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26. Fabrication

19/07/2012

I spent much of last week bending metal in an attempt to see off the making of the majority of the fittings. There are a couple of parts still to be cut out, and some that can’t be finished until the fuselage is assembled, but we should be in a position to finish off the majority of them within a week or two.

Although it’s a huge benefit to have the parts cut out for you, there’s still a lot of rather tedious work to be done. The cut edges are too rough to be fit for use, and need to be de-burred ( the sharp corners rounded off) and the face of the cut ground smooth. The Powerfile is a brilliant piece of kit for this, and I got through a lot of belts. Then most pieces have to be bent.

If there’s just one bend, life is very simple. We bought ourselves a Warco bender, and Warco Magnum Benderit’s proved very useful. You can probably get the hang of the way it works from the picture; you clamp the workpiece under the block in the middle using the two black bolts, and pull up the two yellow handles. The long silver screws are to fix the position of the block in just the right position. If you don’t adjust those correctly, the block will slide away from the yellow handles as you pull them up.

It comes with a series of blocks, each with a different bend radius, because it’s very important not to bend the metal too tightly.

Understanding how metals work is a lifetime’s study, but they are pretty miraculous, and while some of the effects are very useful (like being able to form them in a soft state and then heat treat them to make them extremely hard, for example), there are other unfortunate consequences. One of them is the well-known metal fatigue, and bending the metal too sharply will help to accelerate the onset of fatigue by introducing stress-raisers.

If you pull a perfectly straight, smooth piece of metal, it will take a certain amount of load before it fails. You can imagine the stress flowing in nice straight lines down the metal from one end to the other. But if you make the stress flow round corners, it reduces the breaking strain, and the sharper the bend, the more drastic is the reduction. So if you take your straight piece of metal and make a nice smooth semicircular cut out of one side, it will reduce the maximum stress by more than you would expect if you simply based your assumption on the reduction in cross section.

But if you bend the metal round too sharp a bend, you’ll get microscopic cracks at the ends of the bend; too small to see, but the very, very tight corner at the end of it will make the effect very much worse, and even if it doesn’t fail when you apply a load the first time, it will accelerate the onset of the dreaded metal fatigue, which is a sort of ratchet system that gradually increases the sizes of cracks, and those little cracks will give it a head start.

That’s the reason we smooth off the cut edges, of course; all those little irregularities have the same effect as the cracks introduced by tight bends.

That’s quite enough metallurgy for one day.

Going back to the bender, one of the limitations you come across fairly quickly is the fact that while it’s great for doing a single bend, after that it’s difficult or impossible, because you can’t clamp the workpiece once it’s been bent the first time.

One of the pieces that had us really foxed was the very complicated one at the bottom of the undercarriage legs (click on the picture to get a bigger version of it, then click the back button on your browser to get back to the words).

As you can see from Derek Walton’s excellent drawing, we had the piece on the left, and had to fold it about 6 times. As you’ll know if you’ve tried to fold a piece of paper exactly in half, it’s very difficult to get the fold in exactly the right place so that the edges match up perfectly.

It’s the same with folding metal, only more so, and in the end Rick came up with the solution to both problems – the need to fit the part-folded pieces into the folder, and the need to get the two sides to match up exactly – by making a cut at one side of the bottom, and welding it up afterwards.

There are still some bends that won’t go in the folder, and for these you take the block out of the folder and use it in the vice, gently hammering the metal over using a copper mallet so that you don’t bruise the workpiece.

I ended up with a large box full of bits and pieces, all carefully labelled and tied together with wire, to take to Alan Haseldine, our virtuoso welder as soon as he got back from another job in Devon.

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From → Building, Technical

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