Trouble with Z axis touch off being unreliable

I've had this issue where I home my axes, touch off on each axis, and start my CNC program and it seems like half the time the Z axis will be randomly off by an inch or so. It's always been off in the positive direction (cutting imaginary workpiece in mid air) so I just re-touch off the Z and start over and it's fine.

Tonight it rolled the dice and went negative into my vise. No real harm done (the end mill doesn't look so good) but it's really frustrating.

I don't know how to debug this or even how to detect that touchoff is wrong before I kick off the program. Help!

I'm using LinuxCNC/AXIS 2.5.0.

Might want to start by checking all your offsets on your machine and your program.
wiki.linuxcnc.org/cgi-bin/wiki.pl?CoordinateSystems
wiki.linuxcnc.org/cgi-bin/wiki.pl?Coordi...t_what_should_you_do
Are you using tool offsets?

I don't know how to debug this or even how to detect that touchoff is wrong before I kick off the program. Help!

I'm using LinuxCNC/AXIS 2.5.0.

After you touch off does the Z DRO read correctly?

Rick G

Rick G wrote: Are you using tool offsets?

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Thanks for the reply.

Looking at my tool table, it appears I have Z offsets specified for a variety of tools. I clearly did not know what I was doing at that time. My CAM program is picking arbitrary tool numbers (I haven't bothered to figure out how to control this) and I imagine that explains the somewhat unpredictable results. It doesn't explain why if I re-touch-off after noticing this has gone wrong that it works again.

If I just zero out all the Z offsets in my table will that solve my problem? I'm not doing multi-tool programs right now, I just want to be able to grab an end mill, home, touch off, and press go.

firstly be sure it's not missing steps , this is usually the z axis moving down all the time .
just to remove that from the equasion
clear the tooltable between jobs , it could be your initialisation is not correct at the top of your gcode or in your ini file
what cam program are you using ?

I'm not doing multi-tool programs right now, I just want to be able to grab an end mill, home, touch off, and press go.

The problem there is that if you are going to say mill a 1" square the path the machine must take if you use a 1/8" bit or a 3/8" bit are different.
In your CAM program you can select what size mill to use and it will create the correct path for that bit. If you change mill size the path will have to be altered. You can do this in the CAM program or linuxcnc.

If you are always going to touch off with the tool you are going to use then leave the Z offset in the tool table as zero.

It doesn't explain why if I re-touch-off after noticing this has gone wrong that it works again.

If you have z offsets in your table and you touch off with one tool and do not change tools all is well, However if you touch off with one tool and then your program selects another tool a new z offset will be in effect until you touch off again.

Rick G

cncbasher wrote: what cam program are you using ?

Fusion 360. It has an output processor for LinuxCNC. It doesn't insert tool change code correctly so I output separate programs for each tool and re-touch-off at the beginning of each program.

Rick G wrote: If you have z offsets in your table and you touch off with one tool and do not change tools all is well, However if you touch off with one tool and then your program selects another tool a new z offset will be in effect until you touch off again.

Got it. Will fix my tool table and see how it goes. Thanks guys!