Ask the Stamping Expert: How do I hold washers in the strip in a compound die?
Q: Do you have suggestions for holding washers in the strip in a compound die? High-strength, low-alloy material properties change from heat to heat, periodically causing parts to drop out of the strip. For this reason, we blank up and use a knockout bar to eject the parts.
A: Two significant factors need to be considered here to run effectively: the material properties and the tooling construction. Surprisingly, variations in raw material properties from lot to lot have less of an effect than improper tooling construction does. Even so, you still need to control the material as closely as possible.
You can tighten the variation in specifications for tensile, yield, and hardness by about 25 percent within a given temper specification by spelling out the manufacturing method for the mill you’re purchasing the material from. The cold-rolled material’s mechanical properties will vary much more because of work hardening. Hot-rolled material tends to be more consistent, because the mills can control the final range of mechanical properties more effectively. Both processes yield material within the specifications of a specific temper, but the mechanical properties need to be tighter for this application.
From a tooling perspective, I’m sure there is a reason you are blanking up and using the knockout bar to drop the parts out of the die one at a time. You might try to avoid this to help get the part out of the way for the next press cycle. If by the nature of the process you still need to blank up, try using timed vacuums or timed hydraulic cylinders built into the tool that replace the knockout bar.
A ring sensor in the chute also can help to detect and stop the press if a part has not passed. This is a good application for a servo press, as you can rapidly blank and come up with the ram, then slow down to a speed that will allow you to stop before double material if a sensor has not seen a part eject.
You might try blanking down and pushing the part back up into the strip. Use the strip as it progresses to carry the part to a knockout station and push the part out through the bottom of the tool. As the die closes, the stripper will hold the part strip flush on the die chase while you blank through and then push the part back into the strip for progressing.
In this process, the punch is fixed and the stripper is spring-loaded on the punch side. The blanking station has a spring-loaded shedder inside, so after the press hits dead bottom and blanking is complete, the die opens while the stripper is clamping. The shedder can push the part back in the strip as the punches retract. As the stripper travel ends and the tool opens fully, the strip is free to progress.
It’s important to use standard cutting clearance. The break on the bottom of the strip will act as an angled lead to help put the part back in the strip to be progressed. You should get about half of the part back into the material. Design lifters into the tool to lift the material one-half times material thickness. (This will have to be adjusted to suit the application.) This way, even if the part becomes loose, it cannot come out of the web physically until it reaches the knockout station, because there is not enough strip lift for two material thicknesses.
Often overlooked and key in this design is the spring pressure. The springs on the die pushback shedders must be strong. The springs on the stripper must be strong enough both to hold the strip while blanking and to overcome the die shedder springs. Remember, as the die starts to open from dead bottom, the shedders are pushing parts back into the strip, and the strip is backed up by whatever springs you have behind the stripper.
For sufficient safety, it is best to have two times minimum spring force behind your stripper than in all shedders combined. Use large die springs on the shedders and nitrogen springs to back up the stripper. You will have to adjust the shedder springs. This design will wedge the parts in the strip so well that you actually can generate a galling action that creates a jagged edge condition on the part.
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