Extrusion thickness 2040 vs 3030

Hi, I'm going to approach a new build of a simple machine (pen plotter) and I need to construct a square frame for this machine.

The frame size needs to be at least 841 x 1189mm + a margin so around 1100x1400mm.

The linear rails are already arranged, got some high-quality 'Franke' rails from a friend, but I'm in need of a sturdy frame. It will lay on top of a strong flat piece of wood, and will probably be bolted onto the wooden bottom plate. So it doesn't have to carry itself. The main objective is getting it square and rigid. At first I planned to go for a corexy setup, but now I've settled with a more traditional setup with two belts running the longer X-axis from each side and the shorter Y-axis will be run with a separate belt.

The only benefit I can come up with to use the 3030 extrusion is that it's lower, so the eventual tools/pen/laser will be closer to the build platform. Besides that I think that that is no issue. 2040 is cheaper and suits the purpose very well. Added on top is that I already have brackets for 2040 extrusions so it's even more cost-effective. But I'm wondering, maybe someone with more experience in machining can tell me what would be design choices to use 3030 over 2040?

The belts I'm going to use are still 6mm wide GT2 belts and they will be run next to the extrusions and not internally through the extrusions.

Inside each bottom corner I will use a corner bracket and above that will be a belt guide. The extrusions will have through-holes drilled out on each corner and will be fastened internally to each other with bolts as well like this:


Would there be any reason to go beyond 2040 extrusions?

Also, when making the box frame, should you have the longest extrusions on the outside and bolt on to the smaller ones, like 'sandwiching' the shorter beams. Or should the box be constructed in this way:


Edited 2 time(s). Last edit at 09/26/2020 11:30AM by Ohmarinus.

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http://www.marinusdebeer.nl/

Do you even need the aluminum? Seems easier and cheaper to build a frame out of 2x4 lumber, stick a sheet of MDF on it, and screw the rails straight to that. If necessary, raise the pen holder up from the rail slider blocks rather than raising the rails from the bed.

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dekutree64
Do you even need the aluminum? Seems easier and cheaper to build a frame out of 2x4 lumber, stick a sheet of MDF on it, and screw the rails straight to that. If necessary, raise the pen holder up from the rail slider blocks rather than raising the rails from the bed.

Yesyes, the frame will not lift. The Z-axis has a dedicated miniature rail that gets lifted See my other build here:
[www.youtube.com]

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http://www.marinusdebeer.nl/

Yep, I'd skip the aluminum and screw the X rails straight to the bed. Print motor mounts that screw to the bed as well. If the Y motor rides on the carriage with its own belt to move the toolhead back and forth, then the X motors can be mounted with shafts horizontal rather than vertical, which would allow simple L bracket motor mounts.

Edited 1 time(s). Last edit at 09/28/2020 09:57AM by dekutree64.

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dekutree64
Yep, I'd skip the aluminum and screw the X rails straight to the bed. Print motor mounts that screw to the bed as well. If the Y motor rides on the carriage with its own belt to move the toolhead back and forth, then the X motors can be mounted with shafts horizontal rather than vertical, which would allow simple L bracket motor mounts.

I figured that I probably want to use two nema17 stepper motors where I remove the shaft from the core and then replace the core with a full-length 5mm diameter shaft to keep the motors in sync. I want to use two motors for the extra torque. I'll use TMC2209 drivers that deliver a humongous amount of power while maintaining silent operation so two Nema17 motors can be driven easily with this TMC2209 drivers. Maybe I'll first try it with one motor and a belt as suggested in the other topic.

The question I'm having... Does (18mm thick) MDF stay flat? And doesn't that get much heavier than an aluminium frame? I will use a separate whiteboard as the base layer which is also ferromagnetic so it can hold the work with magnets.

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http://www.marinusdebeer.nl/

It should stay flat if it's screwed to a 2x4 frame, assuming you can find decent wood that won't bend back and forth every time the humidity changes. And yes, it certainly will weigh a ton.

If you want it to be portable, a self-supporting aluminum frame with corrugated fiberglass floor would probably be about as light as you can get. Plus no humidity issues like wood products. The fiberglass is guaranteed to be flat "across the grain" when screwed to the frame, so the only question is whether it's straight enough "along the grain" that the whiteboard on top of it will make a good flat writing surface. Probably would be worth buying a piece and whipping up a test frame out of wood before committing to the design.

For the frame I'd go with 2040 over 3030, since it will have greater stiffness in the gravity-resisting direction. I think the main time side-to-side stiffness would be important is when the carriage is around the middle of the X axis, and then you do a sharp acceleration in the Y direction, causing the frame to vibrate (same effect as "ringing" in 3D prints). I highly doubt it will be an issue with fiberglass to tie the whole thing together, but if you build it without an integral bed like the one in that video then maybe 3030 would be better (or even 20-4040). Torsional stiffness of the structure will be pitiful nomatter what you do, but with legs at the corners, it should just be a matter of sticking shims under them until it's level. Having the frame lay directly on the table would probably work too, but only if the table is perfectly flat.

Oh, and to answer your last question in the original post

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Also, when making the box frame, should you have the longest extrusions on the outside and bolt on to the smaller ones, like 'sandwiching' the shorter beams. Or should the box be constructed in this way:

I can't think of any reason that it would matter which way you do the corners.

Edited 1 time(s). Last edit at 09/28/2020 02:35PM by dekutree64.

I don't think you need to worry too much about things flexing under gravity, but more so under belt tension. My sand table with 45 mm square t-slot had a problem with the long rails -1.8m- bowing outward because of the belt tension.

Are you going to operate the machine with the frame vertical or horizontal? If it's horizontal, you have to worry about the bed sagging as well as the frame rails. If it's vertical, you only have to worry about the two horizontal frame rails, and it will be easy to see everything going on on the plotter. The horizontal rails can be bolted to the bed surface, so very easily stabilized. It's easy to brace a horizontal bed surface so it doesn't sag much, and the pen can be mounted so that it will follow any remaining sag in the bed surface/X axis.

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