I’m using my new MPCNC with a Dewalt DW660. I’m cutting aluminum 4mm thick, using passes of 0.5mm. Currently, I’m milling at 60mm/min. How fast can I go without risking anything? I don’t want to break my bit(3.175mm)
No one can tell you that, it is 100% dependent on your build size, printed parts quality, etc, type of bit, rpm…You would have to give information for someone to even ball park it, a lot of information.
The best thing to do is do test cuts, and listen very closely, it will chatter really bad before it breaks.
Use trochoidal milling on aluminum. The best way to do this is pick a speed, and an rpm and vary the depth only. You should have a look at my aluminum motor mount milling and use that as a kind of MAX speed until you are very familiar.
I used 1/8" 2 flute end mill, .3mm DOC, 5 mm/s, 2mm/s plunge. full rpm No coolant. This was before tricoidal, you should be able to achieve better than this now but use this as a guideline, unless your build is giant and a tall z axis then good luck.
Thanks for your help. I have some other questions:
It’s at 30k RPM. Is this too fast? If so, where can I get a speed controller, and how much do they cost?
What about for wood? Is 30k RM too fast for it? How long can my Z section afford to be if I’m cutting a few mm of it?
https://www.v1engineering.com/speed-control/
Buy it or build it.
Did you have a look at some of my videos, some are done at full rpm and some are a bit slower, single flute endmills do work best with higher rpms. You say 30k rpms, what spindle, some have speed controller’s built in. Details of your intentions and equipment help. I was under the impression you already built it.
Size info, Asking how big it can be is not a good question. most endmills only allow for 3/4" to 1" of cut depth, why would you need more than that? What do you want to do with it? What materials. Unfortunately broad questions don’t get exact answers in a machine that can be configured is so many ways. Specifics help. It is kind of like asking how far and fast can you drive a car, there is no right answer to that, speed limit or car max speed, 1 tank of gas or the life of the car?
https://www.v1engineering.com/assembly/machine-size/
A related question. I’ve seen recommendations to use 1/8" bits for aluminum but, if you can control the spindle speed and it wouldn’t be too big for any interior cuts you’re making, wouldn’t it be beneficial to use a more rigid 1/4" bit?
Spitballing here, but a 1/4" bit will have a much higher velocity at the cutting edge than a 1/8" bit at the same RPM. Even with the extra mass, you might be hard-pressed to dissipate the heat fast enough to prevent re-welding the chips. If you can crank down the RPM so the cutting edge slows appropriately, it probably would be better. But based on using a 660, or at least limited control over the speeds, it’s probably better to stick to 1/8" bits.
At least, that’s my completely WAG/unqualified pontification. 
Exactly why I mentioned having control of the spindle speed. So far I’ve only tried it on the 660 running free so have no idea how effective the HF router speed control will be under load.
From what I understand, it’s not so great, because that particular device is a simple voltage regulator/dimmer. As your tool goes under load, it won’t juice the power to maintain RPMs, it’ll just start to bog down. The DIY PID controller discussed here is an honest-to-goodness speed controller, trying to maintain a set RPM, even under load. Which requires feedback from the tool itself…
edit: And I somehow completely missed the part where you qualified the question with speed control… 
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That was a concern, I was hoping that limitation might be overcome by not taking huge cuts and hopefully finding a setting the machine’s comfortable with early in the cut. In theory running the 1/4" bit at 1/2 the speed of the 1/8" would give equal speed at the edge of the tool. Somewhere I have an old optical tach meant for measuring diesel engine rpm, I need to dig it out and see what the rpm range is.
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The trouble with a 1/4" bit is you have to remove twice the material for it to fit. To do pockets, it can be helpful, but in a slot, or trochoidal milling, you’re just spending a bunch of time turning waste into shavings.
I think there’s a good reason table saws and miter saws come standard with 1/8" blades… But if you know what you’re doing, you should be able to be more effective at pockets with a 1/4" bit.
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I have to confess that’s one thing I hadn’t considered. I was thinking more of cutting a profile and that increased rigidity of the bit would be a plus. I have the feeling there’s so many factors trial & error is still necessary in many cases.
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I buy my bits from Amazon so if I break it, no biggie. I use 1/4 carbide. I can get away taking off 2mm at 50mm/min. I also have a roughing endmill carbide, but I haven’t tested it yet. I also haven’t “pushed” the speeds, so maybe faster? I think it also depends on how big your MPCNC is
Slotting is the hardest the rest can be pretty fast and deep
Curious if you know what alloy(s) you’re working with? That can make a very big difference. When I get ready to play with aluminum I’ll be at a loss as years ago I moved away from a friend’s shop where I was free to sift thru the aluminum stock.
1/4 mills have bigger flutes, but I’m not sure there are any advantages beyond that. It WAS better than some of the 1/8 mills I tried at first because of that, no doubt.
Pretty sure that even the cheap carbide endmills are stiffer than my tool mount, though, and the normal - reach single flute Ryan sells clear chips at least as well.
Takes more power to make cuts with a larger tool, too. MRR is step over x depth x feed, and I think the machine limits these before the 1/8 does, so I’m not convinced the 1/4 would be any faster.
I do have some 6mm hss mills, and I just made room in the teeny tiny. Maybe I’ll try to test it this weekend.
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