The Evolution of Precision Bending

The companies that I worked for claimed they made this tolerance but had never updated there machines!!!

There are several "tactics" one which leads to a very accurate bending is based on a force-displacement measurement during process and a 2 step approach. Precision can be less 0.25°! Since material behaviour is unpredictable (with respect to details) this approach being adaptive can compensate the variations. It is interesting to make the remark that the Young modulus which is "guilty" for the spring back has for steel a variation of ≈300ppm/°C. Of course such an approach can only be used with hydraulic presses with a very precise positioning.

The article referenced by HVAC listed tensile strength, yield point, and hardness as material properties that influence springback. These properties make sense to me. You state that Young's modulus is responsible for springback. This makes no sense to me and I wonder where it came from. Young's modulus will certainly influence the amount of force required to make a bend, but I don't see how it will affect springback.

Hi Welderman,

The above sketch shows how material is loaded at different levels during bending.

As you see it always remains a core which is still elastic. When load is removed the elastic zone under stress will try to spring back and will load the plastically deformed layers. As well the energy in the elastic zone as the "return" of the elastoplastic zones depend on the value of the Young modulus which is depending on temperature.

The required force does not depend on the Young modulus as much as it depends on the strain-stress material behaviour. All steels have, for instance, almost same value for the Young modulus (differences are really minute) but the elastic limit and ultimate stress are much more different and according to them the force requirements.

Makes it sens now?

The values listed in the article define the material "strain-stress" behaviour. Hardness is directly related to yield stress limit so that it has the same effect as it.

The ultimate strength limits the ratio internal radius/ thickness. If the radius is too small the tension stress at the outside layer becomes too big and cracks do appear.

The recommendations r/t are valid for a given material and if extrapolated without taking it into consideration parts can be destroyed.

The higher the elastic limit the thicker the elastic layer and the bigger the spring back for a given ratio r/t.

I hope now it is clear why temperature is important. The people bending tubes know very well that if on the site it is cold or warm the tubes will behave in a different way.

In plants the temperature does not change over the day as much as on a building site so that many times for other operations as machining or measuring its effects are (even could be) neglected.

Sorry not to give an link where you can find the explanations as it is customary for CR4 but I found such a sketch will give more informations even if it takes more time.

I suspect it's not worth arguing the point. In your first post you stated "Young modulus which is "guilty" for the spring back of steel." I simply pointed out that springback and modulus are not related. Modulus is the ratio of stress to strain. While you are correct that it is temperature sensitive, other material properties are responsible for spring back.

I am sorry but I did not try to argue, only to explain. May I try again: A "spring-back" it is only due to parts of the sheet which stay elastic. The Young modulus defines the elastic behaviour of the sheet material thus it is "guilty" for the spring-back. An elastic material has a non zero Young modulus.If the whole section would be in the elastoplastic domain the spring-back would be dependent on the slope strain-stress in this domain which is a lot less than the Young modulus (less 10% of value in the elastic domain, for some materials it is almost nil). If the Young modulus is zero there is no spring back (at least it is what I think).

You write:

"I simply pointed out that springback and modulus are not related."

This means that you consider elasticity and spring-back not related. If it is so why is it any spring back? If you think that I am wrong please convince me with an explanation showing which other properties are responsible for the "spring back" and how they influence it. I am ready to learn and accept any explanation which is physically correct and shall change my point of view if you convince me that you are right since nobody detains the whole truth and every body can make an error.

I shall appreciate a comment in the sens of last paragraph. The Greeks said that discussion brings the truth so that it is necessary to discuss not to argue this is the "dialog" which means the speaking of two.

A sheet of annealed mild steel has very little springback, yet an identical sheet of high tensile spring steel can have close to 100% springback. These two pieces of steel have the same elastic modulus (~30,000,000psi) but entirely different springback characteristics. The difference in springback is caused mainly by the difference in yield strength of the two samples. That's what I mean when I say that springback and modulus are not related.

Hi,

Thanks for the comment.

The difference between what you wrote and what I did write is that you considered the magnitude of the spring-back and I considered the existence of the phenomenon it self.

In fact the spring-back as phenomenon is due to the still remaining elastic layer and its characteristics, which are related (according to my humble opinion) to the Young modulus since it is representing the relation between strain and stress thus being a measure of the material own elasticity. Since the module is function of temperature the spring-back will also be.

I think that you agree that a material without elasticity (thus with E=0) has no spring-back. This was the reason I wrote that the Young modulus is "guilty" for the spring-back. You notice that I did not mention the importance of the spring-back but only its existence. I totaly agree that the magnitude is related to the elastic limit and the ultimate strength as I also mentioned previously.

As you see discussing is important since it makes clear where are the differences and the commonalities.

Enjoyed to have this exchange and hope we will have others.

Are you saying CR4 dont like members posting links to other sites?

Thanks for efforts you have put in to spread knowledge. This kind of info makes it worth while actively being a part of CR4 forum.

I wish I had this kind of info about a decade and half back, when I was struggling to establish manufacturing standard to routinely obtain +0/-.1 doing channel out 0.6 thick 2024 aluminum sheet material for use on bonded panels assembly of a prototype helicopter structure.

Man - what I would have given to know all of this about 20 years ago...

Well, you know what you know when you know it, you know?