July 8, 2012
Our Locost chassis is nearly complete. We finally welded in the round tubes at the back of the frame. After exhausting every possible avenue for getting these tubes bent, including threats, harassment, and shameless begging, we stumbled across a device used to bend electrical conduit. Although conduit is slightly smaller than our 3/4" tubing, and considerably weaker, the bender did the job without breaking or even visibly deforming. Which is good, since we only borrowed it.
We'd been saving a 72" length of 3/4" tube for the main hoop, with the idea that we'd make two 90-degree bends and end up with a hoop exactly 42" wide. Not on our best day would this be remotely possible, but we tried it anyway, and in line with our usual experience, used the wrong measurement in our calculations and ended up with a hoop exactly 40" wide. But we weren't especially annoyed by this because a) one of the bends was kinked, and b) the bends weren't made in the exact same plane, so a certain amount of twisting would've been necessary to get the piece to lay flat on a table.
So we made a couple more practice bends until we had two we liked, and then cut them into 14" sections. These bends were welded to either end of a straight 14" tube, which our now suspect math appeared to indicate would produce a 42" wide hoop. Much to our amazement, measurements after welding confirmed this. The hoop was also impressively flat, another bonus, and after neatly sanding down the welded joints, you could hardly tell we didn't make it out of a single length of 3/4" tubing.
It would be easy to assume from this photo that all of this worked the first time we tried it, and of course as most of us know by now, you would be wrong in that assumption. But we learned something about welding, which is that most of the strength in a flat weld is in the bead, and if you sand away said bead you'll be left with whatever metal happened to get fused, which while considerable and adequate in tensile applications, won't be enough to withstand a crack or two when you try to bend the tube back into shape after the welding process distorts it.
Fortunately, you can't watch billions of welding videos without a few odd facts and procedures sticking to your subconscious. So it occurred to us that we might improve the welds if we beveled the ends of the tubes first. That way, part of the bead would become the tube, and it would never get sanded away. We tested this on a couple of practice tubes, and after sanding it was impossible to get the tube to separate or even bend in the vicinity of the welded joint. Which was great, but would've been even better if it was something we'd known a thousand welds ago.
So there it is, a completed Locost frame, although sharp-eyed readers will notice that we're still missing the two short lengths of bent tubes that finish off the bottom of the frame. We actually made these two pieces, but we're not 100% sure we want to weld them in yet. One of the downsides of using a conduit bender is that the radius of the bend is fixed at 6". On a Locost, this is pretty big. A smoother radius helps a little with aerodynamics, but a sharper radius looks better. And of course looks are everything. Why else would we be building this thing?
The 6" radius thing is okay on top, but really not so good at the base of the frame. It turns out that six inches isn't even enough to make a full 90-degree bend in a four-inch space. So we're not sure what to do there. We've seen a few Locosts with 1" strips of 1/8" steel bent to the proper curvature. The strips matched up well with the flat sides of the lower tubes, and probably made fitting the aluminum skin a little easier. So even though it's not the way the book does it, we might just go with that. We'll see.
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