"Meanie" wrote in message ...

On 7/12/2014 1:08 PM, wrote:
On Sat, 12 Jul 2014 11:21:56 -0400, Meanie
wrote:

A few weeks ago I asked about bending a 2" wide x 1/4" thick x 3' length
flat stock 6061 aluminum bar into a U shape with a 7" radius. I was
concerned about the bar springing back and thus did an experiment. I
made a die using wood to form my radius.
https://www.flickr.com/photos/18223943@N06/14635585195/ and tested a
piece of 1/8" thick aluminum bar.
https://www.flickr.com/photos/182239...n/photostream/
Though, I realize the 1/8" bar is more resilient than the 1/4", the
concern of springing back presents itself as you can see
https://www.flickr.com/photos/182239...n/photostream/

Therefore, what are the possibilities of the 1/4" bar springing back and
what can I do to prevent this? Heat? It's important the bar retains it's
shape after bending.

Thank you
Calculate the springback and overbend, like any metal-worker would do

That's the problem, I'm not a metal worker nor know how to calculate for
overbend. I suppose I'll ask Google.

================================================== ===============

I just took a look at your photos, and I have some suggestions. What you've
made is a bottoming punch and die. There is no way, IMO, that you'll get
enough force with that setup to get the bend radius you want. It would take
tons, springback or not.

If the idea is to have a fairly accurate radius and two very parallel legs,
your wooden die probably is a good place to start -- if it will hold up with
1/4 in. bar. If it will, start your bend with that.

It appears that you have a bandsaw. So I would make some round disks of
wood, starting with 1 in. less radius than your desired end result. After
making the initial bend, clamp one leg firmly and then try bending the other
leg around the smaller-radius disk -- not all the way, but just get it
started bending a bit more than your original pressing.

Check your radius before you've gone very far. If you need a smaller disk,
cut it down some more. Bend some more. If you bend the legs too far and your
radius is not right, you'll probably have to start all over again. Unbending
would likely result in little increase in the radius, but a likely "kink"
where one of the legs emerges from the bend. If the radius is too large,
with the legs parallel, it's unlikely you'll get the radius smaller by
re-bending. The bend will be harder and stronger than the legs. Or you may
get lucky. You'll need some experimenting.

The thing you'll be up against is work-hardening. If you get the legs
parallel but you need a smaller radius, unbending the legs and re-forming
the bend over a larger disk will likely cause some local work hardening that
will prevent you from ever getting it right. It could happen, as well, if
the radius needs to be larger. The tricky thing is getting the overbend just
right so the radius is right, because you will have some flexibility to bend
more or open the legs slightly to get them parallel. That's not true in
adjusting the bend radius.

Your 1/8 in. test piece would be a good start for trying this out. You know
you'll have to overbend to get the legs parallel. You'll also have to
overbend to get the radius you want. Getting that radius right is the tricky
part, so inch up on that radius while the legs are still splayed and you
have some room to adjust.

Good luck.

--
Ed Huntress