The case for hydraulic punch presses

Modern Machine Shop, March, 1994 by Tom Beard

CNC punch presses that offer complete control of the ram stroke deliver obvious advantages in forming operations, and several more benefits you may not expect. Hydraulic punch presses have been around for the better part of 20 years. Though their proponents argue for a host of benefits inherent in the fluid actuation of a ram, these first-generation hydraulics nonetheless staked out functional grounds essentially similar to their mechanical counterparts. In the last few years, however, a new breed of hydraulic press has emerged. Through the marriage of much more responsive servo designs to increasingly sophisticated CNC technology, these hydraulic presses offer a new capability to control every variable aspect of the ram stroke. This type of press is a very different creature indeed, and it offers fabricators some process options they've never really had before.

The most obvious benefit of a fully-programmable ram stroke is that it lends itself so well to forming operations. Both the length and speed of the stroke are set in the part program and executed by the control, and different parameters can be established for every tool in the turret. Don't forget, though, that there are various other benefits that apply to punching operations as well.

But U.S. fabricators are not yet rushing to acquire this additional capability; the majority of machines sold today are still of the mechanical and/or simple hydraulic designs. Maybe that's because of the cost--full ram control will come at a premium. Or maybe it's just a simple lack of awareness. In any case, we wanted to find out more about the promise and the challenges of this technology. So we spoke with Victor Carbone, vice president of engineering at Strippit, Inc., the well-known punch press manufacturer in Akron, New York. This vendor has no particular ax to grind on the subject since they make both mechanical and hydraulic presses. For the rest of this article, just about everything you read represents Mr. Carbone's thoughts on the relative merits of this new generation of hydraulic machines.

Though we will make a fair number of generalizations here, please recognize that there are very significant differences in machine performance from manufacturer to manufacturer, and model to model. There may even be cases where some of the capabilities and/or limitations we discuss do not apply. All we are really trying to do here is provide a general notion of what new-generation hydraulics might mean to your shop. If you are considering buying, be sure all potential suppliers clearly explain the specific features of their machines. Good Form

The one thing that all these new-generation hydraulic presses have in common is that they give users complete control of the entire stoke of the ram--an obvious advantage in forming operations. The length of the stroke is fully programmable in the upward and downward directions, typically in increments of 0.001 inch. Speed is programmable as well. Typically, these parameters are set in the part program, but there are even easier methods, as we'll see in a moment. Several key technologies are converging to make this all possible. Part has to do with more advanced hydraulic servosystems; part with the addition of ram position feedback sensors; and part with more sophisticated CNCs. All play a role in providing ram control that is both precise and highly consistent from hit to hit.

This capability provides the opportunity to take a more scientific approach to forming operations, and to reduce setup. Gone are mechanical screws and shims to set tool length--it is now a numeric dimension indicated in the CNC program. And when that value needs to be adjusted, you simply change the number. With a little disciplined record keeping, future setups with various materials will no longer be a matter of so much trial and error.

Some control systems also allow you keep tool records right on the CNC. This way, the CNC programmer doesn't have to worry about setting stroke lengths for each tool in the routine. At the machine, an operator can call up a tool screen where he enters the proper stroke values for each tool station. When the program is executed, the stroke parameters automatically reset to whatever values the operator has entered. Since the values are tied to the tool, and not the program, they only need to be entered once, as tools are loaded into the turret. Moreover, adjustments for different thicknesses of material can be made automatically once that variable is established in the part program. This method makes programming less cumbersome since there are fewer variables overall to deal with, and it generally gets new jobs running quicker since there is less occasion to make errors in the program itself.

Hydraulics also provide an opportunity to improve the quality of forming operations. For one thing, tonnage is constant throughout the entire ram stroke. With a mechanical press, force (and speed) is proportional to the angle of the driving cam, and will vary as the crankshaft rotates through its cycle. Hydraulics provide a gentle and constant pressure through the stroke, which lends itself more readily to the forming operation. It's also less inclined to tearing the material. Moreover, full stroke control allows a dwell to be programmed at any point in the cycle. At the bottom of a forming stroke, a dwell allows the material to more completely flow to the desired shape, minimizing springback and improving consistency overall.