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Radius cutter for watchmakers lathe

After completing the heavy duty cross slide for my watchmakers lathe I finally have a very stable platform to mount other accessories to. High on the list was to design and build a radius cutting tool for cutting various sizes of concave and convex radii.

I decided that I'd try using the spindle and carrier from the milling attachment I already had. The Unimat vertical table is so versatile attached to the heavy duty cross slide I made that I was able to quickly make a mounting plate for the milling head from 1/2 thick mild steel.
the spindle centers on a short, hardened 1/4 inch dowel pin pressed into a slightly undersized hole in the plate. very secure and easy to mount the spindle.
Radius cutter using spindle from milling attachment

All I needed was a working adjustable head setup to mount a horizontal cutter into. I had already decided to go with an over-center style cutter for stability as that would eliminate a lot of the upward deflection from a standard cutter placement. It would also be easy to adjust the cutter accurately from top to bottom.
The completed radius cutter is below.
Radius cutter for Watchmakers lathe installed on heavy duty cross slide

I found a used small boring head online and purchased it for about $20. With a little machining using the four jaw chuck on my lathe, I was able to turn the 1/2 shank into an 8MM collet with the correct threads and relief milled into it for the key.
Boring head with remachined collet shank

I also made an adaptor out of brass to hold the 1/4 cutter ground from a broken boring tool. The key was to insure the tip of the "V" was centered and that there was sufficient relief ground into the sides and top. Not too much as I intended to use the cutter for both steel and brass and I didn't want the tool "diving" on the brass parts.
Radius cutter made from altered boring head with brass adaptor for cutting tool

Since I needed fine control in rotating the altered boring head to make the radius cuts I used the custom handle from an earlier project that inserts into the end of the draw bar and locks into place using an internal expanding nut. It worked elegantly and really feels good in my hand when in use.
Radius cutter using draw bar inserted custom rotation handle

I've already used the radius cutter in making the die holder project listed elsewhere on this blog. The spheres and ogee came out perfect and absolutely identical across all four spokes. The cut was extremely smooth and free of any chatter. I apparently got the cutter geometry right and the heavy duty cross slide was beyond rock solid. I also used the external chuck bearing just in case there was any deflection. On to the next several projects!!
Radius cutter in action on watchmakers lathe


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