Hey I am about to setup a probe for my router on 2010 Mach 3. Im just in the planning stage but cant get my head around how it can probe for the centre of a cylinder by only four points of touch unless its perfectly round?

Does the probe  contact each axis in several places to find the furtherest point on an inside operation or closest when probing the outside of a cylinder? Am i making sense?

Would it touch on the X, for example, then move slightly in the y direction and touch again to ensure its correct?

Can it measure the centre of an oval shape for example?

Or is it just plain smart and just does it in four sample touches and I am overthinking it all?

You are correct, that it must be perfectly round.

Ha ha thanks! I was in bed and had it rolling around in my head. I sussed it. But I guess if it was not perfectly round it will give a very good place to start to make it round.

Or I guess thats what they made 4 jaw chucks for.

Now I just need to decide whether I spend a couple hours making an electronic probe or buy one for US$79.......

Nice to hear from you Gerry.

In theory only 3 points are need to locate a circle but the math is more difficult. Four is easier if a pair of points fall on the same axis line because you just add the location of the probes axis together and divide by two to get the center along that axis. You don't even need the probe diameter to do this. I pick up hole centers all the time  manually using an edge finder using this technique.