It will be fine for the outer rails. How rusty is rusty? If it’s not too bad, just use wd-40 and steel wool.
Never thought of putting concreat into the tubes, however inserting a secondary tube into my EMT, I’m still considering. No doubt additional support is the way to go for the frame, but what is with the movable XYZ axis, no good way to reinforce them. Of course only suitable if you can source something suitable.
I thought with 3 sets of dimensions it would be an easy task to source qualifying tubes, but then something, maybe the good weather, reminded me that I live in Thailand is easy to come by easy besides beer and h…s.
However we get EMT, even just around the corner, it’s paper thin, so the first build will get a pretty small one. It seems we have a good supply of power tools like the 660 available, I might even start only with a Dremel or a Laser.
However, Project officcialy started today, as I brought two lengths of conduit home.
I don’t know if it’s already been said:
An idea that comes to mind is to put inside the empty pipes of the outside structure a second pipe, heavier than the outside frame, attached with 3d printed spiralized springs that caps the pipe of the outside structure, these might absorb more vibration than the filled pipes you proposed…
I have no math to prove this, but generally speaking this kind of chained weights works.
Did adding the additional bar work for making it more rigid?
My theory was to fill the conduit with 50/50 epoxy and micro balloons. The added strength was not enough to compensate for the extra weight and the deflection actually got worse.
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recent mistake here (I did read the thread before and it didn’t stop me :P): 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 
I located some 25mm / 2mm wall steel tube on the website of a hardware store nearby[1], but I need to go there and see if it’s straight, and if they’re willing to cut the thing into 2m pieces (because using public transit with a 6m pipe is… awkward, and it wouldn’t fit the elevator anyway)
[1] it’s the first time I see such a thing actually, usually they carry inch sizes for water / gas / etc., and those use some funny math - 3/4 in is 26.7mm, no big box store sells stainless, and stainless suppliers won’t even talk to you unless you buy like half a ton (no, really, I added some bar to the cart at one of the rare ones who had an online shop, and the cart refused to let me pay saying “you have 2 kg in your cart, please add 498 kg more”); the joys of living in Eastern Europe, I guess.
If you ask for “pipe” you’ll usually get it based on the inside diameter. So 3/4" on the inside. The 3/4" conduit from the big box stores around here is 23.5mm outside diameter, which is why that’s the original size of the MPCNC.
If you as for “tubing” you’ll usually get it based on the outside diametere. I think a lot of places will either carry 25mm or 25.4mm (1in). It’s not common at the big box stores here, but there are lots of metal supply places around.
maybe in English, in Romanian it’s the equivalent of “pipe” for everything
basically plastic and copper pipe is sold by the outside diameter and measured in metric, steel pipe is generally some sort of large inches (I just measured a piece of 3/4 steel pipe, it’s 26.7 mm outside, 21mm inside); electricals use plastic tubing here, there’s no such thing as EMT, and the (steel) pipe that is metric-sized and measured by OD is probably meant for fabrication
going to pay the store a visit tomorrow morning and see if the thing is in stock and is straight enough
That’s why Ryan made the 25mm size. Steel emt doesn’t really exist in Europe. You guys treat electricity differently over there. You should be able to find 25mm structural tube though.
A very late entry to this discussion - I have only just found MPCNC and am seriously thinking of giving it a go. My contribution is; why not redesign the unit with a different size “Frame Tube”. Some time ago I designed a skateboard-bearing carrier to run on 2" (50.8mm) stainless steel balustrade tubing. It ran beautifully although the design was too light for any serious work. I am not convinced I had the best design, but the 2" SS balustrade was rock solid. I was able to buy 3.9m (12’10.4") for $99 AUD.
I have attached snapshot that attempts to show the bearing layout of my 2" prototype.
Being new to PMCNC, I also have a raft of questions:
- What is the effective work size to rail length of this design ie, a side rail of 600mm would give a Y travel of ??mm.
- What sort of tool travel speed can one expect this this.
- Can anyone who has printed this in PLA tell me what wall thickness and fill density % they used.
Ron, take a look at the links from the top of the page. There is a ton of info, including these answers (I hate telling people to look on their own, but my guess is you just haven’t seen these pages). If you still have questions, start a fresh topic that will pic up some more eyes.
Why not 2" tube? Why not 10"? Because 1" is good enough. You really don’t need anything bigger and you don’t need to fill it with anything.
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Thanks for your response - it may not surprise you I am new to this (blogging).
I have been searching for the link at the top of the page, but can’t find it, could you give me a clue please.
re the 2" tube, I only mentioned this as an alternative to concrete filing as I had already tried this size in my prototype before discovering the PMCNC. The 2" tube would perhaps suit larger builds and I found it for the same price as the 1".
Regards, Ron.
Cancel that request, I just found the link - too obvious to see;)
I concur, 1 in / 25mm should be enough even for large-ish sizes if the wall is thick enough; just don’t use the really thin curtain / wardrobe stuff, or if you do, use extra fine sand to make your concrete filling)
220 / 240V means the standard 16A household supply can deliver at least 3.5 kW without much heat (assuming the outlets aren’t cabled by drunken monkeys - don’t get me started…), and then the walls tend to be made of concrete and / or brick, not kindling 
(if the wires get hot enough for the insulation to melt and short out, the breakers will usually trip long before anything can catch fire; most fires around here are cooking-related, and cooking is mostly natural gas)
looks like I have some yak shaving to do: waiting for a cordless angle grinder ATM so I can then order a 6m length of structural 25mm pipe and cut it quickly in the parking lot (they don’t want to cut it, and the brick-and-mortar store didn’t have it in stock, and I can fit at most 2.2m in the elevator - I did extensive measurements after I had to carry a 2.4m beech counter top up the stairs for 10 floors, alone)
I also noticed that any lingth of tube (stainless steel 2-3mm thick) tend to bend, which may be crucial for heavy duty tasks or lengthy pipes. I ended up with making kinda SBR our of the pipes by printing securings that attach to tube every 15-20 cm.
However, you won’t be able to cure center pipes, that’s why I refrained from MPCNC eventually and moved to my own build similar to rootcnc.
I’m doing my first build and while the parts are printing I had time to overkill my tubes a bit. I printed 20mm sleeves to slip over 1/2" conduit and used JB Quick to adhere them, spacing them equidistant along the length. I then inserted the assembly into the 3/4" tubes and slathered JB Weld (longer working time than Quick) at the sleeve locations. There is absolutely no flex in these tubes now![attachment file=116875]
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That’s neat Dave. Clevee idea.
Can you share some measurements? How long are they? And can you compare the flex somehow by propping up both ends, putting a set weight in the middle and measuring the height under the middle for a normal and a filled tube, with and without the weight? Do you have any normal tubes left?
Nice. One of the better ideas, as far as this sort of thing goes. How far apart are your spacers? Like every 6 inches or so?
Also those tubes look very nice and smooth for conduit. I’m guessing you sanded them?
My build uses 30" x 36" tubes. I plan to measure the difference with my dial indicator but I visually checked by pressing down with two finders on the un-lined and lined 36" tubes. The single wall tube flexed very noticeably, the lined tube did not budge. Like I said, I’ll do a more scientific measurement just for the effort vs return factor. The spacers were 18mm x 18mm with a 14mm hole and I used five in each tube. I intentionally made them loose to allow the JB to fill the gap and bond. The conduit I bought was hot dipped galvanized (all that my local H.D. had) and were very rough. I took them to work and sanded them down on our mini lathe. Since galvanized coating does not receive paint well, I tried my go-to paint for 3D parts: SEM 39143. Its a little expensive but it sticks to everything and dries quick! I fully expect it will wear at the bearing surfaces over time however.
Here’s some pix:
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So… Torsion torus?