Strange Behavior With Laser Mode Enabled

Hi All,

I found this community while researching various topics related to my recent purchase of a cheap laser CNC machine running GRBL and thought it might be a good place to post about some strange behavior I’m seeing.

So for my setup I originally had it set to use $32=0 (disabling laser mode) which was the default. The side effect of this is when running the gcode generated by LaserWeb I end up with faint lines on the travel moves where it looks like the laser remains on.

I tried setting $32=1 to enable laser mode, and this removed the travel lines, however it had 2 unexpected additional effects on my test run (with laserweb laser fill path)

  1. The overall engraving appears less dark/black
  2. The overall engraving is deeper

These 2 additional effects seem contradictory so I’m hoping someone else has seen this and can shed some light on whats happening here :).

Want to post A/B pictures, including as close up as possible, of the differences? :slight_smile:

Good idea :slight_smile:

I’ve attached some images to try to best capture both the depth difference and the darkness difference.

The depth is a bit hard to show in the pictures, but its most noticeable at the boundary of the nose. I also measured the depth with a caliper and it was .4mm with laser mode off and .6mm with laser mode on.

To me it’s interesting that laser mode cut deeper despite using the exact same gcode, while at the same time also produced less black lines… If it cut less deep and was less black I could explain as perhaps laser mode was applying less power to the laser, but to me deeper cuts suggests more power while less black I would think represents less power so I’m puzzled…

What type of laser is this? Diode or Tube? Can you show us the machine?

Did you just activated $32=1 or also changed to use M4 instead of M3?
M4 compensates power on accelleration & decelleration, so that the line ends don’t get burned too much.

The depth and darkness of the engraving is dependant on the focus, material, power, feed, moisture, temp and other parameters.

Was both tests focussed exactly the same way on the same material? Was maximum power and feed rate the same?

By the way: I think this has nothing to do with LaserWeb, but with GRBL itself. @mcdanlj Should we move this post to the grbl category?

The machine itself is a 6550:

And the laser is a diode laser:

I didn’t change anything except setting $32=1, the gcode, focus, wood, etc are all the same between runs and the results are repeatable (for example if I set laser mode back to 0 I see the engraving is darker but also less deep).

I am going to try to find some time to do some more experiments this weekend to see if I can find out more.

Thanks for your response :slight_smile:

Have you asked them to provide you the source code to the grbl build on that board?

I’m wondering whether it has different PWM curves for laser mode on and off…

That’s an interesting idea! I’ll see if I can get my hands on it but I don’t have high hopes. At the end of the day it still works well and maybe I just need to tweak the speeds after setting laser mode=1. Its just interesting the difference between the two modes, and thought I’d see what others had to say about it :slight_smile:

Well, by the license terms if they sold you hardware with it installed, they are obligated by law to give you the source code as well. And China is well known for stringent compliance with international intellectual property law… :roll_eyes: or, maybe not…

A few years ago, before I learned that Tronxy were famous for completely ignoring GPL license terms, I bought a Tronxy 3d printer. I bought it through GearBest, from whom I requested and from whom I received source code for the firmware. So I guess it is possible that your seller will at least try to comply with the GPL.

Haha, yes they are very well known for their compliance with international intellectual property laws…