Measuring insert tip radius

Another thread was seeking ways to measure the radius of a small edge fillet, and it set me thinking again how to measure the tip radius of a turning tool insert. This is an important bit of information for radius compensation in CNC turning for accurate tapers or curved profiles, either for the CAM software or the controller. After some musing I came up with the following approach which I hope might be useful to others.

A simplification is possible since commercial inserts are made with radii that seem to be usually an exact multiple of 0.1mm, so the method doesn't have to be extremely accurate if one is just interested in these.

I made a simple fixture on which the insert can be mounted, just a bit of brass plate with an M2.5 threaded hole near one edge.

These rhombic inserts have a minor included angle of 80 degrees. One then makes two measurements with calipers.

This "across flats", with the caliper held flat to the plate. Call this dimension "h". Then slowly rotate the fixture so the jaws are opened up, keeping the caliper flat on the plate, until they reach a maximum value.

Call this dimension "w". Then the tip radius "r" is given by

r = 1.39973.h - 0.89973.w

For the vakues above this gives 0.413mm so the actual radius is (probably) 0.4mm.

For triangular inserts:

This is h

This is w.

The formula here is:

r = (2 + sqr(3)).h - (1.5 +sqr(3)).w

which for the values above gives 0.378mm, so again the nearest "standard" size is 0.4mm.

So now I can go back and add the tip radii to my tool table - might be useful!

Edited By John Haine on 06/01/2022 17:22:07