Filling tubes - a test

I was going to recommend a dial indicator. I picked one up for setting router bit and table saw heights. Totally over kill, but it’s a fun toy to play with.

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I’m not an engineer but I have spent much of my working life using and building custom machinery (out of necessity rather than desire) Looking at the performance of my MPCNC I have long thought some form of lightweight filling in the tubes will reduce flex and improve performance. I run my wiring inside the tubes so my proposal is to fit a 10mm pvc conduit in the middle of the tubes and then fill the remainder with a mix of fibreglass resin and 30% perlite. For anyone not familiar with perlite it is both an agricultural substance used in potting mix and a building product to make lightweight concrete products. You should be able to get it at hardware or nursery stores. If anyone is going to give it a go please wear eye protection as perlite can adhere to your eyeball and cause great discomfort for many days. When I give this a go I’ll report back on the results.

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I wonder (with no practical experience or plans on trying it myself) whether people have considered placing a vertical “wall” into the tubes rather than lining with another circular cross section. I would think that would maximize the resistance to bending in the desired direction without adding any more weight than necessary.

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We did talk about angle iron at one point. That gives the up - down against gravitaty and the x or y against steppers.

Hello everyone,

I was wondering the same in the same order :slight_smile:

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 edited this reply several times…

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.

A giant cupcake!

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Depending on the fit, I’d try jb weld or even just smearing red loctite on the inner tube.

You could also try heating the outer tube and wicking silver solder in between the two tubes.

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 thought about liquid nails, but it’s fairly flexible after bonding two surfaces.

The JB Weld is solid after it hardens. I’ve used it in paste form to seal a cracked float bowl on a Harley carburetor before.

Larger diameter tubes is the way to go if u want a more ridgid gabtry. I eant to 1.25 and the difference. Is night and day

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I also opened the x tube spread 40mm aswell and added nema 23s for the z and y drives

@darxide Would you mind sharing your modified STLs to fit all this? sounds interesting. Cheers

Wouldnt mind at all. A good amount of the parts are cut rather then printed too allow running the motors at peak amp ratings

I would try putting a suitable size piece of steel rebar in the center of the tube. Steel and concrete might give you much better results.

That adds weight.

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.

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