I recently needed a clean-cutting bit to drill through some old Douglas fir molding. Lacking the funds to buy a good bore, I purchased a cheap spade bit and remodeled it with a flat file, as shown.
First, I used the corner of the file to make two V-cuts near the edge of the bit. Then I removed the area between the base of the new cutting spurs and the pilot with the edge of the file. The resulting bit produces a remarkably clean cut.
Duff Bogen, Seattle, WA