Z-Axis Tool Path is Higher than Z-Axis Zero

Just finished building my first CNC. I'm using the Chinese blue board with Nema 23 270oz motors and Mach 3 (R3.042.020) on a dedicated Windows XP computer (no other software installed). I have a zero plate probe installed on pin 15. Everything on the machine works great except that when I zero the X-axis (manually or via the zero-plate), the z-axis tool path is higher than the zero. For example, I was running an engraving g-code, created in Photo VCarve, and the cut depth was 0.01". After zeroing the machine, I ran the g-code and the tool ran just above the material surface, almost grazing it, but never cutting into it. I had to change my zero plate thickness in Mach3 from 0.060" to 0.075" just to get it to cut into the material. Even then, it still wasn't accurate depth. When I tell it to return to zero, all 3 axis return to where they are supposed to be - correct zero.


I've re-zeroed and re-confirmed zero multiple times, using multiple g-codes, from multiple sources, and the z-axis tool path is always a little higher than the z-axis zero.


It's almost like the z-axis zero is set to the bottom of the tool cut depth. I tried some V-bit letters in a block of scrap wood as a test, and I had to zero the z-axis 1/8" lower than the material surface in order for the bit to cut 1/8" into the wood.


Am I missing a setting somewhere??


I am a total noob to Mach3, and CNC in general. Thanks guys!!

Mike

What auto zero script are you using, and how thick is your touch plate?

Gerry. Below is the code I'm using (exactly as copied from the web). My plate is 0.060" thick

CurrentFeed = GetOemDRO(818) 'Get the current feedrate to return to later
CurrentAbsInc = GetOemLED(48) 'Get the current G90/G91 state
CurrentGmode = GetOemDRO(819) 'Get the current G0/G1 state

If GetOemLed (825)=0 Then 'Check to see if the probe is already grounded or faulty
DoOEMButton (1010) 'zero the Z axis so the probe move will start from here
Code "G4 P3" ' this delay gives me time to get from computer to hold probe in place
Code "G90 G31Z-4. F4" 'probing move, can set the feed rate here as well as how far to move
While IsMoving() 'wait while it happens
Wend
ZProbePos = GetVar(2002) 'get the exact point the probe was hit
Code "G0 Z" &ZProbePos 'go back to that point, always a very small amount of overrun
While IsMoving ()
Wend
Call SetDro (2, .060) ' change .060 to your plate thickness and then adjust for final accuracy
Sleep 200 'Pause for Dro to update.
Code "G1 Z1. F50" 'put the Z retract height you want here, must be greater than the touch plate thickness
While IsMoving ()
Wend
Code "(Z axis is now zeroed)" 'puts this message in the status bar
Code "F" &CurrentFeed 'Returns to prior feed rate
Else
Code "(Z-Plate is grounded, check connection and try again)" 'this goes in the status bar if applicable
End If
If CurrentAbsInc = 0 Then 'if G91 was in effect before then return to it
Code "G91"
End If
If CurrentGMode = 0 Then 'if G0 was in effect before then return to it
Code "G0"
End If


I got that code from this YouTube tutorial: https://www.youtube.com/watch?v=G4DeVa8rzA0

Thanks! ~Mike