Page 44 - MetalForming November 2011

Metalforming Electronics
By George Keremedjiev
What the Die Needs
Since a tool cannot speak for itself, let’s use this forum to speak on its behalf in a court of technical jurisdiction. The tooling can be a progressive or transfer die—we will not discriminate as we come to its defense and clearly state to the technical jury what a die needs, at a minimum, to func- tion properly.
Feed
All dies expect to close on top of a properly positioned strip or blank. I know of no die designer who has a casual, cava- lier attitude about this. On his CAD system, the designer con- ceptualizes, designs and details a die under the assumption that everything outside and inside of its internal and exter- nal surfaces will arrive as specified. Then, the strip or blank must be presented to
the die within these
expectations, in order
for the tooling to con-
sistently form good
parts. In the case of a
progressive die, the
strip must arrive at
the die within the cor-
rective tolerance of the pilots. So, for example, if the feed pitch for a progressive die is 4 in. and the pilots allow for a misfeed correction of ±0.020 in., then the die must not be allowed to close if the feed pitch exceeds that tolerance range. Period. Even if the pilots were to be forced to correct for such serious out-of-tolerance feed errors, the stamper risks compromis- ing part quality and will experience premature die wear.
The same holds true—maybe more so—for transfer tool- ing. Unlike a progressive die, where the entire strip must feed properly by a single feeding mechanism, transfer tooling expects the transfer arms to position each blank so that it seats or nests properly within the respective lower die com- ponents, to ensure proper forming, bending, piercing etc. Once the transfer arms release their blanks, the blanks may
George Keremedjiev has been writing this column for more than 20 years. He regularly consults with metal- forming companies worldwide and provides metal- formers with training on the application and imple- mentation of sensors for die protection. For more information on his seminars and consultancies, con- tact:
Tecknow Education Services, Inc. P.O.Box6448
Bozeman, MT 59771
tel: 406/587-4751 | fax: 406/587-9620 www.mfgadvice.com
gk@mfgadvice.com
become unstable and their movement unpredictable. Once the gripper fingers on the transfer arms let the blanks go, there may be little confidence that they will fall into their expect- ed lower-die nesting positions. Just for a moment, in defense of the transfer die, let’s consider each transfer operation as an independent feed mechanism. This means that if the die has 16 stations, it’s considered to have 16 unique feed mech- anisms, each one independently capable of releasing the blank into an unstable flight path.
Part Ejection
In further defense of dies, let’s address the fact that each and every part is expected to exit its outside perimeter in an orderly and predictable way. In a progressive die, the last sta- tion expects the finished part to promptly index out of the way of the following part. And we expect the same with a trans- fer die—stations must properly evacuate to make room for the next blank. Last time I looked, no die-design CAD pack- age features an icon marked, “Double Hit.” Yet, throughout my 25-plus years of consulting, I’ve continued to hear stam- pers plead their case: “This die can take a double hit,” I hear. How do they know this? Other than a rough guess involving the size of the die and the tonnage capability of the press, how dare anyone insult his or her tooling by stating that it can properly close on top of two or more stacked parts, and consider this an acceptable practice?
Slugging
Our die expects to be mounted within the press with absolutely no foreign materials—slugs, scrap, bolt remnants, etc.—between it and the press bolster, and it expects to be mounted perfectly parallel with the press bolster and paral- lels. Again, there is no CAD simulation package, your honor and members of the technical jury, which includes an icon marked, “Foreign Stuff Under Die.”
Your honor, our die may be mercilessly contaminated with slugs strewn over part surfaces and stripper plates, and embedded within cam-return pockets, among dozens of other hiding places. The die was not designed to be pollut- ed with pieces of scrap metal. It expects the same cleanliness as it experienced during its design phases.
Complex Motions
An array of cams—spring-return and positive-follower types—as well as air cylinders, complex motion translators, adjustable leadscrew stations, etc. all tend to make our die nervous, as each of these elements has its own set of failure modes.
42 MetalForming/November 2011
www.metalformingmagazine.com
All dies expect to close on top of a properly positioned strip or blank.