Construction of A Powered Eight-Rat Treadmill
Keith G. Benedict
This week I decided to work on the side plates for the conveyor frame. The first step was to cut off the material by using my cut-off saw. The material selected for this was 3/4 inch thick, 4-1/2 inch wide low-carbon cold-rolled steel (CRS) plate stock. Cold-rolled steel is made by taking slightly oversize hot-rolled steel (HRS) plate which has been acid cleaned of all surface scale and rolling it between massive rollers down to final size. It is cleaner, stronger, and dimensionally more accurate than hot-rolled steel plate.
The second step was to lay out where material would need to be cut away on the milling machine, so I coated one of the cut off plates with layout dye. Lines were then marked on the surface using a carbide tipped scriber. This allowed the marks to be easily seen.
The two plates were then tack welded together to keep them aligned and clamped in my milling machine vise. A three flute carbide milling cutter was then used to cut away the material where required.
The side frames are made from 3 inch wide channel. The flanges of channel stock have a taper on the inside. In order to remove the taper and ensure that the end plates would fit flush inside the channels, the milling cutter was used to cut away some of the inner material of the flanges. I milled the channel out to 2-1/2 inches wide, which left 1/4 inch of stock on the flanges.
The end plates were then separated and deburred and clamped in place inside the channel sections, where they were tack welded in place.
After tack welding, the end plates were finished welded all around. I then set them aside and started on the inner shafts of the conveyor rollers.
The inner shafts are made from 1-1/8 inch diameter CRS round stock. Because they are so long and slender, the steady rest was used to support the middle to prevent deflection. While this helped reduce the deflection, the length-to-diameter was still high enough to cause chatter and a rough finish, so a light feed and low RPM were required. A high speed steel (HSS) tool with a sharp point and a lot of rake angle was also needed. Machining the inner shafts is a slow process, and I was only able to finish one of them.
Next week I plan to finish the other shaft and continue working on the side plates.