Over the many years I have been penning the “Ask the Expert” column, I have received a seemingly endless stream of questions that focus on two areas of stamping: the need for speed, and how to maximize the number of hits per tool service.
Speed at What Cost?
Let’s talk about speed first. The consensus among stampers is the faster we run our stamping presses, the better. This will result in the most product produced in the shortest time frame, which is good for profits. Of course, that only works as long as the increased stamping speeds don’t bring any unintended consequences. But if increasing your speed also causes more miss-hits, then your actual downtime for tooling service and repair may go up as a percentage of parts made.
Miss-hits occur when the raw material does not advance to the proper progression position in the stamping die for the next press cycle. When the stamping die closes and the raw stock is not in the correct position, tooling may break, cutting edges can shear, and the result is downtime for repair.
Let’s look at three examples when speed can be more detrimental than beneficial:
- When you use sensors to protect your tooling from miss-hits, there is a stopping distance for the press ram that’s proportional to speed. The faster the ram speed, the longer the stopping distance. If you have an end-of-feed sensor, you might not be able to stop the press in time to prevent a miss-hit.
- Piercing slugs tend to pull up more frequently at higher speeds. This is because slug control features like ejector pins and vacuums are somewhat speed-sensitive: The faster the ram speed, the less effective they are.
- Sometimes, because of the design of the part or die, the finished part must be blown out of the die or must slide off the end of the die with each press stroke. In either case, the faster you run the tool, the less time you have to eject the part.
What can you do to maximize your press speed and still avoid miss-hits? One remedy is advanced die strip progression sensing, which monitors the strip to ensure it is moving as intended and that it will complete the cycle. If the strip is not progressing as intended, you can stop the press before big problems occur. To encourage die slug retention, you can add physical features like steps, grooves, and darts that are not dependent on press speed. You can design high-speed die stampings to blank through, and design tooling to facilitate part ejection without depending on air or gravity.
Maximizing Tool Life
Tool life is the number of hits per tool service interval. If it takes five hours to service a progressive die after stamping 100,000 pieces, and you can improve the process to get 200,000 pieces within the same service interval, that’s a good thing—twice the parts for the same amount of labor.
So what’s the best way to get the most life out of tooling? The key is to focus on tooling materials and construction.
Materials. Carbide usually yields more hits per service than high-carbon tool steel, but it is more expensive and not as tough. Vibration causes carbide to disintegrate, so you can triple the life of carbide tooling simply by reducing stamping press vibration by using a robust and rigid tool design and minimizing snap-through.
Many varieties of tool steel are available, offering different mixes of toughness, hardness, and wear resistance. Keep in mind that if you plan on coating the tooling, the heat from the coating process may adversely temper the tool steel.
The stamped part’s base material and geometry, as well as the number of parts needed and tooling budget constraints, all play a part in the choice of tooling material.
Construction. Tool construction often is overlooked but has a huge impact on tooling life. The most common problems I’ve seen are:
- Die shoes that are too thin and flex during the stamping process.
- Weak downstops that cause part variation.
- Tool guide pins and bushings that are too small and not numerous enough.
- Lack of press bolster support in the tool areas that require the most tonnage.