On occasion, CNC punching machines will get the call to process materials 1⁄4 in. or thicker, with applications including automotive treadplates, for example, and part production for agricultural and other industries. At the extreme end, producing electrical busbars sometimes means punching copper or aluminum as thick as 1⁄2 in.
Whether a harder steel or a softer aluminum, thick is thick, and thick material presents unique challenges, with two real possibilities: tool breakage on impact due to shock; and tool breakage during the longer-lasting stripping process. So what’s a fabricator to do? Fortunately, solutions exist, offers Susan Erler, punching product manager for Wilson Tool International.
“CNC punching may include using 16- or 14-gauge material for electrical boxes, computer chassis or cabinets,” Erler says. “Occasionally, you’ll see 10-gauge material and think, ‘oh boy, that’s thick.’ Well, busbar fabricators think that working 1⁄4-in. material is like punching butter.”
Butter it’s not, of course, and the vast majority of fabricators find thick material a tricky proposition. But don’t despair, as Erler offers seven tool tips for successful thick-sheet punching, with this caveat: “Thick, mild steel acts similarly all of the time. Thick, softer materials such as copper, aluminum and brass, react differently. They sometimes require a trial-and-error approach.”
- Check the Tonnage
Material thickness greatly affects the amount of force needed to punch a hole.
The formula for calculating required tonnage:
Land Inches x Material Thickness x Shear Factor x 25
“On thick material, it doesn’t take a very large hole to maximize a CNC punching machine’s tonnage,” says Erler. “For example, a 2-in.-dia. hole in 1⁄4-in.-thick material requires more than 39 tons of force, which exceeds the capacity of most machines.”
- Maintain Proper Die Clearance
When punching material beyond 10-gauge thickness, Erler and Wilson Tool recommend increasing die clearance by 5 percent to provide the punch with room to exit. This means boosting the clearance for aluminum (recommended as 10 percent on 10-gauge) to 15 percent, and for mild steel (recommended as 15 percent on 10-gauge) to 20 percent.
“Too tight of a clearance can cause stripping problems, premature tool wear and even tool failure,” Erler warns. “As the punch scrapes up through that thickness of material as it retracts, all of that material essentially holds onto the punch. An overly tight clearance just exacerbates the problem.”
- Mind the Stripping
While improper die clearance contributes to stripping issues, stripping a punch out of thick material can be challenging even with the proper clearance.
“To help reduce stripping issues, consider switching to a larger station for more stripping force,” says Erler. “If a tool normally fits in a B station, try running it in a C or even D station.” Adding extra back taper to the punch also can help with stripping on thicker material.
“With extra back taper, the punch becomes smaller as it penetrates the material,” she explains, “meaning that less surface area on the punch drags through the material as it’s stripping. This doesn’t help as far as the shock impact of hitting the hole, but greatly assists with stripping.”
Read more: Seven Tool Tips for Punching Thick Material