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We started mounting Doc's engine and got a big surprise. There are no, zero, nada, blast tubes on the baffles to direct air to the carb heat and the cabin heat. Wowser! After thinking evil thoughts about the IA who signed this off, I had to come up a new blast tube.
First thing was to find a piece of pipe of suitable inside diameter to match the desired blast tube size, or reasonably close. The pipe was then cut off at the same angle as the finished blast tube. To complete this form, the inside edge of the pipe was ground to a nice radius. I used a Dremel tool and a grind wheel to do this radius.
Next step was to cut a blank out of 3003 aluminum. I used .040 thick because I had it handy and it provided plenty of thickness to make the stretch at the bell mouth. I had some difficulty rolling a cylinder out of the .040 aluminum, but with a bit of perseverance and judicious smacks with the hammer I got a passable cylinder.
I inserted the cylinder into the form part way and then cut the end of the cylinder to match the diagonal end of the form. I took care to make the overlapped portion of the cylinder be at the lowest part of the ellipse so that as the this area was stretch-formed it would have the most opportunity to move.
I sanded the diagonal end of the cylinder to match the diagonal of the form. Then with about one inch of aluminum protruding from the form I began to bump it out with a small ball-peen hammer. The technique is to flare the sides first so the cylinder in not pushed deeper into the form. Once the sides are about half flared, start bumping the lower part where the most severe curvature must be made.
After the flare was about half formed, I annealed the metal. The process I use is to warm the metal with a torch, smear some Tempil stick on the area to be annealed, then heat the area until the Tempil stick indicated the correct temperature - in this case, 500 degrees F is good for annealing 3003 aluminum.
After the material cooled enough to handle it, I continued forming the flare by hitting it on the inside and also flattening it against the table with a few judicious taps. Too much hitting could cause the form to cut through the aluminum, so I made light hits with the hammer and left as large a curvature as I could.
Once the flare, or bell mouth, was formed, I used snips and sander to clean up the edges. The end of this tube can be opened or shrunk slightly to fit the scat tube as required.
If the above explanation was a clear as mud, here are some photos.
First thing was to find a piece of pipe of suitable inside diameter to match the desired blast tube size, or reasonably close. The pipe was then cut off at the same angle as the finished blast tube. To complete this form, the inside edge of the pipe was ground to a nice radius. I used a Dremel tool and a grind wheel to do this radius.
Next step was to cut a blank out of 3003 aluminum. I used .040 thick because I had it handy and it provided plenty of thickness to make the stretch at the bell mouth. I had some difficulty rolling a cylinder out of the .040 aluminum, but with a bit of perseverance and judicious smacks with the hammer I got a passable cylinder.
I inserted the cylinder into the form part way and then cut the end of the cylinder to match the diagonal end of the form. I took care to make the overlapped portion of the cylinder be at the lowest part of the ellipse so that as the this area was stretch-formed it would have the most opportunity to move.
I sanded the diagonal end of the cylinder to match the diagonal of the form. Then with about one inch of aluminum protruding from the form I began to bump it out with a small ball-peen hammer. The technique is to flare the sides first so the cylinder in not pushed deeper into the form. Once the sides are about half flared, start bumping the lower part where the most severe curvature must be made.
After the flare was about half formed, I annealed the metal. The process I use is to warm the metal with a torch, smear some Tempil stick on the area to be annealed, then heat the area until the Tempil stick indicated the correct temperature - in this case, 500 degrees F is good for annealing 3003 aluminum.
After the material cooled enough to handle it, I continued forming the flare by hitting it on the inside and also flattening it against the table with a few judicious taps. Too much hitting could cause the form to cut through the aluminum, so I made light hits with the hammer and left as large a curvature as I could.
Once the flare, or bell mouth, was formed, I used snips and sander to clean up the edges. The end of this tube can be opened or shrunk slightly to fit the scat tube as required.
If the above explanation was a clear as mud, here are some photos.
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