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Lou Kren Lou Kren
Senior Editor

CNC Punching: Positive Stop a Positive Advantage

February 27, 2023

…for coining, embossing, lancing and more, but just make sure to mind the tool sharpening.

A CNC punching machine delivers tons of force to shape sheet metal as desired. The force transfers to tooling, which smoothly and efficiently adapts that force to produce cuts, bends and numerous features and fine details. For controlled-depth operations such as coining and embossing, positive-stop features on tools provide the fine-tuning needed for quality, repeatable performance while saving stress on the machine.

CNC-Punching-Tooling-Wilson-Positive-StopTo learn more about the importance, performance and maintenance of positive-stop tool features, MetalForming tapped the expertise of Susan Erler, product manager of the punching division at Wilson Tool International.

Reigning in Punch Force

“In practice, say in a hydraulic CNC punching machine, force builds as the ram descends with the control opening the hydraulic valve to supply needed force,” says Erler, explaining typical machine operation. 

In some cases, little machine control is available to stop the ram exactly when needed, and when using staggered tooling inserts, the valve controlling ram force must open and close repeatedly.

“That’s hard on the machine,” Erler says. “Where you need a consistent form—are there any scenarios where you don’t need a consistent form?—you’re much better off having stop features built into the tool. No machines provide the precision that precision forms require.”

Diagram-double-electrical-knockout-tool-Wilson-Positive-Stop- CNC-PunchingOf course, non-full-material-breakthrough applications such as forming electrical knockouts (EKOs) and other coining and embossing applications, for example, absolutely demand stopping capability built into the tooling, where material might require a raise of barely one material thickness. A small gap between the body of the tool and the tool cap, equal to the required material offset of the feature, provides for positive stop.

Positive-stop features are built into a variety of tools.

“We put positive stops in stamped-logo tools, for example, where users require a specific depth,” Erler offers. “We put it in lancing forms, again to achieve a consistent height. Once the tool bottoms out, with a properly set height adjustment, forms are consistent regardless of the pressure supplied by the machine. Most 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

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 undertaking 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.

Ensuring proper performance of positive-stop features demands correct maintenance, Erler stresses.

“During my career in customer service, 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, resulting in a lower positive-stop height. This requirement makes sharpening a single 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, repeatable form,” Erler concludes. “Relying on external variables such as machine programming, while important, really can’t accomplish that.” MF

Industry-Related Terms: CNC, Coining, Die, Form, Forming, Ram, Stroke, Surface, Thickness, Tolerance
View Glossary of Metalforming Terms


See also: Wilson Tool International

Technologies: CNC Punching


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