IV. Mounting

4.3 Fork

The fork is fabricated from 0.250" 6061 T6 aluminum plate, two 0.500" plate, and two declination housing castings. The 0.500" plate constitutes the base of the fork and the 0.250" plate is used for the fork up to the declination housings. The aluminum plate was mated together so that a 1/4" radius weld could be made on the outside. Each seam was also welded from the inside. Accessing the inside for welding was very difficult. Several of the welds were done blind, since I could not always see what I was welding inside of the fork. The two declination housing are butt welded to the 0.250" with a 1/2" "V" grove weld. Two bulkheads are welded into the base of the fork for added torsion stiffness. One of the inner bulkhead can be seen just after the outside plates of the fork were welded together, Figure 4.3.1a. The inside of the fork and the inner welds can be seen in, Figure 4.3.1b.

a  b

Figure 4.3.1. The fork was fabricated with 0.250" 6061 T6 aluminum plate. The inner bulkhead is shown here. The outside pieces of the fork have yet to be welded in place , 4.3.1a. At this point in the fabrication process the fork has yet to be formed into its complex shape. 4.3.1b is the inside of the fork after the fork was finished. Each of the welded seams were welded form the inside.

 A 6" hole was drilled into the bulkheads for access to the polar axis bolts and to allow declination motor cabling to be run through the fork and down the polar axle. The other 0.500" plate is used as a spacer to allow the fork to clear the roller assemblies. The outside curves of the fork tine were bent by heating the aluminum and bending the plate under a pipe that was bolted to the welding table. The inside curves of the fork tines were made by cutting a 2", 0.250" wall tubing into four pieces. It would have been very difficult to bend 0.250" plate through such a short radius. Two of the these pieces were welded into the corners of the inner tine Figure 4.3.2.

Figure 4.3.2. After being welded, the fork assembly  is bolted to the polar axis. The cross section of the fork is a trapezoid shape. This shape results in the complex shape of the fork. All of the bright lines are where welds are ground to provide a smooth exterior. Note that outside of the fork is made in two pieces.

 The fork end castings were butt welded to the top of the fork assembly, Figure 4.3.2. The inner bearing journals were roughed out on a lathe prior to welding. The outer bearing journals were bored for the 60mm bearings. The outer fork plates were made in two pieces since the plate we had was to small to make the entire section from one piece. The light lines on the fork are where welds were made and ground down for a smooth finish.

After all of the welding was done, the fork needed to be annealed. We did not have access to an annealing oven large enough to put the fork in. Instead we used an old trick for annealing aluminum. The fork was coated with carbon from a carbonizing flame from an acetylene torch. Annealing is done by raising the temperature of the fork with the torch until the carbon burned off. The carbon will burn off at about the correct annealing temperature for aluminum. Annealing the fork relieved the stress built up from welding.

After all of the components of the fork were welded, the fork was ready for boring the inner declination bearing journals. The inner declination bearing is a 4" ID ball bearing. The 4" bearings were given to Bill by the local bearing house. The bearings were special order parts that were never picked up. The outer bearing journals were bored to size in a lathe before the castings were welded to the fork. The inner bearing bores were left rough machined from the lathe. Boring the inner bearing bores was very tricky since both of the bore holes needed to be concentric to each other despite the fact that they are some 36" apart. To bore the declination bearing journals the entire fork was put into a Bridgeport milling machine so that the inner fork bearings could be bored, Figure 4.3.3.

a  b

Figure 4.3.3. The fork was placed in a Bridgeport milling machine so that the inner bearing bores could be made concentric 4.3.3a.  After the head was aligned a boring head was placed on the mill and the bore machined to the final size 4.3.3b. Note the scale of the fork compared to the milling machine.

 A new draw bar for the Bridgeport mill had to be made so that two dial indicators could be used to position the milling machine head in relation to the outer fork bearings. The outer bearings were used to align the mill head to both of the inner bearing bores to within a few mills. A dial indicator was placed in the inner race of the outer declination bearing on the top bearing. Another dial indicator was placed to run on the inner race of the lower outer declination bearing. Once the milling machine head was aligned the first inner declination bearing journal was bored. After the first bore was made the fork was rotated 180o and the process was repeated.

The fork was fabricated over a period of about week. A friend, Ken, helped with holding parts of the fork while I welded, helping bend the outer tine curves, and giving suggestions. The fork took abut 80 hours to fabricate. All of the outside welds were ground to make a smooth finish. Grinding the welds took about 20 hours. The completed fork weighs about 300 lbs. Approximately 20 lbs. of 0.030" aluminum welding wire was needed to fabricate the fork.
 

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