Monday, June 3, 2013

Bending 1/4" Thick Aluminum

One of my readers suggested an interesting way of solving the problem of needing to bend 1/4" aluminum into a hexagon: notch the aluminum where you need to bend it.  I could use my vertical mill and an 1/8" diameter end mill to put a 1/16" deep, 1/8" wide notch in 1/4" aluminum plate, then bend the aluminum on the metal brake.

I would only be bending 3/16" aluminum (which the brake should be able to do) at the notch, and because I could place the notch quite precisely, I would get much more exact results than simply marking the line in 1/8" inch aluminum.  I wouldn't be able to use the 1/8" aluminum sheet that I have, but I suspect that it would be stiffer than the layered approach I discussed recently, and perhaps no more labor, since I would not have to drill and tap the individual layers.

UPDATE: This did not work as I expected.  I had some 1/4" aluminum to experiment with, and the results were disappointing.  I used my new 1/8" end mill (which notched the aluminum beautifully) to cut a .125" wide, .125" deep slot, then tried to bend it to a 60o angle.  Even though it was only a 1/8" thick layer that I was actually bending, it produced a slight break, rather than a smooth bend.  I thought that perhaps the problem was that I needed a wider slot relative to the depth, so I next cut a ..19" wide, .100" deep slot -- and that did not want to bend at all.  I think I will go for the two layers of 1/8" aluminum instead.  I can drill and tap the layers for 8-32, 1/4" long screws right next to each vertex, and have something impressively strong.  (The shear strength of a single 8-32 steel screw is > 2700 pounds.)

UPDATE 2: I am beginning to think that what make have happened is that aluminum work hardens, and cutting the notch with the end mill may have work hardened the aluminum underneath the notch enough to make it too brittle.

4 comments:

David said...

Relying on stuff I learned from an old sheet metal worker many years ago for working aluminum...

- plan a bend radius equal to the thickness of the material. For harder (more brittle) AL you may have to get the bend radius up to 1.5X or 3X the thickness.

- if possible always bend across the grain of the metal. Bending along the grain will usually crack the bend.

- You can anneal (soften) the the metal by annealing - look up sooting or soaping techniques. I was told about them, but never tried them. Once bent - air cool.

Sigivald said...

Hmmm.

From my experience with mild steel, I'd expect it to bend.

Plainly, aluminum does not bend like steel. Unsurprising, but...

Clayton said...

Two things:

1. For the same weight, aluminum is actually stiffer than steel. For the same thickness, of course, aluminum is far less stiff, because is a bit more than 1/3 of the density. There are probably differences other than simply stiffness.

2. Aluminum work hardens in a way that steel does not. I wonder if the milling operation in some way made it less willing to bend.

Clayton said...

The article at Wikipedia about work hardening https://en.wikipedia.org/wiki/Work_hardening gives the example of changes to a surface caused by machining.