A small arduino sketch to operate a servo from the Air/Spindle pin from standard GRBL.
It includes a very small serial console to set the up/down position of the servo and saves those values to EEPROM.
If you connect to the sketch whilst running, you can issue commands like
u=20
d=150
?
To set the extents and query current status.
Once the hardware is set up, you will need to tweak Lightburn. Because it takes time to raise/lower the servo, you will need to add a delay after the air/spindle is switched on.
This happens in the advanced tab; setting the start and end times will allow the servo to move, before the machine starts moving. I found somewhere between 250 and 500 ms were about right, but it'll depend on the speed of your servo and the distance it needs to move.
I set my machine up as a GRBL-M3 machine. This results in Lightburn generating M3 (Spindle Start) and M5 (Spindle Stop) GCode instructions at the correct places:
; LightBurn 1.1.03
; GRBL-M3 (1.1e or earlier) device profile, current position
; Bounds: X-142 Y0 to X0 Y1
G00 G17 G40 G21 G54
G91
; Cut @ 50 mm/sec, 50% power
M8
M5
G0 X0Y1
; Layer Border
M3
G1 X-23.417Y-0.165S127.5F3000
M5
G0 X-0.5
M3
G1 X-46.833Y-0.333
M5
G0 X-0.5
M3
G1 X-46.833Y-0.334
M5
G0 X-0.5
M3
G1 X-23.417Y-0.168
M5
M9
G1 S0
; return to starting pos
G0 X142Y0
M2
With this, I was able to connect the two grounds of the arduinos, then connect pin 13 (INPUT_PIN) to the spindle pin of the arduino running GRBL.
It was very useful when testing this out to initially connect an LED to the spindle pin, so that I could see what was happening.
Alternatively, if you have a board like the MKS-DLC32, it looks like the TTL-Output pin may be the right choice, although you'll want to ensure there's a laser output level that holds the pin fully on, not outputting a PWM signal, which would cause the Servo Controller to constantly try to shift up and down!
It looks like in that situation, setting the $30 parameter in GRBL to a value that Lightburn can send will set the PWM output to 100%. e.g. if $30 is 1000, then ou want GRBL sending M3 S1000 to turn the pin fully on.
The other sneaky (but unverified) way to do it would be to tell Lightburn that your machine has a Z axis. That will cause Lightburn to send G01 Z[some number] commands on the start of each "cut" - you could likely disconnect the Z stepper driver and use the direction pin for the Z axis as a signal. That approach would need verifying, as I don't know whether the pin is left on/off after a move has happened.