Stainless Steel - Quick and dirty flex test

Has anyone tried filling a rod with, I don’t know, concrete? Plaster of Paris? Epoxy? You get the idea.

Filling a tube with something doesn’t make it stronger, just heavier.

@Dui, ni shuo de dui
Steel is stronger, but heavier.

Um. You sure about not stronger? Heavier yes. But if you did this only for the frame rails and not the gantry ones…

After all, those yellow steel tube barriers you see everywhere, they’re filled with concrete for some reason.

It would be an interesting and simple test would it not? And as a solution, rather inexpensive and easy to implement (no mods to be made).

They’re filled with concrete for mass. We’re also worried about weight. I misspoke about the strength. With a tube and a solid bar of the same material and diameter, the tube will be less strong. The problem is your weight goes up considerably. That’s why race car cages are tubes. Strength to weight is critical. If you’re going to start beefing up this CNC to use solid rods, you may as well build a conventional gantry style CNC. The MPCNC is supposed to be a low cost starter/hobbiest CNC.

IIRC, Ryan’s answer to square tubes is that it takes 4 points to align them, instead of 3. Or really, 8 instead of 6. He answered it in the MPCNC 2.0 thread, I think.

I built a machine that is just strong/rigid/heavy enough for milling aluminum slowly, that is also just light enough to 3D print slowly, and all the other things it can do. It is not perfect it does all kinds of things well but nothing perfect. Yes you can strap an extruder on a $250k mill but it has such large moving mass it will print worse than the MPCNC.

Tons of reasons for using tubes brought up many times.

What you are not looking at is mass, round is the strongest for it’s mass. Round is also the most true, square is structural not meant for rails (neither is round but it always comes out nicer). Round is also equally strong in all directions, square is not. 33%more, hardware, more expensive, harder to build, heavier, harder to true without introducing expensive things like eccentric hardware. Square is not readily available in standard sizes. Lots of square tubing machines out there, I didn’t want to build another one. I actually went to dozens of shops of a few months time and put my hands on all the material I could, EMT was far and away the winner in all categories, most importantly price.

Roll cages are not made of square stock for a reason…Mass.

Wall thickness, of course it has an effect, that effect diminishes very quickly as you move towards the center. The center actually does nothing. I spent too many semesters learning this stuff. Choices have to be made guys, do you want it ultimately rigid and molasses slow due to moving mass and inertia, or do you want a machined tuned to all of its parts. If you have a perfectly rigid rail system then the plastic is the weak point then you use metal parts and on and on it goes. I feel these tubes match the strength of printed parts wonderfully.

Filling tubes. Yes filling a tube will make it better, but not very much, largest effect is vibration damning and increasing the weight. This has been asked so many times, I think it is kinda funny no one has sacrifice $2 worth of conduit and just tried it.

This is to be built with no power tools other than a 3d printer, or buy them if you don’t have one. This is a fun project to use and learn and grow. If you need more spend the money the next options are about $5k. I built a machine you can do all kinds of things with.

If you change something to make one function better (eg milling) you will sacrifice some other function (laser speed), or more money. Designing a machine has thousands of decisions to make, these are the decisions I have made.

There are a lot of machines that have been reinforced for milling, but they do nothing else with them. Perfect! If that is what you need go for it, that is kinda the point of me releasing all the info that I do. If I just sold a kit you would be stuck with my choices, you are free to make changes.

I love and appreciate all of you guys, I know my replies kinda make me sound like a dick but I promise I do not intend it that way. There is just so much information to get out and I have said it in many other places each time it just gets condensed down. Please do not let this discourage you from trying to help make this a better machine. Trust me I have another Idea in my head that One day soon I hope to get started on, lots of things can be changed and I love to think I learn from all of our experiences with the machine.

Thanks you all again, keep the conversations flowing and I will jump in with my opinions.

Just a quick story to try and bring this all full circle (still under NDA so I will generalize). At my previous job I made things for people for mass production. They usually brought in a prototype and we took it from there. These people had seemingly random stipulations. One of the last projects had a time constraint meaning it had to move in 5 seconds, really for no good reason, I even felt it was too fast. If they would have said sure two more seconds is fine they would have had a much better product, but choices had to be made. If your garage door opener took 4 more seconds to close but was absolutely silent and the drive was half it’s size would it be worth it? That is designing a machine in a nutshell.



He was talking about filling the frame tubes, not the gantry ones. There is no harm in doing that, it will be a bit stiffer. Actually, even the ones in the gantry shouldn’t be a big deal, we are not running our machines are insane speeds so the inertia is quite limited anyways.

Either way, I assume that the difference will be hard to notice… If you plan on making your frame stiffer you better just replace your bars with thicker ones directly. It doesn’t cost much and in both case you’ll have to tear apart your machine anyway…

I don’t know if this is the biggest rigidity issue on your machines anyway. On mine, the weak point will be the middle carriage long before the tubes.

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The weakest part should be the middle x and Y tubes, as there are only one of them, supported by two outer tubes. Making the outer tubes stronger will/should have minimal effect as they are already effectively two times stronger than the tube they are supporting.

At the same time every little bit helps.

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Vicious1 thank you for your designs, and creating this tool. I’m a product developer as well so I completely understand your design process and choices. So I wanted to say thank you for your work and open source approach to this project.


Thanks! Sometimes the choices can be overwhelming and slow iterations, That is the one place a boss comes in handy, forcing a choice.

Not completely open source but I have my reasons…

I used to race go karts. The rear axle of a racing kart is usually hollow chrome moly and the ability to adjust axle flex was part of setup tuning. We did this in several ways, but the one that comes to mind pertaining to this discussion was inserting a tight-fitting wooden dowel into the axle. It reduced flex without adding much weight. The loads seen in racing are huge and this had a noticeable effect both in the “feel” of the handling and the stopwatch. I doubt it is “stronger,” but it does reduce flex and weighed less than an axle with thicker walls sufficient to provide similar rigidity. The reduction in flex was greater than the sum of the rigidity of the steel and the dowel.

Weight is a big deal in racing. We either had to reduce the vehicle weight or stop drinking beer. Necessity is the mother of invention!

Faster times and the need to keep post race celebrations probably spurred many a great invention!


How does extruded aluminum compare to our conduit for the center X and Y rails? I can visualize an advantage to making the center rails stiffer, as long as they don’t also get heavier. A quick set of Google searches says 15/16 extruded aluminum bars weigh a bit less than twice the weight of 3/4 inch conduit, but costs in the 10X range (still affordable). Don’t know about stiffness, but would expect it to be better. I wouldn’t be surprised if wear would then become a bigger factor.

My gut feeling says way too soft, the bearings will sink in in a few minutes.

Thanks again Ryan, tomorrow, I was planning on calling a place here in Alberta about price and availability on both 304 & 316 stainless, and I may still call them, but I know I can get 10 foot lengths of 1" OD EMT locally for about 15 bucks, so I think I will go with that first, hey, what’s the worst that can happen, I’m out a few bucks, and a bunch of scrap EMT pieces :), the rest of the stuff I will get from you, by the way, I am going to a 24" x 36" workable area. Thanks again

Anyone looked into carbon fiber?

Everyone likes carbon fiber but I believe that 300$ would get me far better solutions than carbon tube with 1mm wall thickness

It will also delaminate under the bearings.

If you were really concerned, it seems you could either spend the money to go up to stainless tubing or spend some time and modify parts for making the X-Y Gantry use larger diameter EMT for more rigidity at minimal additional expense. I’m sure Vicious had some reasoning to use 3/4 EMT but i don’t see why a larger size wouldn’t work as well.

According to the calculator posted above, carbon tube would have even more deflexion than steel, for the same thickness.
So, this doesn’t seem to be a good choice here.