What are the constraints on cutting aluminum?

I’m thinking of building an MPCNC. If I were to mill aluminum with it, what are the recommended limitations (e.g. length, width, and height)? I understand that smaller and thinner is better, but that’s just kinda vague. What are the maximum dimensions at which good results can be gotten consistently without needing to be lucky?

The recommended size, the default in the calculator works just fine.

The reason there is no specific number in there is because there can not be. This is not a fully assembled kit, your size can vary as well as your conduit manufacturer or using stainless, ect. Plenty of people have milled aluminum with it huge but if you have no experience the smaller the better.

The best way to build it is as big as your largest part, with no room to spare. It only takes a few dollars in conduit to change size. The total price is easily under half any other machine so you could even just build two. One for metal one for whatever else you are into.

There were far to many posts in the early days with things like “I am building it big enough to mill a door just in case I want to do that one day…What is the best bit for milling a 2"x2” PCB which is my main focus?" Waste of time.

Do not get caught up in “a few extra inches just in case”…build it tiny, learn how to use it. If you need more work area spend another $10 on conduit later and expand. Trust me you will easily save $10 in endmills doing it that way.


OK, that makes sense, but I’m still left with a sense of vagueness. If possible, I’d like some numbers to make this real. So, in that vein, what’s the smallest machine build that’s still practical, and what are the maximum dimensions of aluminum (lxwxh) that can be reliably milled at that size of machine build?

So, for instance, if you can’t mill thicker than, say, 1/4" thick aluminum at even the smallest machine build size, then that would be useful to know, because then presumably the even bigger machine builds wouldn’t be able to handle it either. Correct?

It’s just useful to have a general sense as to what is practically possible, and what isn’t, without having to discover everything purely by trial and error.

It does not work that way. The max depth you can mill aluminum is the usable depth of your endmill, or you can flip it over and double that. You can mill aluminum on the Lowrider up to 4’ wide by literally as long as you want.

As much as people will have you believe otherwise aluminum isn’t that hard to mill, you can mill aluminum on an MPCNC at most any size. Just earlier Barry mentioned a large aluminum piece he cast then milled on a 5’ square MPCNC.

If I say you can do it at 5’ and you can’t figure it out I will get all the blame. Some people can not figure out how to cut wood at the recommended size, Barry didn’t really have any issues at 5’, but frequently says smaller is easier. I have no idea where you or anyone else are on the scale of mechanical adeptness.

I said earlier the recommended size is a good size, smaller is better. That is as specific as you will get from me.

If you read specs on any CNC type machine you will see theoretical resolution, never promised for this exact reason, CAM is 90% of the job.

Oh, I thought a smaller machine build would be more rigid, so perhaps easier for that reason.

Yes, 100%. More rigid=more forgiving for bad CAM.

The pucker factor was huuuuge milling that thing out. Almost everything was done with either trochodial cutting for facing off the top, or peel pocketing for the shapes. This helps a lot for clearing out the chips. Otherwise you just end up bonding aluminum to the flutes, then you break off the end mill. I never finished the touch off plate, haven’t had a chance to remelt the aluminum to start over. Now that I’ve shrunk the mpcnc back down to 2 by 2 it should go better. I was mainly just screwing around with the imperial logo, didn’t actually think it would work, but everything kept working, so I kept going. Once I was almost done I thought for sure the thing was going to break off the build plate or something and mess the whole thing up.

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I wanted to cut some D-shaped holes for SHV connectors (high voltage connectors similar to BNC) in 1/8" aluminum for a lab equipment enclosure. My MPCNC is about 3’x4’. You can certainly cut aluminum with good results, but the rigidity, spindle power, and other factors mean that you have to experiment with the feeds, speeds and machining methods to get it right.

My first attempt was to try a hole on a piece of scrap material. My initial feed rate of 300mm/min was way too high and it chattered a lot. I dropped it to 30mm/min with a .5mm depth of cut and it went slowly and relatively smoothly.

I used a 1/8" 4 flute carbide end mill which worked well at first, but when I did some straight cuts, the aluminum started to gum up the bit and I had to stop and clean the flutes out. So, I’d say that setting wasn’t perfect for doing straight cuts. Using cutting fluid or a slower spindle speed might have helped, but I didn’t try those things yet. A ball end mill probably would have been better too.

All that said, it’s important to use the best tool for the job. If the holes needed to be round, it would have been much faster to use the MPCNC to put little divots/pilot holes at each location and then use a drill press to drill the meaty part of the holes at those locations.

Anyway, with patience, my project turned out great. Many thanks to Ryan (and the community) for this fantastic design and everyone’s efforts.


I’m rather new to the mpcnc world. I have built one and I have done multiple test cuts and wood and I am working on aluminum. I mostly build the machine to mill aluminum and hopefully someday carbon fiber bodies for quadcopters. I have had absolutely zero problem with the rigidity of the machine because I built it small 9” x 9” x 3.5” using galvanized conduit. The biggest thing is just feeds and speeds. But honestly if you want like set dimensions I’m pretty sure you won’t get any problems if you max out around 2’ x 2’ x 4” you could probably even go way higher than that but there a lot more to learn before you can even dare to attempt making a cut that large. When I say lots to learn I mean lots. Speeds, feeds, number of flutes (these first three are a heck of a lot on their own), type of steel the end mill is made out of, rpms, chip clearing, and many other factors. So I wouldn’t recommend building it too big at first because I’ve just been learning about some of those things as I attempt to cut aluminum on my rather small machine and I am doing cuts not even close to a quarter of the size of build volume. Wish you the best luck and please share with me any success or failures you have.


Other than doing my own PCB’s on a 2418, which probably doesn’t count, I’m a complete beginner at CNC. What do I do to ensure “good CAM” rather than “bad CAM”?

What’s needed is a recommended demo that we can all reference. It needs to be specific, and by that I mean, it needs to say a particular machine build, a particular blank of aluminum, particular feeds and speeds, particular bit, a g-code file, a particular spindle, etc. I’d like to start with a reference that I can simply copy to see if I can get the same results. It would be like a “Hello World” for cutting aluminum. That would be a huge catalyst. Once that’s done, I’d feel more confident branching out from there.

Unfortunately “Good CAM” is specific to any given machine. Even the expensive ones that cost $1000’s of dollars. Good CAM is just learning what the right feeds and speeds, depths of cut, stepover values, spindle speeds, etc. are right to use on your machine for the material you want to work. Lars Christensen on youtube has some good videos about CAM related to fusion360. This was one I used to start to dial in some setting for my particular machine. https://www.youtube.com/watch?v=7DIWAqe4wDY&t=689s

Basically just use a small shape to work on, start out slow and increase your speed settings until you start to see/hear problems. Metals have a much smaller window of tolerance for CNC. You have to have the right numbers or aluminum welds itself to your bit/you break a bit/you break your machine/etc. etc. You have to find the right combo to make chips not dust or chunks.

Ryan is spot on when he says start in foam, advance to mdf/wood products and then graduate to metal working. You really have to get a feel for what 30mm/m vs 300mm/m means for your specific machine in any given material.

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Would you mind posting a photo of that? I’d love to see what that looks like.

Is it just the photo or is it reality that some of those holes didn’t turn out very round?

When I’m milling my PCB’s (admittedly, a different activity), I just mount the right size drill-bit and let the CNC do all the drilling for me. They come out perfect. If I have different size holes, then I just run different g-code files for the drilling, one for each size of drill bit.

Can the same be done when drilling holes in aluminum?

For instance, starting at time index 4:52, this guy reports trouble with too much twist in his 6040 (a lot of others have reported the same problem) when it came to milling aluminum:

Was the cause the well known design flaw, as he alleges, or was it “bad CAM”?

That’s the reality - the holes didn’t turn out round. But that was the whole point - they are indeed supposed to be “D” shaped holes to fit the cable connectors. (That way the connectors don’t spin when you twist cables onto the connectors).

You are absolutely right about using drill bits of various sizes to drill precision round holes. That’s generally better than machining circles with a milling bit when possible.

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Is it a design flaw…no, it was a design choice. The machine works just fine, just not as fast as he wants. They used large un-suported rails fixed on the ends, they could have used large supported rails but they would be heavier and more expensive.

You are looking for an absolute answer, welcome to engineering, it does not exist. Every single choice has a consequence, cost is almost always #1.

How does he solve this issue, slow down his cam by going less deep. This lightens the load on the gantry and and all of the sudden, poof, no design flaw, just a slower machine. Or he can rebuild it with supported rails and spend money to go faster.

All of the sudden it makes you wonder if his building choice of putting such a giant spindle on the tiny machine was a good choice. He will never over power it with that machine. Perhaps with a lighter spindle he could move faster and get the job done just as fast with more shallower faster passes?

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Brings to mind the old adage, “Do you want it fast, cheap or correct? Pick two.”

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According to this theory, then, practically any CNC can cut aluminum, as long as it has a shallow enough cut depth and doesn’t go too fast?


I mean, it still needs to finish the job in a reasonable timeframe. Just eroding it over centuries doesn’t count.

Yes. Have you seen one that can’t? They/we all show some sort of aluminum being cut, and PCB’s. The only things anyone every asks about (once and a while people ask about Carbon fiber). “Is it strong enough for aluminum”, “is the resolution high enough for PCB work”, the answer is yes.

That is a tricky sentence. Each bit has a proper chip load range, which is then defined usually by the rpm you are running, that in turn defines the feed rate. The only real variable is the depth of cut…determined by how rigid the machine is…in the case of the MPCNC and 99% of all other cnc’s by how small you can make it. Simplifying some other constants in there, the real variable is still depth of cut.

So now when you watch some videos out the even of just the mpcnc and you see some people doing .5mm depth of cut and some doing 5mm it will make sense. I was super pumped on a 9mm DOC in pine, Kevin comes along and does 5mm in aluminum…so now I have to make a new video so I don’t look so bad


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This does not work. You have to remove material every single pass. You can not just go less deep, this is where most people screw up. In wood if you do not remove enough material it will catch on fire, with aluminum it will work harden and break your bit. Now we are back to CAM, it has to be right for your machine or things go bad.