Hi all, would aluminum 2020 or 2040 extrusion be stiffer than EMT conduits? I have seen lots of topics with ways of stiffening up emt tuning.
Yes.
Also cost an ass ton more. The whole point of this cnc is to be inexpensive.
I don’t think it (2020) would be stiffer. I had similar thoughts prior to building mine and from the online calculators I could find, 3/4" EMT Conduit has about 2.5X less deflection than 2020 extrusion in similar configurations (fixed end beam with load at mid-point of span, 24" span, 15lb load). This surprised me at first, but you have to remember, it’s aluminum vs steel…
2040 does perform better (about 2X better (less deflection)) than the 3/4 EMT - but at that point you’re talking about a .0014" difference. And at those levels, I doubt that rail deflection is going to be the largest contributor of flex to the system.
I did some measurements while building my machine (MPCNC with 3/4" EMT Conduit), which is built to a 12"x18" work area. With the legs screwed to the table, gantry in the middle of the field - I measured ~.0015" deflection of the gantry when adding a 5lb load to it; <.003" with 10lb load. This was actual deflection (downward) of the Z assembly. I was getting about half those deflections at the X/Y (perimeter) rails. Also note that I can introduce over .006" deflection just by physically pushing down on the work surface.
All in all, I have to come to the conclusion that 3/4" EMT is plenty stiff, at least for my size machine…
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I have a 28"x 28" work area with 3/4" EMT. I am happy with this build and so far what I have done in milling oak and hickory. All the 3D printed parts and cheap skate bearings are what keeps the price low. The EMT is very accessible for everyone in the US for sure and that is an entry level design consideration.
Add extruded aluminum profiles and you are going into another level of cost with hardware in addition to the extra cost of extruded.
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1.2 meters is $10. That doesn’t seem too bad.
Thanks for the data. After seeing all these posts about concrete filled thick steel tubes, I was curious.
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I’m not surprised that 2020 is not very stiff. There are different profiles, but if you look at the cross section, there is typically only a very small diameter “tube” in the center. The corners do not really contribute much to stiffness I think. 2040 looks a lot better, but there’s still a lot of material that does not contribute a lot to stiffness.
You could make it a lot stiffer if you could bolt it onto something, like the table. But that would not work for the gantry, and it would require a major redesign of some parts if you try to use it as a frame.