Strange behavior with circles

LightBurn Software Questions
1

I probably have a setting incorrect but when cutting a circle the laser slows speeds up close to 0 degrees and 180.
Well the file will have to be renamed from .txt to .mp4 to see the video . circle.txt (825.6 KB)

2

There are two reasons the software might do this:

  • You’re in rotary mode and the Y axis is your A axis (unlikely)
  • You are using 8-bit GRBL and it simply can’t keep up with the amount of GCode

It’s most likely #2, which you can improve somewhat by increasing the ‘Curve Tolerance’ value in the settings, which controls how many segments LightBurn emits when converting circles into line segments. The default is 0.05mm. If you increase that to 0.1mm, it will emit half as much GCode for arcs and smooth shapes, but they won’t be quite as precise.

3

I don’t think I’m using 8 bit grbl. Its grbl-lpc running on a mks sbase which is 32bit I changed Curve Tolerance to 0.1 and it does the same thing. Strange

4

Do you have rotary mode enabled? Can you save the GCode out for this circle and post it here so we can have a look?

5

No rotary mode is not enables. it does it with rectangles also. I attached a better video file and the gcode.
video : https://www.dropbox.com/s/z940ts93re0wmv4/WIN_20200808_20_34_59_Pro.zip?dl=0k40_pnl.lbrn (28.1 KB) circle.txt (48.3 KB)
circle.tct is gcode file just rename.

lightburn 0.9.16

Thank you

6

Ok I see what is going on but I do not know why.

I put an o-scope on the fire pin and with it working correctly the signal from low to high to low is is 38.4us and from the start of one pulse to the start of another is 79us

when not working correctly low-high-low is 54.4us and pulse to pulse is 64.4 us

in this case $33 = 15000 but dose same thing (different times of course) if $33=5000

I’m trying to get screen shots of the o-scope. Soon as I do I’ll post them

Thank you
Doug

7

There is exactly one F command (feed rate) in that entire GCode file, so it’s not LightBurn doing this.

8

ok thank you. that points me in the right direction. I thought as much since I’m the only one with this problem. This should be fun to track down. Time to dig through the firmware source.

Thank you again

9

I have a Ruida controller and noticed it doing something similar if I set above a certain amount of backlash compensation enabled. Could it be that ?

10

It could be something like. I have some different controllers I can hook up. I have an extra Duet 2 wifi but I don’t think it will work with Lightburn. It’s best to start simple so I’ll put a cnc shield on a uno with grbl and go from there. I’m pretty sure its a firmware issue in the grbl-lpc running on an mks sbase. I just haven’t had a lot of time yet to track it down .
Thank you
Doug

11

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