I sometimes have a problem when i try to zero to my z plate.
when i try to zero the bit goes up instead of down
what is strange this does not happen at all times  sometimes i have to shut down and restart
or have to touch the bit with my zero plate to stop the up movement up  this does not always work
anyone has any idea why this is happening sometimes?
hope someone can help  thanks Paul

Hi,
all touch off routines rely on a G31 probing move.

The general form is:
G31 Z -1 F10

So the machine will drive to Z=-1 inch at 10 inches per minute. There's a couple of things that an go wrong and may explain what you've described.

The first is that the tool needs to be above the surface by a small amount, I can't remember what the number is but I always make sure that the tool is at
least 1/16 inch above the surface. The second problem is what happens if the surface is already at -1.2 inch. Even if the tool is just above the surface as I
recommended above it would still start out lower than it was supposed to finish the move! Believe it, I've done it myself and wondered what the hell was going on!

Double check these two items and let us know how you got on.

Craig

If you aren't in incremental mode, the current position can influence which way the tool moves, as Craig mentioned.

What control card are you using? i found many don't implement G31 properly and i had similar problems to what you describe with the "e cut" usb card and also the "XHc" usb motion controller card. With the XHc card the issue was even related to teh number of times you have called G31 it seems, like the first time would work perfect, then the second one, doing the same thing but not having restarted mach 3, would cause the Z axis to go up until it got to +6mm on the dro every time, or until you hit the probe as it ascends as you said.

Today i went to the machine the bit was 2 inches over the work.  I started to zero the bit the bit again went up instead of down.
i took the zero plate from the material and touch the bit.  it stopped moving.
I then moved the bit back down to about 2 inches and zero again. it worked  did this a few times in diff positions and it worked correct
correct in that once the bit touch the plate it retracted to the correct set position
this is the first time i have been having this problem having been running mach3 for the past 7 years
about 3 months ago my computer broke down  so this is a new installation  i suspect this is when the problem started
I am using a PMDX-122 break- out -board
running a shopbot cnc
I do not have in-depth  knowledge of mach 3    but it does what i need
Thanks again  Paul
Ger21 not sure what  a incremental mode is?   can this be it

Hi,
not sure whats going on.

Incremental vs Absolute....have a look at G90 and G91

In Absolute
G0 X10   will cause the machine to drive from its current location to X=10

In Incremental
G0 X10    will cause the machine to drive its X axis another 10 units from its current location.

As I posted earlier all touch off routines have a G31 probing command. Can you open the button macro and post the code?
I really want to see what parameters have been put into the G31 command.

Craig