Of course, hence the instructions to spray water on surfaces and between layers.
Thank you for reporting your experience.
Concrete has been raised before and it is true that it would carry a big weight penalty. But it might be interesting in plastic pipes because it could provide a way of measuring how much (or how little) stiffness comes from concrete filling.
With a steel tube and a concrete-filled steel tube, the relative difference could be small, making it hard to draw quantitative conclusions. But in a plastic tube, it would be easier to see the difference in deflection under light loads and you could more accurately calculate the stiffness gained by the concrete filling. Then you could definitively say that concrete filling should increase the stiffness of conduit by 3.5%. But that 3.5% is small enough that it’s hard to measure directly.
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Thank you for doing it and not just asking about it!!! This has been brought up so many times.
For my quick and dirty tests I set two point as far apart as possible and set the tubes on them I have used tables and chairs. I put a tape measure right behind it then a camera zoomed in right in front on a tripod. I take pictures before, then hang a 5-10lb weight dead center, take another picture. I take the weight off and take another pic, if the first and third are different I throw that result away. Repeat a few times. DOM only moves a few mm at a 5’ length, so any difference you notice would be significant.
I have my predictions but if you take some better measurements let us know!
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@vicious1 @jamiek @kockie-nl I only intend to do this once, as cheaply as possible. Please help me to do this in a way that will gain maximum acceptance from the community, so one ever has to do this again.
My thought is to use 2m (about 6ft) lengths of the cheapest plastic tube I can find “FLOPLAST PE-X PIPE - WHITE 22MM X 2M”.
Here is my test setup:
Method:
- Measure both tubes’ midpoint deflection when supported near their ends.
- Fill up one tube with mortar (sand & cement mix) and another with foam (after having sprayed the inside with water first!)
- Let both fully cure (24 hours I think)
- Remeasure midpoint deflection (let’s hope I need a weight this time).
- Cut the tubes at the midpoint to prove that they are fully cured.
When people use “concrete“ I hope that they realise that concrete is a mixture of cement and aggregate. The aggregate is sand in this case.
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@jeffeb3 I would love to have your input too. I tried to add you to the post but I can on mention 3 people
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@stpdhurts previously tried filling the tubes with an epoxy aggregate Concrete filled tubing “The added strength was not enough to compensate for the extra weight and the deflection actually got worse.”
Another user found that “I filled the static tubes with epoxy concrete for crush resistance (the only easily available 25mm tube around here is the kind you hang curtains on, chromed 0.5mm wall thickness steel tube; it is flexible enough to be deformed by the bearings, and tends to become a bit sloppy eventually); the filling did achieve the “crush resistant” goal, dampens vibration really well, but I got the concrete composition wrong (gravel too large, not enough fine sand, maybe not enough epoxy), so after a few trips of the gantry the exterior steel took the shape of the underlying pebbles / small voids and looks like the surface of the Moon” Concrete filled tubing
I think a camera setup with a tripod is particularly good because it does not interfere with the system itself. Especially for small deflections and low loads where calipers can get fiddly.
I think another factor is the amount of deflection should be relatively low, ideally in a similar range as the tubes of the CNC machine. It’s possible that the deflection vs. load could be nonlinear, say if the concrete develops cracks it might be stiffer at small deflections and become less stiff at larger deflections. This could raise an objection that the test is showing concrete to be worse than it really is at small deflections that would be realistic for the inside of a DOM tube.
This implies that the measurements should include small deflections, which might require small loads or shorter spans. And this gets back to the optical measurement technique that is easier to feel confident about.
I have some Portland Cement that I bought for something else. I might buy some PVC and try an experiment also. I also have some white silica sand so I can try both cement by itself and cement with silica sand.
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I have bought a metric dial gauge (I have other uses for this so I’m not counting it as a cost - mental gynmastics) so I am confident that I can measure the deflection down to about 0.02mm accurately which should be good enough. I can document the deflection with a camera at any angle then.
As for loading, I suspect that a few kg is all that is needed. I estimate the static load of the CNC mill, plate, motors etc will be less than 4kg per rail. I saw previously that the torque loads are the order of 2kg ish. A maximum loading of 6kg should be enough. I will start small and work my way upwards.
Failure of the concrete would rule it out as a suitable material.
Edit: in any case, the loading should produce a linear stress/strain relationship (Young’s modulus) otherwise you have strayed into plastic deformation or, in the case of concrete, cracking. I think that I’ll also measure deflection of some stainless steel and copper pipes that I have lying around as a control sample.
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I agree it should be linear over the range we’re interested in. If the naysayers are correct, the stiffness of concrete (or concrete within plastic) could be much less than the stiffness of steel tube (with or without concrete), so even a few kg might be too much. Again I agree, it’s good to start small as you said, just make sure you start small enough that you don’t have excessive deflection right from the start.
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 
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)