Awesome machine! I'll be working on one about the same size starting late this year, but obviously different in design. I do really like how you built the gantry, seems very efficient.

I'm wondering about how you fabricated the dimensionally critical portions, where rails mount, etc. Did you have the steel stress relieved and then machined? I've been thinking of using 3/16" tubing and welding solid 1/2 inch bar stock to them to be machined for the rails and wonder if they will need stress relieving due to the unbalanced heat that will generate. Would it be better to use 1/4" tubing and have it machined directly instead?

There are a few ways to do it, I have used most. Yes welding on steel bar works but needs stress relieving.  Yes direct machining on thicker plate or tube works but can get heavy.  I have also done machines where I have bonded on aluminium or steel bar with epoxy and then machined the strip, I made the tapped holes go through both pieces to make sure there is no chance of a debond.  I have also done it by casting a bed of epoxy (filled with aluminium powder) and then machining that. That particular machine was done by making a dam on the edges of the tube and then pouring a self leveling epoxy to make a flat surface that we then just drilled and tapped holes in as a large mill wasnt available (well not a realistic price anyway) for when I wanted to do the job.  If you can get it machined its easier. A precision machine frame level and square comes in handy too.

I can sell you a full set of drawings and components at a reasonable price (or a complete machine), if you are interested pm me.

I appreciate your comments and help, it sounds like we are of like minds! I'd also considered using epoxy as a filler in a couple scenarios, but since there is a great deal  of aerospace here in Seattle, there is also a great deal of very large machinery at various job shops. It may cost a bit more, but I had narrowed it down to either welding a rail platform or having the tubing machined directly with a large metal planer (has about a 16 foot stroke). I have designed a stationary gantry style machine and should be able to fit the entire frame on the planer to machine both the rails and the flanges for mounting the gantry. I'm still looking for a shop that can stress relieve it in one piece though.

If you have access to the machines then definitely machine it in one piece or as few as possible. For stress relief try vibratory stress relief like Metalax.

For stress relief try vibratory stress relief like Metalax.

I love this place!  I learn something new every day.