Non repeatable z=0 setting.

[schweik]

I have a prt 96. 2 out of 8 z= 0 settings are .020 inches above the stock. The other 6 are at zero. The zero plate was mic'd and correct in the sbp zero program. The z-pinion is tight. Any suggestions?

[fleinbach]

Are you sure the plate is perfectly flat on the material? I have gotten into the habit of holding down pressure on the plate and the material. I was getting depth errors some time ago and after paying close attention to what was happening I noticed when I put pressure on the plate the material would move slightly due to being warped. I also noticed if you don't put pressure on the plat the spring cord can actualy cause it to float slightly.

So I always hold firm down pressure when zeroing now.

[jeffreymcgrew]

Are you always z-zero'ing at the same place on your table?

Could your bed be flexing slightly when you z-zero?

[schweik]

Frank and Jeff, Thank you for your input. The table is very stiff, so I don't think it is flexing at all. The measurements are at the same xy position. I will try holding the zero aluminum plate down and see if that gets rid of the non-repeatability.

[myxpykalix]

i HAVE ALWAYS HELD MY PLATE DOWN BECAUSE I SAW THAT THE WAY I WAS POSITIONING IT (oops) the dust skirt was hitting it on the bent up part causing it to move or tilt up some giving me a bad reading.
I don't think it would be flexing of the table because the Z motors don't exert enough pressure to cause that...do they?
To answer my own question, i do see how when i lower the Z on the unsupported part of the plywood end it will bend some, but not anywhere else on the table.

[harryball]

I had a similar problem at one time, I thought my Z pinion was tight, turned out it was not. I only found it by removing the Z axis and taking the pinion off. As I removed it I discovered it was jammed and appeared tight but with enough force it would wiggle. Not saying that's it but that is what I found in my case.

Robert

[schweik]

I have run tests on holding down the z-plate while the zzero program is running. The tool puts absolutely no pressure on the plate when it makes contact. I can move the plate out while it is making contact. Holding the plate down does remove the clearance between the tool and the stock when setting z=0. Without holding the plate down, I found that the clearance under the cutter to the stock was a variable and not necessarily repeatable. Which means putting in a fake thicker plate thickness will not eliminate the problem because sometimes it will be too high and sometimes too low. I am machining a 3D shell which means any clearance on one side is doubled when I turn the part over. It is not always convenient to hold the plate down during zeroing. I think the best thing is to check the clearance after zeroing z, and then make the adjustment to z to remove the clearance. A better solution would be to figure out how to put a known force on the plate from the tool to make sure it makes solid contact. The electrical contact alone has too much uncertainty.