coordinate system

when I use the touch off button to set a coordinate for G54, I enter a positive value and then when I run the program I get an error message that the it exceeds the negative minimum limit. If I enter a negative value for G54 and then run the program the machine moves in the positive direction to the specified coordinate and then starts the program. I am confused as to why. Can someone explain why? Did I not set some kind of setting some where? the machine seems to run fine otherwise so I will continue to run with it this way, I would like to understand what is happening.

It depends on how your axis is set up. Basically if you are in machine coordinates you should input the same value shown on the DRO as your offset including the sign.

For example if your Z axis origin or zero point is at the top of the travel. and your vise top or workpiece is 6units below the origin then the offset will be -6units.

If your Z axis origin is at the bottom of the travel and your workpiece is 6 units above the origin then then the offset would be +6units.

This is assuming the + Z is in the up direction.

Same would go for any other axis.

So if you home the machine and stay in machine coordinates. Move the tool to where you want the origin for G54. Look at your DRO in machine coordinates or absolute coordinates and put the values that are shown in the DRO into the offsets and you should have G54 set up correctly.

This may help...
wiki.linuxcnc.org/cgi-bin/wiki.pl?CoordinateSystems+

When using a mill I normally start by drawing the part in CAD with the reference being the lower left corner of my work as X0, Y0 and the top of the material as Z0.
In this case on the mill I jog to the lower left corner of the work piece and touch off as X0, Y0 and the top of the work as Z0.

Rick G