Tuesday, August 25, 2009
After several coats of exterior polyurethane, the rings, focuser board and finder scope base are finished except for a final coat of varnish. I plan to spray on the final coat after making sure no runs remain. The next step was to make a light baffle to put around the inside of the rings. I had thought about using Kydex, but I didn't want to hassle with it's idiosyncrasies. I opted instead to make it from thin plywood, and using the cutouts from the rings, I made a form. After carefully cutting a lot of beveled slats from thin plywood, I used painters tape to hold them together in a strip. I calculated the angle from the number of slats divided into 360 degrees, then divided each angle in half to get 3.5 degrees. After taping them together, I put wood glue in the gaps and wrapped the assembly around the form. Strap clamps then held the assembly in shape until the glue dried. After sanding, I primed the baffle then painted the outside with glossy black paint.
Sunday, August 16, 2009
After finishing the spider, I needed to determine where to drill the holes for the screws that will hold the focuser board and the base for a finder scope. After laying them out, I drilled pilot holes in the rings with a countersink bit, then enlarged them with a 3/16th inch bit. I used a jig to hold the boards vertically while drilling smaller holes into the wood strips on the focuser board and the base for the finder scope. They were then secured with 1 1/2-inch #10 stainless steel screws. The parts were also sanded to remove left over wood glue and the opening for the focuser was drilled out 5 inches from the top ring to the hole's center. Next I drilled two 1/4-inch holes for the poles seats 4-inches apart center to center, and two inches to center from the line midway between the struts. I also sanded off the edges with medium grit sandpaper. The rings are now ready to be filled and sanded with fine and very fine sand paper in preparation for varnishing.
Thursday, August 13, 2009
Now that all welding for the spider has been finished, I primed it with a gray primer then painted it with ultra-flat black paint to suppress stray light. I've also purchased four 2-inch #10x24 machine screws to secure it to the struts. I've also chosen which screws will be used to secure the rings and struts together, which are visible in the photo above. They are Allen head stainless steel screws whose heads are flush with the wooden surface. The focuser board and finder scope base will be mounted with countersunk stainless steel wood screws. After that, the attachments for the truss poles on the bottom of the upper cage will be made next. Once they are made, the hole for the focuser will be drilled into the focuser board and all wooden parts filled and sanded as necessary before varnishing.
Monday, August 10, 2009
After making the split blocks for the mirror box, I turned my attention to making the secondary mirror cage, which will hold the secondary mirror and it's holder, spider, focuser and at least one finder scope. I was planning to make the rings from Baltic Birch, but found it to be unavailable locally and ridiculously expensive to have shipped to me in the small amounts I needed. The shipping would have cost more than the plywood itself. This plywood isn't as stiff or strong as Baltic Birch, but since I'm not building a very large Dobsonian it will serve. I'm thinking about using 3/4-inch plywood of the same type for the mirror box and the cutouts from the rings as gussets for the inside corners. My Discovery Dobsonian's base is made from softwood core, hardwood veneer plywood and is going strong. The 1/2-inch plywood when double layered will make a nice, stiff rocker box.
So I opted to use a Birch veneer plywood that has numerous thin plies inside, but is made of softwood instead. It has voids that not desirable, but can be filled in. I routed the rings half an inch wider than the plans from the book called for, and may use seven instead of four struts to make the cage stiffer. Or find some thin hardwood plywood and make the inner shell for the cage from that, which would stiffen the rings substantially. I routed out the rings, rounded the edges and drilled the holes and seats for the four struts I have already made. I made the focuser board from a piece of Baltic Birch plywood and left over maple from the pole blocks. I will makes the pole seats from more maple when I acquire it.
After weighing my options, I decided to make my own spider, since I have the means to make it. For the vanes I uses 1 1/4-inch wide, 12-gauge mild steel flat stock. The central hub will be a piece of 1/2-inch square mild steel tubing, and the lugs are 10X24 coupling nuts used for threaded rod. They too are made of mild steel. Welding them to the vanes was the way to go, so I notched the vanes so the coupling nuts would fit in the slots, then welded them in place. The clean metal was fairly easy to weld for a change. A tap cleaned out any slag or spatter than got into the threads, and now all that remains to do before welding them to the hub is to cut each vane to the correct length.
After that I trimmed the vanes to the correct length, then welded them to the hub in the jig pictured, then welded a 3/8-inch washer welded to both ends of the hub. I sanded the hub bright and clean, then welded one side of each vane at a time, allowing the metal to cool to the touch before welding again. That stopped porosity and cracks from forming in the welds, which do result from the weld bead being overheated. The washers I welded onto the hub will help support the nuts and washers on the stud that will secure the holder to the spider in place. When tightened, the nuts and washers on the stud will lock the secondary mirror in position. After welding, the spider will be primed then painted in flat black paint. I could have made the spider from stainless steel, but it would still have to be painted, therefore I took the easy way out this time and opted for mild steel. Stainless steel fasteners on the other hand will be used to hold the cage together after I remove the temporary steel screws used to hold the cage together so I can verify everything fits correctly before finishing and final assembly. After completion of the welding, I wire brushed and filed away left over spatter and slag so I can apply primer then paint to the spider to prevent the onset of rust and blacken it to stop scattered light from ruining the views.