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Work Flow    Gcode 

Downloads Windows and MAC
With Phlatboyz plugin
Camotics Emulator
Upload by SD Card
Universal G Code Sender

Github CNC
Spindle Speed Controller

For a spindle, M3 is clockwise rotation, M4 is CCW, and M5 is spindle off.

Both M3 and M4 can be used with an S### command to represent spindle RPM.
The S parameter can be on the same line, or if M3 or M4 are used alone they will both revert to the last S value.

Start here. Grbl follows the LinuxCNC standard very closely, except in laser mode.
Then, study the laser mode wiki for how its different. For the S parameter, your answer is here.
Spindle speed control GRBL follows LinuxCNC protocols

These units can be bridged to drive external stepper drivers .
MKS TFT32 interface software link 
MKS TFT35 works with MKS  Robin and  MKS Gen-L boards

MKS DLC Board:  DLC32 to right   There is a SD card slot plus connectctor for an sd cable on the Display
The MKS DLC uses the8 pin  MKS TFT24 Board with CNC firmware    The MKS DLC32     uses the 10 pin  MKS TFT32 board and  CNC firmware.
The DLC32 is said to be able to be updated via SD card.
For heavier duty cnc one can  use external stepper drivers. The plug in types are good for 1.2 to 4A with proper cooling.
External drivers which can drive larger stepper motors  can plug into the motor sockets using adapter sockets to feed the signals to the  appropriate pins.