Bi-directional ghosting

[bigstoo (Stuart Mayor)]

Hi all.
I wonder if you could shed some light on a niggle I’m experiencing.
Firstly, a little background. I started with a cheap Chinese laser. A cheap laser that was virtually useless until I bought Lightburn. So thanks to the developers for that!

Over the last few years, I’ve modified my laser, adding things like end-stops, to make it easier to use.

I’ve just finished completely rebuilding the laser, as the bearings had started grinding into the 2040 aluminium, causing dodgy artefacts in the burn when the gantry moved over well-worn parts of the frame.

Anyway, having rebuilt the rig, I’m getting the ghosting/banding below when it does di-directional burning.

(Obvoiusly, this is massively zoomed in. The text is about 5mm tall)

I’ve checked all the belts (tight - but not too tight), pinions, joints, nuts, bots, etc. Everything is nice and tight. No play anywhere. So, having read topics elsewhere on this forum, I thought I would try tweaking the max speed and acceleration settings.

However, my acceleration settings were already pretty low (I can’t remember exactly what it was set to). Setting it any lower didn’t make any difference. So, on a punt, I thought I would try increasing it to something daft Strangely, that seems to have helped. I’ve set acceleration to (IIRC) 1000m/s/s and now get the following

Still not perfect, but much better.
Uni-directional burning is absolutely pin sharp - with either acceleration setting - but obviously I’d like bi-directional working as it should.

Does anyone have any thoughts on what I could try next? Increasing the acceleration seems a bit counter-intuitive. I’m using a diode laser, by the way.
It’s as though the movement of the gantry and the power of the laser aren’t quite in sync. And dramatically increasing the acceleration reduces that gap, simply because everything is moving much faster.

Thanks in advance.
Stuart

[JimNM (James)]

Give OFFSET fill a try - I bet you will get even better results. That wont fix the problem, but it will look nicer

[gilaraujo (Gil Araújo)]

If i had to give a guess estimate
And considering your issue disappears on uni-directional

99% sure your X belt needs some serious tightening or, your X axis eccentric nuts /assembly needs tightening

Is all slack of movement , and the fact is consistent means to me is belts

[ednisley]

@bigstoo

tweaking the max speed and acceleration settings

Do this:

Scanning Offset Adjustment - LightBurn Software Documentation

That adjustment compensates for the delay between the hardware motion and the laser output, which is what causes the offset between successive passes.

If you use single-direction scanning, the entire pattern will look good, but you’ll find it is offset relative to adjacent vector cuts.

You should also tune the controller’s speed & acceleration settings to match your hardware, but that is a separate task.

[bigstoo (Stuart Mayor)]

Blockquote
If i had to give a guess estimate
And considering your issue disappears on uni-directional
99% sure your X belt needs some serious tightening or, your X axis eccentric nuts/assembly needs tightening
Is all slack of movement , and the fact is consistent means to me is belts

Yes, that was also my thought.
However, the x belt is brand new and tight. The whole gantry assembly is rock solid and nice and tight.
But yes, there’s clearly some backlash in the x-axis somewhere. I wonder if it’s in the stepper motor itself?

Blockquote
Do this:

Thanks. Yes, I saw that link when searching on the forum. I thought that applied only to CO2 lasers?
I’ll do some experimentation and see how I get on.

Thanks once again to you both.

[gilaraujo (Gil Araújo)]

@bigstoo

Yes, that was also my thought.
However, the x belt is brand new and tight. The whole gantry assembly is rock solid and nice and tight.
But yes, there’s clearly some backlash in the x-axis somewhere. I wonder if it’s in the stepper motor itself?

3 sources of backlas as far as i know
a) belt itself, but if you say is tight then is tight, could send a image of it for double checking
b) Sproket to motor slack. When the coupling screws of the spoket are a bit loose so theres some slippage
c) The axis itself has slack. So wheels vs frame are not spot on. Easy to do
Hold your module, apply rotation. left right, up down. in and out. do you feel any slack between wheels and the frame?

Without seeing the machine, and JUST by the results and descriptions i woul still say 99% belt issue. is just too perfect to be other type of random mechanical slack

[bigstoo (Stuart Mayor)]

Thank you for your help guys.
I’ve tweaked the Scanning Offset Adjustment, and it’s now working perfectly!

Strangely, an offset of 0.15mm appears to be working regardless of the speed I set. I suspect there is a slight variation but a combination of my eyesight, the lighting and the magnifying glass I’m using isn’t going to get me much closer.

And since I mostly burn wood, what’s a few hundredths of a millimetre between friends?

Thinking about it, a fixed offset would suggest there’s 0.15mm of backlash somewhere in the system, as Gil said, rather than it being an electical/electronic delay between the laser and steppers.