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Maximum sheet metal punching, minimal distortion

Tooling and programming strategies reduce distortion in hole-intensive work

Figure 1
A cluster punch can increase efficiency for jobs with consistent hole patterns.

The turret punch press excels at making holes, and lots of them, but you can’t avoid physics. The act of punching numerous holes, especially a tight perforation, releases stress in the material and builds up heat, and that in turn causes distortion, which includes stretching, deformation, and warpage. You can’t eliminate distortion entirely, but you can reduce its effects.

Some jobs may benefit from stress-relieved sheets that have undergone specialized leveling. Sometimes you may be able to increase the hole size and spaces between the holes, if the design allows for it.

But there’s also a lot you can do at the punch press itself. These techniques boil down to three strategies involving tooling—including cluster punches (see Figure 1) and custom punches— programming, and the mode of punch press operation.

Tooling Considerations

First, be sure you’re using sharp tooling. Dull tooling requires more force to make a hole. That force creates more stress and distortion. Not only should you use sharp tools for hole-intensive work, but you should also sharpen those tools more often, so that you take less material off the punch with each sharpening. This will improve your tooling performance and prolong tool life. Using treated or coated punches also helps improve performance. The coating reduces heat buildup during punching and makes stripping easier.

Next, make sure you have the correct die clearance for the material type and thickness you’re punching (see Figure 2). If you have too much clearance, the material will stretch before the material fractures to produce the slug. This adds stress to the sheet and increases distortion. A clearance that is too tight makes the tooling work harder; more heat transfers to the punch tip and the sheet, again adding to distortion.

Also watch out for punch deflection, especially if you’re working with small hole diameter-to-material-thickness ratios: 2-to-1 for stainless, 1-to-1 for carbon steel, 0.75-to-1 for aluminum. A deflecting punch requires more tonnage to break through the material and, again, increases distortion. It also causes the tool’s cutting edges to break down quickly.

Using fully guided tools can help. These tools have a stripper clearance that is tighter than the die clearance. This means that, along with the punch being guided by the holder, the stripper opening also guides the punch tip into the material. Multiple points for guiding ensure the punch travels straight. This prevents deflection and, hence, reduces distortion (see Figure 3).

You can also try coining, a punching method that entails a custom tool set designed to combat warpage. The die surface is slightly convex, and the edge of the matching stripper plate is slightly concave. When the punch descends, the stripper presses the material around the domed die. This downward force counteracts the material’s natural tendency to warp upward.

This isn’t scientific. Material properties vary, so a die that’s domed to a specific degree isn’t guaranteed to eliminate warpage every time, but such tooling can help in some circumstances.

Finally, lubricate the sheet metal whenever possible. Lubrication prolongs tool life and reduces heat buildup in the tool and the sheet. You can also use pads that lubricate the punch tip with each stroke of the machine. You may need to weigh the benefits of lubricating the sheet against the time it takes to remove the lubricant from parts; this can be time-consuming, especially if you’re working with stainless steel. But the reduced distortion (and, hence, improved part quality) and prolonged tool life may make that time spent cleaning lubricant off parts worthwhile.

Figure 2
Excessive or insufficient die clearance both can lead to more workpiece distortion.

Cluster Punches

If at least a portion of a nest has consistent hole patterns, consider cluster punches, which allow a number of holes to be punched at the same time (see Figure 4). Cluster punches come in insert or one-piece styles. Once-piece-style punches are more expensive but are easier to maintain and highly accurate, especially valuable when holes have tight center-to-center tolerances. Insert-style cluster punches have their advantages too. For instance, if you break one punch insert, you can replace just that insert, not the entire cluster punch.

The larger your cluster punch, the more holes you can make at once and, generally speaking, the more efficient you can be, as long as you have the space in the machine and you don’t exceed the machine’s tonnage capabilities. A large cluster punch also tends to distribute punching forces more evenly.

If you find that you’re close to the machine’s tonnage maximum, you may want to try staggered-punch cluster tools. These have varying punch heights in which every other punch in the cluster is between 50 and 60 percent of the material thickness shorter than the other punches in the tool.

Staggered cluster punches reduce noise during operation as well as the required tonnage, because not every punch tip in the cluster is hitting the workpiece at once. Instead, half of the punches contact the workpiece first and start punching through; the other half doesn’t hit until the first half is at least 50 percent through the sheet. This reduces stress and the potential for distortion.

Be aware that careful inspection and maintenance are important for these tools. Because the slightly longer punch tools descend farther to complete the punch cycle, they’re more susceptible to extra tool wear and galling on the punch points.

Programming Strategies

As an additional distortion-reducing strategy, you can also use a cluster pattern that’s double- or even quadruple-spaced—and this is where strategic punch programming enters the picture.

Let’s say you have a double-spaced pattern of eight punches in the tool, and you need to punch 24 small holes, grouped in a square that’s six holes across and four holes down. The first hit of the cluster punches six of the holes; the tool then moves over to punch the second set of six, in between the holes that were previously punched. It then would follow the same pattern for the third and fourth hit (see Figure 5).

Few jobs have hole patterns that match the pattern in the cluster punch exactly, and you should never repunch through an existing hole to complete a hole pattern, so you can use a single punch tool to punch the remaining holes. You can also use this strategy entirely with a single-hit tool. Following the example in Figure 5, the tool would punch the No. 1 holes first followed by 2 through 4.

This strategy prevents you from having to punch a tight cluster of holes, which can induce a great amount of stress into the material and, again, lead to distortion and deformation. Punching every other hole disrupts the accumulation of tension and compression that occurs when punching features in close succession and in the same direction.

In fact, the direction of operation can be critical. When punching perforated patterns, or any hole-intensive job, start the program as far away as possible from the material clamps, then work your way back toward the clamps, usually near the throat of most C-frame punch presses. This helps compensate for material stretching. Most of the stretching occurs away from the rigid clamps and toward the unclamped sheet edge.

Figure 3
Fully guided tools are guided not only by the toolholder, but also by the stripper, because the stripper clearance is tighter than the die clearance.

To illustrate, consider a job in which you need to make two rows of holes on a sheet that’s 24 in. wide. Measuring from the part edge closest to the clamps, one row of holes needs to be at 8 in., and another needs to be at the 16-in. mark. Starting at the 8-in. row that’s closest to the clamps can cause problems. As the machine punches the row, the material stretches slightly; the clamps then move the material to punch the row at 16 in.—only now, that row isn’t exactly 16 in. from the edge.

When you measure the component, you find that second row now is ever so slightly closer than it should be. That’s because the clamps moved to the true position and didn’t account for the fact that the material stretched. This tolerance will stack up as the machine punches more rows of holes moving away from the clamps.

Conversely, if you start away from the clamps and punch the 16-in. row first, most stretching occurs from that row out to the sheet’s unclamped edge. So when the machine moves the sheet to the 8-in. row, its position relative to the first row remains accurate.

Punch Press Features

Certain features of specific turret or rail-type punch presses can help minimize distortion. For instance, during conventional operation, the punch tip contacts the workpiece surface at full speed. On some systems, the stripper remains about 0.040 in. above the workpiece throughout the stroke. This means that the sheet sometimes can hit the stripper during the punch’s return stroke. These and similar elements of normal operation can cause problems in applications sensitive to distortion.

To combat this effect, some machine-specific operation modes apply pressure to the sheet from the bottom of the stripper before the punch tip reaches the material. The stripper continues to apply pressure to the sheet throughout the punch cycle, until the punch tip ascends and clears the sheet surface.

Some machines also have a soft-punch feature in which the punch descends at a slow speed. Once it contacts the sheet metal surface, it senses the resistance and drives the punch downward with enough force to penetrate the material.

Alternatively, if a sheet has symmetrical hole patterns, you may be able to punch half the sheet, flip it over, then punch the other half. This tends to equalize the sheet stretching of both halves. This strategy also can mar the sheet, though, so it may not be practical if the punched part is cosmetic.

The Big Effect of Minimizing Distortion

When it comes to distortion (or almost anything else in manufacturing, for that matter), you need to consider the trade-offs. Cluster punches, fully guided tools, and custom coining tools help reduce distortion, but may require additional tooling investment. Machine-specific functions help, too, but they can add to your punching cycle time. So can lubrication and the associated washing required.

But a workpiece with even minor distortion can have major implications for downstream operations. A press brake backgauge may be perfectly accurate, but the sheet metal butting against it needs to be flat, not bowed or warped. A warped sheet changes the flange depth dimension and bend line location. This in turn creates a formed part that gives the welder trouble when putting together a subassembly—and the troubles mount from there.

In many cases, the costs of punching with less distortion are far outweighed by its benefits, including improved manufacturing, part flow, and quality.

About the Author

John Ripka

Applications Technician

1295 Lund Blvd.

Anoka, MN 55303

763-421-0230