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.
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and tested a piece of 1/8" thick aluminum bar.
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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
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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.
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.
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and tested a piece of 1/8" thick aluminum bar.
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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
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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.
In commercial press-brake work, springback is compensated by overbending. If you have one of the fancy new CNC press brakes with databases of materials and their springback properties, you can program it all in and you'll come quite close. Otherwise, you do it the old-fashioned way: keep bending it a little more until you get the bend you want.
That's how I do it with my sheet-metal equipment: a very big vise, some angle iron, wooden forms, and some really big hammers. d8-)
Heat and 6061 are not a happy combination, because 6061 age-hardens and moves, and can drive you nuts. You'll also never quite recover the pre-heating strength, unless you put it through a carefully controlled heat-treating cycle afterwards.
Not likely a big issue as IIRC the OP said 6061 was grossly overstrength , structurally, for his application. But for the 7" radius annealing is not required even or T6 or T651. A strong arm is thoug. It is seriously tough stuff!!
I would bend the sides a little beyond parallel and then open them up with a scissors jack etc by slowly increasing the handle turns, backing off and measuring the result. If you know that perhaps one more turn opens the ends by 1/4" you can correct the remaining error without as much risk of overshoot.
Or if you have a hydraulic press stop before parallel and tweak the sides inward. Once you are close to final size you can measure and correct for the springback.
Apply the force close to the ends of the curve or else you will bend the flat sides.
Well, assuming you bent to size, how far back did they spring - how much too wide are the legs now? Just bend that much further past straight, and they should spring back basically to size. You can calculate and predict all you want based on book properties, but in the end you bend and check and tweak even with a cnc brake :-). Oh, it would be best to make the radius of your form a little under, too.
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.
I have access to a 12,000 ton press at work which is where I was planning to use the wooden die. Though, I believe the results of spring back would be similar anyway even if using the press. I'll consider your suggestions. Thank you.
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.
I have access to a 12,000 ton press at work which is where I was planning to use the wooden die. Though, I believe the results of spring back would be similar anyway even if using the press. I'll consider your suggestions. Thank you.
OK, then, ask a press operator. A wooden die probably won't handle bottoming with 1/4 in. aluminum. You'll be air-bending. If your press is a press brake, and your operator knows his stuff, he'll know how to calculate springback with a large radius. What he may not know is whether your wooden die will explode when you try to apply that much force to it. d8-)
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