So I'm milling a lot of aluminum and making a horrible mess. Kept adjusting the feed rates, spindle RPMs, and all that trying to get this silly thing to make some cuts and feeling like I was getting slower and slower and the results just weren't looking right. So finally I remembered that I don't really know much about machining and punted from the internet to see what kinds of settings other people were using.
Long story short it seems hard to find just a simple table that would get a person, you know, like in the vague area of what they should be doing. But eventually I did find some equations.
The results? I was off by about two orders of magnitude on my last attempt. And killing my bits prematurely. Here's the basic idea:
RPMs = feed rate / (number of flutes * chip size)
that's pretty rough and there are lots of variables to take in to consideration (coolant, how much block am I actually cutting, is it a pocket, bla bla bla) but this seems like a good starting place. Re-arrange that for whatever variable you have easy control over.
The idea is simple actually: your chip size has to be bigger than the roundness of your bit (which really does always have a little bit of roundness and usually a chamfer leading to the cutting surface). Otherwise you are just scrubbing the outside of the bit against the aluminum until it wears down or melts/wears away. This is called burnishing. You might as well have loaded the machine up with a slightly roughened carbide rod instead of a proper endmill. A good size for this is 0.004 inches and up on a 1/4" bit. So I tossed together this little chart with the idea that I would be taking out 0.005 inch chips. ipm = inches per minute and all the numbers in white are RPMs for the spindle:
| flutes | |||
ipm
|
2
|
3
|
4
|
0.25
|
25
|
17
|
12.5
|
0.5
|
50
|
33
|
25
|
1
|
100
|
67
|
50
|
2
|
200
|
133
|
100
|
3
|
300
|
200
|
150
|
4
|
400
|
266
|
200
|
5
|
500
|
333
|
250
|
10
|
1000
|
666
|
500
|
15
|
1500
|
1000
|
750
|
20
|
2000
|
1333
|
1000
|
50
|
5000
|
3333
|
2500
|
100
|
10000
|
6666
|
5000
|
150
|
15000
|
10000
|
7500
|
200
|
20000
|
13333
|
10000
|
Now is this some kind of be-all-end-all chart? No, but it's a good place to start and a good place to see if you've really screwed up or not. You can also see some of the trouble of using bits with lots of flutes on aluminum. Aside from troubles clearing chips before the next cutting surface comes around: if your machine isn't very fast it's difficult to move forward fast enough to do more than burnish the aluminum away.
So there's a really simple "this is the sort of feeds and speeds that make sense for milling aluminum" chart I wish I'd found really early on. I might not have been grinding away with a 4 flute 1/4" bit at 0.5ipm and 1500rpm trying to get the darn machine to settle. Oops.
Lots of really good in depth stuff is on here as well
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