Troubleshooting the punched sheet metal edge

Worldwide, CNC punch press operators produce millions, if not billions, of parts monthly. The process appears straightforward, but the subtleties abound. Sometimes part quality is affected by its finished edge.

How do you avoid producing parts with inferior punched edges? As with so many problems in metal fabrication, various factors come into play. The trick is to consider them all before jumping to a conclusion, taking corrective action, and discovering that your edge problem stubbornly persists.

Tool Sharpening
Worn or dull tooling can produce punched edges with more burrs. Increased burring can affect the way parts assemble and can be a safety hazard when handling parts. Dull tools also force machines to work harder to produce the same holes or features, accelerating machine wear and maintenance.
When a visibly larger rollover appears on a part’s punch edge, it’s probably time to sharpen the tools. Sharpening tools regularly will help produce good-quality parts and help extend tool life. Tools should be sharpened when cutting edges are worn to a maximum radius of 0.010 in. (0.25 mm).

To inspect for this radius, hold the edge near a light source and look for reflections as the light bounces off the radiused edge. Also try the fingernail test: Lightly (and carefully) drag your fingernail across the edge; if that’s enough to lightly shave your fingernail, the punched edge is sharp and the tool has some life left. If you don’t see shavings, it might be time to sharpen your tool.

When sharpening tools, it’s best to remove a small amount of the tool surface more frequently rather than large amounts less frequently. Light, frequent sharpening helps extend tool life and improve part quality (see Figures 1 and 2).

After sharpening a tool on a tool grinding machine, use a sharpening stone to remove the small burr that develops on the punch point. Doing this will create a minimal radius, between 0.001 and 0.002 in., on the tool’s cutting edges.

The tools become magnetized when sharpened, so be sure to demagnetize them afterward. Forget this step and you might find punched slugs sticking to punch points and possibly ending up on top of the sheet, where the machine can press them into the sheet and create unwanted marks or dents. Slugs on top of the sheet and close to the punch point might result in a double material thickness being punched, which can damage the sheet and cause premature tool wear.

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