Having discarded the solution of the solid aluminum bar because of the softness of the surface under the weight of the bearings, I thought of a combination of 1. choosing a thicker stainless tube that increases the strength without increasing the weight too much + 2. filling it with a lightweight material with low viscosity and good compression properties —PU expanding foam is lightweight and pretty good at compression but the problem is that the application makes it really inconsistent inside; it will have uneven density, holes, etc. A resin like Smooth-on Feather-Lite would be easy to pour consistently. What do you think of this?
Somebody posted good results from tube - in - tube bonded together a while back. 1/2 inch conduit fits nicely in my DOM so I’m trying it out with measurements.
I wasn’t sure how to bond them, though. Thought I might use some construction adhesive, but I don’t think i could slip the two together without smearing off too much of the adhesive.
I caulked the ends of the emt and taped up one end of the DOM so hopefully I can slide them together and pour epoxy resin in the gap. Maybe pour some in first and then push the emt down into it. I’m just worried about blowing out that caulk plug and filling the emt with resin. That would be a disaster.
Maybe tape one end as well? Not much space though, so I’d have to mash the tube in.
I found a data sheet on it. It has a tensile modulus (Young’s modulus) of 150,000 psi, about 0.5% of steel’s modulus but it is much lighter. As @kockie-nl and @withoutmessage suggested it would give more consistent results than my spray can.
This discussion lead me to this supplier of cheap 2 part foam, which also sells in small quantities. Apparently the material is used is some kinds of mould-making but mostly for buoyancy in boats. This video provides useful instructions.
Man, you always have some good ideas.
Been a long time since I mixed up some jb weld, but I remember it being just runny enough to work it’s way down if I could be that patient.
I’m not sure how hot you can get the galvanized emt before it gets dangerous, but I know I shouldn’t weld it.
Definitely too much of a gap for loctite.
I think I’m committed to the resin at this point. It’s definitely more flexible than jb weld, so i think any stiffness increase will be from the emt and the bonding. Maybe the jb would offer some stiffness of its own accord.
Of course, I’m talking out my rear. Only reason I tried this was because somebody else already said it worked.
Liquid nails (polyurethane adhesive) can also fill gaps and has a pretty long working time. It is a bit sticky though, so you may need to be creative and wear gloves (big ones)
I gave up on the whole tube-filling idea because basic mechanical engineering courses are clear that a hollow tube is much stiffer than a filled one for the same mass per unit length. There is quite complex (complex to me!) mathematics that proves this. And my basic tests showed that any improvement is marginal.
In school (AE) I gained a fair understanding of monocoque structures and how they behave and can be optimized. The thing with thin shells is they are efficient wrt weight, but they buckle easy under compression. The loads on a hollow tube are concentrated at the top and bottom (vertical load only), and the buckling mode on the compressed side will take many forms… it will end up looking like a wave… like what happens when you crush a soda can. This wave shape would require not just a filling that withstands compression (like concrete), it would also need adhesion to the walls to prevent outward deflection. This alone will make most fillers (short of welded fillers) not very effective.
Otoh, assuming the accelerations are reasonable wrt stepper torque, going to a solid bar would be much stiffer than hollow bar. Someone who has access to cheap solid bar could try this out… see if a primo or lr can handle the weight.
Excellent. “Buckling” in my mind is a plastic process but, of course, it must happen elastically first. So, gasp, there’s a trade off. Larger diameter tubes are stiffer from the moment of inertia but thinner tubes buckle more.
In any case, the combined effects of the moment of inertia and buckling must be visible under static loads, so it should be say enough to test.
Tricky stuff, this engineering…
Edit: I wonder what is happening to the pipe under the concentrated load of the bearings resting on top of the pipe of the X-axis (router platform); this is exactly where the buckling loads are highest.
Looking at my DOM tubes and bearings, there is definitely a squeeze acting on the tubes as indicated by the 3 flat marks left by the bearings. Also the hardened bearing surfaces are completely chowdered at this point (cheap bearings). At the end of the day, those core z clamps, trucks, and core clamps are probably well under what it would take to induce DOM tubes to buckle. Even with a heavy router installed the pla will yield first.
That is exactly why I decided to print my parts with 4 walls. That makes the weak points a lot stronger I think.
