August 10, 2004
Poor cutting section design
Cutting problems, such as die sections breaking, slugs jamming and pulling, and burrs, are common stamping challenges. Many of these problems are caused by poor die design. Others are caused by poor maintenance techniques, improper setup, and poor scrap monitoring. Some die design and maintenance principles can help reduce or eliminate these problems.
Design Solution A. Even though very intricate cutting sections can be wire burned easily, avoid trying to trim the entire shape at once. Doing so often leaves a very thin, weak, difficult-to-support portion of the cutting steel.
Better cutting design
Figures 1 and 2 show two different methods for producing a blank in a progressive die. Figure 1shows all of the metal being cut in a single notching operation. This leaves a long, small, thin blade on the lower die section. Fully supporting these weak earsall the way to the die bottom is very difficult. Designing the die so that the necessary cutting can be done in more than one station allows for the use of stronger die steels. The weaker sections then can be larger and are fully supported. Even though the die might be longer, it will be a higher-quality tool and less likely to break.
Design Solution B. For sections that are weak and cannot be well-supported, use tool steels with greater impact toughness, such as S-7. However, keep in mind that tool steel such as S-7 requires more frequent sharpening.
Maintenance Solution. Using incorrect grinding wheels and improper procedures can damage a tool steel cutting section. Avoid using wheels that don't break down and load up when grinding your die sections. Reduce the heat as much as possible by using coolant. If you can see tiny black marks on your cutting sections, they have been burned. This overheating often causes cracking, imbrittlement, and overall toughness loss. Your grinding wheel supplier can help you choose the proper wheel for the tool steel you are using.
Design Solution. Keep in mind that during a cam piercing operation, the cutting slug must travel horizontally or semihorizontally before being ejected from the die. During horizontal slug travel, gravity often works against you rather than for you. A common mistake designers and diemakers make when designing cam-piercing systems is to use a step relief pierce button or matrix. A step relief button, often referred to as a "sudden death" button, works well when direct piercing vertically. Once the slug travels beyond the cutting land or die life (the amount a cutting section can be sharpened before it must be replaced), the slug is free to fall from the die. See Figure 3.
Slug must not rotate but still must move freely
Using a step relief button in a cam piercing operation often is asking for trouble. After being pushed horizontally through the cutting land, the slug often tips over onto its side. After numerous cycles, the slug begins to tumble, which results in slug jamming.
Tumbling and jamming are caused by uncontrolled slug rotation and excessive slug buildup in the matrix. The key to preventing tumbling and jamming is to use a special tapered button that allows the slug to move freely horizontally and keeps it from tipping over in the matrix. The slugs stack up and move horizontally until they hit a special angled tool steel section called a slug shedder. This angled insert forces the slug downward and out of the die. See Figure 4.
Design Solution. Many factors can cause excessive die wear and cutting burrs. Among these factors are poor cutting conditions; hard, abrasive sheet steel; poor tool steel selection; poor heat-treat method; and improper cutting clearance. Another common cause is that dies often are designed so that upper cutting section joints are aligned with the lower cutting section joints. Wherever upper and lower joints align, more erosion occurs. Intentionally mismatch the joint lines to reduce burring. See Figure 5.
Design Solution. Completely eliminating bypass burrs is nearly impossible. However, reducing the cutting clearance to one-third of the normal die clearance at the bypass point often helps to reduce the burr height. See Figure 6. The best solution is to provide a small void or cutout for the cutting steels to intersect. This can be achieved very simply by piercing a small hole in the metal before cutting it. The disadvantage to this method is that it requires acceptance of the small half-moon cutout that will remain on the part. It also requires an additional piercing operation. See Figure 7.