But, as I don't have home switches and still want the machine-coordinates to be accurate, I have to zero the machine manually

This has nothing to do with accuracy. What's the difference if the machine coordinate is 0 or 5.68754?? Your using G54 coordinates. Zero them, and your good to go.

If I zero the G54-coordinates I loose the reference for Z. I need the tool-tip of the G54 Z to be exactly the same after tool-change.

The machine coords are just that coords for the machine to use to keep track of where it is at in 3d space. They are not there for you to use on a daily basis. and that is WHY you cannot zero them as you think you should(;-)

The user coorrds are what you use to do work.  Set up your machine and touch off Z zero it.   Then whatever you do after that tool wise that you simply change tool ,touch off rezero the z and you are done machine away.

You don't base tool height offsets from Machine coords Z0.000 you use the Coords base you are in at the time for example G54 or G56 or G55.

YOu might want to consider getting a good primer on cnc and gcode to get a good understanding about offsets and how they are used in CNC. It will save you a lot of hair pulling

Just a thought, (;-) TP

I know very well this is not how it's supposed to be done. The problem that I see is that I can not always touch off the second tool on the work piece as the surface might all have been machined off. But I guess the best would be to manually, or preferably by a script, be able to touch off each tool on the table or another fixed surface then have the G54 coordinate system update so that its Z=0 would be where the work piece surface would be.

You could use a planer gauge  as a Z =0 reference such that if you machined the original reference you still have a
reference to go back to. Actualy you can use anything as a reference since your manualy adjusting. Everynody works a
little differently.

RICH

But then the code have to be referenced to this gauge level also. I really like to use Z=0 in the CAM as the top of the work-piece. Then, no matter what fixture I use, the code will always be the same.

I think I should look into the offset functions a little more. Perhaps it can be automated so I touch off the first tool on the table, set the offset to zero. Then touch off on the work-piece, zeroing the G54. Then, after tool-change I can just touch off on the table and set the offset to the difference in machine Z. This would also work if I used some kind of pressure actuated switch for semi-automatic operation.

The way I do with manual tool changes is touch off to the TOM(top of material) then set z zero. Now touch off on another spot on either the actual fixture if used OR the table top or some other permant spot. Then write down the number.

Then at next tool change change tool and touch off on the reference point and set the Z to the saved number.

BUT to each his own(;-) what ever works for you.

(;-) TP

That sounds like a good way to do it. Now I just need to make a script so I don't have to manually enter any numbers. Should be possible. Anyway, thanks for your help.

After some research of the vb-scripting, I put together something that actually worked very well. So now I have my machine coordinates set as they are, permanently. Basically, I added an unused M-code in the post-processor that gets run at the very start of the program. This runs a script in Mach3 that sets a gauge-level variable from the machine Z. Then, I mapped a button to another script that sets the current tool-offset to the difference in current machine Z and the previously stored gauge-level.

So, now all I do is move first tool to stock zero and zero fixture coordinates. Then I move to some table-related constant, either the surface or something else and start the program. Upon each tool change, fit the new tool, move it to the gauge-surface and press my custom button and continue the program.

So, this is effectively what BR549 suggested, but without having to write anything anywhere.

Thanks for your help guys!