Now that I have router speed control, it’s time to start thinking about the actual cutting speeds I should be using, rather than just running everything full-out.
I searched around, and this site had a pretty good rule-of-thumb for calculating chip load.
Essentially, fill out the following formula:
Feedrate = Diameter X Chipload X Flutes X RPM
Where:
feedrate is in inches per minute (IPM),
diameter is the diameter of the endmill in inches,
chipload is the constant 0.02 (for a 2% chip load per tooth),
flutes is the number of flutes on the endmill, and
RPM is the rotational speed of the spindle.
As diameter, chipload, and flutes are fixed per endmill, this can be collapsed into a single constant.
With my machine, I can change RPM and feedrate to make the rest of the equation work, but feedrate seems to be my biggest hurdle; at the moment, I don’t set the feedrate above 100ipm, although the machine could probably handle anything up to about 800.
So, RPM ends up being the thing I’m calculating.
At the moment, I’m running either a 2, 3, or 4 flute 1/4″ endmill, so the constants are:
Endmill constant (Em) = 0.25 * 0.02 * 2 = 0.01 (for 2 flutes), 0.015 (3 flutes), or 0.02 (for 4 flutes).
I then have Em * RPM = feedrate. But feedrate will pretty much be 100ipm, so I move terms around:
feedrate / Em = RPM
and I come up with target RPMs of:
10,000RPM for a 2-flute
6,600RPM for a 3-flute
5,000RPM for a 4-flute
to reach 100ipm feedrate.
If I want to run the bit a little faster, I can up the feedrate and modify the RPM accordingly.
Also, in Mach3, I need to set the acceleration to 20 inches/sec/sec, which should keep the bit running nice and cool.
I made a test cut program to test various RPM and feedrates. That should help me home in on a good solution without killing an endmill in the process.