Page 27 - MetalForming April 2010

take to prevent this.
• Add ejectors to the blank punch.
This will prevent the blank from stick- ing to the punch in dies where the punch is magnetized or coolant creates a hydro-lock. Pulling the blank back out of the cutting section can damage the cutting edges as well as create the potential for hitting double material on the next hit when the blank stays in the die.
• Polish the draft on the die section. Polishing the wire-cut surface on the die section will prevent parts from hanging up just below the land. Be careful not to hit the cutting edge.
• Take the sharp edge off of the cut- ting section. Adding a small parabolic radius to the cutting edge of the cutoff punch and the blank section will remove unnecessary stress to either detail dur- ing the plastic-deformation stage of blanking. Dead-sharp corners fracture easily, creating the need to sharpen the sections early in the production run.
• Ensure sufficient punch entry. Shal- low punch entry will cause blanks to pull out of the die section instead of falling free of the die. Entry into the cav- ity should be at least double the mate- rial thickness.
Unfortunately, even seasoned die makers eschew these simple provisions, so part ejection remains a common problem. Monitoring the blank section for part ejection will require ingenuity, mainly because blanks don’t fall out of the die on every hit. We’ll discuss solu- tions for detecting blank ejection toward the end of this article.
Parts May Need Help Leaving
Cutting off the part at the end of the die or cutting a part free from a carrier or onion cut is another common method for separating a part from the strip. When the part falls free off the backside or the end of the die, it can still hang up somewhere, especially when part fea- tures such as hook-shaped forms or large notched-out areas grab onto the side of a chute or ramp. These features also can cause the part to catch on conveyors, but our main concern is making sure the part exits the die before the next hit.
Depending on where the cutoff occurs, the part may need some help exiting the die, which is why many oper- ations use air blowoffs to eject the part before the strip feeds into location for the next hit. Using compressed air is expensive, so timing the blowoff during the stroke is a necessity. To prevent the use of excessive air, stampers can use a programmable cam output from the press controller or a rotary cam switch
to activate the blowoff. These devices can be programmed to close a solenoid connected to the airline and allow air to blow the part off during a certain num- ber of press degrees or for a length of time in milliseconds.
For example, consider a part cut from a strip at 180 deg. or a few degrees before. If the part has cleanly exited the die by 220 deg., there is no need to continue blowing air at the cutoff.
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