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Inline Punching and Cutoff Applications in Roll Forming

Sunday, May 01, 2005

Hydraulics are becoming prevalent in roll forming as more operations are done inline at faster line speeds with smaller footprints. This review compares the latest hydraulic technology to current mechanical and air press applications used in roll forming.

A hydraulic press once meant a slow-moving ram with a cycle time of a few seconds, or a pause at the bottom of the stroke before returning to the top. In theory, integrating hydraulics into a roll forming line doesn't make much sense unless a positive stop or some type of accelerator system is used.

But hydraulics have been a part of punching and cutoff dies in roll forming lines for years - not as the main actuator for the dies, but in secondary roles that incorporate pickup mechanisms, material gripper units, pull back systems, and cam actuators.

These systems usually run at low pressures, where cylinder cycle time or overall cylinder stroke doesn't cause much concern. The continuing evolution and reliability of hydraulics and electronics now makes it worthwhile to add these high speed, versatile systems to existing and new roll forming lines.

Hydraulic Presses
Benefits increase with a correctly-sized hydraulic system. Hydraulic systems improve accuracy with open-loop measuring by producing extremely fast, repetitive ram action with a cylinder. This provides more measuring flexibility through closed loop, positive stop, flag trip and pick up finger mechanisms. Hydraulics shrink the overall line layout by cost-effectively buying tonnage and die space. They normally improve noise levels by operating more quietly than the constant pounding of an air press.

Hydraulics can dial-in an accurate bottom stop for forming and/or embossing, allowing the total die stack up to be locked in. Other types of presses need clearance at the bottom of the stroke so die movement is not restricted and material won't buckle between the roll mill and press.

Hydraulics punch or form features in a part from top and bottom or at different angles, producing parts with a roll design that is more beneficial than previously possible. Hydraulics perform ‘outside the box' applications that were never considered in-line before due to press limitations associated with selfpiercing nut assembly or in-die-tapping applications. Hydraulics provide full tonnage through the complete stroke of the cylinder for punching at different levels of the part and with longer strokes, all without worrying about tonnage loss incurred with other types of presses.

Correctly sizing the hydraulic system requires basic information to set parameters. First, identify the minimum and maximum part lengths or features being punched or cutoff, plus the desired line speeds at the minimum and maximum lengths (to determine whether the system can be sized). Be sure to include any parts that might run through the line in the future. This prevents costly system redesign and rework later.

Next, calculate the tolerances of each part feature and length to run in the system. Tolerances identify the required up-and-down cycle time and the type of measuring system that can be incorporated. Total cutting or forming tonnage and the die stroke are also needed to determine the cylinder bore size and running pressure of the system.

Using this information, choose the correct system components and consider the die types that are available.

Die Styles
General die types used with hydraulics in conjunction with roll forming lines include selfcontained style dies that don't require a stationary or flying press frame along with the die. Their flexibility cuts die design costs and saves overall line space.

The overall weight of a self-contained die is less than flying press frames. This helps in open-loop measuring applications, in turn saving money on closed-loop systems. Changing over a self-contained slugless cutoff die is simple and quick because only a few screws are removed to disassemble the cutting sections from the close-fitting pocket type holder.

With slug-type cutoff dies changeover time might increase due to hydraulic hoses that must be removed from the cylinder. The cylinder may also have to be removed from the die and the die disassembled, which takes longer than a die in press frame or conventional type press.

A flying fixed press with a die inside runs more styles of dies with faster changeovers. This press is ideal when using multiple dies because standard punching and cutoff dies are cheaper than a self-contained style. Initial sizing of the system is crucial with this press. The tonnage, stroke and speed must be correct for all applications being run. The weight here may slow down the overall line speed in an open-loop measuring system. Costs increase with a closed-loop system due to a larger servo motor moving the heavier mass.

A fixed press with a die flying inside offers flexible placement of the press system with the design of larger tonnage systems. The press can mimic an air or mechanical press on an adjustable base at the end of a roll forming line. Another option is to place the press right on the roll forming base, in-between roll stands. This saves room compared to splitting the base and installing a larger mechanical press inbetween.

However, the ability to build a larger system creates a need to design the press and the mounting surface so they can handle the tonnage and force generated from the system. Some of the same problems occurring withconventional presses can also present themselves with this style.

Incorporating hydraulics into a roll forming line has some disadvantages. With high-speed cycle times (up-and-down under .25-sec), the required large tonnage and/or long strokes might increase the sizing of the hydraulic system to the point that internal shock or cavitation occurs from oil flows being so great. This shock is usually more damaging to the hydraulic system itself, and may cause the hydraulics to leak from different areas of the system.

Custom high-speed hydraulics are not versatile enough to add drastically different part configuration to an existing line. The new part may require the system to have a different cycle time or longer stroke. A hydraulic system currently sized for a 30-ton application may not work for a future 30-ton operation due to speed or cylinder-stroke differences. Also, most hydraulic systems cost more than compatible tonnage air presses. Even weighing these possible disadvantages, hydraulics can still be a viable solution for the application.


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