Ask the Stamping Expert: How do we solve problems with die springs in hinge stamping?

A: In my experience, most problems associated with die springs are not caused by the springs but are related to their application in the tooling.

Unfortunately, little consideration typically is given in the design phase to balancing the spring load, where one spring is used for lifting or forming, and the spring pin (which transfers the force) and the spring are not precisely in the center of the load during actuation. In some applications, it is nearly impossible to determine the exact center of the forming forces. The best approach in this case is to design in two springs, straddle the force with a bar, and transfer with a spring pin centered on the bar. For heavy applications, use four springs to create a square, cap it with a square plate, and transfer the force with a pin in the center of the plate.

In many cases, springs are viewed at as secondary items, unlike the punches and dies that are designed to the tenth of one thousandth of an inch. I have seen $100,000 progressive dies for which the tool assembler was allowed to choose the springs for lifters, behind pilots, and under pressure pads. The right approach is to engineer and purchase drop-in-ready, off-the-shelf springs from a reliable supplier. If you need a cut-to-length coil spring, spell out the wire type, diameter, free length when cut, and spring OD.

When determining which springs to use, calculate the force requirements. Add a minimum 2x safety margin, or 3x if there are no negative ramifications. It is easy to reduce the pressure in development, but increasing spring pressure—if it’s not designed in—can require a substantial rework of the tool. Compensation for opposing springs often is overlooked in the calculation of required spring pressure.

I know a stamper who struggled for weeks with breaking punches until we discovered an error in the spring calculations. The stamper used a standard three-plate tool design with a spring-loaded stripper to stamp 0.04-in.-thick high-carbon steel. The part had to be blanked and pushed back in the strip. This took significant force, so the stamper used a shedder in the blanking die with very heavy springs.

As the tool passed dead bottom and headed upward, the punch retracted (it is attached to the top die shoe). The stripper is spring-loaded and compressed some distance on the downstroke, so it doesn’t move for the same distance on the upward stroke. This allowed the punch to retract fully behind the stripper face as the coil strip material was held in place, and the die shedder pushed the part back in the strip.

The stamper could not push the part 100 percent flush to the bottom of the raw stock. As the punch retracted, all the shedder’s upward pressure was transferred through the stamping stock and pushed against the stripper face, again because the part is not flush to the bottom of the stock and wedged tight in the strip.

The stamper used four 350-lb. nitrogen springs in the shedder (total 1,400 lbs.), and the stripper had eight 125-lb. commercial die springs (total 1,000 lbs.). The shedder overpowered the stripper springs and was cocking the stripper during every press upstroke.

Consistency problems often are caused by die spring force increasing almost in a linear fashion with compression distance. If the amount of compression is dictated by the design, and you have limited space, you may be forced to compromise on an optimal design because you need the spring travel. The more travel you need, the less preload is available. So when springs start to do “work” in the tool, the force is underrated. You can overcome this with nitrogen gas springs. They achieve five to 10 times the pressure on contact compared to die springs in almost the same space, travel permitting, of course.