Construction of A Powered Eight-Rat Treadmill
Keith G. Benedict
The next two steps in the construction process were (1) turn the conveyor belt rollers to size, and (b) drill and tap the aluminum plate that the belt will slide over. In order to turn the rollers I needed to find a shop with a lathe that could handle a long piece of material. My lathe can only hold a piece 50 inches between centers and the longest shaft was almost 58 inches long. I decided to call my old friend Alan Moore, owner of Moore Machine & Gear
I gave Alan a general description of the rollers and then asked if he had a lathe big enough to do the job and how much he charged. He said he had one large enough and the rate was $30/hr. As I was mulling it over Alan said "Of course a little B.S. from you goes a long way. You can use it for nothing if you do the work when we aren't using it." Alan is the salt-of-the-earth type, bless his kind heart!
We arranged it for Friday morning. Alan showed me the machine I could use. It was a classic machine, a 1940-ish Monarch engine lathe.
Don't let the cosmetic appearance fool you. The machine had character and was as smooth and steady to operate now as it was 50 - 60 years ago. It was one of the top notch American metal working machines made during or after WWII, when American manual machine tool technology reached a pinnacle of perfection. I didn't see a tag on it, but I have run similar machines in the past that were designated "War Reserve", "Property of U. S. Government", and similar, all dating from WWII.
The photo below shows the shorter of the two rollers being turned to true it up. Note the difference in appearance in the cut area between the end of the roller and the tubing used to make the body. The slight difference in material composition and grains structure can be readily noticed.
I spent all day at Alan's place, taking light cuts (to prevent deflection) at low speeds (to prevent chattering and vibration). I just wanted to remove enough material from the roller body to make a true cylinder. What I didn't realize was that the type of tubing I selected for the roller was the wrong type. It was welded tubing, made from a flat strip of steel that was folded around a mandrel and welded along a continuous longitudinal seam. This causes three problems during machining: (1) The material is out of round, (2) the stresses induced by welding causes warpage during machining, and (3) the tube stock is not very straight to start with. The end result was that after machining, the first piece still was not trued up all over and the wall thickness was way too thin. After I left Alan's I decided to discard the piece, and the other roller with it.
I thought about the problem all that night and decided to redesign the roller. I would use a piece of seamless tubing with a thicker wall. I went back to Alan's the next day to get my tools and thank Alan for his courtesy. When I told Alan of my new roller design he suggested that I look behind his shop in his storage area. I did, and found exactly what I was looking for. Alan cut two 50" long pieces from the 20' bar and just gave them to me. Thanks again! I owe Alan for all his help. Maybe one day when he gets swamped I can help him out.
I decided to work on the aluminum plate next, since I had borrowed a set of transfer punches from my nephew and I wanted to return them ASAP. The hole centers were punched through the subframe into the aluminum plate while the two pieces were clamped together. The two pieces were then moved as a unit to the drill press, where two 3/16" diameter holes were drilled in diagonal corners. Roll pins of the correct diameter will be pressed into the holes in the aluminum plate and will align with the holes in the frame when the two pieces are finally assembled.
The sub frame was removed and the 51 holes were drilled into the aluminum plate.
The holes needed to be chamfered or countersunk before tapping threads. Chamfering puts a small bevel at the top edge of the hole, enabling the tap to start straight into the hole and preventing burrs at the mouth of the hole. This was easiest done using a hand electric drill with the proper tool.
The holes around the outside were tapped using a striaght-line tapping tool. Aluminum is soft and tears easily, so a sharp tap and the proper cutting fluid are necessary to cut good threads.
The holes in the center of the piece had to be tapped by hand, using a T-handle tap wrench. Care was used at this point to make sure the tap was started straight into the hole before tapping.
After tapping, the plate was set aside. I started on the new rollers. The new design would be a two piece assembly, consisting of a large roller with concentric holes bored in each end, and an inner shaft that would go through the large roller and be clamped to it with setscrews. This would allow me to do all of the work on my own lathe. Two plugs were turned on my lathe and inserted into the end of each piece of tubing. After squaring them up I welded them in place. I would chuck up on the end of the tube that didn't have a plug in it, then turn the piece all the way from the welded end to the lathe chuck. I would then turn the piece around, chuck up on the cut end, line up the other end with a lathe attachment called a steady rest, and turn it to size. I would turn the inner shaft in the same fashion. That's next week's job.