Spar Radius Issues, DIY Metal Brake

[Frank]

This version of Dave Clay's bending brake has more photos than the Sport Aviation article. Especially shows the important tip for bending past 90 degrees.
davesbrakeplans copy.pdf

[Camflyer]

I have been thinking about this topic for the past few days. Most of my experience forming long pcs is with heavier gauge materials. One issue that is common is you might be over bent at the ends and under in the middle. (This is using a hydraulic press brake) Is this a concern when bending the spars? Im thinking probably not? I have access to a 10' cnc brake at work that I can hopefully make work with the tooling available.

[NSLaw]

Nice job on your brake NSLaw and thanks for revisiting this topic. I too am at that stage where I need to bend the spars and joggle the ribs to match. Because of the joggling and testing, I didn’t even try to find a local fabricator to do the job. Instead I started with John Bannon and others who followed Dave Clay's “Build-it-yourself: sheet metal bending brake” which can be found at EAA Sport Aviation archives or by googling.

For me it has been a slower more difficult process than advertised, but is progressing nicely. My problem is I tend to sabotage myself. Using 3/16 steel for the hinged pieces is considerably cheaper and easier to handle than 1/4. But beware: this hot-rolled steel isn’t necessarily straight!

The clamping angle (the “foot”) is 1/4 steel and straighter, but first I tried making a foot using MDF which I could make perfectly straight and with a precise 0.1 inch radius. This has proven problematic for a couple of reasons so I removed the MDF and currently testing the steel angle as the foot. String and eyeball indicates one edge of that angle is straight.

But the proof is in the pudding, and that will be full 96 inch bends which are straight. Haven’t tested that yet. It’s too cold in the shop to paint.

One thing I did which I think is an improvement is to clamp the pieces together and then drill and bolt the hinge in place. In my case I used two 48 inch hinges. The advantage here is clamping takes out the bow and the hinge keeps it straight. Also you don’t need to take out the hinge pin and try to put it back together. When you open it up, though, the hinge projects above the clamping surface. So I used strips of 1/8 primed fiberboard to raise the surface which has the advantage of being smooth. (The aluminum drags across the surface of the “bender” as you raise it up.)

Still need to work out the joggling blocks, precise dimensions of the spars to match, confidence in the long bends. But I still have confidence this may work. Seeking out a sheet metal shop to do the bending will be a reluctant last resort.

Thanks Frank - looks good so far! I just bit the bullet and went with 1/4 angle, seems to be working great. Impressive getting two hinges lined up perfectly, I was worried I'd have a 1/16 off or something and the resulting bend would be crooked. Looks like you nailed it. Smart to clamp everything together for drilling.

Re: shops - I live in Salt Lake City so plenty of options. Unfortunately with today's market a few immediately said no to the small job, and some were just unwilling to chance their brake set up to accommodate the .10 R we need.

I didn't see the longer write up of Dave's brake, good idea for going past 90. Thanks for posting the photos of your brake!

[sjt]

Even with a 10 foot brake there will small differences. For me, I never precut center ribs. If you save material you will have cut the center ribs at various angles relative to the grain of the 2024 sheet. When you go to bend the center ribs around the same mdf form using the same technique they will come out slightly different.

I sorted my center ribs for size. The different size center ribs had better fitment at different locations along the spar.

You have this option if the center ribs are not precut for capstrips until they are selected for that location.

I also wanted to keep the clean routed edge that is parallel for the main spar and rear spar. It helps keep the spars aligned the correct distance apart during assembly.
rib.jpg

[Frank]

Thanks for the tips. I'll check my ribs for size variations as you suggest. Also, when I trim the ribs to length upon assembly I'll consider notching instead. Worth thinking about.

[Chewie]

For a little perspective: On the old factory-made brakes the hinge axis is right in line with the bend axis of the part itself. (Pic attached) This is because those things are so heavy and solid the hinges are just two pivot points on either end of the brake. If you use a full length hinge you're forced to locate it below the part, causing the leaf to slide against the part as it's lifted.

Does this hinge offset cause any issues? It doesn't look like it from seeing the results posted here.

I was lucky enough to borrow this brake. It's 10' long. With that long a span, despite how heavy it is, you can still get a slight variation in stiffness between the ends and the center. Sometimes after the initial bend you still have to manually push the center part of the spar in with your hands while it's still clamped to get an even angle all the way down the flange.

[nborer]

Yeah, I've noticed that about the hinges on factory brakes as well. As an aside, that is an awesome-looking brake. I want one! (But I don't want to lift it!)

[Frank]

Question: springback! When I bent the rib flanges, 10 degrees of springback was a close guess when I beveled the formblocks. Testing the .032 on the brake I haven't come up with a formula yet. Seems like several variables may be at work: thickness of the material, angle of the bend, and perhaps the length of the bend. So, to bend 12 degrees past 90 degrees for instance, what is the angle to shoot for so it springs back to where you want it?

[Frank]

This helps:

https://www.thefabricator.com/thefabricator/article/bending/bending-basics-the-hows-and-whys-of-springback-and-springforward

[Frank]

I'm finding complex formulas for the calculation of springback. But I imagine for me it will be trial and error. One basic question: will the springback be the same for different lengths? Can I determine the bend angle on a 4 inch test piece and then assume it will be the same for a much longer piece?

[Chewie]

Depends on the brake. Apply trial and no-error: undershoot the first lift to a reference (mark on the brake hinge, measurement of handle to floor, etc) and check the flange. Then lift handle a bit more beyond the reference point, check again, see what that got you, repeat. Creep up to the final angle. You can always bend it more... Don't want to un-bend from going to far.

[sjt]

I used 11 degrees over bend but as you surmised it changes along length even with the best brake.
It is not the sort of thing that can be done accurate to 1 degree. You can get it close.

When you have figured out the number you should write it down so your next airplane will be perfect.

You will end up with a table.
There is the number that you wish you knew when you started which is the first bend number.

You will also have a number for what angle to shoot for if you already work hardened it from the first bend that was not quite enough.

If you are a gluten for punishment you will have a number for third attempt bends.

I think Mary Poppins had it right - Enough is as good as a feast. Which can be adapted to:

Just get it close, we are forming the ribs with plastic mallets. The airplane wing is a smooth integration of a bunch of stuff that is not perfect.

The bugs smashed on the leading edge will probably have more impact on laminar flow then slight errors.

images.jpg

[Frank]

I'm definitely not gluten free, so bring it on. Most technical articles I've read about bending usually end with the same caveat: if you get it close then you're grand. My tech counselor said if the fuselage from firewall to tail post is 1/4 inch out, you're actually dead on. Most people say perfection is the enemy of completion.

Some good tips from this thread. For instance: leave the piece clamped in the brake and measure the resulting angle. Be prepared to adjust and bend by hand either the central area or edges before releasing from the brake.

Another good tip: don't freak out!