Creating correct corner radius when bending sheet aluminum

[Mark Spickard]

I tigged up these mandrels that fit in my 48” brake. I can now create essentially perfect corner radii when bending aluminum. The radii of the mandrels are 0.079 (2 mm), 0.125, 0.1875, 0.25. This will allow at least a “4x thickness” inner radius on stock up to 1/16.

[Chris Werner]

I recently bought a 48" brake and need to make bars like this. My mentor suggested getting some sheetmetal to bend and leave over the fingers.

[alaskabearhawk]

That's what I did. Just got a piece of .050 and left it on. Had a few other sizes too. Simple and easy.

[Bcone1381]

.....or fabricate several that will stack on top of each other out of thin material for a controllable bend radius .

[Mark Spickard]

Tried that. Bought a piece of 18 gauge (0.048) mild steel, cut an 8” strip of it and tried to put about a 120 deg bend it. My brake isnt strong enough to bend it, especially to get a nice, even bend with consistent bend radius across the entire 48”. Thought about using 28 gauge (0.015”) but I would then need many overlapping sections in order to get up to a 0.25 radius, which is what is needed for bending 1/16 alum. If you want to put about 3x thickness radius on thin stock (less than 0.032) and about 4x thickness radius on thicker stock, and you want to bend it precisely and with a known radius across the entire bend, I think you need a mandrel that is better than just a bent piece of steel over the foot.

[jim.mclaughlin924]

I made a jig and machined a .100 radius on the end of the fingers using a milling machine. This works well and allows the fingers to be removed for box bending.

[sjt]

If your brake looks like this:
1.jpg

I used steel banding material (green) to make up various length brake shoe - as shown in Bob's little book.


To get real utility out of those 4 foot brakes you need to modify them. Adding shoes requires them to have much more adjustment.


A real brake has something like this in the back:

a.jpg

With a grinder ,two mild steel plates, a drill press, and a file, it is possible to make the equivalent:

grind this off
2.jpg

add replacement plates
3.jpg


I did a write up for this mod

The EAA free for member builder log allows for uploads of pdf and pictures
I hope this link works.


Download the file jun.odt from the link below

https://eaabuilderslog.org/?blprojph...t=&e=7XGUzxbsX



One thing I learned the hard way, is that red metal on the brake is tool steel. If you cut it fast or use a torch, you will harden it. Cut slowly, no torch.

[Mark Spickard]

I have a Woodward Fab brake and its eccentric adjuster has about 0.420” of adjustment. The problem was that when adjusted full forward, the tip of the feet were about 0.27” forward of the bend line…….0.27” of the adjustment was not usable. In the full back position there was only about 0.15” clearance between the tip of the foot and the bend line. This is not sufficient to get a 0.25” radius on a 0.063” sheet. So, I just cut off the support brackets, moved the entire foot assy back about 0.27” and tigged it back together. I can now fit my 0.25” mandrel (0.5” dia half-round) and 0.063” material no problem. Then, for quick adjusting, I tigged some allen wrenches to the eccentric locking bolts, threaded a bolt shank into the eccentric and put big wing nuts on the clamping rods. Everything works great now.

[sjt]

I liked your foot idea. I have been buying 3 foot long O1 drill rod in standard sizes from machine shop vendors. I do not harden them. I also had to make up feet in an array of lengths for use on a 10 foot brake when needed.

I use packing tape to tape the various lengths of shoes to the brake so that they stay put.
tape.jpg

[RVC]

How much work is accomplished with a brake? I do not own one and wonder if access to brake is sufficient. I suspect ownership is best to efficiency. What is 'Bob's Little Book'?

[jaredyates]

How much work is accomplished with a brake? I do not own one and wonder if access to brake is sufficient. I suspect ownership is best to efficiency. What is 'Bob's Little Book'?

I would say that very few Bearhawk builders own a brake big enough for bending spar parts. There's no need to own and house a 2000+ pound machine to make just a few bends. I would say that most builders probably do have some kind of smaller brake, even if it is the 18" bench-mounted model. Something like that can be really handy for minimal investment. Some builders have made a side hobby out of building a big brake, similar to how some make a side hobby out of building the airplane in computer design software, but neither is necessary to build the airplane. If you were to substitute the time and money required to research, design, build, or otherwise procure that big of a brake, you could have found one to borrow or buy time on, and saved almost all of those resources. But being that the whole plane-building pursuit is a hobby, there are several different avenues on which to have fun.

As for the book, when Bob ships the plans, he includes a book that is consolidated from the first few years of the newsletter. This has some details about how to build the airplane that are not drawn on the plans.

[RVC]

Thanks Jared. Plans building is the most likely option for me but I have noticed they offer a wing component kit which I do not consider too costly. I think this may be a suitable tradeoff in the time/cost analysis for me but it is too early to conclude anything at this date.