Construction of A Powered Eight-Rat Treadmill
Keith G. Benedict
More work on the side frames and the roll-around stand. The first thing to be made were two clevises which would connect the roll-around stand with the conveyor roller assembly. There were five steps involved in making these pieces:
1. Cut off two 4" long pieces of 1-1/2" x 1-1/2" square steel bar stock.
The first step was done using a horizontal cut off saw. Step two was done in the milling machine. Since the clevis slot needed to be a little bit wider than 3/4", two milling cutters were used. The first cutter was a four-flute 5/8" diameter ripple-tooth end mill; the second was a 3/4" four-flute convential end mill. A series of cuts 0.100" deep was made with the first cutter, feeding it into the piece from the end. After the slot was cut all the way through, the second cutter was used to widen the slot out to 3/4". The table was then moved 0.015" to either side of the center line to widen the slot out to about 0.780".
The third step was to square off the shank end of the piece to be a snug fit inside the tubing of the stand. This was also done on the milling machine, using a three-flute carbide face mill. The four sides were cut down to about 0.005" smaller than the inside of the stand tubing.
Step four was also done on the milling machine. The pivot pin hole centers were located using a needle point shaft in a wiggler body held in the drill chuck.
The piece was then center drilled
The hole was then drilled using two different diameter drills.
After deburring, the pieces were slid into the ends of the frame elements to check the fit. Note that the upper corners have not been chamfered off. This will be done later, prior to welding them into place.
The clevis pieces were done for the moment. The next task was to create the holes and slots in the side frame ends for the bearings that will support the conveyor roller shafts. After locating the proper position for the center point of the bearing, the hole was center drilled and then a series of three drills (1/4", 5/8", and 1") were used to drill out the hole. This hole must provide clearance for the roller shaft, which has an outside diameter of 0.990" so it will be made larger in another operation.
The next step was to locate, center drill, and drill through a 1/4" hole at the center of the bearing bolt slots. This was done to allow easier down-feeding of a 3/4" end mill, which was used to make a slot 0.200" deep. This would provide a recess for the head of a 5/16 carriage bolt.
A 0.312" wide slot was then cut by using a 1/4" end mill. The slot was cut all the way through on the center line and then the table was moved 0.031" either side of the center line to enlarge the slot to the proper width. A carriage bolt has a square section under the head that is the same size as the outside nominal diameter of the bolt. There are two advantages to cutting the slots to the right width: (a) the bearing bolts would be free to slide in the slot to permit adjusting the roller to be tangent to the top surface of the conveyor skid plate; and (b) the square head will not allow the bolts to turn, permitting one hand/one wrench assembly. Note the carriage bolt laying on top of the milling vise.
The center hole was then enlarged using a 5/8" ripple-tooth end mill to make a 1-1/4" square opening. This would provide for clearance around the conveyor roller shaft.
Shown here are the two sides, one showing the outside surface where the bearing will be located, the other showing the inside surface and the slots for the bearing bolts.
Next week I plan to continue working on the sides and the roll-around frame. The adjustable bearing plates which fit on the other end of the frame have to be milled out identically to match those on the fixed end of the frames, and the parts which will make up the tension adjustment mechanism need to be fabricated. That reminds me, I need to order some springs... better get hold of MSC Industrial Supply Co..
I'd planned on the original timeline to do a full test of the conveyor mechanism in about three weeks. I think I can make that deadline.