Posted by Matt Wilson [172.69.68.246] on Monday, January 13, 2020 at 01:15:11 :
I was working on a welded steel mount for my dual-circuit master cylinder, and I built up too much weld material adjacent to a couple of bolt holes that I had already drilled. To get the surfaces true again for the bolt heads to sit flat, I needed to spotface those areas, but I don't have a milling machine, and I don't have a tool to do it on my lathe. So after some thought, I figured what the heck, I'll try a forstner bit from a cheapo set that I bought probably 15+ years ago. For those not familiar with forstner bits, they are made to drill clean precise holes in wood, or you can even make a flat-bottom hole, rather than going all the way through.
Anyway, I pulled out the size bit I needed (3/4" dia), put on my safety glasses, stuck the bit in the drill press and with gentle pressure at 400 rpm, and lots of patience, I was able to make two decent spotfaced areas. I just eyeballed the centering of the bit over the existing holes. I sprayed water as a coolant periodically. I really expected it was going to ruin the bit immediately without getting anywhere, but instead I was shocked when it worked out well. I knocked off a couple of large lumps of weld material with the bit, and then proceeded to remove around 0.030 - 0.040" from each area, leaving two nice flat, relatively smooth areas.
I'm sure this was a little hard on the bit, but I was able to use it to drill a flat-bottom hole in a chunk of wood afterward, although it seemed a little less aggressive than it normally would have.
The steel I am using is just the standard stuff you buy in bulk at the metal supply shop, probably A36, so nothing very hard. Still, it was surprising to me to have this work out so well.