Tuesday, November 27, 2007

Simple bending brake

This is a simple 2' bending brake I built recently. The goal is to validate the overall design approach in preparation for building a 4' version.

The leaf and bed are 1x4 oak from Home Depot. The bending bar is made of two pieces of 1x2 hard maple from a local lumberyard, topped with a piece of 1x4 pine, and a 1/8" radius is created with a beading router bit. The bed is screwed down to a large 2x6 for stability; the latter is clamped my workbench during use. (The reason the bending bar is in several pieces is because I didn't want to buy a wide -- read, expensive -- single piece of maple. I'll splurge for the next brake.)

The construction is pretty obvious. I ensured proper alignment of the edges of the leaf and bed with the edge of the piano hinge by "match drilling" each side separately while fixtured to a base, like this:

For the #8 wood screws I used, I match drilled using an 11/64" drill for the unthreaded portion, ensuring reproducible alignment. I also drilled a deeper, pilot hole for the threaded portion. (Note that, in so doing, I ignored the pre-drilled holes in the hinge.)

One problem I encountered -- perhaps because my pilot holes were too small, or not perfectly centered, or whatever -- is that the (admittedly, small and questionable quality) wood screws would torque off at the junction between the threaded and unthreaded portions.

I added setback stops to provide a repeatable location for the bending bar:

I aligned the stops by putting some scrap into the brake, raising the leaf to the angle I expected I would need to get a 90 degree bend taking springback into account, then snugging the bending bar forward evenly:

This is the bend line being set up for making a 2" wide channel. I figured out from some experimentation that my parts came out 1/32" undersize, so I needed to "steal" 1/64" or so from each flange, which is why my alignment is just a smidge to the left of the line here:

After bending, this is as far as I could go towards 90 degrees. This is due to inadequate leverage in my leaf -- I need to attach a handle:

But a few taps of a mallet put me all the way to a right angle. Note that the long 3/8" dia. lag bolts that you see pointing upwards and engaging the wingnuts are just about to get in the way of the channel if I bend it far enough. Also, you can't see this, but it's really pretty painful to tighten the wingnuts without mechanical assistance (hence the pliers you see), so one might as well just use regular nuts. In any case, the result is that, at one end, my channel is pretty exactly 2" wide:

But it's about 1/32" too small on the other end:

This is within spec for my (forgiving) uses, so I'm happy, though I will run some more metal through this to see how reproducible (or improveable) these results are.

The following are the inspirations for this brake:
  1. Dave Clay's brake, made of steel angle sections; and
  2. Murray Johnson's "Home Depot" (wood) bending brake.
The following are the things I would/will do differently next time:
  1. More leverage for the bending leaf;
  2. Make all 3 working surfaces (leaf, bed and bar) out of maple;
  3. Use larger and more durable wood screws that won't torque off;
  4. Make the bending bar out of one wider piece of maple;
  5. Secure the bending bar with bolts tightened from the top, as with Dave Clay's brake (above).

Thursday, November 15, 2007

Of Recreational Vehicles and Army Navy Hardware

Last Sunday, I visited my friend Paul Eastham, builder of an RV-9A aircraft, at his hangar at South County airport. We chatted about riveted aluminum, and went on a short trip to Watsonville for lunch. He very generously let me take the controls and boy, I tell ya, that was a blast! He is building a camera mount, so he also took the opportunity to teach me how to drive solid rivets, and I learned about rivet smileys. :) All in all, I had a great time.

As a parting gift, he gave me some leftover hardware (mostly AN3) to experiment with for my own projects.

Now, meet my son, Aden. He is a nut (so to speak) for AN hardware. It was like showing a bag of diamonds to a jewel thief. He had to have an RV-9A. It had to be made of "real, lightweight" aluminum just like the real thing, and it had to be made with AN bolts. These were, so to speak, the design constraints. Here is the result:

You may notice that it ended up being an RV-9 instead of an RV-9A. That's life, I guess. You start out trying to build one airplane, and you end up building the other. It just happens.