I've been using this machine with Mach 3 and Gecko G540 for almost a decade.  This problem may have occurred before, but isn't every time.   Today the code was created by Vectric V-Carve Pro 9.0.  After setting the X0, Y0 and Z0, I went to run the code.  Z always goes to home position first, but it went past the limit switch and stalled mechanically.  Letting the code run had Z go too low, past tabs and into the table.  It seems the stalling affected the Z0 setting position.

I went to the toolbox screen and the limit switch is working.  If I use "home all", it works just fine and does not mechanically bottom out.

X and Y do not "home" at start of code, just Z.  Is this something in Mach or with Vectric?

When things work, I don't tempt fate.  What is the intent of line 140: N140 G00 G43Z1.7424H1

The G-Code is as follows;

( File created: Tuesday October 10 2017 - 02:03 PM)
( for Mach2/3 from Vectric )
( Material Size)
( X= 9.875, Y= 4.250, Z= 0.755)
()
(Toolpaths used in this file:)
(Profile 1)
(Tools used in this file: )
(1 = End Mill {0.25 inch})
N100G00G20G17G90G40G49G80
N110G70G91.1
N120T1M06
N130 (End Mill {0.25 inch})
N140G00G43Z1.7424H1
N150S18000M03
N160(Toolpath:- Profile 1)
N170()
N180G94
N190X0.0000Y0.0000F35.0
N200G00X1.5456Y3.1342Z0.9550
N210G1Z0.5033F30.0
N220G1Y2.6407F35.0
N230G1Y2.0157

Steve.

I found something.  If I set Z0 as the last item, then zero all and run the code it all works.  For some time I had had it run from multiple points and Z would home, but not mechanically hit, and it run properly.  Maybe lucky those times.

It still seems odd that only the Z would try to home and go past the home limit switch.
Steve.

The real problem was the CAM software was putting in a "safe Z" higher than my axis was able to move.  So it went through the limit switch until a mechanical crash trying to get to the "safe Z."  I corrected the CAM software and all is good.