Bending Steel Plates

I would say as the bend progresses due to work hardening

In order to fully bend the parts, you have to hit "bottom" in your tooling. This would be the greatest force applied.

Define a "fully bent" part. If I need a bend angle of 30 degrees which does NOT require my tool to bottom out, does your theory apply? Just curious.

No, but your bend will likely be "lazy" instead of being real crisp and consistant.

I always had the understanding that the beginning of the bend required the most force but hadn't any means of substantiating this. Thanks!

No wonder you have the picture of that guy. Neither of you know/knew what you're talking about...

Please substantiate your response. You accuse them of not knowing what they are speaking about, yet offer no valid proof.

But, my friend, there is sufficient proof out there (for who wants to find it...) that the great Einstein had no clue what he discovered, and he blundered for the rest of his life trying to come up with some inane unified theory. As far as our friend's comment concerning, he presents a molecular (molecular?) theory of plasticity, that revolutionizes the field knowledge. If he keeps going like that, he might become a Nobel prize candidate for something... these prizes are pretty easy to get nowadays...

What he meant by plasticity is that when the metal is forced to bend it becomes warmer and as such it becomes softer and more pliant.

It has nothing to do with turning metal to plastic.

If you want to disagree with someone then post your valid information instead of nitpicking on someone's wording. You actually made yourself look stupid by knocking on someone's valid statement.

Furthermore, get registered. It will add to your credibilty more then just being a "Guest" on a nerd rage adventure.

I thought the point of posting a question to this forum was a means for those "who want to find" the information, to actually "find" it? Either way, you still have not offered validation to your response. Please do so as I am interested in the truth.

We're probably not going to hear from that "Guest" again.

That my friend is an excellent answer.

Where do you guys hail from?!?!

Your arguement contradicts itself.

I hope we can agree that you are elastic before plastic.

Lets say - you bend some steel with 50 lbs and you remain elastic.

Then lets say - you apply 75 lbs and you hit the plastic range.

Which one had the higher force?

I would also even say the original question may not be as accurate as intended. It might be better asked "ONCE yielding (bending) starts does it take more or less force to continue bending" in which case POST #1 is correct. Refer to any elementary stress strain diagram.

as much as I don't like referencing Wikipedia, it can provide quick succinct answers instead of wading through other references.

http://en.wikipedia.org/wiki/Stress%E2%80%93strain_curve

#1 is right.

"LET'S SAY- YOU BEND SOME STEEL WITH 50 LBS AND YOU REMAIN ELASTIC."????? BROTHER!! GET FOCUSED!!!

IF YOU REMAIN ELASTIC, YOU DID NOT BEND ANYTHING.

THE HIGHEST FORCE IS AT THE POINT OF YIELD. THAT IS THE BEGINNING OF "THE BEND".

WE DO AGREE THAT WE ARE ELASTIC BEFORE PLASTIC. BUT THAT IS ALL WE AGREE ON.

AS FOR THE REST OF YOUR COMMENT, YOUR LOGICAL APPROACH IS VERY GOOD IN ONE RESPECT.

THAT IS: THERE ARE ONLY TWO WAYS TO HAVE THE CORRECT ANSWER TO A QUESTION. YOU GIVE THE CORRECT ANSWER, OR YOU CHANGE THE QUESTION UNTIL THE ANSWER YOU GAVE IS CORRECT.

You have now removed all doubts that you don't know what you are talking about/grasping at straws. The original question was never changed at all,

is the greatest amount of force required at the moment the plate begins to bend or as the bend progresses

you also contradicted yourself again!!!

METAL IS DISPLACED BY FORCES IT IS IN ONE OF TWO DEFORMATIONS; ELASTIC OR PLASTIC. THE ELASTIC STATE WHERE IF FORCE IS REMOVED THE METAL WILL RETURN TO IT'S ORIGINAL SHAPE

when a metal is "displaced" by forces - it does bend ELASTICALLY (until plastic) just as you noted, and then retracted, and are then probably going to confirm again in some obtuse jargon.

Look at the stress-strain curve in wikipedia - POST #1 is correct.

I am not going to respond anymore to this thread because it is getting ridiculuous, and trying to argue with you is pointless.

BENT OR NOT BENT? THAT IS THE QUESTION. IN THIS CASE, I GUESS YOU GOT A LITTLE BENT.

This person is a jerk an idiot and obviously has an inferiority/persecution complex all wrapped up in one.

Just ignore their obviously inflammatory baiting.

It's easy to spot. The idiot poster is too inferior to register and participate like a normal person.

This isn't the first for this slime ball.

Is there a way to distinguish one guest from another so as to identify repeat slimeballs, or must one infer on the basis of idiosyncratic verbal styles?

I think I have a fair idea what you grasp on a regular basis. Grow up.

Congratulations, that is the answer I was going to post. My background is in the structural metals industry so I can confirm with some authority.

Al Gore did!

YOU ARE ABSOLUTELY RIGHT AND THE FASTER THE BEND THE MORE ADVANTAGE YOU GET FROM THE DYNAMIC CONDITION. THE OTHER REASON THE FORCES ARE HIGHER AT THE START OF THE BEND IS THAT THE POINTS WHERE THE PLATE CONTACTS THE DIE GET FURTHER FROM THE POINT WHERE THE PLATE CONTACTS THE PUNCH AS THE BEND GETS DEEPER. THE DIRECTION OF THE FORCES AND THE LENGTH OF THE MOMENT ARMS GIVE THE PRESS MORE MECHANICAL ADVANTAGE THE DEEPER YOU GO INTO THE BEND. THE LEAST MECANICAL ADVANTAGE IS AT THE START AND THEREFORE THE FORCE NEEDED IS THE HIGHEST.

Many brake presses have a pressure gauge on the hydraulic line. Try looking at that is you have one

Our press is mechinical...no hydraulics....would have been a good idea though.

Thanks for responding!

Try adding an amp-meter to your motor, and record the current draw as the process takes place. You should be able to construct a current draw vs. time chart to coincide with the actual bending process in real time.

The ram on the press is operated by the energy stored in a large flywheel, the electric motors job is to bring the flywheel back up to RPM. The current draw vs. time chart would reflect this energy transfer. The time that the punch is in contact with the plate wouldn't be much more then a second.

Not all press brakes work in that manner. The Hydropower press brake is hydraulic and exerts constant pressure throughout it's cycle. It gives you more control over its decent.

At the end,of course.

I have bent many flat steel parts using a 55-ton hydraulic press and home-made dies. When I observe the pressure guage on the hand pump, I see the pressure increase to the highest level when the male die approaches the female die and the effort is focused on forming the plate to match the surfaces of the dies, which creates the 'crispness' that an earlier poster mentioned and is the most important part of the bending.

If one were not interested in that 'crispness' and uniformity, one could heat the bending zone with a rosebud torch and beat the bend in with a hammer over and anvil. I have done this many times as well and the results, while maybe satisfactory for field work or repairs, certainly do not match those provided by a proper brake or mechanical bending device.

If the bend progresses slowly, strain rate is not a factor. High speed forming is another animal. At low strain rates, the stress and therefore the required force continues to increase as the material yields. Past the yield point the metal won't return to its original shape and the incremental force required to futher deform it goes down, but it still requires more force to move it.

OP poses a legitimate question and desrves more than personal attacks on each other. The question is a little vague because op doesn't define which force he is referring to. Mechanical presses always employ some type of mechanical advantage which boils down to a lever. If one excludes the input to the lever and agrees that it is only the force on the bending punch that we are considering, and if the bend progresses slowly enough to be considered isothermal, then I maintain the force on the punch will continue to increase as the bend is made. If one considers mild steel, the yield strength might be 40ksi. As that stress is exceeded a permanent bend will begin. If the punch is unloaded during the bend (let's say at 50ksi), the bend may be partially formed. In order to continue the bending process, the metal will need to be loaded past 50 ksi before additional plastic deformation will occur. This information can be read directly from the stress/strain curve. Under the stated conditions, the required bending force increases as bending progresses.

To fill in the blanks for you, we have a 520 ton press brake. The energy from the flywheel is transfered through an air actuated clutch to a gearbox which turns a heavy duty crankshaft. The ram is raised and lowered by the offset journals on the crankshaft. As the crank journals travel through the lowest point in their travel the ram (or punch) is in contact with the material that is being bent. The length of time that the punch would be in contact with the plate wouldn't be more then a second. The plate that started this debate was a 10 foot wide piece of 400F with a tensile of 175 ksi and yield of 145 ksi. being bent approximately 10 degrees.

According to the press brake tonnage calculator the force needed to make the bend would be 940 tons, 80% above the rating for the machine. The operator had been told that he was only bending the plate a small amount so the tonnage necessary to the make the bends would be allot less and would be with in the rating for the machine. Hope that this helps.

THE PRESS IS RATED IN THE BOTTOM 45 DEGREES BEFORE BOTTOM DEAD CENTER. IT WILL DELIVER HIGHEST TONNAGE IN THE LAST 10 DEGREES BEFORE BOTTOM DEAD CENTER. SINCE YOU ARE BENDING 10 DEGREES YOU WILL BE USING CLOSE TO THE MACHINES CAPABILITY WHEN YOU START THE BEND. IF THE TONNAGE NEEDED IS THAT MUCH MORE THAN THE PRESS IS RATED FOR AND YOU ARE IN THE LAST 10 DEGREES WHEN YOU HIT METAL, YOU WILL EITHER STICK IT OR BLOW IT UP. IF YOU TRY IT AND ANYONE IS STILL ALIVE LET US KN0W WHAT HAPPENED.

Nothing broke, blew up, or shattered! He would have made 20 or so bends in the plate....and got away with it!! I'm trying to prove to the supervisors that bring the work out that they are wrong in their thinking regarding how much force it takes to make..."just a little bend" Thanks

HOW THICK IS THE PLATE?

1/2"

WELL, YOUR TONNAGE CALCULATION IS RIGHT. YOU GOT AWAY WITH IT BECAUSE YOU WERE CLOSE TO BOTTOM DEAD CENTER WHEN YOU HIT METAL. THEORETICLY THE TONNAGE DELIVERED BY THE PRESS GOES TO INFINITY AT BOTTOM DEAD CENTER. I THINK THE CAPACITY RATING FOR THE PRESS IS AT 45 DEGREES BEFORE BOTTOM DEAD CENTER. THE FORCE NEEDED TO BEND THE PLATE IS STILL HIGHEST AT THE START OF THE BEND. IT'S FORTUNATE FOR YOU THAT YOU ARE USING THE PART OF THE PRESS STROKE THAT DELIVERS MORE TONNAGE THAN WHAT THE PRESS IS RATED FOR. DANGEROUS BUT EFFECTIVE.

If you open your hymnal to http://www.custompartnet.com/calculator/v-bending-force you will find a force calculator. There are others available on the web. According to these calculators, you should be able to make the bend with your press if your die ratio (width of V die opening/plate thickness) is large enough. Some of these calculators also state that making a 10 degree bend takes less force than a 90 degree bend, but not much less.

Thanks for responding! I had never been on that site before, it has allot of interesting information in it. The calculator that I like to use is at the following address: http://www.jorgensonmachinetools.com/press-brake-tonnage-calculator

It also takes in to consideration the punch radius as well.

At bending 6 mm thick steel plate with ultimate strength 470 MPa with die width 50 mm and punch radius 3 mm the force per meter versus angle is:

YOU CAN PROVE THIS DATA WRONG WITH METALLURGY, DISREGARDING PHYSICS. YOU CAN PROVE IT WRONG WITH PHYSICS, DISREGARDING METALLURGY. OR YOU CAN USE PHYSICS AND METALLURGY AND PROVE IT REALLY REALLY WRONG.

Punch of bending machines moves slowly and don't create essential heating.

Actual bending force depends on many factors including punch radius.

The same steel plate at bending with the same die (width 50 mm), but with punch having radius 20 mm gives the force per meter versus angle: