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# Bending force query3

## Bending force query

(OP)
Hi All,

I've been trying to figure out a way to calculate the bending force required to permanently deform a steel plate.
The process involves pressing a 300mm wide, 2mm thick s275jr plate into a shaped die. The desired outcome is to form a 90mm radius that passes along the entire width of the plate.
The bottom die will have a radius of say r90+thickness, and the punch will have a rad of r90. The plates internal angle after forming will be 90^.
I have looked up a few equations, but they don't seem to make sense when I pick them apart, and the forces calculated are way too low in my opinion.

Would the correct way be to approach this as a deflection and calculate the plates second moment of area, or do a cross sectional calc and sweep this around the curve?
Any help would be much appreciated

:)

### RE: Bending force query

You are thinking elastically, you need to think plastically. Work out the plastic moment of the section. Assume the plate is giving Sy in tension on one side of the neutral axis and Sy in compression on the other side. Then do a work calculation.

If I sound a little abrupt this is first year stuff.

Cheers

Greg Locock

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### RE: Bending force query

it sounds like you mean a "press brake". If you have one, you should know what it'll handle; if you don't then talk to someone who does "can you bend this for me?".

what's the value in knowing the force to bend something if you don't have the machine ? If you have the machine, why do you need to know the force (either it will or it won't be able to do the job).

### RE: Bending force query

I ran into this once when trying to determine forces for dishing plate.
"What's the value in knowing the force to bend something if you don't have the machine?" The answer might be "to know what machine to buy".

Anyway- one possibility, figure the total work that must be done to form the plate, figure distance the dies have to move in doing that, figure peak force is at least double the average force, and see where that gets you.

That plate size is small enough that it wouldn't be too hard to do some testing- cut dies out of wood and try a strip 1" wide or so first.

Actual metal forming for that application may involve putting it through rollers rather than pressing.

### RE: Bending force query

You probably need an application like Blankworks or similar to get a decent answer.

You could send the part out for a die quote and the toolmaker will tell likely you tonnage required.

### RE: Bending force query

The answer might be "to know what machine to buy". If I was looking to buy a machine, I'd go to several suppliers saying "I want to bend this, what machine should I get?". Sure, knowing the force puts you in a better position to negotiate with the suppliers (always trying to up-sell).

### RE: Bending force query

You will always get the salesman's answer, If you need a 90 ton machine to bend this. What are you going to do when the next part is an 1/8" thicker ?
B.E.

You are judged not by what you know, but by what you can do.

### RE: Bending force query

(OP)
Thanks for the responses.

### RE: Bending force query

seriously it can go both ways, but must sheet metal fabricators know what the press brake can handle from experience. then again it would be great to know the tonnage required to yield the plate.

### RE: Bending force query

"You will always get the salesman's answer" ... hence get several quotes.

### RE: Bending force query

Just for kicks and grins I looked up the bending force required for a 300mm square plate 2mm thick with a 90mm radius at 90 degrees out of mild steel at a friends shop, it came out to approximately 12,5 tons. The program also told me that the bend would not be true. and that a rubber forming die would give better results. This is on an Accupress Pres brake program with an installed data base.
B.E.

You are judged not by what you know, but by what you can do.

### RE: Bending force query

#### Quote (mfgenggear)

sheet metal fabricators know what the press brake can handle from experience. then again it would be great to know the tonnage required to yield the plate.

In my experience they always know what the press will do once, but rarely know what it can do an unlimited amount of times without damage.

Come on guys, straightforward question here. Help the guy learn something.

### RE: Bending force query

(OP)
HI,

Thanks again for the responses.

Kind regards

### RE: Bending force query

berkshire

By "the bend would not be true" do you think the program was saying it would tear, or just not be a smooth curve across the both 300 mm sections?

### RE: Bending force query

Racookpe,
It would not be a smooth curve, it would tend to bend more in the middle because the die would not support it.
B.E.

You are judged not by what you know, but by what you can do.

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