M3, M4, M5 Spindle & M7, M8, M9 Coolant Codes

Spindle and coolant are the workhorse M-codes — identical across every control in the table, which makes them the most portable lines in a program:

• M3 — spindle on, clockwise, at the modal S speed
• M4 — spindle on, counter-clockwise
• M5 — spindle stop
• M19 — oriented stop (lock at a known angle — tool changers, back-boring)
• M7 — mist coolant on
• M8 — flood coolant on
• M9 — all coolant off

T1 M6
S12000 M3        ; speed first, then direction — spindle spins up
M8               ; flood on
G0 X0 Y0 Z5
...              ; cutting
M9               ; coolant off
M5               ; spindle stop

S sets speed but starts nothing; M3/M4 start rotation but carry no speed of their own. The pair on one line (S12000 M3) is idiomatic. A new S while running changes speed on the fly — canned tapping cycles rely on the control managing exactly that.

#Timing: does the machine wait?

Most controls hold the program until the spindle reports at-speed before the next motion (or the post inserts a G4 dwell to the same effect). Hobby controls often don't — GRBL fires the PWM and moves on immediately, which is why GRBL posts add a dwell after M3. A tool entering material at 10% speed sounds exactly as bad as it is; if your first cuts chatter for half a second, check the post's spin-up handling.

#Common mistakes

• M3 with no S word anywhere. Depending on the control: alarm, zero RPM, or a stale speed from the last job. State S in every program.
• Wrong direction for the tool. A right-hand end mill under M4 rubs instead of cutting and work-hardens the material before it breaks. If chips aren't forming, direction is the 10-second check.
• Plunging while the spindle is stopped. Programs edited by hand sometimes lose the M3 line. A simulator that flags feed moves with the spindle off catches it before the machine does — this viewer's spindle telemetry shows exactly that state.
• Assuming coolant lines exist. M7 on a machine with no mist system is silently ignored (fine) or alarms (annoying), depending on setup. Posts have flags for which coolant outputs the machine actually has.
• Forgetting M9 before program end. Harmless to the part, hard on the shop floor — the pump runs until someone notices. M30 resets coolant on many controls, but not all; explicit M9 is good manners.