Primo: 1" tube vs 3/4" conduit?

Mostly Printed CNC - MPCNC Advice
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I built my Primo several years ago, right after the design was published. At the time, the only US design was using 1" tubing, which I did. It was DOM tubing, purchased locally, and unusually thick walled… maybe even 0.125"? Because it was not stainless, I primed and painted it.

That Primo build did NOT use the dual end stops, which I have come to really appreciate in my LR3 build. In addition, painting the tubing has not really worked very well… the bearings have worn tracks in the paint, badly degrading the appearance and possibly affecting performance, even though the tubing has been rotated occasionally.

On the other hand, the accuracy seems to be decent: a few days ago I used it to cut out a rectangular hole in a piece of hardboard. Over 16 inches, it was apparently out of square by about 0.050", which for that purpose was adequate. I’ve also been able to do a small amount of aluminum milling, which is also nice.

However, because of wanting to switch to the dual endstop configuration, and, to address the damaged tubing paint issue, I’m toying with rebuilding this Primo, which is primarily re-wiring, but could include new tubing. (My controller can handle the dual endstops, so that just requires uploading a new firmware version. )

But, I’ve noticed that after I built my unit, a new design was added that used the 3/4" conduit instead of 1" steel tubing. Obviously, from the cost and convenience standpoint, the conduit version is desirable. However, it’s clearly not as robust as 1" steel tubing.

So, my question is, are there significant compromises inherent in using the 3/4" conduit design? I realize that all the 3D printed parts would be new, but that cost/time is not a big deal for me. Or, is the 1" tubing enough stiffer that it is still the preferable solution?

My Primo has a work area of about 24" x 24" and I would not be changing that. Thus, all the belts, motors and bearings would be re-used in either case.

Any insights into these tradeoffs would be greatly appreciated.

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The 1" OD tubing is going to be a more rigid solution. Especially because you can purchase different wall thickness. You can get .120" thick wall tubing if you want. Even the .065 wall is plenty strong for a 24" x 24" I believe.

I went with .083" wall on my 24" x 36" Primo and I haven’t noticed any issues over the years. If you have the funds to go with stainless tubing then I would suggest to do so.

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what you are considering is going from a Cadillac to Hyundai. I would advise against it.

Drawn over Mandrel (DOM) is the stuff they build racing buddies and roll cages from. There is different grades of it, but even at its worst it is significantly stronger in every way to Conduit.

If you wanted to trade your Cadillac in for a Ferrari, there is Chromoly. Buyer be warned, Chromoly has a Ferrari price tag. I think DOM steel is the best balance of strength vs cost for most applications. But the conduit is significantly cheaper which is where something like these machines that are designed around low cost of entry and easy to source parts come in. These machines have been running for years on conduit and it works.

Perhaps the solution would be to strip the paint and use gun blue or something like a cerakote that is designed for rugged use.

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Yeah, this is possibly a case “if it ain’t broke, dont fix it”.

I haven’t tackled setting up the dual endstops yet, but the paint continues to chip off, so I SHOULD do something.

I’ve never tried using gun blue, but, it would involve stripping the paint, thoroughly cleaning and then bluing 6 pieces of ~36" long tubing. Based on the effort that was required initially, I’m not set up for this.

Interestingly, at Speedy Metals, things are on sale for a couple of days and the required 304SS is only about $11 more than the same amount of DOM tubing. So, if new tubing is a preference, SS is an easy decision.

A quick check of pricing indicates that the materials for the stripping, cleaning and blueing might be a much as $50, plus time, so that’s not necessarily clear cut.

On the other hand replacing with conduit would only be $20, but would require re-printing nearly all parts so another spool of filament or two would increase that to $50 or $60. Plus time.

Still thinking about it…

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Bench grinder on marketplace with wire wheel for 20 bucks and do one at a time.

Gun blue was just an idea because it’s easy wipe on wipe off application. Wiping them down with oil would prevent rusting as would some of the nano polymer .

I’m just thinking outside the box here to give you ideas. I think you have ruled out the conduit because of the printing cost and the down grade in quality from what you have now. Just be careful if you buy new DOM or Stainless that you dont go to a thinner wall. I’m sure there is some room to play, but i wouldnt go much thinner than you already have.

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Personally, I’d just refinish the tubes that you have. A spray can of paint stripper and a bit of elbow grease will deal with most paint on steel, unless you used a really good etching primer, a bit of tool wax to keep the corrosion at bay and call it a day. It’s the cheapest option (You don’t even have to use real tool wax. Paraffin from a cheap wax candle actually works very well. Re-apply about twice a year.)

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Well, I did attempt to use a good primer, but who knows how good it really is. Certainly the bearings are chipping all the way to the bare metal.

But, I’m getting pretty convinced that the time will be better spent getting the dual endstops in place and getting some stuff made in preparation for the big show this spring…

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Thanks for your comments. I actually have a bench grinder so a wire wheel would help make short work of this effort.

Time to get rolling.

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Stainless steel tubing is not that expensive, I’d think about using the printed parts you’ve got with new SS tube. In my area, a 20 foot length of 1"x.065 stainless steel tube costs about $75 Cdn (approx $57 USD). In Canada, where I am, 3/4" conduit is ridiculous expensive at the local big box stores, for example, at Homedepot Canada, it costs 3X what it does at Homedepot U.S. :face_with_symbols_over_mouth: Fortunately, it is available for a reasonable price at specialty electrical suppliers.
FWIW, I’m in the process of building a primo, I’m using 1" SS and going with the largest work area possible with a single 20’ length.

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Thanks for your comment. I’ve not found stainless steel to be anything close to your cost. 6 pieces of 3 feet long is going to run over US$100, whereas a 10 foot piece of conduit is only about $10 at our local Home Depot.

If the conduit design had been available when i built the Primo, I would have used it. That’s what I used for my Burly and it was never an issue. But, given that I HAVE the 1" x 0.120 DOM, it’s hard to just walk away from that. But, SS would sure be nice.

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I would go with what Dan said about reusing the tubes. A wire wheel and a can of Johnson’s paste wax and you’re good to go. My Primo is about the same ago and size, built shortly after release with .083 DOM, and I’ve had to service the tubes exactly twice. That’s even only starting using the wax after the second service, which was yesterday. All it took was a rubdown with some 600grit, and then a coat of wax.

As for SS being “better” than DOM, in terms of stiffness, rigidity, etc. (I’m hesitant to use specifics because the exact terminology is like when you say something is in “good” condition, you mean its “not bad” but certain folks take the term “good” as a very specific condition :sweat_smile:), I believe testing has confirmed that the DOM is struturally better, and is more apt to resist bending and sagging.

Just my $.02.

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Thanks for your comment. It sounds like the need to avoid rust on the DOM steel is very manageable (especially here in CO where our humidity is sometimes in single digits! ) so, this is essentially an “if it ain’t broke, don’t fix it” situation.

It’s clear that my time is going to be better spent stripping the paint and getting the dual endstops set up rather than printing an entirely new set of parts for conduit.