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Tips for creating a whole lot of holes

Face perforating challenges with cluster tools and back-bending tricks

Image courtesy of Wilson Tool International®.

It is the Swiss cheese of metal fabrication. Manufacturers make dryer drums, HVAC components, lawnmower mufflers, architectural façades, and just about anything in between out of perforated metal. Creating perforated parts is a common practice among job shops and OEMs alike, as any type of finished part or product that needs air circulation or heat release requires perforated metal to deliver that result.

While a common practice among manufacturers, perforating sheet metal can deliver many production challenges and unnecessary costs. Sheet distortion, the need for secondary operations, shortened tool life, machine wear and tear, and extreme tonnage are some of the complications that rear their ugly heads during perforating applications. Throw in the extra time and labor required to mitigate these dilemmas and fabricators can face significant machine downtime, which translates to lost production time and increased maintenance costs.

The Devil Is in the Distortion

A typical and highly frustrating problem, sheet distortion that occurs during perforating operations often delivers a sheet that’s oil canned, warped, bowed, deformed, or stretched – and completely unusable. This leads to wasted materials, disrupted part flow, and the need for secondary operations to reflatten the sheet.

A challenge inherent to all punching operations, distortion during perforating occurs when the punch naturally brings the material inward during the shearing and penetration process. The oil canning or warping effect is caused in part by the combination of heat buildup and stress release in the sheet as the punch pierces the metal. This process creates an asymmetrical growth or reduction between the top and bottom of the sheet, which is due to the inherent draw caused by the clearance between the punch and the die.

While completely eliminating oil canning and warping is somewhat unrealistic, certain punching techniques and tools can assist manufacturers in alleviating the sheet distortion that occurs with perforating applications and counter negative effects. What’s more, these tooling systems and tactics can help fabricators increase tool life, reduce wear on the punch press, and confront the challenges brought on by extra tonnage.

Cluster Tools: Tips and Tricks

Using cluster tools for perforating can help minimize distortion and its negative effects.

While manufacturers can perforate with a single punch, this highly repetitive process creates significant wear and tear on the punch tool and the punch press. Cluster tools provide a punch with numerous tips, allowing them to punch a pattern that requires close center-to-center holes.

These tools can be designed with as few as two holes or as many as 120 holes, depending on the application. With the addition of replaceable inserts, cluster tools offer cost and time savings.

Along with implementing a cluster tool, manufacturers can employ a few tips and tricks to maximize the efficiency of the tool in perforating operations. One simple option when using cluster tools is to create more space between each tool tip. This technique minimizes the pressure on the sheet by increasing the surface area of where each individual punch penetrates the sheet.

However, the part design often does not allow for increasing space between the tooling. In these cases, reprogramming the machine to strike random hits as opposed to linear hits can help alleviate stress on the sheet and in turn, reduce oil canning and warping.

Offsetting the application so that the punch tips hit every other row is a similar technique. The cluster tool can be manufactured with one row of punch tips missing to achieve this result.

Another option for reducing oil canning and mitigating the effects of extreme tonnage in perforating applications involves staggering the height of the tooling inserts in a cluster tool. This process requires adjusting the height of every other insert in the cluster by a measurement equal to one material thickness. When inserts are placed in this staggered formation, the delayed piercing action of the shorter inserts can lessen the distortion by decreasing the stress on the sheet. Adding a rooftop shear to the punch or individual inserts also can provide the desired effect.

Cluster tools are available for thick turret, fab/turret, and various styles of punch presses. Because of their complexity and customizable design, the tools often take longer to manufacture and cost more than single punches, but many manufacturers find the added lead time and costs outweigh the headaches inherent in perforating applications.

Back-bending Techniques and Dies

Overbending or back-bending techniques and tools can help neutralize the effects of the oil canning and warping that distort the sheet during perforating. While this approach can be tackled using a number of methods, two common techniques are often the most effective.

The first technique for performing a back bend on the sheet involves relieving the die and allowing the stripper to bend around the relief. This process causes the sheet to bend back into the die.

The second approach uses an optional back-bending die. Many manufacturers offer an optional back-bending die adaptor, which incorporates a smaller die and allows the stripper to overbend or bend back into the die.

Coatings for Perforating Tools

Better lubricity for punch tooling can cut down on distortion and contribute to less wear on the tools, which extends the life of the punch, especially during perforating applications that require so many hits. Using punch tooling without proper lubricity causes the material to pull up during the perforating process. This effect can exacerbate the oil canning and warping and further distort the sheet.

Adding a coating to the punch tips helps create a smoother shearing and penetration process. Options such as titanium nitride and proprietary coatings offered by many tooling manufacturers can provide longer life for punch tools, as well as better performance in perforating applications.

Perfing for Your Operations

With the many options and techniques available for countering oil canning and warping during perforating operations, combatting distortion and extreme tonnage doesn’t have to be the headache it is for many fabricators. The key to confronting these challenges effectively is finding the right tooling and techniques that best suit the operations and the particular job by weighing potential drawbacks with desired outcomes.

Whether using cluster tools is the right solution for one job and overbending is the best choice for another, taking the time to temper sheet distortion during perforating can boost operations in the long run by helping keep production up to speed, prolonging tool life, and eliminating sheet metal waste on the floor.

About the Author

John "J.J." Johnson

Punching Product Manager

12912 Farnham Ave.

White Bear Lake, MN 55110

800-328-9646