Gerald, there should be a "center in hole" routine located in the SBParts folder. It works great at centering in a cup, I use it for my highest precision centering on a fixture. On my bot, I get a 0.001 repeatability on that center point. Usually 0.000.
Additionally after a long cut, if your cup is still on your fixture you can check the center in hole again and find out how accurate you were able to relocate the same point after a lot of jostling. Its a good way to test shopbots claimed position repeatability of 0.002".
I have found the cheapest way of making this test is to embed a copper sweat fitting end cap into a fixture. They are cheap and I can leave them in the fixture if desired. I use both 1/2" and 3/4" sweat fittings depending on the bit size I am using. Also there is a 3/4" sweat fitting pocket in four locations on my spoilboard so I can check for changes in X/Y squaring actual move distances, etc after months of operation.
To hook the sweat fittings up to the ZZero plate I just hooked up two car-battery-size alligator clips from the local auto-parts store with a wire. If you like I can post some photos.
D
"The best thing about building something new is either you succeed or learn something. Its a win-win situation."
--Greg Westbrook