Profiles diameter for aluminium frame

I am planning to build a coreXY machine (about which I have been polluting already the forum here and here) with a cubic aluminium frame and a printing volume of about 300x300x300mm, so that the frame will be between 500x500x500mm and 600x600x600mm.

What would be the right diameter of the aluminium profiles for such a length, in your experience? I was thinking about 30x30mm: they have a moment of inertia almost 4 times that of 20x20mm profiles (so they are roughly equivalent to 20x80mm profiles in the weaker dimension).

I plan to fix the profiles with corner brackets. Should I add diagonal supports (such as plates) or this kind of corner supports may be enough in your opinion?

Edited 1 time(s). Last edit at 08/24/2015 06:08PM by cristian.

This is my printer's frame:



It is made from 1.5" 8020. There are no corner brackets or gussets anywhere, just ends butted together and secured by bolts. Though the machine weighs about 60kg, I can transport it laying on its side in my car, stand it up, and start printing without any adjustments. You should not need additional hardware for a machine that is smaller and has lower kinetic energy than my beast. The key is to have the parts cut then milled square and to equal lengths so that when they are bolted together the frame will be square. Aluminum extrusions are designed to be joined this way. Please see the link in my signature, below, and look at step #3.

I purchased all the 8020 for the machine at a local scrap yard for $2 per lb. The frame probably cost about $80 for the aluminum extrusions. I did the cutting and milling myself at Milwaukee Makerspace.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

I wish there were a milling machine at Lille makerspace... However I have been reading good reviews of the seller where I am going to get the profiles cut to length.

1.5" should be about 38 mm, so to be sure I should probably get 40x40mm profiles...

There is only one naturally stiff shape. A triangle. You drive over a large bridge, what do you see? Did they make all those triangles because they thought it look cool? Look up at the ceiling beams in Home Depot. Why didn't they just make rectangles, it certainly would have been easier? You were impressed by 30mm vs 20mm stiffness. Do you have any idea of how much stiffer a rectangular frame of any material is when you add a diagonal? I understand you have a boatload of money and don't care about the cost, but if you are adding material you are adding cost. Remember too that metals and plastics are ussually stronger in tension than compression because tension has no buckling forces to deal with. So crossed diagonals of a thin inexpensive material can make a far stiffer frame than larger material will.

Yes, you can add diagonals, but you're forgetting that you have to be able to access the parts of the machine for maintenance and repairs and the prints that it produces. You'll be reaching around (and cursing) diagonals to make every adjustment and to remove every print. I don't know how you'd go about making a tensioned frame that is rigid without something rigid to anchor it to to provide a counter force to the tension. A rectangular frame made using 8020 type extrusions of adequate dimensions, simply bolted together, is stiff enough for most printers without adding diagonals (or even corner gussets) that get in the way. I don't care if adding material adds cost- it makes a more rigid frame and that improves print quality and reliability.

When I started designing and building my machine I put its performance first. I figured out what I needed to do the job right then looked for reasonably priced means of doing it. That's why my machine is full of scrapped industrial parts that I bought for a tiny fraction of their new prices and why it took me a couple years of searching and trying different things to end up with the printer I have now.

If your goal is to not spend money then sit on the couch and watch TV.
If your goal is to build the cheapest possible printer so that you can spend all your free time adjusting and tweaking to coax Yoda heads out of it, then buy the cheapest stuff you can find that sort of works, or get a $300 kit that someone else has gone to the trouble of locating the cheapest stuff that sort of works.
If your goal is to build a quality 3D printer to enable producing parts for other interests and activities with minimal screwing around, then do what it takes.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

In the end I will probably build a much smaller printer, but in any case I will do some deflection tests with a dial test indicator to see if the frame is stiff enough. Thanks for your suggestions!