M3 means "start the spindle at a specific speed".
A spindle is a controlled motor that turns a tool ( endmill ) at a given speed to allow it to remove material from the workpiece.
The M3 command must be accompanied by a S parameter which sets the speed of rotation.
M3's evil twin is the M5 command which turns the spindle off ( it is equivalent to M3 S0 ).
The command is used as such :
Which means : start the spindle and make it turn at 1000 rotations per minute.
|S||The spindle must turn at this speed in rotations per minute||M3 S1000|
The spindle functionality of Smoothie is configured via the Spindle module
These resources are used as references for Gcode :