A customer recently reached out to me and asked if I can machine slides for the RMR plate mount. I said, of course, I can, but then I paused for a second. The RMR footprint is really simple, except it has the 3″ radius at the front. That’s right, 3″ radius, not diameter! So how do I machine this radius?

An obvious answer would be to use a rotary table. But all I have is a very small 5″ rotary of a very questionable quality, and properly mounting a slide on it would be very difficult, if not impossible. An easier option would be just to hold the slide in the vise, find the centerline, remove the metal from the top until the rear sight dovetail is gone, and then sneak up on the radius with a 6″ cutter until the required dimension is established. But where can I get a 6″ cutter? Well, silly, I can make it!

I started with a 6″ aluminum plate I found in my metal bin. It is pretty much square already, so I just found the center and drilled and tapped it for 5/8-24 thread. Then I drilled the 1/2″ hole for the boring bar and M5 threaded holes on the opposite sides of that corner for the set screws to secure the boring bar in place.

A 3/4″ shank was machined and threaded on the lathe.

I then held the entire assembly in the collet chuck on the lathe and rounded the corners a little bit to reduce the overall mass of the cutter.

I then secured the slide in the vise and found the center line and after touching off the front edge of the cut I started moving the cutter about .008″ at a time, spinning it at about 400 RPM. The chips were flying everywhere but it did the job!