Maybe I missed it in the docs, but I’m still uncertain what tube I need for my Lowrider build. I’m planning on a 3’ x 5’ build area and have a few questions.
I can at least answer this one: No, it isn’t too heavy. I use it for instance. 
If you’re US based, the recommendation is to hit Lowe’s or Home Depot for this:
3/4-in x 10-ft Metallic EMT Conduit
To be clear, you have choices as to what size to use. From the Lowrider Documentation page:
There are three different brace sizes provided - Brace 25.3mf, Brace 25p4.3mf, and Brace 23p4.3mf which correspond to the diameter of the X rail in millimeters. For US based builds, the recommended X rail size is US ¾” EMT conduit which has a diameter equal to 23.4mm, so print the Brace 23p4.3mf parts for this size X rail.
As mentioned by others, 3/4" EMT is one option, especially for North American builds, as it is readily available and pretty cheap. Use the 23.4 mm braces. Choose the least bent piece you can find at the big box store.
1" SS Tubing is another fairly popular option in NA. There are different wall thicknesses available (.060" and .120"), Either thickness still has the same OD. Use 25.4 mm braces. Usually available at Metal Supermarket or similar metal suppliers. Arguably stiffer and stronger (although also heavier), and definitely more expensive.
25mm tubing is more of a European standard, and probably not applicable to your build, unless you are outside NA.
Note also that the Y rail also adds the choice of 1/2" EMT. Again, you need to print the correct version for whatever choice you make.
Opinions on some of your other questions:
Yes you can, and yes, it is the “recommended” or default option, based mostly on availability and cost (I believe). It is never a bad idea AFAIK, but it can bend a bit easier than SS Tubing (although there aren’t really any forces in the LR3 that would cause that, more likely from trying chin-ups or dropping the gantry from height)
No, you have options. See the reply above.
You don’t “need” it. Depending on the OD, it may be an option, but I personally would avoid welded seam tubing.
That depends on your definition of helpful. It can add stiffness and strength, while also adding weight and cost. The weight isn’t a huge issue (IMO), but the cost may be (depending on your circumstances and budget).
Thanks so much, that makes total sense. I am in the US (in Seattle, WA). I was checking MetalsSupermarket, and all the tubing for my build was coming out to $350, so knowing I can use EMT is a relief!
Just out of curiosity, does welded vs seamless make a difference for SS? If I did welded, that brings the cost of SS tubing for my build down to $200.
Edit: NM, see my question about welded was answered. Thanks!
That’s surprising. I bought 12’ of .120 SS 1” tubing for less than $120 CDN ($90 US?).
You don’t need SS for the Y axis, the EMT is probably preferred there, as there’s no stiffness or strength requirement for that rail.
Steel prices can be quite volatile, and highly dependent on location.
I would second the opinion onnusing conduit for the Y rail, it’s more than adequate. In fact the 3/4" conduit for the X beam is good. (I use 1" DOM because I can get it cheap.) It isn’t as shiny, but after you start using the machine, will it still be shiny?
That’s a good point, I was pricing for all the tubes as SS. It’s better if I just do the X axis. Just to offer a datapoint, for me in Seattle shopping at metal supermarkets, it’s $200 for three 36" SS tubes (seamless 1.000 x 0.065).
There are various marine suppliers that are more expensive. I couldn’t find DOM steel tubes on metal supermarkets, but online metals says $65 for three 36" tubes, which seems totally doable.
Do you have to worry about DOM steel tubes rusting? Would you need to paint them?
What is the build size? You only need two sections for the X axis rails.
If it does not say either stainless or galvanized, it can rust. If it says something like “mill scale” finish in the listing, it may be decently protected already. Painting is doable, but the path of the bearings will likely rub paint off where they travel. A coating of wax may be as good or even better than paint. The EMT seems resistant to rust, as it has a galvanized finish.
I used DOM stainless steel tube on my first LowRider (a v2) because v2 needed more stiffness from the tubes as it did not have braces and struts providing “webbing” strength like the v3. And because I already had it, I used it on my first v3 LowRider. I built a second v3 LowRider (for plasma) and I used the recommended 3/4" EMT on it, and it has a gantry well over 4 feet, and it’s fine. I don’t have a router on it now, but I did. I thought (imagined?) I detected a little bit of twist while with a router, but I did not measure it and cannot confirm it.
It’s amazing how much steel prices vary from place to place. There are multiple steel producers not too far from me so I can get steel comparatively cheap, a local supplier is asking $75 Cdn for a 20 foot length of 1" x.065" stainless steel tube. And the local branch of Metalsupermarkets.com is asking $120 Cdn for the same.
I feel silly - I thought the gantry had three tubes. At least I’m not the only one to make that mistake.
Thanks for all the help - it sounds like SS isn’t bad as I feared, but I also feel really good about going with EMT, expecially since I’m planning a shorter gantry.
At least you caught it beforehand. I can visualize myself doing this and holding the extra piece in my hand wondering where the heck it was supposed to go!
For a while I thought the same thing too. An assembled one does look like it has 3 tubes, a bit anyhow.