It's been a couple of weeks and it seems everything I touched has not worked or broken so I wasn't too interested in building a prototype. As John said, the proof is in the pudding so I went ahead and started one today. (As proof of the above snake bite period, I didn't get the post a pic on my last post! The block showed up here so I will try to edit that post later.) More proof! :p When I was sizing the tubes, I forgot that the adjacent sizes "nest". Therefore, a 1/8th outer tube is needed with a 1/16th inner tube. Of course, I didn't order any 1/8th but found them at the "local" hobbyshop (only 65 miles away). After I got home, I remembered that I also needed some 5/32nd size, that is why I didn't finish the prototype today. I did buy the DuBro #785 tubing bender that John mentioned and I also got the K&S spring style bender set. The DuBro works great and bent a 1/8th tube into a 180* turn without kinking with an outside dimension of 1/2 inch. As John said, take it slow.
Try to follow the pic below along with the description as I might get it complicated. I was able to hold the 1/16th tube tight in the die with a finger and get a .365 outer dimension for a 180* turn. Not quite to my desired size, but good enough for a first try. (I haven't tried the K&S bender yet.) The hole in the 1/16th tube is a #65 drill (.0345). I hand filed and sanded the tip to a hemisphere looking shape and chamfered the inner edge also. Then I moved off to the 1/8th tube. Using a 3/32nd tube as a spacer, I decided where the static holes would go and drilled 4 #68 (.0310) holes at approximately 90*. I was able to use my Dremel for these holes as I have a 3 jaw chuck attachment for the tip. I used a #54 drill and hand spun it to deburr the holes. The inner dia of this tube just fit a #41 drill so, about 1-1/2 inches from the other end of the tube, I made a #41 hole to "vent" the static air. I assembled this much using a short piece of 3/32nd to center the 1/16th in the 1/8th and soldered the 3 together . You can see part of the 3/32 sticking out of the 1/8th in the pic. Then came the first design change! I started to do the same to the cut tubes at the other end when it was apparent that the 1/8th could have extended to the end of the 1/16th and been sealed the same way. This would have provided a 1/8th OD for the flex tube to attach to. I will cut this section off for the same result, but will lose about an inch of length. When I get the 5/32nd tube I will be able to "tee" off the 1/8th for the flex attachment there, but I'm not ready for another 130 mile drive just now. So far, no problems with construction and if the "tee" arrangement works, I'll be finished in no time! Yeah, right! ??? Then, of course, it will be test time! Everybody got a spoon for the pudding?
Doug
BTW - The straight tube shows how the 3 sizes nest. I added a black band at the end of 2 so you could see where the joint is.