This collection of GRBL settings gets the MPCNC hardware up and running:
| $$ | |
| $0=10 | |
| $1=255 | |
| $2=0 | |
| $3=2 | |
| $4=0 | |
| $5=0 | |
| $6=0 | |
| $10=1 | |
| $11=0.010 | |
| $12=0.002 | |
| $13=0 | |
| $20=0 | |
| $21=0 | |
| $22=1 | |
| $23=7 | |
| $24=500.000 | |
| $25=1000.000 | |
| $26=250 | |
| $27=2.000 | |
| $30=1000 | |
| $31=0 | |
| $32=0 | |
| $100=100.000 | |
| $101=100.000 | |
| $102=400.000 | |
| $110=6000.000 | |
| $111=6000.000 | |
| $112=3000.000 | |
| $120=1000.000 | |
| $121=1000.000 | |
| $122=1000.000 | |
| $130=650.000 | |
| $131=475.000 | |
| $132=100.000 | |
| ok |
Conveniently, the $$ command (in the first line) produces output in exactly the format it will accept as input, so just pour the captured file into GRBL’s snout. I used ascii-xfr with a 250 ms line delay:
ascii-xfr -s -v -l 250 MPCNC-GRBL.cfg > /dev/ttyACM0
Now, to be fair, the MPCNC hasn’t yet done any useful work, but it moves.
Notes:
Setting $22=1 requires home switches to be installed and working, with $23=7 putting them on the negative end of the axes, which may not work well in practice. In particular, having the Z axis homing downward is just plain dumb.
The step/mm values in $10[012] require 1/16 microstepping with 2 mm belts on 16 tooth motor pulleys. The MPCNC’s Marlin config uses 1/32 microstepping, which doubles the step frequencies and (IMO) doesn’t provide any tangible benefit.
The speeds in $11[012]=6000 seem aggressive, although they actually work so far.
The accelerations in $12[012] may push the motors too hard with anything installed in the toolholder.
The travel limits in $13[012] depend on the rail lengths you used.
The Smell of Molten Projects in the Morning 
As Galileo said: “E pur si muove”!
Along those lines, I’ll put in another plug for Galileo’s Daughter by Dava Sobel.