of drilled holes.
Most drills sharpened in a normal manner will drill a hole about .003” to .004” oversize. Photo 1 shows a standard drill point with a sharp corner. To reduce this oversize effect, simply hand-stone a small radius onto the corner of the drill (Photo 2).
When stoning the radius it’s necessary to provide some back clearance to the radius cutting edge. To achieve the back clearance, just eyeball the upper flute’s cutting edge to set it a slight angle (Photo 3).The angle shown is much more than necessary – about half, or even less, than shown is all that’s required to obtain the desired result. For a comparison I’ve added Photo 4, showing the upper flute’s cutting edge sitting vertical.
The radius needs to blend to the side of the flute, and this is achieved by starting off with the drill’s side virtually parallel to the honing stone.
If even better hole sizing is required, first drill a pilot hole 1/64” smaller than the desired hole and finish off with the doctored drill. Use a slow speed and cutting oil to obtain a smooth bored hole. I use this technique instead of reaming holes to accept dowels.