Maximum Wall Thickness?

I would like to use 1" OD DOM steel tube for my MPCNC, which I would get case hardened for maximum durability. From what I’ve read, DOM had very good tolerances, and when case hardened should be even more durable than stainless. My heat treater says that the can straighten all my tubes after treatment for only a few dollars extra, but at 0.49 wall thickness they say its likely thy will only be able to get a 36" piece of steel straight within 0.035". They said if I can beef it up to 0.1" that they should be able to ensure a much better straightness.

My question is, how thick/heavy can the rails be before you start to run into problems with fast movements or belt stretching?

Weights of different DOM tubes, (two pieces, 36" each)

1.09" is 6.22 lbs
0.083" is 4.87 lbs
0.65" is 3.93 lbs

I can’t comment on DOM, I have only tested stainless and conduit. Be careful, Stainless is nice and rigid and won’t rust and pit, most steel is not and has lots of flex.

Weight is a moving mass issue, belts are not going to stretch before you break plastic. The heavier it is the slower your accelerations, and cnc’s are basically all acceleration plus and minus. It has a huge effect on cut time. If you go too big it will also sag under its own weight.

What will the primary use be for your cnc?

I’m almost positive that 1020 should be very close to the same rigidity as 304. I know the risks of rusting, I plan on keeping everything well oiled. Have you noticed much wear with stainless rails? I know that smaller stainless rods wear down very quickly when subject to steel on steel bearings. I’m planning mostly on doing light aluminum work (plates and brackets) and engravings, and if I can get away with it I’d like to be able to make simple molds for resin casting and injection molding. I also need a few relatively large things I need 3d printed (new Z-carriage for my main printer).

The good thing is that since DOM is very cheap, and since I pay the same amount per batch for heat treating up to 50lbs, I could just buy enough steel tubes for whatever configurations would be best for each process, and then harden them all for the same cost.

EDIT: 1020 actually looks more rigid than stainless.