I believe either drill rod or gas pipe is acceptable for a machine as you have titled. ENCO
www.use-enco.com has drill rod consistently on sale along with acme rod and nuts. Just make sure you size appropriately. Many machines have tried to use 3/4" DR for an X axis to support a moving gantry (my 1st attempt also) and 1/2" for Y and it just isn't enough. 1" drill rod is $14 there, 1.25" is $17. 7/8" is $12 and 3/4" for $8. The one thing about drill rod is it's easy to upgrade the machine to precision shafting of the same diameter it a person wants more precision. Same can be said for 1" gas pipe using roller bearings. my 2 cents