Engineering Decisions, Rants 
10 Comments Strike the Restrike
It seems as though stampers that are running anything but 1008-1010 steel want restrike dies for flanges. This is flawed.
Restrike, or spank, dies are intended to sharpen feature lines and plussed radii. Radii that is too sharp to be formed home in the draw or form operation.
Somewhere along the way, it became acceptable then fashionable to use restrike operations to compensate for springback coming out of flange dies.
If you need to compensate for flange springback, do it in the flange die. In other words, if the flange die does not work, fix the flange die.
I mean, if the trim die did not get the developed trim within tolerance, you would not add another trim die to retrim after the first trim would you?
Hell no. I believe they would take away your Journeyman’s card for something like that.
You would rework the trim steels until the developed trim works. The same rational thinking goes for flange dies. Too much springback in the flange die? Rework the flange die to fix the problem right where it starts.
Working with HSS is like the Ronco “Showtime” Oven: Set it and forget it.
With restrikes a real kick in the pants is that if we try to use a restrike to correct the springback is that we have to assume that the condition coming out of the initial flange operation is repeating. Only then would a restrike die even come close to working.
However, what needs mentioning is that springback is only an issue that we all keep coming back to because it IS variable. The restrike die only masks the issue a little bit, but we should never forget that all springback is driven by the amount of strain (or deformation) in the material as we form it. For any forming to work regarding springback to work we have to assume that we know how much to over-bend it (whether it is the first hit or the restrike).
But if we built our restrikes to hit a flange that was 10 degrees out one day, ext day the flange could be 8 or 12 degrees out; then that same restrike tool will not induce the same amount of deformation as before. The process will still provide a scattering of results. I guess people are just HOPING that this scatter hovers closer to the product intent shape than without.
Exactly.
So, because of the variation, and assuming the variation exceeds the assembly function of the stamping, then you HAVE to rework a die anyway. So, why not rework the source of the problem instead of working worked material in another die? It is just another die with another set of problems.
The more I think about it, my analogy to having a retrim to correct trim problems makes sense.
It seems to me that the reason that the restrike is more appealing and rational than the trim line correction…
by creating an adjustment point closer to the final destination (the conveyor belts and the racks) they are able to affect the “illusion” of control over some of the variation. To address the initial form or flange die may require the definition of a different tip angle, or inclusion of a CAM that was not originally part of that planned operation. Additionally, fudging a “calibration” hit or tweaking the restrike may have far less negative impact on the overall behavior of the panel.
I am not defending it, but I can see the safety-blanket-like appeal of using the purpose-built restrike die as a control point. A tool that if the attempt at re-cutting or shimming or welding up or just plain hard hitting the tool away from original design intent will not have as broad or potentially negative impact on larger portions of the product. severely mess with the restrike and you are not messing with the product.
Similar to the concept of double draw beads used in large panel draws. We know that square lock beads can effectively lock out all material flow, and that grinding on those square beads can affect all gradients of restraint until the bead is gone. However, we still see double draw beads and the significantly larger blanks that they require in production. Why? Because if I have to make adjustments to the material flow, it feels like we can safely grind and weld on one of the two drawbeads without drastically altering the die. While grinding on the ONLY drawbead feels more risky.
Rational yes, but still an illusion of control.
What folks need to be able to do more effectively during their engineering of the PROCESS and all the tools in the entire process, is to meaningfully evaluate not just what the die does in the best/worst/nominal cases that they might be simulating as their standard. But they need to consider what happens when the world changes slightly around them-as it will. In springback there is no Best or Worst case.
Different is Different. And when it comes to springback any difference can end up being wrong. The restrike die just gives us one more bite at the apple to fix what we could not stabilize in the initial hit and when it appears to fix the problem at buy-off via tweaking, it actually becomes a bit of a self-fulfilling prophecy. We will prove by having it that we needed it. Vicious cycle, the only good news is that we might get paid to make the restrike die, so I don’t foresee the world doing away with them.
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Rather than focusing on correcting the problem once it’s in the panel, it’s ultimately more cost effective to avoid the problem to begin with. Yes, the blank may need to be larger (in the case of a channel section where you can use a double-break flange) or you may need to engineer the stroke to give what is called “post-stretch” (locking out material flow near the end of the stroke.) But the cost/effort usually pales in comparison to the time/cost/effort needed to apply a Band-Aid (TM) to the wound. Not to mention the cost of poor quality associated with the panels that were not dimensionally compliant.
Well spoken Danny! There also needs to be a reality check on the automotive stampings. The GD&T in many cases is simply unreasonable and unnecessary in a functional build environment – but I will save that rant for another day.
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