Construction of A Powered Eight-Rat Treadmill
Keith G. Benedict
The four end pieces for the rollers were finished on the lathe. The tubing has a bur inside from the seam welding process which needed to be ground out. I removed the bur using a pneumatic die grinder. I turned the OD of the end pieces to give a 0.005" clearance between the end piece and the inside of the tubing, allowing easy insertion while providing for accurate alignment.
The end pieces were tacked in place in four places around the circumference. After checking them to ensure accurate alignment, they were welded in place using 60 ksi wire in my old Linde 160 wire welder.
After welding, I ground off the excess weld to be sure I had 100% fill of the joint. I found a few places that might not "clean up" during machining, so I rewelded all the suspicious places and ground off the excess again. After another inspection, I decided that everything was OK so I annealed the weld joint with a "rosebud" tip on my oxy-acetylene torch. This is necessary to get rid of hard spots in the weld joint, to prevent chipping and premature tool wear during machining.
Localized heating caused by arc welding has a tendency to cause a crystalline structure known as martensite in the areas adjacent to the weld. The carbon in the base metals migrate toward the weld joint; rapid cooling as the heat is sucked out of the weld joint by the cooler material around the joint causes martensite. Annealing, which requires heating the material in a broad area around the joint and allowing it to cool slowly turns the martensite into ferrite, which is easily machined.
The next step was to begin work on the platen/tubing frame. The problem was that the plate, a piece of 3/8" hot rolled steel, was warped. If this were being done in a big shop, I would have used a hydraulic press to put a counter bend in the plate to straighten it. Problem was, I don't have a multi-ton hydraulic press! I tried to straighten the plate out with my truck, by supporting the ends of the plate on some wood blocks and driving over it, but with little success. Time to get out the heat and the heavy duty C-clamps.
The plate was put up on my weld table and clamped down on one end. A bar was placed under the bent areas and Rosie applied the heat. Heavy duty C-clamps were then used to pull the plate down while it was hot and somewhat plastic. By careful work (and a few sledgehammer blows) the plate was straightened out enough to be OK.
(Sure wish my good friend Bill Leth was here! He's an artist with two MFA degrees, one of them in blacksmithing. I've seen him do things with heat and steel that is hard to believe. I first met him when I was a student at USI almost twenty years ago, when he threw me out of the school's machine shop. He didn't know me then, and I was doing something he hadn't seen done before and which he thought would damage some of the equipment he was responsible for. We soon worked things out, and later collaborated on a project of restoring the striker mechanism of the Vanderburgh County Courthouse bell. I'm sure that if he had been around the job would have come out better and taken less time.)
The plate was then flipped over and the 2" x 2" square tubing was clamped in place and welded. I used a "skip" weld, with two inches of weld and two inches of space between the welds.