Used the search function to see if anyone had experimented with this: nothing relevant to this found.
I’m building my first MPCNC, in the suggested 2x2 format. I doubt rigidity will be a problem. But upscaling could generate issues.
I’m wondering if anyone had played around with increasing rigidity by stuffing the conduit with Organic Carbon Fiber Foam (OCFF): you know… wood.
If no one has tried, I’ll be your huckleberry. Seems like the fiber strand orientation might help for longer runs, without the cost of going to steel. Sand to fit and rout for wire runs where necessary.
Thoughts?
Not sure about shoving a dowel through the conduit.
On other posts, it’s been determined that anything stuffed into the conduit causes more issues with the added weight then they gain with the added regidity.
If you want a super rigid frame, it’s recommended to go with the stainless steel rails I believe.
Hard to believe doug fir dowel would have weight issues. Was looking for actual experiments and results, if any.
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I’m not a materials scientist, nor a mechanical engineer, I don’t even play one on TV, but like Og, I’ve got an idea forming in my 'ead…
If you just slip a dowel in the rail, it’ll slide around and not provide much help at all. You’d have to epoxy it in for it to be any real help, sort of a tubular torsion box. Then, the fibers of the wood could actually add to the stiffness of the whole. Then you’d run into issues of the wood swelling/shrinking and causing issues for the straightness of the rail.
The best thought-experiment I’ve read or come up with is to get square tubing that fits snugly inside your rails, and somehow get JBWeld on enough of it to add to the structural stiffness. Of course, even better would be triangular stock, so you could reinforce the bearing runs…
No kind of engineers myself. But my thought is that if a 3\4 dowel is too loose, I go to 1 inch and sand, which is doable on lathe or by hand… The idea is a tight fit that doesn’t have the gap you’re describing.
Someone will have actually tried this, I’m sure.
So I am an engineer, but I’ve been working as a manufacturing engineer for so long my design and mechanics of materials chops are rusty to say the least…
My opinion is that the wood would do nothing for you UNLESS the stiffness of the solid wood is greater than that of the thin steel tube. Youay get a bit of an additive effect, but not sure it would be worth the weight penalty. I think the wood would just bend right along side the metal.
I do think that the idea of epoxying the wood inside might get you something. As the metal wants to bend, an ATTACHED bundle of wood fibers (the dowel) would have to stretch. In this direction (axial) the strength of the wood may be significant enough to make the difference.
That being said, I wonder if carbon fiber tube wouldn’t be a better (albeit very expensive) choice.
my career path never took me into hard core design. (Which is unfortunate cause that what I really wanted to do when I graduated)
Lots of good speculation. But I think I’ll just test it. Run dowel down one, measure deflection with weight. Then measure deflection without dowel but same weight. Be the end of the month before I can do this, but I’ll post when done. Probably use longest EMT I can based on longest dowel.
Cause now I gotta know.
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But what about CONCRETE?!?! 
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I’ll run my experiments, you run yours. Yogurt, if it suits your fancy. But you gotta measure, or it ain’t science.
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reference to the much-maligned “concrete-filled-tubes” thread…
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I like it. Do you think it’s worth it to measure the deflection of the wood alone, before doing any shaping? If it deflects way more than the emt, it might save you a lot of time.
I am imagining having two sharp tables 1m apart, and placing a straight edge next to it. Then attach a string with a weight exactly in the middle and measure the protusion under the straight edge. Does that sound about right?
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Yeah, that’s the idea. Maybe a 4 foot section, since I’m sure I can get 4 foot dowel and maybe not ten foot, like the EMT comes. My theory is that the combo would be stiffer, but have to test to be sure.
Measure deflection of wood alone, EMT alone, then EMT plus dowel.
I run wires through the tubes. Might be hard with a wood core.
I’m using Cat5 on mine. I figure a little dado, 2 blade widths, would accept the wire. Probably before inserting the dowel. I’ll be testing 4 foot sections, as that’s the length of the dowels available. And I can try the dado and measure rigidity again.
Well settle this one way or the other. Wouldn’t be my first crackpot idea.
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I had an idea recently for making a square 1 1/4" tube stronger by inserting a 1" tube inside with some washer spacers for centralising it (there is a 1.6m ish gap when using 16g tube). I told this idea to a friend of mine who is an aircraft mechanic and he said that is how they used to make the wings in the spitfires strong but light. ie. a circular tube within a tube. That said i think another thinner tube with plastic printed or metal spacers might make for conduit with a little less deflection and the brucey bonus is that you still have a hole in the middle for your stepper wiring.
My original idea is for adapting my class 1, 1 1/4" curt hitch receiver on my car to the cheap bike carrier which comes built from 2" pipe for class 2 hitches. I intend to make the bottom piece doubly reinforced internally and weld some pieces to increase the width to make up for the 3/4" that i am missing by removing the 2" pipe. hopefully it will be laterally stiff enough for my heavy electric downhill bikes.
Mechanical design engineer here. The two things to look at are the bending stress equation, and the young’s modulus (stiffness) of the material.
Bending Stress Equation:
Sigma = Mc/I
M is the moment, which is the moment arm times the perpendicular force. We want to decrease this number as much as possible. Short conduit runs and light weight gantry (For X Axis) help keep this number down.
C is the perpendicular distance from the force applied to the neutral axis. The neutral axis is the center of the cross section, so basically OD/2. We cant change this number much, since all 3 versions are basically the same size. If you like to split hairs, choose the smallest OD option.
I is the second moment of area. This is determined by the shape of the cross section. There is a set equation for an annulus (a circle with a center cut out) and a solid circle. The I for a solid circle is larger than that for an annulus, making the bending stress smaller. That’s the effect of filling the middle of the tube with something. I did a quick calculation for the difference, and the solid circle for our size would have an I of 2x over the conduit alone.
Young’s Modulus (stiffness):
It’s a known value in a lookup table for different materials, although I know it’s hard to find for specific types of wood. Steel is way stiffer than wood, and it is not common in engineering to add a weaker, more brittle material inside a stronger more ductile one, usually it’s the opposite.
For a composite material, you have to make sure there is no slip at the interface of the 2 materials, they have to move like 1 unit, so press fit, glue, whatever you have to do.
In the end, I say try it! Data speaks volumes over speculation (as long as your experiment is accurate).
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I’ll do my best to make an accurate test but I’ll publish the conditions and results.
We’ve grown skeptical lately of the models used by “science” but engineering is ACTUAL science, not this politically driven nonsense. We build huge civil works and launch people into orbit, and it’s all models the first time. So thanks for the science.
I echo your suspicion that rigidity will increase with the insert, but I have to try it.
I’ll be a couple of weeks gathering these simple materials. Budget you know.
I am going to throw my $0.02 in.
I am betting since wood is more flexible than conduit, I bet any return will be small. Maybe the returns will be higher if the wood is epoxyed into tubing.
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Or tightly fit, with no slippage.
We’ll know for sure when it’s tried.
Here’s a nice little beam deflection calculator for a variety of tubes…