Concrete filled tubing

Do you thing that filling the outside tubes with concrete would have a noticeable impact on the overall rigidity of the system? You would not want to fill the tubes that cross in the middle as it would add a lot of mass that the motors have to move around.



This gets asked often enough it should be stickied. It will add some, but not really worth the hassle. Mid span supports are way easier.


How it works isa single set of mid span supports add more than 4x’s the rigidity, concrete would not and make a huge mess.

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Is there a STL file for mid span supports?

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I just used what’s in the second picture. Just drilled a hole in the tube, and used a locknut on the bottom of the tube to hold it. Nothing inside the tube is really needed, it just keeps it from sagging.

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Hey Barry what’s your working area?


It was almost 4 feet square. I’ve put it back to about 2 feet square since building the lowrider.

Barry, how well did it do with the 4x4 setup? Asking because I’m thinking of doing the same. Also was it hard to load a 4x4 in from the top since the midway supports are in the way

It worked okay. Works better smaller though. I use the lowrider for bigger things now.

This is what I just did this afternoon to see if it would help. 1/2” steel tubing screwed down to the X / Y tubes to see if it would give some risgidity.

And? The suspense…

Hey guys, i’m new here so I apologize if this has already been addressed… Regarding the rigidity and the square tubing attached to the the conduit, i’d be curious to hear some feedback…

I may be massively overlooking something obvious here, but why not just buy 1" round tubing with ~ .125 wall ? I called my local steel supply and they have 3 sizes of steel tubing in true 1" OD, I forget exactly, but roughly they were .060 .090 & .125 … It was only about $1.20/foot of .125. So the cost for the framing would be about $25 worth of steel as opposed to $10 of conduit, but it should add a bit of rigidity and eliminate the wear of the galvanized coating where the bearings ride?

Has anyone done this? I feel like I must be overlooking something, because it seems to easy and cheap to add rigidity this way, unless it introduces other problems (or isn’t more rigid for some reason… I havent had my hands on the conduit so maybe it’s stiffer than I assume given it’s lightweight)


So the 0.125 is cheaper? How is that possible? Is it just that your local guy has too much of it?

w.r.t. trying it, I don’t remember anyone doing that and posting it here. There would be a disadvantage to the extra weight, and I’m sure some mechanical engineering thing that I don’t intuitively know about the thickness not mattering as much (Ryan, help, it’s not software!).

I can say that I have 60" pipes on my low rider, and I have the 0.065" thickness, and it’s definitely rigid enough. I did an experiment where I drilled some holes to a certain depth at different locations on the CNC and they were about 2mm more or less across a 36"x48" work area. I posted the results here somewhere. Maybe in the LR hardware development forum? The low rider’s parts are designed for that long span, and the MPCNC is designed for smaller, more versatile CNCing.

If it’s cheaper, I would try it for my machine, but if it’s more expensive, I wouldn’t bother.

For thickness, anything above 0.065" is fine. It is stronger than you think. If you can check out a long bar at your supplier. I had the chance to just handle a 10’ section of various thickness. Look at the difference in sag vs weight. I know 10’ is far beyond anything we use but it is an easy way to physically notice a difference.

The non software answer is, diminishing returns. Above 0.065" the added weight is outpacing the added rigidity, usually the price as well. Heavier means slower accelerations (not a huge deal at our speeds) but to take it to the extreme a solid bar would probably sag under it’s own weight on a lowrider, but maybe not on a 2’x2’ mpcnc. So there is a sweet spot, in my opinion 0.065-0.09" would be great anything above that is just weight.

If you can check out a long bar at your supplier. I had the chance to just handle a 10’ section of various thickness. look at the difference in sag vs weight. I know 10’ is far beyond anything we use but it is an easy way to physically notice a difference. I noticed little difference in sag for a massive weight difference. I am sure there are some calcs online but it is far easier to just pick one up physically.

It’s very possible that when he says “steel tubing” he doesn’t mean “stainless steel tubing”.

Sorry for the slow reply (I had to figure out how to find my way back to where I posted the question… new to this forum, but i’m getting my legs about me:)

Ryan, I should have but didn’t hold two 20’ sticks of .090 & .125 and see the difference in sag. I think what you say regarding the extra weight washing out the added stiffness at a certain point is probably a good point.

Also, as Bill mentioned I did not mean Stainless, I was referring to cold-rolled 1"OD. It’s not much more than EMT and is quite a lot heavier/stiffer (I suspect). I just picked up 20’ of the .125 today, but after reading this i’m wondering if .090 wouldn’t be totally adequate for handling a larger build area (3 or 4 ft sq.) and not impose possible accel/decel issues with the added weight. I didn’t think of the weight as too much a factor since the majority is the perimeter frame, but I guess there is still a few extra pounds which would probably be noticeable swinging around on the gantry… Thanks for the thoughts and feedback. I will be near the steel supply again tomorrow, maybe i’ll try to run by and as Ryan said just visually compare sag on the .090 and .125 to see if there is any real gains to be had.

I think after seeing so many HUGE CNC’s using massive steel tubing or alum. extrusions, it’s hard to imagine that the MPCNC would work as good as does using the lightweight tubing. At least it’s cheap and easy to scale the weight down by switching out the two gantry tubes to .090 if I need to later.


You will have to check the last regular steel I looked into was actually many times more “wet noodle” than regular conduit. Weight is not really a factor it just plays into the equation a bit.

I think it is best if you do get your hands on some. I spent way to many hours and miles driving around looking for the “ideal” rail I did not want to believe conduit was the answer, my original idea was uni-strut. You could imagine how stiff I thought it was going to need to be. This is not to say you don’t have something better in your area, so taking a look is good.

20’ is pretty rough to gauge, at that length everything flexes a lot. 5-10’ tells a better story.

Thanks Ryan, it is funny how things sometimes turn out not to be as you would think (i.e. thicker walls being better…)

So yesterday afternoon I dropped back into the steel supply and set 2 pieces of 20’ 1" tubing (they didn’t have any pieces cut so I could only look at the 20ft sticks w/out buying something to cut it) … results are this - at 20’ the .090 had slightly less sag (not much difference, but hands down a winner in that regard) … Although I still wonder that when it comes to a 3’ x 2’ machine frame the sag should be 99% eliminated on that short of a run and I suspect that really most tubing would be fine for a build size that small. I would like to be able to work w/aluminum as i’ve seen others do so with good results, which is really why i’m wanting to go a little ‘heavier-duty’ than EMT. I guess what I’ll do at this point is just build the machine with .125 and if it seems to underperform compared to what others are getting i’ll maybe swap out for some .090 , .060 or EMT. I suspect that this will work fine though with what I have.

Ultimately i’d like to probably build the low rider and keep an MPCNC for smaller / aluminum projects … (the desire to stiffen everything up was in pursuit of larger build volume, but I think the simple and easier answer probably is just 2 machines :slight_smile: )

My only hang up with the low rider is garage space … i’ve been toying with modifying the design very slightly so I can turn the machine up at a 60 degree ish angle and put it against the wall similar to a Maslow design, but with much better accuracy and more complete design of the low rider… Maybe i’ll post more about this idea in another spot on the forum, but i’ve got a few thoughts on how I might achieve this. However, one area where i’m totally guessing is the X stepper motor … i’m wondering if a Nema 23 upgrade would be strong enough to pull the router up said 60 degree slope while cutting through wood? maybe 2 -23’s or gear reduction? I think it’s solveable problem, i’m just guessing at what would be sufficient for the first iteration. I’m working on MPCNC now, and I suspect i’ll learn a lot which would help with the modded low rider concept and take some of the guess work out.



Almost every single build is standard EMT, there are very few stainless builds. Trying to fix something that isn’t broken can result in a machine never getting built, or costing more than it needs to. I have added a FAQ section on this topic.


Another assumption…to do this you need to use a larger power supply and drivers. To do that you need a different control board, to do that you will probably need Mach 3-4 ($$$$) …heavier and stronger. Would a 5HP Stepper really make this build even better? It is a balance, everything is the right size, solid diamond carbon fiber rails with 100hp steppers would still run up against, spindle power, endmill capabilities, processing power, part strength, bearing quality, pulley runout, etc. As soon as you “upgrade” one part, you literally have to upgrade everything. This all comes up at least once a month, there are tons of threads about all this, this one was started last year.

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