Z Moves Not As Expected

I’m a little confused on how this works. I have a machine with a z axis controlled by the Ruida controller. I have it set to home the z axis. When the machine is reset it homes the z axis by bringingg it all the way up until it hits the limit switch and then sets that as Z zero. So far so good.

Now I go to device settings and enable z axis control. According to the documentation I need to be careful to set a material thickness becuase if the thickness is set to zero it might bring the table up and crash into the head. Makes sense. So to test it out I created a layer, set the z axis to 10, and clicked start (z axis currently all the way up at 10).

As I expected it dropped the table to 10mm and ran the program. If I hit start again to run the program again it drops the table ANOTHER 10 mm down to 20mm and runs it. Do it again, 30mm, etc. I would expect it to take the table to whatever the material thickness is set to, not incrementally drop the table by the material thickness every time I start the job.

Am I misunderstanding how this works?

To clarify, when I say I press start I mean I press the start button in Lightburn, not on the control panel. If I send the file to the machine and run it from there it will indeed always move the table to the material thickness and will not increment it with each run of the job.

Do you have ‘Relative Z moves only’ set in the device settings?

No, I do not. If I check the relative z moves only then the z doesn’t move at all as I don’t have a z offset set for the layer. It’s a single layer, no z offset in the layer, material thickness of 10. Send it to the machine and it increments/increases/moves the table down by the material thickness every time I tell it to start the job. And again, I’m using z homing. So at startup the table moves up until it hits the limit switch and then sets the table location to zero on the Ruida. At least it is moving the table DOWN by the material thickness. But it is definitely not setting the table to the absolute value of the material thickness.

What is also interesting is that it obviously knows that a positive or increasing value is used to move the table down. However if I set a z offset for a layer I have to make the offset negative to move the table down.

If you zero to the top of the material then LightBurn can’t set the material height, because that’s now where zero is. If you zero to the top of the bed, then LightBurn can set the material height. Most people with DSP systems don’t use the material height, but just focus on the material and use relative moves from there - it’s less to worry about.

The material height stuff is most useful for lasers that have a fixed bed, movable laser head, and no easy manual way to set focus.

Hmm… not sure I’m following you. I’m not zeroing to the top of the material. I’m zeroing to the top of the bed. I don’t have an autofocus probe. I’m zeroing to the top of the table travel with a limit switch. I turn on the machine or reset z which causes the table to move all the way up and sets 0 when it hits the limit switch. So zero is set each time the machine is turned on just as it is set for x and y.

So I’ve turned on the machine, z zero is set to the limit switch (all the way at the top), I have a file in lightburn with the material thickness set to 10, and I click start. The machine then moves the bed down 10mm and starts the job. Fine. If I click start again it moves the bed down another 10mm (to 20mm) and runs the job again. And so on. From what i’ve read in the documentation the bed should move to the absolute value of the material thickness. Am I reading it incorrectly?

If I don’t click start in lightburn but instead send the file to the Ruida and then run it from there, it will indeed move the bed to z=10 anytime I start the job. If the table is at 0 it moves it to 10 and starts. If the table is at 100 it moves it to 10 and starts. Only when I’m clicking start in lightburn does it incrementally move the table by the material thickness each time I start the job.

My use case for this is that I know that my focal distance from the nozzle to the table is z=4.2. If I’m going to run a project on a piece of material that’s 10mm I can simply set the material thickness to 14.2 prior to running the job. I’m just wondering if it’s working as intended. If it is, no problem.

The other thing that would work nicely is if there was some way to move the z axis to a given position as there is with x and y. I could easily just go to the move section and tell it to move z to 14.2 before starting the job. As it is I have to read where z is, figure out the move amount I need to make, put that in the distance to move box, then tell it to move. Just looking for a way to send the z to an absolute value from within LB.

I’m also curious if moving the table down should require a negative offset for the layer. Seems counter intuitive. My z 0 is the table at its highest point and triggering the limit switch. Any positive offset should move the table down but I’m having to enter a negative offset.

Z-per-pass uses a positive number to go down (into) the material, so I made the Z offset use the same sign. I’ve got “In” and “Out” displayed beside the numbers so you know which way the value goes.

The way you’re suggesting the Z should work when you run the job is correct - I’ll have to check to see if I’m doing something wrong with the Z handling on Ruida.

So you’re saying if I set the z increment on a layer to a positive number the bed should move down assuming my zero is the bed all the way up/to the top? I see the In/Out there but that seems a little confusing. Up/down might make more sense.

What I’m seeing however is the opposite. If I set the z increment for a layer to a positive number the bed moves up, not down. I would expect it to move down.

I can make a video of this if you think it will help.

And I’m not talking about the z per pass, I’m talking about the z increment for a layer so that the z moves down (or up) for that layer. I haven’t tried the z per pass but obviously the use case for that is cutting thick material where you want to incrementally move down into the cut to bring the focus point down into the matieral.

My use case is a focusing test with the same pattern repeated at different z heights. For instance, I think that my best focus will be at z=7. I use several layers with z increments at say -2, -1, 0, +1, +2. When I do this the -2 layer is bringing the bed down while the +2 layer is bringing the bed up.

Why not just use Tools > Focus Test?

And yes, a negative Z-offset should pull the laser up / out of the material, and a positive offset should push down into the material.

Well, because I didn’t realize that existed until about a day ago. :smiley: I did try the focus test functionality and it works great. The only suggestion I have for improvement for the focus test is to use a single line font for the numbers.

My original issue/question still remains however. If I set the z increment for a layer it moves the bed up if I use a positive number for the increment. If that’s the intended functionality that’s fine but it just seems counter-intuitive.

And I also still have the issue with the z being moved incrementally the amount of the material height each time I start the job from the software.

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