At some point you need to do the math: the theoretical milling speed of your milling machine running a milling bit at high speed. And guess what we just mentioned the 2 variables to te equation:

- Spindle speed
- Milling bit

So you have to measure 2 things and then you’re able to calculate te IPM. Coming up: theory versus practice.

## Spindle speed

Fair enough most spindles will ave ample documentation describing te maximum RPM. Mine as well but I wasn’t all that convinced so I bought a tachometer.

Theory versus practice.

In theory the spindle would run at 13000 RPM maximum. Measurements show a maximum of 11400 RPM. So the spindle speed was exagurated by 15%.

Fl = R/min * Ft * n

So here it is:

Fl = 11400 * 0.3 * 1 = 3420 mm / minute = 57 mm /second

From a pcb milling forum (it off course depends on the type of bit used) : Advised speed on milling a pcb is 20 to 30 mm/second on 40000RPM. Proportional reduced to 8 mm/second for my 11400 RPM spindle.

From pcbgcode: As a rule of thumb, a safe feedrate for very small milling bits (under 20 mils diameter) is about 1% of the diameter per revolution.

Using the rule of thumb approach. The bit is 0.3 mm V-tip and the RPM is 11400:

0.3 * 0.01 * 11400 = 34 mm / minute = 0.5 mm /second

### Conclusion

“Your milage *will* vary”

The calculated speed seems very high. From other sources and depending on the bit used te feed rates are muc lower. The deafult on the pcb-gcode setup screen is 254 mm /minute (4mm /second). It appears that te spindle speed cannot be entered there so I assume it will be higher then my 11400 RPM.

First I’ll stick to the 4mm / second but I might have to reduce that, after giving it a go.