Calculator for conduit length?

I’m going to start printing parts for my first MPCNC this weekend. I want to build the unit to be able to handle 24"x24" sheet material, so having 25"x25" working area is probably what I’m after, but I need to know how long my conduit needs to be when I go to Lowe’s to pick some up. Is there a calculator for this?

Welcome to the forum. There’s a lot of helpful information, including the size calculator in the Docs section (which is linked at the top of the web browser forum interface.

as @ttraband stated, there is a calculator there, and the nice thing about it, it will give you the actual lengths of each piece of conduit you need for the given/inputted size desired.

Thank you. Looks like 2x 10’ lengths of conduit will do me.

Conduit is the cheap way, but I’d recommend DOM tubing

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I’ve not heard of it. What’s the difference?

Dom Tubing is a material that is “drawn over Mandrel” (no weld seam) giving consistent OD and ID of the tubing. It is considered a “structural” tubing and has a much thicker wall than EMT. The common wall thicknesses are 0.95" and 0.120" (which I used). I have 2 MPCNC’s that are larger than yours. Having used EMT in industry I know what it can and cannot do, its only advantage that I can see is its lower price. I have used DOM for years, and know it is more rigid, and is harder to deflect, which is good traits for the structure of the Primo. I can run my machines at speeds most would be shocked at, yesterday I was running some test cuts at 240 in/minute, with max deflection on the Z at 0.001" with 3/16 step downs.


DOM is steel tubing (not pipe) that comes from a steel metal supplier. It is measure by the outside diameter and is 1" (25.4mm) in North America. The extra diameter (over 23.5mm conduit) adds some rigidity. Stainless steel tubing also comes in that size, but is more expensive. DOM is not stainless, and can rust. You can coat it with paste wax to keep it from rusting.

Conduit is a champ too though. DOM is harder to get and more expensive. In a beginner CNC machine, conduit is a cheap, available, and works well. It is covered in zinc and won’t rust.

None of the printed parts are the same between a 23.5 and a 25.4mm build. So it isn’t easy to swap.

The 25mm tubing is more common outside of NA. The 0.4mm difference is enough to need a third size of parts, so the 1.05" pipe options in NA will not work.

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Thanks Jeff. I’ll be using 1" tubing. I do think I’m going to start with galvanized conduit, and if I want to upgrade from there, it really shouldn’t be too difficult. I figure if I can overhaul the rails and bearings on a Prusa, I can swap pipes on a MPCNC.

Thanks again to all of you.

There isn’t any galvanized conduit with 1" OD.

Conduit is pipe and it’s measured by what it can hold (inside diameter). 3/4" conduit has 3/4" ID and 23.5mm OD.

So if you start with conduit, it is 3/4", and the 23.5mm printed parts.

If you want to change a conduit build to a 25.4mm build, you need to print or buy all new printed parts.


Are you sure? Lowe’s carries “1-in x 10-ft Metal Emt Conduit” (Item #72715) It’s galvanized, which certainly might add a few mils to the OD, but it will be within what the printed parts can handle.

When looking at the part sizing there’s a trailing letter that indicates the outer diameter of the tubing the parts will fit:
C = “Conduit” - US 3/4" (internal diameter) electrical conduit, which has a 23.5 mm outer diameter
F - “Foreign” - European/international 25mm outer diameter tubing
J- “Jumbo” - US 1" (i.e. 25.4 mm) outer diameter structural tubing (either stainless or DOM)

As @jeffeb3 states, if you change your tubing, you’ll need to have the proper plastic parts. Generally, “tube” is specified by outer diameter, and “pipe” or “conduit” is specified by inner diameter (e.g. capacity or flow rate).

Fromt the DOCS pages:

Printed Parts Sizes

There are 3 different sets of printed parts C-23.5mm, F-25mm, or J-25.4mm (1 inch). The measurement is for the Outside Diameter of the conduit/rails/tubing. Please measure your rails before printing! 23.5mm fits ¾″ EMT conduit in the US. Anywhere else you must physically measure first. Some things are sold as Inside Dimension (ID) (conduit), or Outside Dimension (OD) (tubing).

Hardware store steel EMT conduit works well and is inexpensive; an upgrade would be .065” (max is 0.120”) wall thickness stainless steel tubing or DOM. Stainless steel tubing is more rigid and smooth, but also much more expensive, Dom is less expensive than Stainless but requires some coating to prevent rust.

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Welcome @pruckelshaus !

Lowes website specs only mentions 1" (bit vague and misleading), but Questions section says 1.163" is the O.D." (outside diameter)

HomeDepot’s website product specs do a better job of detailing inside and outside diameter

Personally spent bunch of time deliberating what to do for my build. Ended up going with common choice of 3/4" EMT for my LR3. Can always upgrade if needed and am heavily using my machine for tougher jobs.

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This is not a “correct” option for the MPCNC. The outer diameter is too big to fit any of the designed plastic parts. The conduit version of the MPCNC is designed for 3/4" (inside diameter) conduit. Try this link.

Because this is conduit, it is specified by internal diameter, because what’s important to those selecting it for its primary designed purpose is it how much (wiring, other tubing, etc.) can fit inside of it.

My name is Ryan, and I approve these messages.


Good work everyone. Probably saved a lot of future heartache.

Peter, at 24x24 3/4" EMT is perfect for wood, no worries. If you have a local metal shop of a few extra bucks you can get DOM online it could allow for a bit faster cutting and should make metal cutting easier, but for wood it is not absolutely necessary.

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1" dom, 25.4mm J parts. See the speed test vid I posted today on the FaceBook V1 group. I doubt very much that speed would be possible with Conduit, as its rigidity is a lot lower.
The problem with conduit is it is based on ID, not OD, and its wall is very thin, that why a conduit bender works. So trying to start with the files for 1" conduit, and then trying to upgrade later makes for a frustrating ordeal. Better to start with 1" DOM, and save the frustration.

That my opinion, not gospel

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Your recommendation for the expensive tubing is based off a large build with a very tall Z axis. That Z of yours is compounding any flex several times. A normal-sized 2x2 build with the minimum Z screams through wood. If you look at my speed tests vs yours, since it is the same tubing, you will see how much faster I can cut than you just with a normal sized build. You needed the DOM mainly because of your choice to do an extreme Z. CNC Drag Race, 0:39! - YouTube

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