Extrusion size and milling (Prism)

Hello everyone.

I have been reading the forum since September, and plan to build a Prism variant soon.

Does anyone think it would be a good idea to use larger extrusions? Do you think the current 20mm size limits printing speed or accuracy? Would standardizing on 30mm help with high speed printing or light milling?

Using Misumi parts, 30mm has about 3.85 times the stiffness of 20mm, or 4.67 times 20mm if you go for the thicker walled "G" series solid 30mm extrusions. The price estimator puts these at the same $4.00 a piece as the 20mm parts that actually go for about $2.00 each. I am not sure what the real price difference would be. Another benefit and drawback would be the use of stronger screws and less parts compatibility.

Alternatively you could go with the 20mm class extrusions, but go with double the thickness on a few dimensions that experience the bending or twisting loads. That would give you 12 times the stiffness in bending, not that simple beam bending is the problem with these machines. The added distance from the center of the beams would help the brackets deal with the loads trying to distort them, especially in the X axis. This would also add a little more room below the moving bed. The price does go up to an estimated $8.00 each for the 40mm extrusions though. Given that the 20mm parts cost about $2.00 each this makes sense, as you are getting four times the material.

I am thinking the modified BOM would be two 40x40mm (sides), six 20x40mm (front, back, and the diagonals) and maybe one 20x20mm for the top, unless you change its bracketing so that it could fit something else.

I will likely build two frames, one normal and one larger, and buy some extra extrusions for playing around with because they are cheap.

I think the biggest improvement to its stiffness will be the use of aluminum or fiberglass corner brackets and support pieces.

Let me know what you think.

Edited 1 time(s). Last edit at 03/06/2015 12:30AM by MattMoses.

The thing that limits the stiffness seen by the tool is the belts and stepper motors I would think, rather than the frame.

As far as the frame is concerned it can be made much stiffer in the directions it needs to be by using the Mendel90 style layout (inverted T) instead of a triangular prism, which is only stiff in directions that don't matter. See [forums.reprap.org]

---

[www.hydraraptor.blogspot.com]

I'm not a mechanical engineer by trade so have had to study a bit.
The Mendel90 is not a simple inverted T but one stabilized by the 2 back pieces of dibond.
The Prism is not just a prism but one stabilized by a heavy vertical steel rod.
An architectural structure that has stood the test of time is a King Post.
[en.wikipedia.org]
It combines a vertical beam with a prism shape.
To answer the original question, I would say that 15mm extrusion is adequate to the task as it has sufficient strength in the needed axes and, as nophead has pointed out, the belts are the weak point.
The structural design has held up this bridge for a long time:
[en.wikipedia.org]
I've taken this thing apart and remade it a few times. I'll finish it one day hopefully in July.

The Prusa i3 has been modified by sgraber in a way that partially incorporates the principle:
[www.thingiverse.com]
The excess Z height allows for the addition of braces to stabilize the X axis if needed.

Edited 5 time(s). Last edit at 06/02/2014 07:13PM by Ralph.Hilton.