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If you don't love tools you shouldn't be building an airplane. You just can't have too many of them. One of the things I enjoy doing is figuring out a tool to do a certain challenging job. Airplane building is full of challenges! 8-Foot "Home Depot" Bending BrakeI read somewhere on the internet that an 8-foot bending brake absolutely had to be made of steel, and rather heavy gauge steel at that. I just couldn't accept that. There aren't a lot of long bends on the CH701 but there are some - and they need to have a 1/8" radius. This is not something that the local aluminum siding guy can do for you... I thought to myself, "if strong airframes can be made from flimsy sheet metal, then there's gotta be a way to make an 8-foot brake out of building supply store materials at a low cost". So I borrowed some ideas from structural engineers and came up with exactly that.
"Ah", you say - "door hinges have an offset point of rotation. A bending brake needs to be hinged so that the bend line is in the center of rotation." Actually, I found out that it doesn't. Because of the offset in the door hinges the metal will slip across the bending face as it is rotated, but it still bends just the same. To avoid the "banana effect" caused by the bending surfaces bowing out in the middle we just use several hinges along the width. I built the bending brake you see here from my own design. It took about four hours to put together. If you look closely at the pictures you should be able to build one for yourself. Some important things:
If you look at the bending bar you'll see that there's an 1/8th inch radius on the nose. The bar is a piece of maple that I ran through the saw and cut in the radius with a router. A piece of steel angle and some C-clamps gives it the rigidity it needs to stay put when the bend is made. When you consider the total cost of the brake (about $60 Canadian), the properly radiused bending bar and the fact that I can use it any time I want... I'm really quite pleased. So there - don't let those long bends hold you back. |
There
are two problems to overcome. The first is maintaining stiffness on the bending
surfaces over an 8-foot width. This is easily done by building a sandwich out of
3/4" MDF board and 1" x 2" and 2" x 3" lumber, in a manner
similar to the way aircraft wings are constructed. The second problem is keeping a
straight axis of rotation over that same width. This is something that common door
hinges do quite well.
How well does it work? Not bad, actually. This picture
shows a darn-near 8-foot bend in 0.025" 6061-T6 for the elevator spar. The
angle isn't quite 90 degrees, but a mallet can fix that using the clamped bending bar as a
form. In this case, the bend needs to go another 9 degrees past 90 anyway, so that's
not a big deal.