Page 27 - MetalForming Magazine March 2022

punch face to seat over the workpiece. “Any less than 80 percent will unevenly load the punch and lead to premature tool wear, burrs and shav- ings,” Visser says, “and you may start to pull slugs. During nibbling, you’ll seat 100 percent of the tool face on the first hit, then perform a series of punch- es where perhaps only 10 or 20 percent of the punch might seat on the mate-
rial—not a best practice.”
Tools used for nibbling generally last
about 5000 hits, Visser says, meaning that the tool may require maintenance every 3.125 days (based on 200 hits/ min.). “The proper tool could last at least 10 times longer,” he says. “Every time you replace a tool the machine is down for at least 5 min.—lost productivity.
“When touring sheet metal fabrica- tion shops, I’ll often look into the scrap bins and find evidence of hits made where less than 80 percent of the tool face contacted the workpiece,” Visser
“Generally, parts smaller than 3.5-in. diagonal or diameter should be punched out in a single hit,” recommends Peter Visser, regional sales man- ager for Mate Precision Technologies. Unfortunate- ly, in this case the fabrica- tor used nibbling to punch out the rectangular open- ings in the sheet in four separate hits, wasting valu- able machine time, leaving scalloped edges and quickening tool wear.
continues. “Your managers should do the same, looking for shavings and pulled slugs, and go back and ask the programmer to develop new punching routines to follow the 80-percent rule.”
Visser, in fact, recommends that shops place two scrap bins near their machines—one for planned scrap such as setup sheets and initial scrapped parts at the beginning of a run, and one for unplanned scrap such as slugs and parts with excessive burrs, “so that they can track the amount of scrap that could or should have been avoided.”
Minimizing Unplanned Scrap
...often comes down to avoiding dull tools, and a great way to do that: Start each shift by punching a sample template with every tool in the turret.
“Many shops use a standardized tur- ret, both a good and bad practice,” Viss- er says. “On one hand, it can mean less time spent changing tools and, there-
fore, increased machine run time. On the other hand, it may mean that the shop never removes the tools to inspect them, until something bad happens.”
The best shops, says Visser, start each shift by clamping a test piece of sheet metal into the punch press of the same type and thickness that they’ll be fabricating, and make one punch with each tool in the turret. They then inspect every hole in that template and feel for burrs, and look for any other indication of dull tools.
“Some of my best customers run two or three of these tests per day,” Visser says. “You also can use these templates as a way to train new oper- ators, so that they learn what good punches should feel and look like.”
When it comes to sharpening their tools, many fabricators don’t have grinding equipment on hand, so they either run the tools until they don’t work and throw them away, or send
Fabrication: Audit Your CNC Punching Operations
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