Wednesday, September 2, 2009

Altitude Bearings







Now that I completed the upper cage as far as I can until I can buy the rest of the parts, I am now making the altitude bearings and pole seats. To make the altitude bearings, I set up my router with a circle cutting jig I made to route out a plywood arc 23 and a half inches in diameter and 3 and a half inches wide. The altitude bearings will be hollow arcs to avoid interference with two of the poles and pole blocks, and I can use a pie shaped piece of plywood on each bearing to stiffen the unsupported end if the need arises. Since this is not a very large Dob, I doubt the need for that will materialize, but I can add them if it does.

I then routed out the outside curve by running the spiral cutting bit to the inside of the pencil line drawn with a compass made from a piece of scrap plywood. Then I routed the inner curve with the bit running along the outside of the pencil line but stopping just past the line the compass pivot point was placed in the exact mid point thereof. Then I cut out the arc on the table saw, which also yielded smaller semi-circles that can be used for bearing on a smaller telescope.

I will glue them together with another sheet of 1/4-inch plywood to get the 1 and 1/4-inch thickness I was planning for them, then I will trim off the rest with a trim bit. Then I will locate the holes for the bolts that will attach them to the sides of the mirror box, then drill and countersink them. I'll make a template that will also be used to position the bearings and the holes for their bolts on the mirror box correctly. Everything will be made in a manner that will allow disassembly for either repairs or upgrades if necessary. The outside edge will be covered with Ebony Star Formica and the rest of the bearings will be coated with four coats of exterior grade polyurethane.

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