View Full Version : bending 4130 sheet

06-17-2009, 03:07 PM
Greetings everyone,
I have been lurking on the forum for a while, being a newbie to metalworking, and soaking up all the knowledge I can. I have a question on 4130 steel and hope it is not a dumb one.
I am making a small hinge bracket and the plans call for normalized .050 4130 sheet. I have to make a 90 degree bend. Can the normalized 4130 be bent on a small radius without losing strength or does it have to be annealed first and then brought back to N condition?
Thanks in advance for any help?
Dave A.

06-17-2009, 03:53 PM
I've always thought that "normalized" is more or less equivalent to "annealed," but I wouldn't be astounded to find out I'm wrong about that.

06-17-2009, 06:25 PM
Normalizing is performing by heating above critical and air cooling, annealing is done by heating the same but a slow cooling process. For some steels with a very low carbon content and little alloying components normalizing is the same as annealing. For something like thin 4130 it's akin to a half-hard state.

I'm not sure of the minimum radius that you could take that to in the normalized state, but a guess would be about 1/32" 4130 is pretty tough stuff, it's going to have a lot of springback.

06-17-2009, 07:29 PM
Normalized 4130 is about half hard and is definitely not annealed. It work hardens quickly and should be bent to a radius of around 2 1/2 to 3 times thickness to avoid cracking.

06-17-2009, 11:58 PM
Thanks for the info! I thought the N condition was a lot stronger than annealed and I had to specifically ask for it in ordering material. I was afraid that I couldn't fabricate hinge brackets in that state.

The brackets secure a flight control surface (elevator) and thus my being a bit obsessive. I have a small brake and can choose the bend radius so it looks like I am good. I will just have to be careful not to work harden it.

Dave A.

06-18-2009, 12:57 AM

Don't bend it into a sharper turn than you have to. The tighter the bend the more distortion of the sheet structure you induce, (cold work).
Also such is really making more difficult challenge for the bending equipment.

Any reason why you would need to go less than one thickness inside radius?
How about two thickness inside radius? One heck of a lot easier to bend.


06-18-2009, 03:24 AM
Is this for an ultralight? I used to build and repair certified aircraft. I have seen some very poor engineering in ultralights. What is the configuration of the hinge you are making?

These are the common configurations in order of worst (A) to best (d).


06-18-2009, 02:20 PM
The hinge assembly is similar to "D" but with a rod bearing inside. Here is the hinge assembly in question. Its the outer hinge on the horizontal stabilizer.


Even though it was supposedly drilled and riveted in place at the factory, it is off by half a bearing. Here is a picture of the hinge with the elevator temporarily put in place.


The aircraft kit is from a well known and successful company and though they have had a few issues, I think this is probably been engineered well, supposedly for aerobatics, 6G's etc. The problem is that I am the second owner of the kit; the first guy gave up. Even though the ownership was transferred, the second owner loses some of the warranties. Plus, its an old kit. So, despite a couple of phone calls to tech support and an email with pictures, I haven't got much help.

Here is a photo of the plans drawing and an extra standard-sized hinge.


The radius on the factory bracket is probably 1/16" (5/64" gage shown)


I could go a little bigger on the radius without difficulty.

The positioning and alignment of the hinges on control surfaces is pretty critical, especially the elevator. Excessive control stick pressures, constantly oversteering and constant flexing and bending at the misaligned areas are potential problems. And if you lose one, you become a lawn dart.

The options to fix this are:
1) Build a new elevator with the bearing offset to match the stab, i.e two wrongs to make it right. The company's solution.
2) Build a new horizontal stabilizer and mount the hinges correctly so the elevator will fit. Expensive and time consuming as the stab has already been aligned and mounted on the fuselage.
3) Fix the hinge assembly. Moving the standard hinge over and drilling new holes in the stab will weaken the rear spar by some amount. The best solution as I see it is to remove the hinge and install a new hinge with a slightly larger base to accomodate the relocation and still use the old holes in the spar, which is my plan. That will take some tedious work but it can be done. Making the hinge properly and drilling the holes correctly is the key.
Probably a lot more than you wanted to know. <g>
Dave A.

06-18-2009, 03:18 PM
First, I spent a lot of time building parts for homebuilts as well as all possible airframe repairs on certified aircraft.

Those rivets were placed using a flat faced snap in a rivet squeezer instead of a proper tool. That alone is enough to fail inspection on a certified airframe. Because it is a flight control surface there is absolutely no leeway for mistakes.

Rivets shouldn't be used in the first place to fasten steel to an aluminum spar. It's poor practice especially since the spar member isn't protected. It should at least be zinc chromated to prevent corrosion. If rivets are used in that manner there should also be a steel backing plate on the back side of the spar to prevent pulling through.

There are a number of approaches but I would drill out the spar along the entire length and make a new one. The hinges should be installed first either with Hi-Lok fasteners or with certified aircraft bolts and nylocks with a backing plate and the proper anti-corrosion treatment. You will need to take into account any extra weight even though small as it is a long way from the CG.

Hi-Lok fasteners are by far the best way to go for this application. See here:


06-18-2009, 04:48 PM
Your comments are noteworthy.
The glare in the photo could be throwing you off(I jury-rigged the lighting); the rivets (470AD-4) may have been squeezed, instead of riveted, but I'm pretty sure a universal head was used. You can see evidence of that on the bottom row. I have asked dozens of people about the strength difference between squeezed and riveted rivets and other than widely varying opinions, no one has ever cited data to support their position. I even called Alcoa at one point but they just referred me to their company website.

In defense of the manufacturer, there are at least 5,000 of these planes flying, some for over 15 years without any structural problems in the rear spar, and this tail was built "per plans".

I agree with you about primer (which is almost a religious issue), and although the factory uses Sherman Williams wash primer on all the parts in the photo, I am not sure it is as good. None of the primers except for epoxy-based primers provide the best protection without paint over them. No question steel on bare aluminum is not good. The surfaces need to be protected.

I have used Hi-Loks before and they are really good fasteners and must be applied with much more skill than rivets.

This particular model of aircraft is known for its propensity to be built with an aft CG, especially at low fuel loads, so every thing done in the back has to to be watched carefully.
Dave A.

06-18-2009, 05:12 PM
I have no issue with the strength of squeezed rivets. The main problem is easy to quantify and that is because rivet squeezers don't tighten the skins as well as a rivet gun. If there is any gap it will likely still be there after squeezing a rivet. That can be avoided of course by proper technique and preparation, mainly by using plenty of Clecos or machine screws to hold things in alignment while riveting. If not done the skins will chafe and corrosion is much more likely at the seams when the cladding is worn away.

5000 flying and metal? Must be an EvansAir Volksplane.

[edit] Poor memory, that one is wood. it's been a long time since I worked in the aircraft business.