Page 26 - MetalForming April 2010

Tooling Technology Sensing Strategies for
Part Ejection
This article is excerpted from Drew Stevens’ 2009 book, Die Protection
for Lean Manufacturing, available from the Precision Metalforming Association; 216/901-8800; www.pma.org/mfmarket. Stevens also writes a column on
die protection for the monthly Tool & Die Authority e-newsletter. To subscribe, visit www.metalformingmagazine.com/toolanddie.
BY DREW STEVENS
Arguably one of the most important events in the stamping process is the stamping exiting the die. Dam- age caused by hitting double material thickness can range from simply shear- ing the die to destroying cutoff sections and breaking or bending strippers.
Third-party sorting, express ship- ping of replacement parts and repairing die damage can get very expensive very quickly, so preventing double hits is a necessity. This is another application dictated by the material, part geometry and how it is ejected from the die, so let’s first examine some of the prob- lems caused by improper part ejection. Then we’ll look at solutions for error- proofing the exit side of the die.
Stampings made from thin materi- al often have high aesthetic quality requirements. Examples include elec- tronic connectors or parts used for reflection or refraction, such as auto- motive headlight components. Slight scratches or shallow slug marks can create a quality-assurance issue that many times won’t be detected by the opera- tor, due to rushed periodic checks in a pressroom with less-than-adequate lighting, so preventing scratches and slug marks in the first place is critical.
Thicker material—in excess of 0.125 in.—poses a threat if the part stays in the die after cutoff. Although cutting
clearance becomes greater as material thickness increases, hitting one part on top of another in the cutoff section not only can destroy the punch and die section but also can damage the bearing cages or the press crank, in a worst- case scenario. Obviously, these are not situations conducive to a lean environment.
Part-out sensors such as this model from Banner Engi- neering Corp., Minneapolis, MN, features response speeds as low as 0.8 msec., making them ideal for applications where high-speed miniature object detection is required.
24 METALFORMING / APRIL 2010
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Granted, some of
these issues can be
caused by foreign objects inside the die. Part counters are often based on the number of strokes made by a press, as opposed to the number of parts actual- ly exiting the die. Therefore, monitoring the press for parts that fail to escape the die also can aid the operator in keeping accurate part counts.
until enough parts have pushed it past the land and into the drafted area of the die section, where it can fall freely. If one part hangs in the cavity or becomes cocked, the rest of the parts behind it can stack up, eventually resulting in several bent parts blasting out of the bottom cavity or cracking the die sec- tion to relieve the built-up pressure. This can create unnecessary downtime and, in the case of an inherited tool, manufacturing a new die section with- out drawings can pose a time-consum- ing challenge.
One of the most obvious solutions in this case would be to ensure that blanked parts never hang anywhere in the die section. Here are some steps to
Part Ejection
Many elements contribute to parts staying inside the die, whether they remain in the working area of the tool or pile up on the die shoe. Blanked parts often behave as do pierce slugs when exiting the die. Because of the land in the lower blanking section, a blanked part can remain in the cavity