Hole probing for endmill diameter measurement?

[Julien]

@neilferreri, I re-read the thread where you posted the CNCjs macro to find the center of a hole. I get that this is useful for e.g. making a probing cycle independent of the endmill diameter, which is fantastic.

Since the hole diameter is known, and the procedure determines the center X0/Y0, and the X/Y values are known at each of the four touch points, we can compute the actual diameter of the endmill (i.e. X- chord is at offset Y, therefore it must be D long, but since we measure M, therefore tool diameter is M-D)

My question is: what kind of precision do you think we could expect on this endmill diameter measurement ? I guess machine calibration ($100/$101) would severely limit the accuracy of the result, and we would also include backlash in the measurement, but I have no idea what ballpark accuracy we could get, and whether this makes any sense.

[PaulAlfaro]

I’d imagine its dependent on the resolution of the machine and if any upgrades have been incorporated (screws/motors/etc).

I couldn’t find specs for the shapeoko, @WillAdams ?

I sorta documented the resolution on the Nomad to line up with the specs listed on their site. Will made a good point regarding motors and just the stock hardware being the limiting factor in general (unless upgrades);

Nomad Tool length Probe UGS

One consideration is the machine can only hold full and 1/8th step positions — the variation might be caused by how closely the metal rod and its spring are aligning with the nearest position — it is possible that at certain temperatures this is more/less repeatable. The numbers you’re getting exactly match the specified mechanical resolution: Mechanical Resolution .0005 in (from: https://carbide3d.com/nomad/specs/ ) and are better than the expected repeatability/accuracy

[Julien]

Assuming the shapeoko error would be around twice that, that would make it relevant to check whether a 0.25" endmill is actually e.g. 0.247"

[neilferreri (Neil Ferreri Ω)]

@Julien I didn’t use the hole diameter or center macros for endmill diameter, but I do have one posted to calculate the diameter of the endmill with a probe block of a known size. It might be better to do it using the hole (if that’s accurate) because you wouldn’t have to worry about the block being parallel to axis motion. When I tested the endmill diameter calculating macro, I was always off by a bit. After my experimenting with the tool change probe macro (see thread @PaulAlfaro linked), I may have to revisit the tool diameter calculating macro.
I sometimes find myself chasing down solutions to problems I don’t have.