Page 32 - MetalForming March 2023 - PMA Chairman Jeff Aznavorian

Special Section: FABRICATION CNC PUNCHING
machines have a ±0.005 to 0.010-in. tolerance on the stroke from hit to hit, but a 0.005-in. variation on form height often is unacceptable.”
Sharpening can be Tricky
Ensuring proper performance of positive-stop features demands correct maintenance, Erler stresses.
“During my career in customer serv- ice, fabricators would call and explain that after sharpening an EKO tool, the required knockout height no longer could be maintained,” she recalls. “What happens then? They’ll try hitting the tool harder, which is not a good idea—the tool will break. If it doesn’t break, the tool still can’t achieve the needed height.”
This occurs because the sharpening process removed material and altered the positive-stop gap distance in the tool.
“On an EKO tool for instance, tool- maintenance personnel must remove an equal amount of material from both the punch tip—cutting surface—and the back of the ejector to maintain that gap,” Erler says. “When they remove material on one surface but not the other, the gap becomes shorter, result- ing in a lower positive-stop height. This
This diagram of a double electrical knockout tool, used to raise material by about a material width and then cut the circumference of the raised material while leaving tab attachments, shows the positive-stop tool feature that stops the ram at a consistent, repeatable height.
requirement makes sharpening a sin- gle EKO tool a complicated process. Now imagine needing to sharpen a quad or tangential EKO tool. That’s why we recommend that users send the tools back to us for sharpening. It’s not as intuitive as just sharpening
the dull surfaces.
“Again, positive-stop features are all
about providing a consistent, repeat- able form,” Erler concludes. “Relying on external variables such as machine programming, while important, really can’t accomplish that.” MF
Sharpening a Positive-Stop CNC Punching Tool
Sharpening a positive-stop tool can be tricky, so fabricators should send the tool back to its manufacturer for sharpening to preserve tolerances. For those undertak- ing the task themselves, here’s an example provided by Wilson Tool International that walks through sharpening instructions.
Upper-unit punch-sharpening instructions: Disassemble the upper unit. Sharpen surface A; record the amount removed; then remove an equal amount from surface B.
Lower-unit die-sharpening instructions: Disassemble the lower unit. Sharpen the top of the lower-unit surface D; record the amount removed; and match the angle and plunge-grind this surface until the same amount has been removed. Grind the same amount off of the die-cap surface C; then reradius and assemble. Do not allow cap screws to extend above the die cap when reassembling.
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