To get the holes in the tabs and the edges of the 2 halves to line up I made a template from some scrap steel. The template is 1/16" larger than the hole in the form so it just touches edge of the corner radius. To locate the template I mark several spots along the edge of the radius on the inside of the part..
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The edge of the template is lined up with the inside edge of the marks. This gets it well centered on the part even though you can't see the part while you're doing this.Then using the duplicating punch (no nib) in the Whitney punch, hold it all tight and punch the first hole. Ok, if you look close you'll see 2 holes on each tab of my template. I mislocated the first ones and just had to be careful to use the correct holes.
Put a cleco (3/16") in the first hole. Make sure it's still lined up at the other end and punch the second hole. You could clamp all this but I didn't want to risk scratching the tabs. Because the template sits just in the radius it wants to stay where it belongs making this easier than it seems.
I made a second template 1/16" larger than the first to mark the cut line . The holes are duplicated on it from the first template. You could do this with just this template but it was hard to get lined up correctly. The first template locates the holes, this one locates the cut line. Cleco it in place and draw a line around it with a felt pen. The wide line was easier to follow with the nibbler than a pencil line.
The part is ready to cut the outline.
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The nibbler works best if you push the metal at it with a steady force, you'll get a nice smooth line. I started on the inside and saved the big corners to make the end tabs which go between the shells. OK, I have a lot of steel but wasting those corners is just not in my nature.
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The finished part from the inside and a comparison with the original part.
The parts still need a notch at the trailing edge for the 3/4" tube and at the leading edge for the 1" spar tube.
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