Sunday, July 12, 2009

Split blocks


Since I have idle time on my hands, I decided to makes the split blocks for the truss poles. After testing and rejecting the idea of welding aluminum conduit to a piece of aluminum flat stock, I opted to make wooden pole blocks from Hard Maple in the same manner as the Obsession telescopes. I found 1-inch aluminum conduit and 1-inch OD tubing to be of such a loose fit that I opted to take an easier way out.

With a 1-inch Fortsener but and 1-inch outside diameter aluminum tubing, drilling a hole that holds the truss tubes firmly yet allows me to insert and remove them easily is a snap. After buying some Hard Maple, a tough hardwood that resists splitting, I laminated it by cutting it into three sections then gluing it together to form a thick piece. After the glue dried I ripped the wood to width, then planed it to the correct thickness before carefully cutting eight identical blanks. First they were beveled with a router and a 45 degree chamfer bit around all the edges except for the back side.

Then I set up a temporary jig to position each piece on the drill press, and drilled a 1-inch hole slightly deeper than 2 and 3/4-inches. To make the collimation consistent, I had to make all the pole holes the same depth. Next I drilled the holes for the screws in each corner then counter sunk them.

The next steps were to drill a hole in all eight blocks for a bolt and knob at the same spot on each block. The bolts and knobs will be used to secure the poles in the block. Then I used another jig to hold each block in the correct place to cut the slits in each block while keeping my body away from the blade. I sectioned each block into thirds with the middle third being used as a cam to grip the poles. The slits were cut into the block all the way to the far edge of the pole hole from where the bolt will go. Then I hollowed out the back of the block along the middle section just enough to expose the pole hole to allow the middle portion to flex and hold the pole tightly.

After that, I sanded the blocks to clean up the wood in preparation for varnishing. They will be coated with three coats of exterior glossy polyurethane and set aside until the mirror box and upper cage are complete. Once I determine how long the truss poles will have to be, I will know how to position and angle the blocks on the mirror box, to which they will be secured with stainless steel machine screws, nuts and washers to allow removal in case repairs are needed in the future.

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