Laser cutting 1% oversize


#1

So, when I ask my cutter to cut a 400m square, it comes out 404mm wide.

The front-panel display suggests the laser thinks it is cutting at 400mm, so I think this is a settings problem in the Ruida controller.

In Lightburn, if I go to Edit / Machine Settings and expand ‘X axis settings’, I can see
Step length (um) : 8.965

And similarly for the Y axis I see
Step length (um): 4.476

I presume that’s the source of the problem, and I should set those to a value that is cunningly calculated from

  • Steps per revolution
  • Microstepping factor
  • Pulley tooth count
  • Belt tooth spacing

Before I go find all those facts, I’d be interested to know if there were conventional values for these step lengths that would save me the trouble.

Ta!
R.


#2

Just done some measuring, spreadsheet here: https://docs.google.com/spreadsheets/d/1YRT-U1aTXw5LR80Vh7ZdEXB9ObUfCnn3PZG7hBUiQaQ/edit#gid=0

I presume it just inherited some old, bad settings from someone calibrating it empirically. Will try the new settings and report back.


#3

… That fixed it. microns/step is now 9 for x and 4.5 for y, and things come out the right size.

GOK how the bad number got there, but I blame Previous Owner.

Anyhow, working it out from first principles wasn’t too hard. I had to

  • measure 10 tooth-spacings on the belt, which was a nice round 30mm, so it is 3mm belt.
  • Read the DIP switch on the stepper drivers and look up the steps/rev in the table printed on the driver.
  • Count the teeth on a belt pulley (mark one with Sharpie, then pull on the head to rotate it, counting as I went.

R.


#4

Hello Richard

That’s a good bit of detective work. I spoke with Oz at lightburn who made the alternative suggestion to fix this in software (more of a workaround for the less mechanically minded) who suggested checking the vendor settings in Lightburn via Edit/Machine Settings/Read (then checking and fudging the settings) and then Write.

Another article I saw that might help others in this situation was this one on the Lightburn forum which runs through checking Absolute Co-ordinates

Not the fix that was needed but perhaps helpful for those looking at this thread but with different issues.

Dominic