probing an arc

[kendo]

I've got a part about 3 inches wide and 25 inches long with a big 19" radius arc segment cut into it and so Im wondering how to probe 3 points and find the actual radius.

If I manually do a circle measure and hit three points will that be correct?
Is there a way to do it automatically in an MMS unit?

Thanks,
Kenny

[christinandavid]

Seem to recall that even dedicated CMMs have trouble with small section/large radius measurements, accuracy improving with each point probed...would be surprised if you could get consistent results using only 3 points.

DP

ps Ignore that post as I may have got the wrong end of the stick and thought you were probing a radius over the 3" width

[kendo]

thanks for the reply,
you're right, the radius is cut into the long side so I've got a section about 20" long, but the three points will be probed in the same +Y direction and I'm sure it'll be a little off.

The first pic shows what Im doing, the second shows the error I'm expecting.

[christinandavid]

A quick calculation (roughly scaling your drawing and using the error as a guide) I reckon the machine will think the radius is .1" smaller than it actually is if you probe those points in one direction.

Unless of course it can compensate for the probe knowing the angle it is at, in relation to the origin of the radius.

DP

[Wheelz]

Don't know if this will be of any help or not, but assuming you're still not at the automated stage, we use this 3 point macro on our Kitamura 2Xi with very good accuracy results.
Our machines are all metric so the variables will likely differ but the math and layout should be the same.

Be sure to move on both axes on all 3 points. (Else divide by zero error)

Had to zip the file as file with no extension is considered invalid.

Pete