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Understanding Key Process Variables

During the panel investigation, a formability analyst will try and capture as may process input data points as possible (Figure 1). If the stamping process is stable (repeatable), there are no unacceptable quality issues, and the formability status is safe, those inputs and process settings form the baseline of the recipe for that specific part. Think of it as the zero point, if all the material comes in with all the same mechanical properties, surface roughness, blank size and shape, lube type and amount at the same temperature, what would we have to be concerned about? How about the wear of the die or the stamping press after so many hits?

 



Figure 1
 
Since there is variation in the material, lube, press, and the die, we need to better understand which input variable effects the expected output (a quality part) of the stamping process. Think of it this way, If I make an adjustment to this control point, can it effect the part? If an input has variation (which it does) how much variation can be tolerated before it effects the process. To truly understand this methodology, we first must identify how to measure the variation. Mechanical properties such as yield strength, tensile strength, and elongation can be measured using the tensile test. We can get an accurate value from this type of testing. What do we do with that information other than say it’s within or not within the specification?
 
Let’s look at the output value of Yield strength from the tensile test and examine it a little closer. If the specification for Yield strength for part x is a minimum value of 170 MPa and the plant receives material at 160 MPa what would happen? It would be rejected because it does not meet the minimum requirement and the plant could have issues with running it. Now, what happens if the plant receives material at 190 MPa? Its above the minimum specification, so the plant would put it up on the de-stacker and try to run it, right? If the part splits out or does not run in enough, would there be some adjustment made? Adding some lube, or adjusting the cushion pressure may quickly produce an acceptable output to the process but what do we need to understand about this variation?

 



Figure 2
 
The process is not robust enough to handle that 20 MPa variation of yield strength without some sort of adjustment being made. There is a very specific measurable (yield strength) from the tensile test, identified on a lift tag or coil certification, and it is trackable. Using this information to understand the frequency and severity (Figure 2) of this issue can allow the plant to enhance the robustness of the tool to absorb the variation inherent to the yield strength variable. If the plant just makes an adjustment to the process to get the material to run, the result is a process that will quickly become out of control. If we understand, by measurement, the current level of variation we can gain a better understanding of which inputs are Key inputs to the forming process and what control plans or improvements should be performed. This is the basis for continuous improvements and will result in increases of overall efficiency.