Reducing GRBL minimum feed rate to 5mm/min

I understand that GRBL has a minimum feed rate (1/8 microstepping) of around 45 mm/min. For some laser work I have planned, I need a feed rate of 5 mm/min. I’ve tried editing config.h to comment out the adaptive smoothing (AMASS) , but that seems to affect my laser at low speeds (e.g., laser appears to pulse). I’ve also tried to increase microstepping from 1/8 to 1/16, but that only cut the minimum rate to around 20 mm/min. Is there some other way to attain a feed rate of 5 mm/min using GRBL to control a laser? Thanks in advance for any help!

That is a much slower rate than I have seen before. My guess is that the problem is that a single step has to move at least some distance, so even if you step less often, it is really more like 20mm/s and then zero.

This isn’t answering your question, but can you just do multiple passes instead of going so slow?

More directly answering your question, you probably need to get a gear ratio in there. If 10 turns of the motor ended up as 1 turn of the pulley, you would be there.

Thanks for the input! Yes…I can make multiple passes and may end up doing that. It’s just not my preference if there’s a viable alternative. I’ve thought about mechanical solutions like gear ratios, but I’d hoped for an electronic solution of some sort.

What about combining this with turning off the laser mode? Laser mode does something to adjust the intensity based on the actual speed l. Maybe that is turning it off and on.

But if I turn off laser mode ($32=0) won’t that prevent the laser from running at all? I’ll try it to see.

Laser mode in gentle is a set of adjustments to PWM controlled spindle speed behavior. As I understand things, laser mode enabled ($32=1) does 3 things:

  • Assumes intensity changes are instantaneous (laser disabled leaves a short pause for spindle rpm to stabilize when starting/stopping/changing tool rpm)
  • Lowers laser intensity automatically when tool velocity slows (as when decelerating at corners) to prevent burning
  • Turns laser off during G0 travel moves

I don’t think losing that final function would be worth the possible slower speed.