Tangential Fly-Cutter

After making my tangential tool-holder for the lathe and reading the Mike's Workshop page about a tangential fly-cutter, I thought it would be interesting to have a go at making one myself. I made it out of aluminium so that I could make it fairly quickly and test whether the idea worked. I'll probably re-make it in steel fairly soon, but I've got some ideas for some design tweaks to do first.

The design is somewhat different to Mike's fly-cutter. Unlike Mike's design, it has two cutting bits (both came from a 2 1/2" long piece of 5/16" square HSS, which I cut in half). One of these cuts on a diameter of 105 mm and has a very sharp tip. The other cuts on a diameter of 100 mm and has a heavily rounded tip. The rounded cutter is set 0.1 mm lower than the sharp tip. As the sharpened tip cuts at a greater diameter, it reaches the workpiece first and can take very deep cuts from the surface of the workpiece. The rounded cutter reaches the workpiece later and takes a very shallow (0.1 mm) cut. The intention of this is that the rounded cutter produces a better finish.

In use, I've found that fly-cutter is able to take fairly deep cuts in mild steel: I usually limit myself to 0.5 mm cuts but it feels like it could take more. I have been less impressed with the rounded cutter though. Although it produces a reasonably good finish, it isn't much better than the finish from the sharpened cutter. If/when I re-make the fly-cutter in steel, my plan is to replace the rounded cutter with one ground from a piece of 8 mm round HSS. This should make a much better radius than I was able to grind by hand from a piece of square HSS.


Without a tilting vice, it took me a while to work out how to machine the complex angles involved in the tangential tool holder. In the end, I used my high-profile clamps to hold a piece of scrap aluminium at 12° to the table (set with a digital angle gauge). I machined the top surface flat and drilled/reamed a 12 mm hole in the middle of it. I then put a 12 mm piece of silver steel in the hole and placed the machined piece of scrap aluminium on the table at 80.5° to the X-Axis (having calculated the angle required for 12° to the tangent to the circles of 100 mm and 105 mm diameter, I found the angles to the X-Axis should be 81° and 80° and I figured that it probably wasn't too critical so used the average for both cutters). I used some standard table clamps to hold this at the right angle and then mounted a piece of 38 mm × 25 mm aluminium with a 12 mm hole reamed in the middle onto the piece of scrap. This was held in place with the high-profile clamps and one standard table clamp on the end that wasn't being machined. I could then machine one half of the fly-cutter, turn it round and machine the other half. The photos above shows the set-up prior to cutting the first side. The precision vice you can see on the right-hand end of the table was mounted square to the table and used as a reference for the protractor when setting the 80.5° angle.

A 3D Model

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