“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.”
3. 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.”
4. Punch Construction Matters
“Use punches made from powdered metal,” says Erler. “They’re stronger and adds shock resistance upon the punch first hitting the material. Given advances in metallurgy, many tool suppliers offer powdered-metal punches at economical prices.”
Also recommended: adding a coating to a punch.
“When punching soft material such as aluminum or copper, coating should be considered a must-have,” Erler says. “Soft materials become gummy when punched, which causes stripping failures. Wilson Tool, for example, offers several coatings that greatly reduce stripping problems in softer materials.
“For mild steels, the value proposition from coating becomes less obvious,” she continues. “If stripping isn’t a problem, the added cost of coating may not be worth it when considering that tool life is reduced when punching thicker mild steel.”
Noting that punching any thick material stresses the tooling, Erler recommends placing a shear on the punch to reduce required tonnage when just punching a hole.
“A rooftop shear is the best bet for hole-punching. Any shear will reduce needed tonnage, but a rooftop shear is strongest and best handles a full hit.”
For parting in thick material, Erler recommends a concave shear.
“Using a rooftop shear on a slitting tool, for example, requires that least 75 percent of the tool length penetrates the material on every hit,” she explains. “This doesn’t always happen in the real world, as software designed to optimize the number of hits may instruct only a 25-percent length on the last hit.”
Here, a fabricator may be advised to use a bridge hit—skipping the second-to-last hit to make the last hit, then returning to hit the remaining material with the center of the punch.
“But my experience tells me that not too many fabricators will go through the hassle of programming for bridge hits,” Erler says. “Using a rooftop shear on a slitting tool for thick material creates a side load that is very tough on the punch. Consider a concave shear, which still reduces the needed tonnage while better inverting the stresses of that high-tonnage application.”
5. Properly Maintain Tooling
There’s no getting around the fact that punching thick material results in a higher tool-wear rate.
“Check the punch more frequently for wear,” says Erler. “Though seemingly counterintuitive, more frequent sharpening actually extends tool life. In addition, dull tools increase the tonnage required to punch a hole. Punching thick material typically means running at higher-than-ideal tonnage, and a dull tool only will increase that tonnage.”
Also important in extending tool life: proper turret alignment and lubrication.
6. Sacrifice Grind Life for Strength
Consider reducing the straight-before-radius on a punch tip, recommends Erler, as the stouter punch provides added strength for punching thick material.
“This results in reduced grind life,” she says, “but the reduced risk of catastrophic failure often makes this sacrifice worth it. The stout punch decreases the odds of a punch breaking upon impact.”
7. Consider a Tool Insert
Implementing some or all of the advice above can improve the success rate and tool life when punching thick material. Take it to the next level, recommends Erler, by switching to a tool-insert design. Advantages include significantly lower replacement costs than with full-body tooling, quicker setup times and lower storage requirements.
“Even when doing everything right, tools do not last as long when used on thicker material, so reducing costs by using inserts just makes sense,” she says. “And, if adding a coating, it’s on a smaller area, which also reduces costs.”
Bonus Tip: Radii on Corners
Beyond these seven tips, fabricators can improve tool performance and life when punching thick material by re-assessing the hole shape.
“Whenever possible,” Erler suggests, “place a radius on any sharp corner—perhaps a few millimeters or a millimeter radius on four corners of a rectangle. Something as simple as this helps increase tool life.” MF
See also: Wilson Tool International
Technologies: CNC Punching
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