Laser cutting 1% oversize

jarkman · February 21, 2019, 10:20am

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.

jarkman · February 21, 2019, 2:24pm

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.

jarkman · February 21, 2019, 6:12pm

… 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.

Smokey · February 22, 2019, 8:35am

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

github.com

LightBurnSoftware/Documentation/blob/master/CoordinatesOrigin.md

[Return to main page](README.md)

----

# Coordinates and Job Origin

There are a couple different ways to tell LightBurn how to cut the project within the work area of your machine.  You choose them in the "Start From" box on the Laser tab:

![Start From](/img/CoordinatesStartFrom.png)

## Absolute Coordinates

**Absolute Coordinates** is the simplest - The page grid you see in the main editing window represents your machine's work area.  Anything you place in that area will be cut in the corresponding place on your machine.  Users with small lasers like the K40 will likely find this the simplest and most intuitive option.

In the image below, the two circles placed in the middle of the work area will be cut in the middle of the machine work area.  The green square in the lower-left of the image represents the Job Origin, and the red square in the same place shows the Machine Origin.  In "Absolute Coordinates" these are always in the same place.

![Coordinates Absolute Position](/img/CoordinatesAbsolute.png)

## Current Position

This file has been truncated. show original

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

Dominic