Here's a quick-n-dirty
Forstner Bit

© Frank Ford, 2007; Photos by FF

This is the kind of job I find myself doing from time to time when we need a special wood drill bit for the repair shop at Gryphon. One of the guys was getting ready to install a new bridge on a solid body electric guitar, and he needed to drill two 9.0 millimeter hole to press fit the special bushings.

Because this was to be a quick job, I thought I'd time it. . .

I mounted a piece of 3/8" diameter W-1 drill rod in the lathe collet, and turned about an inch of length down to nine milimeters, or 0.354"

At the end of the rod, I dished out a bit of it, leaving a sharp rim and a little point in the center:

Both features will serve to center and stabilize the bit as it starts into hardwood.

Here, I have the turned rod mounted in a collet block in the mill vise, and I'm making a couple of cutting lips, with a 90-degree cutting angle:

Back on the lathe, I turned the shank down to about 5/16" to provide chip clearance and to avoid having the cutter rub too much in the hole:

I used a flat file to provide a bit of clearance on the perimeter of the cutting faces:

And a carbide disc in my Foredom tool to cut relief behind the bottom of each cutting lip:

I didn't measure any of the angles or amount of relief - I just wanted some clearance and relief. I wasn't making this tool for sale or heavy long term use. In fact, it might be a one-use thing for all I know at this point.

Next, it was time to harden the cutting edges. I stuck the bit in the propane flame until it was nice a bright red-yellow:

A quick dip in the pool:

And my cutter was basically done.

A diamond pocket hone and a few strokes to hone the cutting edges completed the job:

Hey, it's ugly, and ill tempered, but it's ready to cut:

That's right - I did nothing at all to temper the hardened bit. So what if it's brittle? We'll use it on wood, and we'll hardly test its resilience that way.

Here's a first test cut in a piece of maple die board:

A nice flat bottom hole:

Oh, and the entire job took 47 minutes, start to finish, including setting up the photo shots:

That's just short of the time it would take to drive to the Woodcraft store and buy a bit, if they have such a thing in stock.

Back at the guitar repair shop, the job went well, with no chipping of the finish:

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