Tooling by Design
Forensic Investigation--Press-Shop CSI
Forensic engineering may be defined as the investigation of materials, products, structures or components that fail or do not function as originally intended. Vital to the field of forensic engineering is the process of investigating and collecting data—through collecting evidence, performing inspection, taking measurements, creating numerical models or performing experiments.
When a problem with a progressive die occurs, or when it becomes damaged, it is essential that engineers thoroughly investigate and analyze the strip. Unfortunately, many decisions are made in the press shop based on inspection reports, a die-maintenance log, past experiences, a supervisor’s opinion or any number of other reasons, rather than interrogating the eyewitness—the die strip. The strip can provide invaluable information with regard to the stamping process:
• Is the material feeding properly?
• Are the pilots positioning the strip accurately?
• Is the pitch length (the distance between the pilot holes) growing or shrinking beyond design specs?
• Is a stretch flange splitting due to a mismatched cut?
• Are extrusions splitting due to a burr?
• Is the die hitting too hard or not hard enough?
• Are deep-drawn features splitting due to restricted material flow or an incorrect radius?
• Is the carrier design appropriate?
• Is the die hitting level in all the stations?
• Is die timing correct when the die is fully loaded?
• Do cutting clearances change when the die is fully loaded?
Setting the pilot or feed release is an important step during progressive-die setting. The pilot release must release the coil strip after feeds into the die so that the pilots can properly position the strip before the die closes on the strip. Most coil feeders use feed rollers to feed the material forward in the die. The feed rollers clamp onto the strip and rotate a specific amount, usually through the use of servo motors. This causes the strip to feed one progression forward. For the pilots to correctly position the strip, the feed rollers must unclamp the strip before the pilots fully enter the strip.
One of the most misunderstood part of die setup is setting feed-release timing. The strip material can slide backward after the feed releases the material and before the pilots have entered the pilot hole. This will be evident in the die strip, as the pilot holes will appear distorted or heavily marked on one side of the hole where the pilot tried to pull the strip back into progression. To avoid this situation, make the first pilot with an undersized nose longer than the other pilots (Fig. 1).
Short feeds (or misfeeds) likely cause most of the die damage occurring in the press shop. Take care to ensure that the strip starts correctly into the die. Improper positioning of the lead end of the strip can do more damage to the die in the first 10 strokes of the press than the next 10,000 strokes.
Here are a few tips to help minimize short feeds:
• Minimize the use of round stock lifters notched on one side for threading the strip through. While inexpensive and easy to install, they will provide less support for the coil strip as compared to a rail-lifter system. Also, the need to hand-feed the stock through each lifter presents an opportunity to make a mistake when starting a new strip. A good rail-lifter system will minimize feed-related problems, reduce setup time and allow higher press speeds.
• Ensure that the lifter systems (or individual stock lifter, if used) are set at the same height with the die open. This ensures that the strip is supported in a level plane as it feeds forward.
• Ensure that the strip does not sag between the lifters. Otherwise, it will be pulled out of its correct die position (off progression). A rail system can help reduce this effect.• Provide extended guide rails with large lead-in angles, to assist the press operator in starting the strip and make one of the stock rails adjustable to compensate for any variation in strip width (Fig. 2).
Look for more press-shop forensic tips in next issue’s episode of Press-Shop CSI. MF
Related Enterprise Zones: Tool & Die
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