How to calibrate the Z axis?

[MDCochrane]

I have an old, but new to me, PRT96 with the Vg upgrade. I'm new to ShopBots and have been learning by doing. I've carved several attempts on a prototype model that always gets spoiled on the second side (2 sided carve). Turns out my Z depth calibration is out of whack and I'm losing 20% of my model from the middle.

Yesterday I:
1. Z zeroed at the material surface using the Z zeroing plate and routine.
2. Told the router to plunge to -20mm. (I'm working in millimeters)
3. Measured the hole with a depth gage. It's 25mm deep.
4. I repeated the process with different bits and collets, zeroing with the Z zero plate each time. Every hole that should be 20mm deep is 25mm.

I read the posting about the "random" z depth issue that was fixed by cleaning and greasing the Z gantry. I'll do that, too, but there's nothing random about my problem. A 20mm plunge always gives me 25mm.

What is the recommended procedure for calibrating this machine. I can see that it was wrong of me to assume that it would be accurate after buying it and moving it.

Thanks!

Mike

[billp]

Are you sure that your unit values are correct?

[MDCochrane]

Yes, the model, a 7 foot kayak paddle, carves out beautifully except for the depth issue. Yesterday I lied to the machine and got my best result. (... feeling a bit guilty) I raised the Z axis to 4mm above the material surface and set 0 there. The center of the paddle turned out just about perfect. The top and bottom, however were somewhat flat, since the model fills the material boundaries completely and the tool passes to the top and bottom levels were air cuts.

The Z zeroing routine always sends me a warning. "You are working in units other than the machine was set up initially. Do you want to convert and continue" (something to that effect). I always answer "yes" and continue.

Mike