Suggestion: Construct Frame from Conduit

Using steel conduit instead of the solid rods used currently would change the Darwin in several ways:

Neutral
The outside dimensions of the unit would be larger to still have the same work area

Positive
Weight savings (or is this actually a negative?)
Conduit/pipe is easier to cut than solid rod
Material is more readily available

Negative
Might "cheapen" the appearance?
The brackets would have to be quite a bit larger to accommodate 3/4" or 1" conduit requiring more plastic and print time to create

Has this been considered?

Another negative:
Having to support an imperial and a metric variant.

But if it gives us the rigidity to also do CNC routing, we should consider it.

SebastienBailard Wrote:
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> Another negative:
> Having to support an imperial and a metric
> variant.

If this thing is made in the US there will be SAE versions no matter what. It might not be the "offical" version but it will happen. The only way I see metric gaining the upper hand here is if the entire populace were compelled to take a year of college level physics

Is conduit an american name for pipe, or is it something else?

greenarrow Wrote:
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> Is conduit an american name for pipe, or is it
> something else?


Typical usage in the states refers to piping, plastic or plastic-lined metal that contain electrical and communications lines. Specifically, though, it can also mean a pipe that carries one or another type of fluid.

Like the stuff used for waste water from a sink?
Steel tubing would probably work pretty well with the current darwin design, as I would have thought it would still be strong enough

American conduit is usually defined as thin walled piping for holding electrical wires. It is usually made of aluminum, steel, or PVC. The internal diameters start at 3/4 of an inch and go up to at least 8 inches. The one to two inch diameter stuff is the most common. By thin wall, I mean that the 1 inch (27mm)steel conduit has a wall thickness of 0.315 inches (6.4mm). It bends, cuts, and threads easy, but resists crushing flat.

With that out of the way, the exterior finish of the conduit is smooth, but not as smooth as the solid rods. So there could be some issues with the z axis slider.
The only other problem that immediately comes to mind is that conduit tends to carry vibrations - which may or may not be an issue.

Thanks for the explanation Cris. The main thing I would add is that your dimensions for wall thickness are about 3x as thick as applies for steel conduit of the smaller diameters.

[www.wheatlandtube.com]

This page lists the wall thickness for 1" as 0.126" or 3.20 mm.

All the steel conduit I have ever seen has been galvanized so it wouldn't work for sliding rails. I was mainly putting the idea forward for the structural frame (uprights and cross braces) as it is light and inexpensive and ought to provide good rigidity.

An even lighter model could use conduit for the uprights and small cables with turnbuckles as diagonal tensioners.

The RepRap is either going to become more complex or the $400 price is going to continue to come down. Actually both at the same time would be best!

Rodzite is correct, the wall thickness is 0.126" or 3.20 mm.
I forgot to divide by 2 when I measured the conduit beside me - duh! ;-)

I'll agree that it could be used and if you can find someone that does conduit work to give you their scrap pieces for free it would be great; otherwise, conduit can get expensive.

I've heard of 1/4" ID aluminum conduit, which should have an OD close to the M8 rod that darwin uses, but I have no idea how much it costs. Maybe you should give it a try.

1/2" EMT (Electrical Metallic Tubing) Conduit is quite cheap at McMaster-Carr. 5 feet for $3.05 10 feet $3.73.

By way of comparison the 8mm 12L14 Carbon Steel rod is $11.72 for a 6' length. Even 5/16ths "High-Strength Easy-to-Machine 1144 Carbon Steel" in Unpolished (Mill) finish is $6.00 for a 6' length (give or take 6 inches!).

The Darwin needs about 8.5 meters of rod so close to $60 worth of 8mm rod vs. about $12 worth of EMT... Seems like a reasonable cost savings to me.

JohnWasser Wrote:
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> The Darwin needs about 8.5 meters of rod so close
> to $60 worth of 8mm rod vs. about $12 worth of
> EMT... Seems like a reasonable cost savings to
> me.

Short pieces of conduit are readily available as construction site waste. i haven't seen much 1/2" being thrown out but 3/4" is readily available. I have a bunch in my garage right now as a matter of fact.

Wow, I didn't realize conduit was that cheap. I must remember to ask the contractor I use why he is charging us so much for conduit.

That is a pretty good cost savings.
Thanks for edumakatin me.

How strong is that conduit? When I've used it in the past, it seemed to be really weak, very easy to bend accidentally. Seems like that might be a problem on a Reprap.

I've already had problems with my 8mm stainless steel threaded rod bending inadvertently on my Darwin, despite being pretty careful with it. The 8mm smooth rod I found is extremely tough and strong though; I found a box of old 8mm inkjet printer rails at a surplus shop, it's a very tough alloy, not stainless as it's magnetic, and perfectly smooth. A bit of a pain to cut, but a Dremel did it with some effort.

A dumpster tour could find a lot of inkjet printers these days, if you really want to save money. You might find 5/16" rod cheaper too - it's more readily available in North America, and close enough to 8mm.

If you want cheap 5/16" or 8MM steel rod and studding, go to your local steel supplier. (From the phone directory.) Carefully calculate the total length you need. They'll cut it to 10 ft/3-4 m lengths*, or smaller if you want it, so plan your cuts before you order. Fax or call in a request for a quote, and go pick it up. You probably want mild steel, rather than drill rod.

This will be less expensive and much more efficient than going to a hardware store or buying online. It is unlikely the hardware store will have enough on hand for the project; it would be best to call beforehand.

*Assuming this works with your mode of transport.

I don't find the idea of buying rod prohibitive to the project although it would be nice to use chromed steel or stainless to avoid having to paint it. It just seems that if stiffness is important that you get a lot more rigidity for an equal amount of weight if it is in the form of a hollow tube than a single solid rod.

I suspect someone could assemble a super-rigid repstrap by welding tubing together.

EMT/conduit may not have the proper ratio of strength/size but I'd be curious to see how it measured up.

How much does a completed reprap weigh anyway? Weight may not even be an issue.

Another thing to think about is that once these things start branching out for other applications someone is going to make a larger version. What will its frame consist of? 2" conduit or even better 2 5/8" steel fence post material would be much cheaper than rod of equivalent stiffness over multi-foot runs.

Maybe we could stuff some of the ugly wiring inside the conduit, that would also make for a more profesional look :-)

That is the same thing we thought...

oitreprap.googlepages.com

but it turned out bad. The wiring, I think, has gotten a bit of interference from the steel frame and maybe from the other wires. Not so good as you can't drive the motors. Perhaps using shielded cat cable?

Demented

Demented, still having trouble with that? Try making the output wires from the stepper driver boards as short as possible (ie, move the boards closer to the motors), and make sure you don't have any control wires running close to the stepper motor wires. The chopped output of the stepper drivers is essentially a noisy AC signal; running those wires near your control wires can create an ad-hoc transformer, maybe even a feedback loop.

Metal on metal loose connections can create lots of interference too; make sure everything is tight, and poke around with a scope.

Wade,

Turned out that we had accidentally--how I don't know--magintized the wires and they were acting as an inductor and causeing all those problems. Re-did the wiring harness in shielded cabling and the problem no longer exists. It was, I think, all due to the steel frame. No good but with the right cable--ie shielded--not a problem.

Demented

Copper wire magnetised ?

or was it a steel braided screened cable ? Interesting and I would like to know more.

I'm all for an SAE version. Would it really be much more difficult to provide two sets of CAD drawings. The cost saving would be significant for those in the U.S.A.

Actually, regarding the SAE version, Sebastien mentioned above that 5/16" is practically the same as 8mm. And as all the captive nut and bolts holes have a bit of play in them, the build documentation mentions you can substitute SAE for metric pretty much everywhere. No one's bothered to work out the exact sizes, but it should be no problem if you buy plenty of nuts and bolts. It'd be nice to update the docs if you do end up trying a SAE version, but I wouldn't hold my breath waiting for someone to do it for you.

The only place you'd have a problem would be if you have a set of Ian's cast parts with the captive nuts cast in place, like my Darwin - then you'd have to find metric grubs for those parts, but that's not a lot. I think he's switching to laser cut parts though, so I can't comment on the new kits.

I used all metric parts because I like working with them better, but even in Canada I had a little trouble sourcing stainless steel metric drill rod. I ended up finding a box full of old printer guide rods at a surplus store cheap, which worked perfectly. I was really, really disappointed when I found out McMaster Carr no longer ships to Canada, but I'm making do.

Squintz Wrote:
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> I'm all for an SAE version. Would it really be
> much more difficult to provide two sets of CAD
> drawings. The cost saving would be significant for
> those in the U.S.A.

Not if you order your parts from McMaster, MSC, or another similar industrial supplier. Most of them will actually be quite happy to take your order, and if you start doing any sort of serious tinkering you will find their selection to be enormously beyond even the best HW store. You might also have an industrial supplier in your area where you can do cash and carry.