Cutting Out Control Horns

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.

Elevator Horn Forming

Having figured out how the Elevator and Rudder horns were made, the next task was to form the streamline shaped halves for welding together. We need 6 halves from 20 gauge cold rolled steel. The blanks are cut 16" x 6". I couldn't find small pieces anywhere. Dillsburg Aeroplane Works used to sell it but got rid of all theirs, probably not a fast mover. The day before our last big snow storm I drove over to Richmond and bought a 4' x 8' sheet. Do the math I have plenty left if someone needs some.

The tools are: form blocks, clamps, air hammer, rivet gun, leather shot bag, 1/2" straight rivet sets, and some hammer tools made from 1-1/4" & 2" dowel. The form block is made 8" x 16" from 5/8" particle board shelving. I made it wider than the blank to give it a little extra strength. The hole was carefully cut and sanded to have nice straight edges and a 1/16" radius for the metal to draw over. Three tools were made from dowel by cutting 6" long pieces and drilling a 1/2" hole 1-1/8" deep with a flat bottom. I thought the dowel would need to be glued to the rivet set but it was a tight fit and as it gets used it warms up and sticks in. Glue might have solved the problem of the hole deepening and the need to cut off the hole end after each part was made. For six parts I just kept cutting them shorter. The 1-1/4" dowel was oak, the 2" poplar. Oak is clearly the right wood since it did not need as much rework. The 2" tool was less than 4" long when I was done but, it never split. I had one of the 1-1/4" tools split along the hole. It was easier to just make a new tool than worry about how to prevent splitting.

The ends of the tools were rounded by hand using the belt sander and some hand sanding with a light casting shadows on the high and low spots. One of the small tools was cut to 3/4" wide to get into the corners. The 2" tool is almost flat on the bottom with about a 1/2" radius on the corner.
I found these vise-grips (made for holding bolt heads) worked perfect for getting the rivet set out of the wood. It really gets stuck tight. They grip without leaving any marks on the shank.

The rings are most of the ends cut off the tools as the rivet set smashed it's way deeper into the wood.
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The sheet of steel is clamped between the form block and backing board leaving about 1/2" beyond the trailing edge point. I did all the hammering with the form block setting on the shot bag to reduce noise through the house and prevent damage to whatever was under the clamps. They do not touch the work bench (floor).













The first tool used is the corner tool. The corners seemed to stretch better if done first. I formed each corner to it's full depth and then used the same tool to form the radius all around the edge.




















The next step was to use the 1-1/4" round tool to form the leading edge radius. This was blended into the corners to form a smooth transition. The middle part of the trailing edge was left unformed. It takes the least work.

The trailing edge was then formed with the 2" tool to give a nice streamlined shape. The rivet guns faster rate gives a much smoother finish. The trigger also has better control over speed and force.

Note the noise cancelling head set.

Once the shape looks good and is relatively smooth the part is removed from the form block for some gentle smoothing with the 2" tool in open areas and the 1-1/4" tool along the edges.

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I actually improved this part a little more after taking this picture. The slight bumps at the left were easily removed.