January 11, 2005
Even if the scoring is the reason for the split, that doesn't mean that resolving the galling issue will ensure a robust process.
I recently had the chance to engage in some very interesting conversation with the head of the sheet metal formability lab for a very large automotive stamper. During our discussion, we looked at numerous parts and talked about common formability problems such as cracks, necking, wrinkles, loose metal, and surface defects. We also both spent a great deal of time talking about how to correct some of these ongoing problems.
We both agreed that some of these problems started on the drawing board during the product design stages. Other problems were the result of poor processing methods, poor die maintenance procedures, improper lubrication, nonconforming sheet material, and other typical underlying factors.
Our conclusion on how to solve some of these problems was to take corrective action based on part data. The head of the lab referred to this process of analyzing the parts as "letting the part talk to you." We found that "listening" to parts and making data-based decisions are far more desirable than just guessing at corrective action.
Obvious defects such as splits, cracks, and wrinkles tell you that the metal is excessively stretched or simply not stretched enough. I like to refer to obvious defects as "baby talk." Yes, the part is talking, but it's not saying much more than "Ouch, I'm messed up." These basic failures do not give you enough data to determine the necessary corrective action.
To make a similar comparison, imagine going to the doctor. You walk into the doctor's office and complain that you have a constant problem with headaches. The doctor takes a five-second look at you and decides that you need brain surgery. Scary, right?
Obviously, a good physician would ask you a lot of very detailed questions and conduct numerous tests before deriving the solution to your headaches. Yet how often are decisions made about dies without sufficient data? All too often, toolmakers are eager to make major changes based on insufficient data.
If you have ever been the parent of a teenager, you know that they know everything. If you don't believe it, just ask one. You (the parent), on the other hand, know very little about the ways of the world and make stupid decisions.
Of course, adults know that teens often base this conclusion on very little true-life experience. They communicate in a more advanced way than babies, but they still often miss the necessary data to make good, sound decisions.
"Teenage talk," as far as stamped parts are concerned, includes the obvious baby talk but also is accompanied by more finite data, such as scoring and orange peeling of the metal. It also may be accompanied by a little data on the sheet metal, such as an American Society for Testing and Materials (ASTM) number or a commercial label like "draw quality."
The deformation of each circle, expressed as a percentage of change, is plotted on a forming limit diagram.
This additional data may make you think you know the reason for the failure, but it is in no way definitive data. Today's stampers not only have to make good parts, they also must be able to use good data to prove that the process in place is robust.
Figure 1shows a split in a part, as well as scoring. The split is the "baby talk," while the score is the "teenage talk." Even if the scoring is the reason for the split, that doesn't mean that resolving the galling issue will ensure a robust process. And after you take care of the split, you still won't be able to determine how close it is to failing using basic visual tools. To resolve these issues, more advanced data is needed.
When adults want to solve a dilemma, they look not only at the fundamental problem, but also the signs of the problem and a permanent means of resolving it.
To solve for the root cause of a stamping problem, we must first take a look at the issue as a whole, derive basic visual data, and then proceed to derive more finite data. To procure the necessary finite data, we must use some type of formability assessment tool.
Circle grid analysis (CGA) is one of the best methods for collecting finite data. In this process, the blank is etched electrochemically with a pattern of perfectly round circles and later deformed using the same production variables used to produce mass quantities. The deformation of each circle then is measured using either a special camera system or a simple Mylar® scale.
The deformation of each circle, expressed as a percentage of change, is plotted on a forming limit diagram (FLD) (see Figure 2). The FLD is a representation of the metal's deformation limits based on two primary values: the metal's thickness and its n value, or work-hardening exponent. By knowing how the metal has been deformed as well as its forming limits, you can adjust process variables to ensure that the process is robust.
Circle grid analysis is a way of measuring the forming severity of a formed part. It not only can help solve issues such as splitting, but it also can serve as a tool to let you know how far the part is from failing.
In simple terms, CGA is a way of measuring the forming severity of a formed part. CGA not only can help you solve problems such as splitting, but it also can serve as a tool to let you know how far you are from failing. CGA also can tell you metal flow patterns and how the metal is deformed (see Figure 3).
My formability lab friend was absolutely correct in using part data to solve problems. When presented with a part, he often specifically requested to be shown only the deformed etched part because he didn't want to know where it was likely to fracture, or where it was the thinnest, or any other data.
Why? The part doesn't lie. By using advanced "listening" techniques and some good formability assessment tools, he could tell a finite amount about that part, deriving splitting potential, stretch and strain direction, strain distribution, and other attributes.
Work with your formability lab to solve problems, and make good data-based decisions about your parts. Using good finite data not only can help you reduce problems such as splitting and wrinkling, but it can serve as a tool to help you solve your problem once and for all. Listen to what your parts are telling you!
Until next time ... Best of luck!
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