Ask the Stamping Expert: How can I maximize parts per service?

Q: We are stamping a relay spring from 0.008-inch-thick stainless steel. The part is about ¼ in. wide, 1.00 in. long, and stamped across the material grain out of 1.250-in.-wide stock. There are several bends over the length of the spring and holes on each end for staking. The operation runs 24/7. Our problem is the formation of inconsistent burrs during the stamping run. After a sharpening, we sometimes run for several days, but other times we are lucky to make it through the day. To compound the issue, the burrs that form on the edge of a part are located in areas that have the forms. So as the burr grows, it affects the bends, and then we have to make many adjustments during the run. We cannot figure out what is going on. Any thoughts?

A: Without seeing the operation, it would be a long shot for me to guess. But let me give you an overview of the areas I would consider. Remember, if just once you can successfully complete a production run that meets your expectations for the number of hits per sharpening, then you know it can be done! So something is changing in the process.

I will assume the raw material is not the issue, because the failure mode (number of poor hits per service) does not change with coil changes. I also assume the punch and die materials and die construction are not the issues, since the failure mode seems to turn on and off at die service.

So what other variables in the process affect hits per service? In addition to raw material and tool construction, tool performance is influenced by the press, lubrication, tool guidance, and surface finish from sharpening. These variables often are overlooked, but they make a big difference in tooling life.

When you sharpen the tool, you remove it from the press. If the press guidance is worn, this directly affects tooling life. The die guide pins and bushings will be forced to work hard and override a sloppy ram as the tool opens and closes. As a result, you will have to rely on the luck of the draw to hit the sweet spot when bolting the die back in the press after service.

To ensure the best lineup:

• Set the die in the press.
• Bring the ram down on the top of the shoe.
• Bolt the top die shoe to the ram.
• Manually close the tool to bottom dead center.
• Rock the ram back and forth through bottom dead center very slightly to let the top and bottom shoes align.
• At bottom dead center of the press stroke, bolt the bottom shoe in place.

Be sure to measure the guide posts on the die set to confirm they are not worn. If you are using ball cages for guidance, change them at regular intervals based on the number of cycles.

Make sure to apply lubricant consistently and evenly over the entire service area of the strip, top and bottom. Stamping issues often are caused by improper lubricant application. Pads and wipers do not always coat the entire stock strip effectively, leaving intermittent dry streaks. Spray nozzles that are not fixed on location sometimes are knocked out of alignment, so the spray pattern does not cover the entire strip. Be sure to check the setup to ensure all lubricant application equipment is functioning properly.

The guidance system on your tools can make all the difference. Bronze bushings are good for lower speeds, up to 300 strokes per minute (SPM). I recommend ball cages for higher speeds. If you are running at speeds of 400 to 500 SPM and your presses are older and have some ram slop, then I strongly recommend you use roller cages. Ball bearings have an extremely small area of contact; even though there are hundreds of balls, the total area of the guiding surface area is small. Roller bearings, in contrast, are contoured to the shape of the guide pin, and the difference in rigidity compared to ball bearings is incredible.

So why not use roller bearings for every application? They cost twice as much as ball bearings and cannot run at high speeds, as they will wear out prematurely. At Micro Inc., we were getting 100,000 hits per service using ball bearings for stamping 0.050-in.-thick, half-hard stainless steel. We switched to roller bearings and immediately began yielding 200,000 hits per service, with no other changes to the process, and this improvement has remained consistent.

The surface finish of the cutting tool also makes a big difference in hits per service. Even after sharpening, under high magnification, the cutting edge of a punch can look like craters on the moon—and that can have a big effect on the life of tooling for light-gauge stock (0.004 to 0.015 in. thick).

A surface finish of 32 versus 8 (using the microfinish scale) will more than double the hits per service of your tooling in the right application. When you are using grinding to sharpen a tool, add a simple final pass with a very fine-grit wheel. Also try lapping punch and die cutting surfaces to a mirror finish. Again, the lighter the stock gauge, the greater the benefit. With proper die clearance and meeting all other conditions noted here, you should be able to achieve 20 million hits per sharpening on a 0.005-in.-thick stainless steel stamping. Very impressive!