Construction of A Powered Eight-Rat Treadmill
Keith G. Benedict
Several significant activites took place this week. The first thing on the activity list was to turn the second inner shaft. This took about 5 hours from start to finish. (No pictures of this; I've probably put up enough boring lathe pictures for a while.) One comment - I cut the size of the shaft-to-roller diameters and shaft-to-bearing diameters to be about 0.0005" oversize. I plan to remount the shafts in the lathe prior to assembly and use several grades of abrasive cloth strips to hand-fit and polish off the diameters. I will also use several grades of abrasive cloth strips wrapped around an air-grinder mandrel to polish and lap out the roller bores. I want a snug fit that will yield maximum concentricity of the roller outside diameter to the bearing's rotational center. The less eccentricity, the smoother the conveyor will run. I'll post pictures of these operations when they occur.
The second activity was to cut and assemble the pieces that would form the main elements of the roll-around frame that the rest of the components would sit on. The frame is made of 1-1/2" square 11 gauge wall thickness steel tubing. The caster wheel mounting plates were cut and drilled during the first week of the project. The first step was to clamp down the wheel plates to the welding table. The design specifications call for an overall width of the machine to be a maximum of 32". For maximum stability, the wheels should be as far apart as possible, so this dimension determined the maximum width of the outside of the wheel plates. Care was taken to ensure that (a) the required distance was held, and (b) the wheel plates were square and parallel to each other. A cross tube of the required length was aligned, clamped down on the wheel plates, and tack welded in place.
When the two end sub-assemblies were done, they were mounted on each end of the table at a distance determined by the size of the conveyor frame connection points. Again care was taken to ensure that the ends were parallel and square to each other. (Geometrically, I wanted a rectangle, not a trapezoid or a parallelagram.) To achieve this I used a sixty-plus year old carpenter square, a legacy of my grandfather, to square up the frame. As a double check, I measured each of the diagonal lengths of the wheel plate corners and compared the measurements to ensure the squareness of the frame. Tubing of the proper length was clamped in place and tack welded to the wheel plates.
Two of four required corner columns had been cut off and were the next pieces to be tack welded in place. Diagonal braces were clamped in place to hold the columns perpendicular to the rest of the frame. Pictures of this are shown here.
I ran out of tubing so I will have to get some more and deal with the other two columns next week.
The next activity was to cut the pieces needed to make the front conveyor roller supports and to modify the other end of the conveyor side frames. The pieces were laid out using layout blue and marked with a carbide-tipped scriber. At the top of the picture you can see one corner of the frame and a wheel plate.
Two of the pieces are cover plates that are 3/8" thick and needed to have about 3/4" cut off one edge. For this I used a fifty-year old vertical bandsaw that I recently overhauled to cut off all but about 1/8" of material.
The pieces were then cut to the proper width (2.700") using the milling machine. (No pictures of this.)
The conveyor side frames were then mounted in the milling machine vise and milled out to form a rectangular pocket 2.505" wide. A ledge was then milled into either side of the frame 0.760" above the bottom of the pocket. The length from side-to-side across the ledge was cut to 2.705". This allows the cover plates to fit nicely. When they are welded in place they will form one side of a rectangular pocket. The bearing support plates are made from 0.750" material, and will be milled to a width of 2.500" on one end. This should allow a enough clearance to prevent binding but still be snug enough to keep any rattling and vibrations to a minimum. The overall concept is permit movement in one direction of the idler (not the driven) conveyor roller to take up the slack, apply tension to belt, and ensure proper belt tracking by using an adjustable spring-loaded tensioning mechanism. This will be seen in the upcoming weeks.
Next week I plan to mill the bearing support plates, weld the cover plates in place, and cut off and weld in place the last two frame columns. There are also several small pieces that need to be machined that will make up the motor mount, and the bearing support plates need some drilling and milling work to be done where the bearings are to be mounted.