Thursday, March 26, 2009

Tailgate fabrication

Now that I have the jig made, I bought the steel then cut and fitted the parts for the tailgate. For the side rails, I opted to use 1 and 1/2-inch by 1/4-inch steel strap. After cutting and trimming them to exact length, I drilled two 3/8-inch holes for the mounting bolts into both side rails, into which will be inserted 5/16-inch machine bolts to mount the tailgate into the mirror box. The heads of these bolts will be countersunk below the surface of the mirror box's sides. This will prevent the bolt heads from scuffing the inside of the rocker box and allow for smooth and easy motion.

For the top, middle and side rails, I opted to use one inch square steel tubing with a wall thickness of 1/8-inch. This will provide enough metal for the collimation bolts to grip the threads without the risk of them becoming stripped. Finding the tubing has been a bit of a problem since all the steel suppliers here will only sell it in 20 foot lengths. So I bought a 20 foot piece of tubing and had them cut it in half so I can get it home. Then it was simply a matter of cutting three 16-inch long pieces then, laying out, marking with a center punch then drilling all the holes for the bolts that will be added to the tailgate at a later time. Using a center punch and steel ruler, I got the holes marked and drilled in the right spots with my small drill press.

Then it was back to the jig where all the pieces were arranged in their correct positions for tack welding. I then used a square to check that the assembly was square, which it was. After tacking, I then checked again to make sure it was square. Then I welded one side of each rung at a time until each rung was welded to the side rails all the way around. After touching up the welds where the bead was undersized I partially filed away one spot with a little porosity and re-welded it. The metal was fairly clean so the voltage at the arc may have been too high for the welds I was making. I turned up the wire feed speed to compensate. The welding wire I am using seems to be on the troublesome side for porosity effects, or my machine generates higher amperage and voltage at the arc than expected. Then I filed down the butt welds where the top of the gate will fit into the mirror box.

I'll probably dress the welds with a grinder later, but both diagonals are identical and that can only mean the tailgate is square. All that's left to do now is to thread the holes for the collimation bolts then prime and paint the tailgate with flat black rust preventing paint.

Wednesday, March 18, 2009

Construction has begun

Now that I have a source of power for my welding machine, I built a steel welding table on which I can do my welding from scrap pipe, tubing and a piece of old plate steel. I needed a place to work where I wouldn't have to worry about setting something on fire, and a plate steel table top is the place to do welding. It took a substantial effort to cut and fit the steel because I had nothing but a reciprocating saw to cut it with. To fish mouth the tubing I used my grinder and a "bastard" or coarse mill file to shape the ends of the pipe and tubing to the required curvature. The steel was heavily rusted and because of that I had to grind it off to bare steel in order to weld it at all. Even though I was using flux-core welding wire that does not require gas shielding, tests show it will not tolerate heavy rust on the base metal. Because of my lack of practice I had lots of trouble with porosity, which required me to grind out the bad portions of the welds, then re-weld the defective areas. It was good practice because I will be using new, rust free steel to build the tailgate for my telescope. The next step was to build a jig pictured here on top of the table to hold the rungs and side rails in the correct position so I can "tack" them together. I used dimensions from a book by David David Kriege and Richard Berry, "The Dobsonian Telescope" to design and fabricate both the jig and the tailgate. The jig will allow me to cut and fit the parts precisely, which ensures the best possible quality welds. Trying to weld parts with large gaps between them is difficult and could result in weak, crack prone welds. Use of a jig will prevent this problem, and it will allow for clamping each part in place to ensure everything is square. Once tack welded the tailgate will be removed and the welding completed. I chose to use leftover plywood because it will not char badly when the rungs and side rails are tacked together and the jig will be less likely to warp. A crooked jig produces a crooked part, so it staying flat is imperative. If the tailgate is out of square, it will result in major problems when it's time to put it into the mirror box, so I will get it right before proceeding further. Before welding all the necessary holes will be drilled first. After welding, the holes for the collimation bolts will be threaded with a National Fine tap for smooth and precise collimation upon completion of the telescope. After that, it will be primed and painted to protect it from rust. Although I could use stainless steel or aluminum, both materials are very expensive. I will probably use stainless steel for the rest of the primary mirror cell components, but mild steel for the tail gate itself is strong enough. All told, the tail gate will require three feet of 1/4 by 1 1/4-inch or 1 1/2-inch flat steel stock and at little more than four feet of 1-inch heavy wall square steel tubing to make.