I have moved the site and been adding content there and not here

Amazingly I am still getting traffic here. Even though I have not added any new content in months.

Here is what you have missed:

• Die Process: Process Planning die standards, feasibility, process validation  5/3/10
• Get valuable stamping experience  4/28/10
• Tool and Die Futures Initiative:   4/12/10
• Cost focus at all costs???   3/17/10
• Stamping Engineering: The blind leading the blind?… FEA, manufacturing  3/15/10
• Free Webinar: Stamping Simulation  3/10/10  •Stamping Materials: We get different material at t…   2/8/10
• Stamping Materials: What we don’t know about mater…  2/2/10
• Die Planning: 3 steps to getting the wrong blank …   1/27/10
• Springback Compensation (part 4 of many)   1/20/10
• Die Standards: substrate materials- what we usuall…   1/19/10
• 4 out of 5 Simulation experts agree: “let’s not agree  1/11/10
• Stamping Feasibility: 3 new definitions   1/4/10

Where do the new posts go?

Self-Proclaimed die Expert.  http://kam-stampingguru.blogspot.com/

Has moved. For little more than a desire to monetize my efforts with the blog I now make all my new posts there. The WordPress site was a good learning platform, but their insistence on not letting me advertise on my own content was a little too much. Even just in principle. (disclosure I have earned $0.04 to date on the blogger site) Though I am using the blogger experience as a test bed for learning. I find a little more freedom to try things over there.

Surprisingly I have noticed that this site gets a higher ranking on Google, even though the other blog is served by Google. Funny (not like ha ha, but funny as in weird)

• Please come on over:  http://kam-stampingguru.blogspot.com/
• Also for the free-lance engineering consultants: http://tech-ho.blogspot.com/
• And goofy travel and photoblog enthusiasts: http://stringgirl.blogspot.com/

In any case, trying something new, even though it looks to be a losing proposition. The idea that this content is worth anything and the others would pay me to keep it up.

But in either case I am trying it. See you at the new site.

The problem with some software is that they tell me things I don’t want to know!

Now here is an interesting problem: Everybody who makes stuff in a factory environment will have some variation. We spend lots of time and money in attempts to make tooling for these manufacturing processes relatively resistant to the effects of variation. We through lots of statistical analysis at identifying this variation when it happens, we force our suppliers to pass statistical analysis tests prior to buy-off on installation.  So it is clearly something that people care about.

AutoForm Sigma offers the ability to simulate a lifetimes worth of stamping variability

So, tell me why we can’t get the up front engineering departments to embrace stochastic simulations? We show them the software; ours in this case is for sheet metal stamping, it allows the user to simulate the stamping die the same way we do with traditional FEA, but we simulate is under a variety of conditions meant to simulate the amount of variation likely in the inputs to the REAL stamping plant. The say things like “Cool”, “great idea”, and “that could be really useful”. Only later we find that they are reluctant to buy the software. Many in my organization are quick to ask why.

The answer is quite simple: They don’t want to know it is not repeatable.

They sell dies, in the old days before simulation and circle grid and thinning analysis and CMM, all they had to do was prove that the die made parts that “held water” (no visible splits). Later they had to pass circle grid and thinning analysis, then CMM checks for repeatability, and now before we even cut the die we have to show passing simulations. But a simulation is still just a single panel, one hit wonder. That if we manage to align all the stars and get the exact same conditions that used in the simulation to match then we get the same results. But that is very very unlikely. They will tell me, “but we passed that simulation with the worst case scenario inputs”. But when it comes to variability different is different and there is no worst case or best case. They are all different than the conditions you used in your simulation and you should be concerned what happens to your process in that instance.

So maybe the solution is to come up with a software that rather than tells people the truth, just validates what we want to see . Or what we thought should happen, that would really sell!

Okay, maybe not really. But the problem is that we are presenting the software to the wrong people. This solution benefits the stamping plant, not the tool and die provider. Unfortunately, they are the traditional runners of the simulation. And currently most production stamping houses, are happy to leave their futures in the hands of people who clearly have a conflict of interest when it comes to disclosing the repeatability of their process. Here is the shot across the bow for anybody who runs sheet metal stamping dies, get real about the statistical repeatability of your processes. That 50 part study we do at die tryout/buyoff is still smoke and mirrors–after all how much is going to vary during the one shift we set aside for the run-off?

Die Cost vs. Die Price; Prediction vs. Reality

(disclosure I work as the Product Manager for a Sheet Metal Stamping Die Cost calculation product that is commercially available)

I have been having some really enlightening discussions with people lately about the applicability of this application to there business and what our product can/can’t do. It goes a little something like this.

act 1:

Prospect and I run through a sample part (similar to but not from the prospects parts). Software allows for import of the CAD data and with the use of either a predicted process or user input process determines resources and costs for those resources.

• me: so you see if I import the part data and define the stamping process, we can calculate a cost. Cost is based on a predicted number of hours, consumables, and mass of casting, etc and the rates for those items
• prospect: Wow, that’s cool…. But the cost here seems high. I don’t think I could sell the die for that cost
• me: Don’t worry. The database is fully editable, we can input in your hourly rates for Engineering, die makers, machine utilization, consumables, etc. Then we would recalculate and get a more realistic cost for you.
• prospect: Cost of machine utilization? Per hour? For like presses and things?
• me: yes.
• Prospect: but I don’t have those costs
• me: then how do you compute your costs today?
• prospect: we use an Excel spreadsheet, where the quoter puts in the estimate for number of engineering hours, number of stations in die/line, and number of weeks for construction and tryout. This generates the cost.
• me: that sounds like a price, not a cost
• prospect: that is  the cost. And yours still seems high
• me: don’t worry we can calibrate it still if you can tell me how many hours in each category for engineering, labor, mass of casting, or number of consumables were needed to complete each project. And from there you can calibrate the costing program to predict your usage, and therfore we can arrive at costs.
• prospect: sorry we don’t seem to have that kind of data
• me: so how DID you decide that my calculations were too high?
• prospect: I know that we can’t sell a ________ die for that price
• me: using your current system last year, were you profitable?
• prospect: uh nope, but again no body was profitable last year

act 2:

A paid customization/calibration project is undertaken. Several parts from the Customer (former prospect) are used to create data set for customization. Several parts of the same category but different projects are used. Customer also digs up the historical costs.

• Prospect: Here take a look at part A. Our cost for part A was 600,000 and you predicted 905,000. That’s too high even after we adjusted the hourly rates to match ours.
• me: Wow. That is pretty far off. How are we for the reported hours and mass of tool?
• Prospect: we don’t have that data.
• me: hmmmm. Okay we’ll look at that later. What else do we have?
• Prospect: Well now this is really interesting. We tried it for Part B and that cost was 1,200,000 and yor program predicted only 897,000
• me: wow again? But in the other direction. That is a big delta again. Any chance we know the labor hours or material for that job?
• prospect: nope.
• me: So we were really high on one job and really low on the other?
• prospect: yep. And to top it all off they were both inner wheel houses. Same customer, just different model variations. And you grossly over predicted one and underpredicted the other. I don’t think your program works at all.
• me: What? they were both wheel houses. How similar were they?
• prospect: Nearly the same size. just one is for the coupe and the other is the sedan.
• me: So for two wheel houses, with similar features, similar sizes we calculated each at around 900K or roughly 1.8 million for the two. And you built two dies for a combined 1.8 million.
• prospect: more or less….
• me: so what is the problem?
• prospect: you would not have properly predicted these cost?
• me: could anybody? What were your predicted costs?
• prospect: we quoted these jobs out at 750,000 and 800,000 respectively
• me: so not only did you lose your shirts, but you are not sure why?
• Prospect: look if you can get your program to quote +/- 2% then we can use it
• me: HUH?  but +/- 2% compared to what. Your not so good quotes, or your unpredicted losses
• Me: SILENTLY TO SELF. Basically, I know my costs are correct if they don’t match yours then right?.
• Me:Apparently you  accepted variation in your process in this case of nearly 100% accuracy for quote to real. I think we matched that…..
• Prospect: Sorry your costs are not close enough to reality for us to trust.

So yes. If you build a better mouse trap the world beats a path to your door. Except, they can’t admit that they have a mouse problem.

And if you do show them that you are catching mice. Those can’t be their mice. That mouse in the trap is dead, ours was alive and eating our cereal. (?????)

OK that analogy sucked. But really I can’t find any parallel for this at all. We can demonstrate through even the simplest definition that we provided benefit and that the benefit is real, and needed. But because the outputs don’t match the results they expect (results that they admit are subjective and perhaps even flawed, if not totally wrong) they can’t get on board with the software.

Stirring the pot

So I wonder that has caused this minor uptick in hits:

• bombastic controversy?
• Name calling?
• content?
• The magic of MetaTags?
• shameless self promotion on other sites?
• Pornbots?

In any case it has been entertaining having dialog with others who come across the content whether through the:

• SME-Forming and Fabricating Technical Community
• LinkedIn- SME Forming and fabricating group
• DIEGUY
• http://blog.toolanddieing.com/

Or maybe it is just Tim, using a bot to drive up the number of hits here so that I continue to make an fool of myself by pretending that I have something worth saying.

My press runs at 42 strokes per minute will your simulation take that into account?

It was bound to come up sooner or later, and I even surprised myself by optimistically leaving of my list of rants and potential future blogs. But really can commercial simulation really predict the effect of press speed variation on the forming of parts?

• yes and no (or more appropo no, but kind of)

Funny enough this has been brought up in several contexts recently. 1st by a colleague who just addressed the issue with a customer for tech support, 2nd a colleague who ran into this question during training answered it correctly only to have another more technically minded person give a roundabout not quite correct interpretation, and 3rd in a conversation with a thrid party who was relating stories of how our competition answered this question.

To break it down fact by fact:

1. commercial simulation codes assume a single hardening curve for the entire simulation
2. most material properties reported by steel vendors will also assume a constant and singular strain rate
3. mild steel has a positive strain rate (which means it stretches better fast than slow)
4. many steel parts will split if run fast in the press (and may form better in inch modes or slower press speeds)

The top two facts go to the point that most of the time when people run a stamping simulation they will not see any effect of press speed, and therefore the strain rate will not be a consideration. For example, if I run a simulation at a press speed of 1mm/sec vs 1000 mm/sec the stretching in the simulated die will show no effect (OK, if you are running a dynamic explicit solver you will see something different happen, but that would be a result of inertial effects and not a material performance issue). The rules that govern how the material is formed will refer to the hardening curve (stress/strain curve) of the material to determine how the material responds to deformation. However, since only one curve is provided there can be only one answer.

The next two facts are more interesting as they lead to people thinking I have mispoken myself and contradicted the 3rd statement with the 4th. But in fact both statements are correct. Mild steel has a POSITIVE STRAIN RATE, which means that during successive tests of the same heat of steel at different speeds of deformation, the steel will show a higher resistance to strain (deformation) when pulled faster. If the steel demonstrates this increased resistance to deformation as speed increases it means that the material will have more uniform distribution of stretching when pulled fast, and therefore would perform better fast than slow (less likely to fracture, which is a sign of non-uniform deformation).

Brief aside (that should be a footnote, but I don’t know how to do that here):

This is entirely different than the explanation given by some “dieology” gurus, or other self-proclaimed-die-experts, that steel is like silly putty. Silly putty as you might know is a material that when you stretch it slowly it stretches very far without breaking, and when strecthed fast seems to fracture quickly. BUT this is a major mistake, this is the typical thought by those who confuse formability with ductility. The slow stretching of silly putty starts a cycle of non-uniform deformation that we in stamping would consider a fracture/smile/neck. All that deformation of silly putty at slow speeds is the part failing, getting weaker and weaker because we stretched it slow. On the other hand if you attempt to stretch Silly Putty fast it fractures with almost no necking.  They offer this a an explanation as to which some parts only make when the press is run slow, but that has nothing to do with the behavior that is observed. SORRY

So does steel like being deformed SLOW more than fast? NOPE. Steel has a positive strain rate. Attempts to deform it fast result in the steel stepping up and acting stronger in the deformed area to prevent the onset of localized necking. This is GOOD, So stamping steel parts fast should be good too. In fact, if we did go so far as to load a simulation code with the appropriate steel strain rate curves, and run simulations at various speeds we would see that the faster deforming simulations would show better distribution of thinning, more uniform deformation, and therefore better forming. Again SORRY.

Another disappointing revelation. Strain rate sesitivity manifests itself only at greatly varied speeds (i.e. 10m/s to 100m/s to 1000m/s) bu the effects at relatively modest changes say 0.25m/s vs 0.35m/s won’t be noticable. Now consider, just how fast does the ram of your press go, and how much faster will it go if the cycle rate doubles. If you said double, SORRY try again. Press ram velocity for mechanical presses, will vary throughout the stroke. The ram has no velicty at the top and bottom of stroke (where it reverses direction) and will move fastest at midstroke. In most stamping operations the work of the press is done in the bottom few inches (mm) of the press stroke (usually not more than a few degrees of crank angle (maybe 15-20) and therefore will not be near the area in the stroke when a doubling of press cycle time = double ram velocity.

doing the math for a press with a 1 meter stroke at 10 spm to 20 spm. I’ll even spot you average press speed. at a rate of 10spm the ram covers 2m each 6 seconds (or 0.33 m/s). When we double the cycle rate to 20spm thats 2m each 3 seconds (0.66m/s).  You would be hard pressed (no pun intended) to find data on the variation in strain rate sensitity for such a low strain rate variation.

So why do my dies produce splits more readily at these high cycle times?

In the above question you find the answer. It may be the dies, not the steel.

Your dies don’t behave well at higher speeds than they do at slower speeds. Sorry to say that everybodies favorite scapegoat: the material. might not have anything to do with this problem. More likely your press and die are not behaving in a consistent manner from fast to slow:

• press alignment is more likely when we cycle slowly, at high rates the press can remain crooked and unevenly distribute forming pressure
• Die alignment is better slow than fast
• Your lube might work better at slow sliding velocity than fast
• The die doesn’t dissipate heat as well when cycle time is fast
• you pressure system (nitro springs, etc) might show speed sensitivity
• Your part location system might be unstable at high speeds
• Your automation might locating the part different at higher rates (especially pneumatic systems which can’t drop as fast as we like just cause the press is running faster)
• trapped air under the part or in die can affect forming more in fast speed than slow (venting issues)

just to name a few that come to me at the top of my head.

Can simulation give me any indication?

indication yes. but not a direct answer.

If we run a system of simulation runs (stoichastically) allowing for slight variations in binder pressure, friction, Lube, bead effect, blank location, material variation, and thickness variation. We can discover if a process is prone to variation in results for these changes. If a process is fully insensitive to variations due to the changes then we know that the process should produce favorable results at nearly any speed. But if the process shows that results are highly variable (goes for safe to splitting) for minor changes in bead effect, or blank location, or lube (friction) then we can recognize that the process will not be robust, and could be vulnerable to variation if something like press speed were varied.

What is Compensation?

Springback compensation is one of the hottest topics out there in the stamping field. Whether we are looking at Simulation solutions, Scanning and reverse engineering solutions, or plain old fashioned dieology. In all these cases compensation is a geomtry adjustment applied to the tool to accomodate the elastic deformation that prevents attainment of the desired shape of the product. i.e. If springback causes a flange on a part to spring outboard of desired shape by 6 degrees–one compensation strategy might me to over bend the part 6 additional degrees beyond the intended shape so that when the part springs back it lands in the right spot.

This strategy of geometric compensation is the most common approach to addressing the springback problem in todays stamping industry. It has historically proven to provide reasonable results and there is much anecdotal and scientific evidence to support it. However, there are many issues with geometric compensation:

1. the altered tooling geometry needed to achieve might not be feasible (the overbend in the example is now not possible with given geometry)
2. the fix costs alot of money (new CAMs added to the die to achieve the over bend are more expensive and complex than production allows)
3. The mode of springback will not be beneficially adjusted through geometric compensation
4. the compensation is, in fact,  a new deformation mode that it self induces a different springback (now that we have adjusted the process it springback more, or less, or just different)
5. The springback is not a repeatable outcome so sometimes it is too much compensation (the part now is 1-2 degrees closed) other times it is 1 degree open.

It seems though that whenever discussing springback compensation with potential customers, too often they assume that when we say it is not possible to compensate for some mode of springback that it is that our technology can’t and that somebody elses can. It is perhaps our own fault for honestly portraying the capability of springback compensation (geometry based) as an imperfect solution. Because there are some springback behaviours that just can not be compensated for EVER. I know it is unpopular to say, and even more insulting to point out that there are just some things that the magic of geometry manipulation just cannot get done.

For this rant let us focus on issue # 1 from the above list. (altered tooling geometry results in an infeasible tooling condition). For all those who have been recently initiated into the world of stamping Advanced High Strength Steel (AHSS) for structural members–you will appreciate this. On significant springback effect with such parts is side wall curl. If the wall of a “hat” section of the rail is formed by sliding over a radius (i.e. draw/die radius from a binder) then the springback behavior is often times a curling effect (not unlike curling ribbon over the edge of the scissors when we wrap christmas presents). If the design calls for a stright 90 degree wall, this curling will cause a significant assembly issue and must be resolved. If we feed this mode of springback into a “compensation module” of nearly any software that offers the function, we will recieve a recommendation to counteract the springback effect by curling the wall in the opposite shape.

Geometry Compensation of sidewall curl (blue design, red wpringback, green compensation)

Such compensation geomety would create an impossible (nearly) forming condition in the die. Yet this would be the approach that a geometric compensation tool would recommend (whether a simulation code derived, reverse engineering/scanned method, or good old fashioned guesswork). As you can hopefully see this is not an option.

• The direct forming action of the press can’t work under the backdraft area
• To address without using CAMs would mean two separate tipping stages, forming the legs of the flange in two separate operations
• To use CAMs we would need double collapsing cams for each side and a tremendously strong tool to induce the deformation required to overcome the AHSS strength
• If that part ever sprang back less than predicted then the part would stick to the bottom tool
• The deformation behavior will change with the compensation greatly, and the part will show a different behavior

The conclusion here is that compensation is not possible. But it may be possible to provide a countermeasure to the springback, which may require design changes to the product, adjusted assembly processes, or even an entirely different concept for the part. But sadly NO SILVER BULLET. No perfect answer for springback compensation. No instant answers.

Sorry.

Putting together the pieces (springback)

Now with the Springback 101 under our belt let’s put it all together to understand why springback is not so EASY to predict (for ANYBODY).

• Springback is a variability problem, you cannot over engineer for it.
• Stamping is a variable environment, everyday the incoming material changes (a little), the lube changes (a little), the press and die change (a little), get the pattern?
• The relationship between the amount of deformation we get in the part, and the amount that is “springback” is not linear, at some levels of deformation nearly all the in-die shape will springback, at other levels the proportion will be much lower
• Minor changes in the strein level of the part can result in large changes in the stress level, which in turn result in large changes in the springback

Every stamped component that we design and produce, will undergo so amount of springback during production. It is unavoidable! Steel and Aluminum (the most commonly stamped materials in Automotive stamping) are elastic/plastic materials which means they WILL and MUST have some elastic deformation when formed at room temperature.

Are we screwed then? Will we never be able to produce good parts?

Buck up! Yes springback can be dealt with, but it is a pet peeve of mine that people out there think the problem can be made to go away. Instead, the best we can do is find the way to deal with it best. And in many cases that means spending some money in engineering, spending some money on tooling, and compromising on expectations in some cases. But we cannot continue to have people who insist that the part/process can proceed with no funding, no tooling or design concessions; and still produce a NET perfect part repeatably.

Follow-up discussions:

• compensation vs countermeasure
• springback repeatability
• robust springback compensation/contermeasures
• Non-compensatable springback modes
• functionality vs. perfection

So why is Springback hard to predict, if the calculation is so easy?

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.

Springback 101

Of the technical issues that face stampers today, springback consistently appears among the leading roadblocks. Whether stamping small precision components, structural members or large automotive panels, predicting and dealing with springback is near the top of every stampers To-Do list.

After achieving a safe panel, no easy task in itself, stampers are faced with the task of getting the part bought off. Springback is often the last and greatest hurdle to clear after spending weeks, and perhaps months achieving a split free panel. Addressing springback is a multi-layered and complex issue, which for many is a mystery—black magic and witchcraft.

Springback defined

Springback is another name for elastic deformation. Elastic deformation is inherent to all stamped sheet metals.  Dies are an attempt to impart permanent, or plastic, deformation into the sheet metal product. Steel and Aluminum—two common stamped materials—exhibit both elastic and plastic deformation tendencies.  Elastic deformation is the non-permanent deformation that allows a spring to return to its original size, plastic deformation is the permanent shape change intended for the part.

To illustrate plastic and elastic deformation, review the stress-strain Diagram (Figure 1). In the illustration, measured strain and stress relationships are charted for a sheet metal part deformed in a simple die. As the die imparts shape into the panel the material is stretched. This stretching is “engineering strain” illustrated along the horizontal axis of the chart. As the metal is strained, it resists deformation by reacting with “Stress” against the tool—the vertical axis.  Once the die is fully closed the forming strain and stress in the part can be noted.  The vertical dashed line in the figure shows the amount of strain in the part at Bottom Dead Center, where the panel and the tool have roughly the same geometry.

Figure 1. Springback is a function of the forming stress and Elastic Modulus

As the tool is opened, any stresses in the part will relax—the stress level in the part will return toward zero. In the chart, note that as a stress/strain path that follows a line with the slope of the materials Young’s modulus (E).

Springback, or the elastic strain, is then simply the amount of strain returned to the part as the stress returns to zero (Figure 1). And can be approximated by the formula.  However, the forming stress at any location is not easily known, however a proxy stress can be used.  We can simplify the forming stress (S) by assuming that all parts must be stressed beyond their yield point:   therefore. For a part with an expected Yield Strength of 210 MPa the minimum springback response can be found to be 1 millimeter per meter of product length (Figure 2).

Figure 2. Simple membrane springback

The springback illustrated in this example is often called membrane springback.  The membrane springback may make the part seemingly shrink, about its center of mass.  For flanging of sheet metal, springback often shows itself in another mode—bending springback.  Figure 3.

Figure 3. Bending Springback

When bending is the primary mode of deformation the strain through the thickness becomes more important. As the part is flanged it will undergo various degrees of tensile and compressive modes of deformation through the thickness.  The outer fibers of the material (formed under tension) will shrink as a result of the springback, and the inner fibers of the material will expand as a result of springback—causing the flange wall to open outward. Bending Springback usually manifests itself as an opening of flange walls (towards the flat state).

Elastic deformation is unavoidable in all metals stamped at room temperature, and therefore springback should be expected in all cases. Likewise, it is certain that if a tool matches the net shape of the part, any stamped part delivered from that tool will not match net.

The Problem with Springback

In looking at these simplistic definitions and images one might wonder then what is the big deal? These examples look at only single dimensions for the springback (elastic deformation) but in reality we will have to consider the multi-dimensional interaction of many different areas on the part and how their elastic deformation interacts with adjacent areas (introducing new strain). Additionally, the part will not undergo a constant amount of stretching during deformation, ad will therefore not have the same strain in all areas. These strain differences result in disproportianate stress differences, which in turn drives the springback amounts.

Bigger discussion, needs more time and space to cover. Maybe next time.

(Borrowed from one of my own papers from some trade conference)