Beyond Punching

Machine design, tooling and programming software combine to make today’s punch press capable of complex forms. By Dan Caprio, punching product sales manager at LVD Strippit

The introduction of the hydraulic press drive decades ago brought new versatility to the punch press – allowing the punch stroke to start and stop at any point along the ram path. The flexibility of the ram control coupled with automatic tool rotation technology opened the door to a broader range of punching and forming capabilities. The control and programming system’s ability to integrate special functions allowed users to take full advantage of the latest tooling designs, such as a wheel, hinge or bend tools.

Fast forward to the twenty-first century: the modern punch press is more capable than ever when it comes to forming, including the processing of complex, 3D parts. Today’s punch press can create bends in sheet metal that until recently only a press brake could produce.

Being able to form a part on a punch press can help significantly reduce parts costs. Bending is one of the most common bottlenecks in the fab shop, which is why shortening or eliminating the bending operation makes so much sense.

Not Your Conventional Punch Press

Tolerances are extremely tight, but operators can change
parameters to suit different thicknesses.

To form on a conventional turret punch press, you might have a feed clearance of only .984 inch or less. Part of that space is taken up by the form die, which raises the material slightly, and then you have the material thickness.

Some tools allow you to use a significant portion of that clearance, but as a rule, you can form reliably in a space that’s only 50 percent of the total feed clearance minus the material thickness. That’s not much.

New punch press designs, however, have clearances that take forming into account. Some systems make room for up to three inches of forming space from the lower dies to the upper punch. This allows for significant forming and bending, including the forming of flanges up to three inches high.

These punch presses don’t have the traditional turret setup, but instead use what is known as a tool-changer design. In the tool-changer-style punching machine, the lower carousel is underneath the brush table, and dies emerge and retract through a die to move down and out of the way between hits.

Bending Tools

This punch press design opens the door for more forming possibilities, and not just for ribs, louvers and other short forms, but also the kind of tall flanges that would normally be formed on a press brake. The bending punch and die in a punch press are a hybrid between a panel bender and a press brake, with some unique attributes. The punch looks like a miniature hold-down tool on a panel bender, while the die has a V geometry similar to what’s found on a press brake die.

The die body actually rotates during the bend. This rotation folds the workpiece against a stationary upper punch, and the die’s degree of rotation determines the bend angle. The radii you can achieve depends on the V-die design, which can be determined when ordering the tool from the manufacturer. Or, if you need to achieve a certain radius, such as for a profound-radius bend, the die rotates at certain degrees to bump the metal as the piece progressively moves forward. It’s bump bending, punch press-style.

Accounting For Thickness

When programming a punch press, taking into account
how exactly the parts slide down the chute is important.
A large, heavy part with a high flange may be rough enough
to change its bend angles slightly.

Tolerances are extremely tight, both in the positioning accuracy of the machine and the machining accuracy of the tool, similar to the tolerances available on a modern press brake with precision tooling. Press operators also can input changes in thickness. Say one batch of material is on the lower end of the thickness tolerance window, while the next batch is at the high end, such as 0.055 inch for one batch and 0.061 inch for another batch.

This can make a difference in the bend angle, but as long as the operator checks the sheet thickness and makes the parameter change in the program, the machine can account for it. A change in the program code is made that tells the ram how far to come down before it performs its operations.

Besides the 3 inch height limitation, there are other constraints to consider. Unlike a press brake operator, a punch press can’t flip a part over, so a part with both positive and negative bends can create problems. Also, the angle of bend is usually limited to 90 degrees or less; acute bends greater than 90 degrees complementary aren’t practical, for the most part (depending on the tooling you have). And because of tonnage limitations, the material can be only so thick. This varies, depending on your punch press and tooling, but typically it is up to about 0.118 inches.

Programming Strategies

When you bend on a punch press, your programming options abound. Traditionally, you program the forming sequence at a point whether it won’t interfere with any other part. This usually means you’re forming near the end of a nest’s punching sequence, after most or all of the flat-part punching is completed.

At this point you may decide to bend all the flanges in a part at once. You cut the profile, leaving tabs connected to the nest to ensure part stability’ bend the flange, then perform the final punching to cut the tabs and release the part so it can slide down the chute. This strategy can work well if you want to evacuate the formed part from the nest as soon as possible to avoid collisions with the tools.

Alternatively, you can punch the profiles (minus the material for the tabs) on multiple parts – say, all the parts in one row – form the bends, then send them all down the chute with the final punches that cut the tabs. This strategy reduces the number of tool changes and so can reduce the cycle time, but works only if there is no danger of interference between the flanges and the tooling.

Consideration Of Tabs

Tabs keep the part stable during bending, but where exactly you put those tabs, their width, how many, and how they’re cut depend on the flange geometries. Some pieces may call for only a few or even just one tab at a flat section of the part. Other times the bending operation itself can break the tabs. This can be useful when bump bending. During such a sequence, the microtabs holding the part in place break, and after the last bump, the part breaks free and slides down the chute.

Programming also needs to take into account how exactly these parts slide down the chute. For example, if a large, heavy part with a high flange slides down the chute incorrectly, its landing may be rough enough to change its bend angles slightly or it may land on other formed parts with enough force to change their bend angles. You can overcome these problems by making changes to the program.

Software Makes A Difference

It’s possible to program these variables manually, but it can be complicated and time-consuming. There are plenty of details to consider, including which way to rotate the bend tool (the tool set rotates 360 degrees to align with the programmed bend lines); how to position and sequence everything to avoid interference; and which width of bending tool to use, depending on which tool you have in your library and the bend length you need.

In more challenging cases, manually programming may not be very efficient, and it actually may take you less time to form the flanges on the press brake, especially if those parts are heading to the brake anyway for a few remaining bends.

Determining Sequences

This is where the final piece of the puzzle comes into play: software that can automate the task of determining the punch and bend sequences. With such software, you can feed the 3-D model of the part you want to bend on the punch press to the software, and it will unfold the part and suggest strategies to punch and bend it, based on the tools available on the machine.

The offline program works similarly to offline bend programming for press brakes. It sees the interference points, knows just how the tool needs to rotate, and sequences it in an efficient way.

As a programmer, you can either accept the software’s recommendation or tweak it manually to suit your needs.

APMEN Sheet Metalworking, Dec 2016

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