Self Proclaimed Die-Expert

So I delved into some shaky territory last week/month/post when I proposed that calculation of springback is an easy formula. Simply take the stress at the time of forming and divide by modulus of elasticity and you will get a result of the elastic strain at the time of forming. EASY PEASY-Mac and Cheesy.

Bust not so fast. There is more that we did not disucss, I will try to enumerate succinctly and precisely:

Stress at Forming

1. not so easy to predict
2. will change from hit to hit
3. will change from run to run
4. is affected by multitude of die conditions
5. changes with incoming material
6. changes with lube
7. changes with coating
8. changes with phase of moon (not really just checking)
9. is not constant throughout the part
10. is not constant through the thickness
11. and so on, and so on ‘ad nauseum’

To make matters worse, there is another fact that many fail to recognize. That unlike other phenomena we might try to predict, springback cannot be over engineered for. Take for example building bridges, if we predict that a bricge will fail if it is loaded with more than 100 tons, then we tell everybody that the load limit is 50 tons. That way even if people ignore our recommendation by upto 100% we feel the bridge should be OK. If I know the wiring in a circuit will melt at 50 amps I rate the circuit at 15 amps (just to be safe). Over engineering works in many disciplines.

But Springback is one of those issues where any difference in value is a problem. So if being over or under by some amount is not the desire, but instead to  be exactly right. And just how often can that happen?????

So let’s breakdown one of these issues in simple terms. Why might springback be difficult to predict? And we will focus on why forming stress is not easy to predict. Incoming material will be todays guilty party. If the incoming steel today is fed to a tool that induces the same amount of deformation during forming (we will call that strain). Then we could observe the following behavior.

all other things held equal, stronger steel = more springback

Observe in the illustration here that when we see that the incoming steel is stronger, then the stress level at the time of forming will be higher (if not we would not call the steel stronger). With that higher stress at forming we would observe that the same tool would impart the same deformation when the tool is closed. But what happens is that the steel with the higher strength (stress) would show that a larger proportion of that deformation (strain) is not-permanent. e(springback) in this equation is the elastic portion of the strain.

So what is the conclusion? In the exact same tool, if the steel were to come in 20% stronger, the springback would be 20% larger. Snap! that is not good.

How frequently can we expect our material to be 20% stronger than expected? More often than you might realize. Since the vendors are required to supply steel that fulfills our minimum strength requirement, they will mostl often aim high. Why tak a chance that if you ordered 50 Ksi steel that I might send you something weaker. Would it not be prudent to send in 55Ksi or 60Ksi steel?

Additionally, we must not forget that the tool might not function properly if it is suddenly running 20% stronger material it can’t be assumed that the deformation will work exactly the same, so even the simplistic concept illustrated in this diagram is not so reliable. If we saw the strain in the part change as a result of some change in the tooling behavior we could see other changes relative to the stress at forming.

Stress levels will change if the strain in tool changes also

In this simple example the strain in a similar material, was off by half. This change in part deformation when tool is closed results in another drop in forming stress, which in turn, changes the springback. So even if the same tool is running the smae material all the time, any other changes that affect the way the metal stretches or deforms in the die will also alter the forming stress–and the sprinback.

Are we helpless? NO

Is this a real issue? YES

What else do we need to consider? Still more to come on the topic, but now back to work.

Filed under: engineering | Tagged: sheet metal, Sheet Metal forming, Springback, stamping |