Issue link: https://www.e-digitaleditions.com/i/1197609
from Home Depot. I used 1/4" for attach- ing fins to 13mm tubes, 3/8" for 18mm tubes, and 1/2" for 24mm tubes. A picture of some of the jigs my family used over the years is shown in Figure 1. At the top left of Figure 1 is a jig for 18mm tubes, and at top right is a jig for 24mm tubes. Both of those use a metal top piece. At the bottom left is a jig for 13mm tubes made from 1/4" balsa. The bottom right is my latest incar- nation. This one uses a 0.250" gage block (sometimes called a Jo Block) on top of a "tight tolerance" 1/8" thick stainless steel flat. The key concept to this jig is that the lower surface of the flat plate and the top surface of the gage block are parallel. I was concerned this may not be the case with my earlier jigs, so I spent about $50 at Mc- Master Carr for these two metal pieces of the latest version. To make this jig, I placed the gage block on the steel flat so it was 3/8" from the edge of the flat and then I used CA to attach it in place around the backside. The gage block has a tolerance of +/- 0.000006". The stainless steel plate has a tolerance specification of +/- 0.002". That is the thickness tolerance, which isn't critical to us. We just want the surface to be flat. Anyway, with these tolerances, the alignment of the fins to the body tube will certainly be better than attaching them us- ing the eyeball method. To use the jig, first the body tube is taped to the jig as shown in Figure 2. The plate is wide enough so the bottom side of the body tube rests on it, yet the edge of the tube sticks out far enough that there is clearance so that an earlier attached fin can descend down past the edge of the jig when the final fin is attached (more on that later). One other concern I had was how to insure the tube was round and not deformed when taped in place. I've been using a 13mm engine or Balsa Machining long engine block tube to stiffen up the body tube and keep it round. A steel or aluminum mandrel may be better. As you can see in the figure, the tube is taped se- curely in place and is resting on the plate. The fin will be placed on the gage block, which is precisely parallel to the body tube at quarter inch elevation from the stainless steel plate the body tube is resting on as shown in Figure 3. To attach the fin, a thin line of CA is SPORT ROCKETRY JANUARY/FEBRUARY 2020 11 Figure 3. The first fin is put in position on the jig for fin attachment. Continued from page 7.