On high speed stamping and forming lines, the push for improved quality is really changing things around. Certainly, a key technology in this flurry of activity is statistical process control.
“In statistical process control,‘ said J.M. Prencipe, Jr., manufacturing metallurgist, the Chevrolet-Pontiac-Canada Group of General Motors Corp., Grand Rapids, Mich., “the objective is to take a piece of sheet metal and to make a usable part out of it without waste. What are the variables? The incoming metal is one. The metal can be broken down into surface topography or surface characteristics, thickness, coil width, physical properties. From there, we try to take a look at blank size. What do we have to do to produce that same blank size from every coil that comes on that job?
“We can measure the press tonnages or the force exerted on the blanks from the punch. We are also looking at the lubricants and the tool itself. The tool includes the size of the radii, the depth of draw, metal scoring and the overall design. We’re trying to control all of the critical variables of a given draw process so we can make every piece without failure (without a wrinkle, without a split) so that we do not produce any waste at all.’
By using statistical methods and analyzing metal, C-P-C is finding out that it has some suppliers who can make a very consistent, very formable metal. The steels processed in the Grand Rapids facility include hot dip galvanize, cold rolled steel, galvanized, commercial quality and AK quality.
In discussing quality from an overall standpoint, William G. Brazier, divisional materials specialist, Stamping Quality Control Dept., Ford Motor Co., Allen Park, Mich., said that there are a number of ways you can look at quality. One is rejects by supplier and how you rate suppliers in terms of your purchasing decision. The Ford approach is to use combinations of quality, delivery, performance and cost, in terms of whom they buy it from. Quality is 50 pct and delivery is 30 pct in the formula.
“If we take a stretch part, the characteristics of most importance are stretchability and friction. Big changes in friction will cause surface quality or split. As a result, the property of friction has become much more important recently, especially in panels where they all have to be kept at the same size.
“I see all kinds of potential to improve the quality of the product,’ Mr. Brazier noted, “by tying it into formability and identifying the suppliers who can give you the quality that is required. And, it will become all parameters, not just one or two. Formability is a term that will go away, I think, and you will begin to look at frictional, stretch, draw characteristics, internal cleanliness, age control, frictional control and flowability. All these parameters will be looked at in terms of their specific characteristics as required for a given application.’
Mr. Brazier went on to say that he will be examining those characteristics which are specific to formability. Those properties will vary, but he added, ways will be found of identifying them, of making the proper weighting and then rewarding performance accordingly.
“This way we can buy the product from the companies that do the right job and recognize what it costs us when we don’t get the quality that we need, without over-specifying for the quality we don’t need. All of this would be impossible without the computer, of course. These things are all technically feasible. It’s just a matter of putting them all together.’