Z AXIS moving to correct height

So I searched through the forum and could not find an answer for this:
I am using GRBL 1.1F with a Open Builds controller and gantry.
My Z axis is mounted to the gantry and set to Home in an UP direction. This all works properly.
I am confused on how to tell the Z axis to move down to a Burn position ( as though I was burning a sheet of paper).
Do I need to know what this distance should be and load it somewhere?
I understand that material height should subtract from this position, or not lower as much.
I am missing something???

Some further info:
Z Homes in an up direction. GOOD
Z moves down to Material thickness (set to 50mm) GOOD
Z moves UP when I have it set to move 5mm per pass NOT GOOD.
It wont accept a negative number for moves per pass, how do I make it go lower per pass not higher??
Relative moves is OFF

Where is your Z zero location? It should be at the bottom, not the top of Z travel. That way, when you specify a 10mm material height, the Z raises 10 mm.

I have it set to home UP, this is to clear the bed so I can place the target.
I do not see where to specify a Z zero position. I thought the software assumed zero is the HOME position.

You can use ‘Relative Z moves’ and ‘Reverse Z Direction’ to flip the Z around.
The software does assume zero is the home position, and most lasers zero to the material.

Ok, have to do some rewiring of the motor and figure out how to mount the home switch on the down side. will make it hard to get thick material under the laser head though.

It occurs to me that you could offset the Z after homing, similar to how we offset the X & Y on CNC systems where they use negative workspace. You’d have to set $10=0 to report machine position in workspace coords, and use G10 L2 P1 Z<##> where <##> is a numeric amount to offset the Z by, typically the travel length of the axis as a negative number. So if your Z was 50mm long, you’d use:

G10 L2 P1 Z-50

…after homing, and that would put Z zero at the bottom of travel.

That worked once., It would move down 100mm
I have it set for 3 passes, lower 5mm per pass.

It dropped down 100 mm pass one
Pass 2 it raised 5mm
Pass 3 it raised another 5mm
End of the job Z moved up and down and stopped at the original 100mm down?

Not sure why there is no Z position at the end of the job like there is for for X&Y?
Not sure why LB is not set up to deal with a home to top and not bottom??

LightBurn was originally written for DSP controllers, not GCode systems, and they place next to no emphasis on Z control during a job. Several don’t allow Z control at all.

GCode systems and Z were added later, and even relatively few GCode based lasers have Z axis control. That’s why.

So I reset everything to home in a DOWN position.
Set material height to 10mm, 3 passes with a 5mm change in height per pass.
It homes, moves Z up 10mm, and continues to move up with each pass.
May have to look for a different controller.

The Emblaser 2 is a gcode machine that ships with LightBurn. It homes to a limit switch at Z=70mm (up), with zero at the bed of the machine. If z-per-pass moved the head up, as you say, much of the functionality required for the Emblaser would not work, and I know it does work, so there’s something weird going on with your setup or settings.

I think I have it solved… Here are the settings I ended up with. .
It now Homes UP
Moves UP for material height.
Moves Down for mm per pass.
Just need to get my steps/mm set correctly (will figure that out )
Is there a command that will issue at the start of each job to move the Z axis Down a pre set level??
My settings:

Setting a material height (mm) value will do that, if you have Z enabled, and relative Z disabled.

Right now, setting material height will raise the Z axis.
I need it to lower a specific amount to the focus point, then the Material heights and mm per pass will be correct. I know it is a G10 Zxx setting, still reading.

If your Z0 is at the bed, instead of at the top, G0 Z10 (move to a height of 10mm) should move it to 10mm above the bed. That’s all the material height value does.

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