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).

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.

Curta calculator

Yesterday, at the Harvest Festival at Ardenwood Historic Farm, I saw an exhibit of antique surveying instruments. Among those was a Curta mechanical calculator. Fascinating little thing. I subsequently found out that there's a Curta simulator in Flash; that these things go for about a kilobuck on eBay; and that someone out there had the chutzpah to disassemble theirs.

Back in the early 1960s, my dad was studying at the École Nationale Supérieure de l'Aéronautique in Paris. My mother told me stories about him doing his homework late into the night, while she listened to the clicking of one of these things.

Fragment of bracket detail

I recently put together a fragment of this design just to get a feel for how things go together. This is also the first time I'm using the Tempo zinc oxide rattle can primer.


Notice that the bracket, made of 1/8" thick material, is not a complete "T" shape. This is because I just happened to have a thin strip of the stuff, so I cut whatever I could and worked with what I had. This Is Only A Test.

Notice also that I had trouble getting the primer to go on uniformly. It was scratch resistant on the sheet material, but seemed to easily de-bond from the 1/8" plate. I think the latter was because I didn't slap it on thick enough. Surface prep was to scuff with brown Scotch-Brite, wash with warm water and Dawn dish soap, dry, then apply the coating.

You might think these random pieces of stuff I make are useless. Not so! I'll have you know that this latest creation of mine was used as a scoop to rescue a crawfish from the neighborhood street. My wife tells me that the handle on the side was helpful.

On bondage

Folks have asked me about bonding (perhaps with backup rivets) versus just riveting. Elsewhere in this blog, I've mentioned that I am currently pursuing a "riveting only" strategy. The question is, "why"?

In order to be useful, a design must fulfill a purpose, or "market" niche. The purpose need not be monetary -- the market in question may be that of making folks happy, spreading Peace and Love, or winning a competition just for the sheer challenge of the thing. In my case, my purpose is this:

Hypothesis 1. Traditional aircraft monocoque aluminum construction, using thin sheetmetal and large cross-sections, occupies a useful niche between welded space frames and carbon fiber monocoques. It is competitively light weight and rigid while being easy for beginners to build.

There is also another claim, less easily made:

Hypothesis 2. The methods of Hypothesis 1 can be used as the basis for selling parts to homebuilders allowing them to design and build their own configurations.

With that in mind, the question is: should we just use plain rivets, or should we mess with bonding?

The aircraft industry has been using rivet bonded construction for many years. Thus it is instructive to follow their trajectory. The original F-18 A-D had a multi-part metal fin. The newer E-F models have a single piece, bladder molded, heat cured composite fin. Sound familiar? Bikes like the M5 Carbon High Racer and the Velokraft bikes all use this method, as do many upright carbon frames. Clearly, if you have the equipment, this is The Future. In addition, absent bladder molding, people like Garrie Hill, Jim Scozzafava, Tom Traylor and a whole host of others have shown that, if one is willing to do layups, carbon construction rules the roads. Hence, once again, the niche is to find something easier than carbon, but lighter than space frames.

There seem to be conflicting notions out there about how much one needs to prepare a metal surface for bonding. However, among people who rely on it for a living (or whose companies will fail dramatically if their structures come apart in service), the consensus seems to be that it's not easy. Check these out for starters:

• Hysol® Surface Preparation Guide, Loctite Aerospace [PDF]
• Adhesive Bonding Surface Prep Qualification Considerations, Jim Mazza, Materials and Testing Directorate, Air Force Research Laboratory [PDF]
• The Windcheetah manufacturing process
  

In general, it seems to me that proper surface prep for bonding in order to achieve the rated strength of modern epoxies involves at least lots of care, and sometimes toxic and/or caustic chemicals.

Thus my current direction is to just rivet. Every Schmoe can build one and it will very likely stay together. It's easy to see a crappy rivet and, conversely, if a rivet looks pretty, it's probably adequately driven.

Easy Racers Javelin clone

My current design is a clone of an Easy Racers Javelin: 700c rear, 451 front, LWB. I figure this is an easy way to get started -- later on, I can try more fancy designs with integrated seats. I'm trying not to "overmodel", so here is a sketch of just the parts I need to build a simple prototype of the rear wheel attachment. I would certainly not build such a long structure to the right, but I want to give an idea of how it would fit together:
The dropout is made from 1/4" 6061-T6 plate. The stays are 5/8" diameter, .035" wall 2024-T3 round drawn tubing, flattened at the ends and attached with two #8 MS27039 machine screws:
The stays are attached to the body by (roughly T-shaped) brackets made from 1/8" thick 6061-T6 material:

They are held together at the middle by a spreader piece made from 1/16" 6061-T6:
Removing the stay and spreader piece, we can peek in to see that the 1/8" bracket is built up on each side with a 1/16" thick spacer, such that the stay is flattened on both ends to an inside dimension of 1/4". This ensures that it has a nice radius without cracking:
The approach I show here, with a thicker bracket and a stay flattened onto the bracket, is a lot simpler and more symmetrical than either riveting the stay from the sides or inserting the stays into the structure. Note that I'm relying on the brackets themselves to resist side-to-side forces since the 1/8" plate has significant bending strength (though I haven't done the math on that part to know for sure, mainly because I am not sure what the design lateral loads should be).

Early monostay design

This early monostay design was far simpler than my subsequent ones. Here is the overall view:Removing the right side skins, we can see inside. Each monostay side is made up of a top and bottom channel piece; these come together in the middle and are attached to top and bottom plates. The skins hold the whole thing to the main body of the bike.
Looking back, this design is somewhat appealing. Perhaps I should return to it one day.