Ask the Stamping Expert: Stamping thin, hard stainless steel

Q: We recently received an order for domed spring washers stamped from 0.0010-inch-thick 301 1/2 hard stainless steel. The parts are about 0.250 in. OD and have two 0.006-in.-diameter pierced holes on a 0.150-in.-dia. circle centered on the part. We usually do not stamp anything less than 0.010 in. thick, so this is new to us. We are having all kinds of problems: burrs, punches, chipping, slugs. We cannot make it work. Any advice?

A: Although simple to look at on paper, this is not an easy job. Here are some basic concepts you need to follow to make it work.

Let’s start with the tool construction. The part will be stamped from wide material, allowing for pierces and pilots to ensure proper strip registration. Let’s assume the part progression will be to pierce the two holes, form the dome, and then blank the part through.

When building the tool, follow the proper sequence of manufacturing to ensure perfect alignment. Alignment is essential, because the clearance between punch and die will be less than 0.0001 in. This method can be used for almost any type of die construction, with slight modifications. Whether you use a box or spring stripper, the design must accommodate dowel pins and ultraprecise roller guides (not ball cages!) through the die chase and stripper plate for alignment.

Let’s start with the die chase. All the cutting will be done with carbide inserts in a steel chase. Finish the die chase to print and leave undersize the dowel and guide pin holes by 0.005 in. to allow for later jig grinding. For the same reason, make the inside cutting diameters of the cutting inserts undersized as well. Install the cutting inserts snugly in the chase. Finish the stripper plate to print, leaving all cutting, piercing, piloting, and dowel/guide pin holes undersized by 0.005 in. to allow for jig grinding.

Next, set up the die chase on a jig grinder. Place a set of parallels about 0.125 in. thick on top of the chase. Set the stripper plate on top of the parallels on the die chase. You need these so the grinding wheel has room to exit the stripper during jig grinding and not contact the die chase. Align the die chase and stripper to each other with undersized pins through the undersized dowel holes and clamp them in place. Use a separate set of clamps to clamp the stripper in place; this will prevent the die chase from moving when you remove the stripper.

Make sure the parallels between the two plates support the maximum surface area and are under the point of clamping so you do not bow the stripper under the clamp pressure. Remove the undersized dowels and jig grind all the features in the stripper, including dowels, pilots, pierces, and blanks. Remove the stripper, being careful not to disturb the die chase, and jig grind all the features in the die chase. The punch holder can be jig ground in a separate setup; 0.0002 to 0.0004 in. or so of misalignment does no harm because the punches float and the stripper does all the guiding.

When servicing the die, never remove the cutting inserts. Grind the entire chase down. Grind the tool and stripper stops as needed to maintain the shut height. The tool will get shorter at every service.

When designing the 0.006-in.-dia. punches, use a larger body size and grind down the tip to size. I would use a 0.062-in. punch blank. The punch will have three diameter steps: one for the punch holder, one for the stripper, and the tip, which is the pierce size. Grind down the body length that is guided in the stripper to about 0.0150 in. Grind the piercing tip diameter at the same time that you grind the body length to guarantee perfect concentricity. Be sure to keep the pierce as short as possible for maximum robustness; 0.010 to 0.015 in. long is more than enough.

Finally, mount bazookas to the bottom of the shoe for all piercing, scrap, and parts. Use pass-through bazookas for maximum suction. Remember to guide all the scrap and parts as they exit the bazookas through tubing (plastic is OK) into screened baskets. This allows for unrestricted airflow through the vacuum and provides a clean method of collection.

The two considerations outside the tool are feed and lubrication. It is possible to push-feed 0.001-in.-thick stainless steel if the material is totally contained precisely from the feed rollers right up to the tool entrance, but you still will have problems. The better choice in this case is to pull-feed. Mount an off-the-shelf servo feed to the exit side of the tool.

The lubrication must be perfectly controlled and consistent. The only way to accomplish this is with a high-end system that applies lubricant and then vacuums off the excess. This leaves a perfect, light, even, and consistent coating. Without this controlled lubrication, slug pulling will continue to be a problem.