Successful stamping: It's a 'we' thing
As a consultant and educator, I travel to numerous die shops and stamping plants, and almost all of them have the same basic request:
"Can you show these tooling guys how to make the dies run better?"
I hear comments like "We sure have a lot of downtime and scrap, not to mention the usual problems with burrs and product quality. Our progressive dies are being overfed, we've got miss-hits and splits in our drawn part, and we can't seem to hold critical part dimensions. Obviously, our tooling guys don't have a good handle on the proper techniques for die troubleshooting and maintenance!"
Sound familiar? Sure, certain stamping problems are the result of poor troubleshooting and maintenance methods, but a large percentage of problems are not related to the tooling at all, but rather to other factors. Often individuals working in material purchasing, tooling purchasing, press maintenance, product design, and die setup don't realize how their decisions affect productivity.
A lot of problems in the stamping shop start with poor product designs. Parts are designed with unrealistic features that cannot be obtained in the number of operations allowed; products are designed to be made out of materials that should be annealed in between forming operations; and unrealistic part tolerances are specified.
Consider this typical scenario: A product is designed with a very tall extrusion in the center of a large blank. The process engineer calculates that it will take five forming stations to achieve this feature. These five stations will need a press with a 100-inch bed. The company has a press with a 70-in. bed. The typical solution is to remove two stations and wing it—and the typical result is scrap.
What can the tool and die department do to fix it? Buy a bigger press, add the two missing stations, and fix it right.
The fact is, when sheet metal meets physics, physics always wins. Very simply, you can't make a silk purse out of a pig's ear. Yes, those shops with forming simulation software may be able to design the process to run in a robust fashion with only three stations, but in most cases, the process would turn out marginal at best.
Product designers need to understand how drawing and forming work in the die. Product designers need to work with the die experts to help develop a product that will suffice fit, function, and formability.
The old saying you get what you pay for isn't too far from the truth. If you purchase a die made totally of unhardened cold-rolled steel that is supposed to run millions of parts, you'll be lucky if it makes even a hundred parts.
Tooling should be viewed as an investment. Often we look at the tooling cost as opposed to its value. Downtime caused by cheap or poor tool steel; lack of die inserts; thin, weak die sections; no adjustability; no annealing; and die deflection can exceed the cost of a better tool. The success of a stamping company partially relies on the quality and performance of its tooling.
I often hear material purchasers say, "It's my job to purchase the coil material. As long as it is within spec, I can buy it from any vendor with the cheapest price. If the stamping shop has failures, it's not my fault. I've done my job."
These are good points, as long as you know how to spec out the material. Unfortunately, material often is specified using an American Society for Testing and Materials (ASTM) number that allows for major variation in the coil material. Although the average mechanical property may work in the dies, material on the low end of the spec may cause failures.
When ordering material, consider the following: Who defines your material specs? How are they defined? Has a formability study been conducted on the formed part to determine if the material is suitable for the process when it is on the low end of the spec?
Material purchasers should work with the tooling and product designer to determine the minimum mechanical properties (elongation, n value, r value, and yield strength) that are best-suited to the stamping process. Ordering unsuitable material is very costly.
I was once told that dies make parts, presses don't. My response? Bullticky. The press is equally as important as the die. Using a jewel of a die in a glorified 200-ton trash compactor will produce trash.
Presses must be carefully maintained, with periodic adjustment and maintenance of ram parallelism, counterbalance cylinders, ram gibbing, and cushions.
A perfect die set incorrectly in a perfect press will not perform correctly. When installing a tool, the die setup person must consider shut height, cushion and gas pressure, lubricant type and application, feed release timing, die parallelism to the feeder, feed line height, and pitch length. If even one of these variables is incorrect, the results could be excessive downtime and severe damage.
The success of a stamping shop relies on the cooperation of all parties involved. And it is equally important that the tooling personnel fully understand the tasks and duties of others in the company. It is only through a fundamental understanding of how our job affects others' jobs and through full cooperation that we can be successful and competitive.
Until next time ... Best of luck!
STAMPING Journal is the only industrial publication dedicated solely to serving the needs of the metal stamping market. In 1987 the American Metal Stamping Association broadened its horizons and renamed itself and its publication, known then as Metal Stamping.