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%.

Theoretical Feed rate

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.