Selecting the right punching tools may eliminate secondary operations
February 28, 2012
Eliminating secondary processes in a fabricating operation is a way to reduce labor costs, which is good news for both the metal fabricator and, potentially, the customer. Modern punch tooling can duplicate some of those secondary processes while the material remains in the punching press, which reduces excess material handling and those downstream fabricating activities. Fabricators, however, have to realize just what is possible, so that they can take advantage of the tooling advancements.
Every CNC punching press has hidden power—it allows fabricators to eliminate many secondary operations. Discovering that hidden power requires the right tooling, proper process planning, and optimal operation of both the press and tooling.
Today’s tooling is more advanced than ever, providing fabricators with the opportunity to streamline operations and reduce machine downtime. For example, the latest tooling designs allow fabricators to both improve edge quality of punched parts, while simultaneously reducing the chance for troublesome consequences if the machine is not operated properly. Just as important, special-application tools can create new part features previously not possible on most turret presses; this gives process planners one more tool to use when it comes to finding ways to cut manufacturing costs.
It’s all about knowing what tooling is available and how to use it.
Smooth, consistently punched edges are no accident. They are a constant challenge for every fabricator. The best initial insurance for good edge starts is using long-life tooling. This type of tooling, made with premium tool steel, has grooves that help to distribute lubricant evenly and eliminate the possibility of slugs. That combination helps to ensure that tooling life is maximized between grindings.
Of course, every toolmaker makes its own claim to “long life.” The best advice is to check with successful fabricators to get the inside word. Follow their direction, and see if the same benefits result.
Another must is fully guided tooling, which adds the most assurance for quality edges on punched parts. Even when punching thick material like 0.025-inch mild steel or thin gauges of harder material, such as stainless steel, fully guided tooling directs the punch to the exact spot where material will be sheared and supports the punch so that it is centered relative to the die (see Figure 1). Because the punch is fully supported at the point of contact with the material, the punch tip does not enter the die off-center, creating a clean shear.
Once you have the right tooling in the turret press, thoughtful tooling usage will ensure quality results. Excessive burrs, poor hole quality, punch breakage, poor punch stripping, and galling can be caused by improper die clearance. So by setting die clearances right the first time, these common problems can be avoided.
Proper die clearance is determined by calculating the percentage of material thickness and adjusting it for different sheet metals. For example, mild steel might call for a die clearance of anywhere from 15 to 25 percent of material thickness, depending on just how thick the sheet—or even plate—is. Typically, die clearance charts provided by machine tool builders or tooling suppliers can show you the proper clearance quickly.
Dull punches and dies are another obvious but sometimes overlooked contributor to poor-quality punched parts. Regardless of how durable your tooling is, it was never meant to last forever without sharpening or being replaced. So refer to the press hit count indicator in the punching press at predetermined intervals to flag needed punch and die inspection and a timely trip to the tool grinder.
As previously mentioned, lubrication grooves designed into tooling can help to ensure that the punch is always protected as it hits the metal. For thick-material applications in which normal lubricant delivery is challenging, lubrication pads have been developed to keep punch tips lubricated. They insert easily into thick-turret tool assemblies and provide lubrication throughout the punching process, preventing galling. The lubricating pads also help to reduce heat buildup in the punch, which extends tool life.
When burrs or rough edges are unavoidable, special tools help. One such proprietary tool relies on a special ball in both the upper and lower part of the tool to push the burr away, creating a slight radius that is smooth to the touch on the punched edge. The rolling ball tool works in conjunction with the programming capabilities of the punching press to ensure that the ram is down as it traces out the contour of the punched edge. Even contours such as small, tight corners can be processed with this type of tool (see Figure 2).
Some materials are more challenging than others. The key to punching success is knowing what the tooling and press are capable of with a particular material.
One example is copper. While punching thin copper sheet is not an everyday fabricating challenge, doing it correctly points the way to good results when punching other challenging thin materials.
While very soft, copper is especially abrasive and can cause excessive wear on typical punch press tooling. Copper adheres easily to tooling components, particularly to punch flanks during stripping, causing all kinds of grief. The buildup of copper can distort punched parts and cause tools to wear prematurely. Extra tool cleaning and sharpening may be required to avoid this grief.
Most challenging when punching copper are narrow slots in sheets that are only slightly wider than the punch width. This type of part most often is used in printed circuit assemblies. With traditional, nontreated tooling, it would be impossible to punch extremely narrow slots in thin copper, but newer tooling designs with advanced coatings make it possible and relatively worry-free. This type of tooling has a reduced clearance between the punch and stripper of 0.0016 in. (0.04 mm)—traditional tooling’s clearance is 0.0031 in. (0.08 mm) or greater. The tighter clearance and tooling coating allow for clean punching of the narrow slots at high press speeds—up to 700 hits per minute.
In this application, the tool coating is a multilayer zirconium titanium nitride that is hard, wear-resistant, and lubricious. Like all coatings, it acts as a barrier between the punch and the sheet metal. The coating is suitable for this type of copper punching application. The combination of tooling style and coating eliminates secondary operations by preventing burrs, bad edges, and distorted parts. It also eliminates expensive scrap parts caused by excessive material buildup on the punch (see Figure 3).
Because welding is a costly fabricating operation and finding good welders is increasingly difficult, fabricators always are open to eliminating downstream welding if possible. They should be interested in what’s possible in the punching press.
A tool is available that produces self-locking, spring-loaded tabs. They snap together securely into prepunched holes. No additional downstream assembly operations are needed.
The tool eliminates the need for welding connecting joints in the fabrication of electrical boxes, for example. Through preplanning, multiple components can be decreased to a single prepunched component that folds up and locks together. This tool also can be used on prepainted material, which would otherwise have to be painted following welding. This helps eliminate additional finishing operations (see Figure 4).
In some instances, punch tooling can help speed up the assembly process.
For instance, tooling has been developed that fabricates forms that align with and hold hexagon-head bolts. It is an inverted, spring-loaded, triple lance and form tool system that can be used with a variety of head bolts and nuts in many thread sizes, including metric. The tooling provides a reliable and secure method of holding threaded fasteners in sheet metal for assembly.
The form accepts either a hexagon nut or the head of a hexagon bolt. A nut allows sheet metal parts to be assembled with a corresponding bolt from the reverse side of the part. Alternatively, the head of a bolt can be installed with the thread protruding through the sheet metal. In either method, the fastener is gripped securely between the punched forms to prevent nut rotation as the assembly is torqued together securely, eliminating the need for lock washers (see Figure 5).
Today’s punching presses and tooling have features that eliminate several secondary operations, such as creating hinges or threading holes. The key is knowing what’s available and how to use it.