Understanding press feeds
A primer for the nontechie
Major technology shifts in how stamping presses are fed have allowed press feed technology to evolve, enabling stampers to realize increased processing speeds, improved processing flexibility, easier setup, and better quality and reliability. Even with today's advancements, a press feed must meet three basic and important criteria to be successful: Setup must be flexible. It must deliver the material with sufficient precision into the tool and die. It must feed at the correct time. Advancements in feed technology include pilot release, space-saving line configurations, transfer/progressive operations, and scratch-free processing.
|A press feed must not only move the proper amount of material into the tool, it must position it correctly into the die—front to back, side to side, and square with the tool. In this case, it must also gently handle prepainted coil to prevent scratching it.|
Over the last decade or longer, major technology shifts in how stamping presses are fed have occurred. As press feed technology continues to evolve, stampers are realizing increased processing speeds, improved processing flexibility, easier setup, better quality and reliability, and numerous other positive end results. Even with today's advancements, a press feed must meet three basic and important criteria to be successful:
- Setup must be flexible.
- It must deliver the material with sufficient precision into the tool and die.
- It must feed at the correct time.
Basic Press Feeding Principles
- The feed setup must be flexible enough to accommodate the full range of applications that will be run on the line. The feed must allow adjustments to cover all setups respective to feed length, material width and gauge, and die heights, as well as feed and pilot release timing (see Advancements in Press Feed Technologies Sidebar). If the feed is used in a dedicated system, these variables will be fairly limited. More often than not, however, feeds must adjust to a wide range of applications.
- The feed must deliver the material with sufficient precision into the tool. It must not only move the desired amount of material into the tool, it must position it precisely in the die—front to back, side to side, and square with the tool. Misalignment results in slippage and strip buckling, which cause material binding, misfeeding, and short feeds. Short feeding results in bad parts and broken dies. For this reason, it is important that feeding equipment is installed on-center—square to the tool—and rigidly mounted. Proper installation ensures that no movement can take place between the tool and the feed.
In addition, for each setup the tooling must be positioned precisely. It is recommended that some sort of registration device, such as positive stops or keys on the bolster, be used to ensure consistent tooling placement. Without good-quality material, proper straightening, and precise alignment, a feed will have problems regardless of which type is used.
Nearly any new feed that is properly installed and set up can deliver an acceptable length accuracy for most applications. Generally, it will retain that accuracy if properly maintained.
- The feed device should deliver material at the proper pace so it keeps up with the speed of the operation. The time that a feed actually has to deliver material is determined by the amount of time for one complete press or shear cycle, minus the time that the tooling is engaged, and minus the time required to detect a misfeed and stop the press. This means that the longer the die engagement, or the faster the speed of operation, the less time there is to feed.
Many other important factors determine just how effective the feed will be. Some of them are the amount of time and skill required for job setup, the cost of energy to operate it, and how the feed interfaces with the system as a whole.
Two basic feed types are roll feeds (press-driven and servo-driven) and gripper feeds.
Compact coil lines that combine unwinding, straightening, and feeding into one machine can reduce feed line length by as much as half.
A roll feed can be either powered by the press (typically viewed as older technology or for high-speed, dedicated press applications), or it can be powered by its own self-contained drive system (the majority of today's applications).
Press-driven Roll Feed. Roll feeds that are press-driven, such as rack-and-pinion or cam, are always synchronized with the rotation of the press. The feed's motion begins at a predetermined point in the press cycle and finishes at another predetermined point regardless of press speed or die engagement. Although the index speed must increase or decrease to keep pace with the press, the feed can draw as much power as it needs from the press to accomplish this within the limitations of the mechanical coupling of the feed to the press.
This synchronization feature makes press-driven feeds suitable for high-speed indexing, in-die transfers, and for use with unloaders and other applications that require feed motion to be tied to press rotation to prevent a collision.
Drawbacks to most press-driven feeds are their difficulty in adjusting the feed length, lack of inching capability, and absence of controls interface. Most require that gear sets, rollers, or mechanical linkages be changed to adjust feed length. Because press-driven feeds are coupled directly to the press rotation, they lack the ability to jog the strip for threading. Additionally, because of the lack of electrical controls, mechanical feeds cannot accept setup information from or provide feedback to press controls or automation systems.
Servo-driven Roll Feed. The concept of servo-driven roll feeds involves the use of a closed-loop positioning drive, usually a servomotor, to control the index position of the feed rolls. A servo-powered unit begins its motion in response to a signal from the press. It is capable only of a finite, minimum index time, which is based on the amount of power it can deliver and the load that it sees. As a result, a servo-powered unit's finishing point can vary with the press speed. The faster the press runs, the later in the stroke a servo-powered unit finishes. Their top speeds still are limited by the available drive power, as opposed to press-driven feeds that can run as fast as the press and tooling.
This combination transfer/progressive line's primary feeder feeds oscillating shear die and feeding blanks to a transfer operation. A second servo roll feed rolls into place when progressive-die operations are used.
Servo drive technology has been used in press feeding applications for more than a decade, and it has now matured to the point that these drives are more reliable and less expensive than they were in the past. Servo systems' modularity and self-diagnostic features have improved their functionality.
Servo-driven roll feeds have many of the same advantages press-driven feeds have, such as minimal space requirements, low maintenance, and high speeds.
However, servo feeds also provide benefits that press-driven units cannot because a microprocessor-based control gives them capabilities such as programmable move patterns, self-diagnostics, autocorrection, and the ability to communicate with automation. Servo-driven roll feeds are available in several configurations, including:
- Conventional two-roll units
- Four-roll units (sometimes specified when processing thick materials)
- Feeder/straighteners (for space-saving requirements)
- Push-pull units (typically for light-gauge materials)
- Zigzag units (for better material utilization when stamping nestedround blanks)
Servo feed control packages vary in sophistication, from simple-to-use, single-setup controls with thumbwheels or keypads to complex systems that allow programming of elaborate multiaxis move patterns, control of auxiliary functions and devices, and varying levels of job recipe storage memory and communications capability. Most servo feeds offer some degree of integration with the press control to provide constant contact with the press operations:
- "Hand-shaking" on job setupsand recipe storage
- Simplified operator interface that allows single point of data entry/verification and a touch-screen display that delivers operational prompts and diagnostic information and help screens
- Faster maintenance reminders or fault information via a single control station
- More control of peripheral equipment such as in-press transfer units, safety equipment, and die monitoring systems
Scratch-free threading tables can be lined with nylon materials to avoid damaging surface-sensitive materials.
Gripper Feed. A lower-cost option, a gripper feed uses a linear motion, rather than rotary action, to move the strip. Gripper feeds utilize a pair of clamps. One is a stationary clamp called the retainer, and the other, called the gripper, moves infeed and return strokes. During the feed stroke, the retainer releases the strip as the gripper clamps and moves it forward through the top half of the press cycle while the tool is open. On the return stroke, the gripper releases the strip, and the retainer holds it while the gripper retracts from the press through the bottom half of the press cycle while the tool is closed. Because it usually takes about as much time for the return stroke as it does for the feed stroke, gripper feeds are limited to a 180-degree feed window at maximum operating speed.
The gripper and retainer clamps can be air- or hydraulic-powered cylinders, or they can be one-way roller mechanisms that hold the strip in one direction but allow it to roll freely in the opposite direction. With cylinder-powered clamps, the timing of the clamp and release is critical to accurate feeding and can be a limiting factor in terms of speed. If the timing is not correct, the strip can be free at times to fall back, resulting in short feeds. Clamping is actuated by solenoid valves or air logic valves. Timing can be controlled either electrically or through valve porting.
The pulling force for the gripper can be provided by an air or hydraulic cylinder, hydraulic motor, or by a servomotor. The gripper usually is supported by guide bars or rails and is driven by cylinder rods, chain and sprockets, or ball screws. With air- or hydraulic-powered units, the feed length is set by adjusting a positive stop. The gripper moves between the adjustable stop and a stationary stop and employs a cushion of some sort to soften the blow at the end of each stroke. Feed length adjustment may require the use of tools and often involves some trial and error, which usually results in longer setup times.
For applications requiring low to moderate speeds and limited feed lengths, relatively inexpensive air-powered grip feeds generally are used with pull-through straighteners to provide a cost-effective alternative to roll feeds with powered straighteners.
Some gripper feed limitations are:
- The longest feed length requirement must be anticipated at the time of purchase. The disadvantage is that each additional increment of length costs more money and requires more valuable floor space. If there is ever a need to run a feed length that is longer than the machine was designed for, it must perform multiple cycles on each press stroke, commonly referred to as multistroking. This capability requires an optional and more expensive controls package, and because of the time required for the return stroke, the press usually must be operated in the single-cycle mode when multistroking.
- Its low initial capital cost may be offset by longer setup time and higher maintenance and energy costs. Compressed air often is an expensive energy source because of losses due to leaks, pressure drops, and contamination. Because of the many moving parts and wear components, maintenance costs can be quite high. These machines require timely maintenance to sustain good accuracy and performance.
Gripper feed sizes can run the gamut from compact, press-mounted models to large, cabinet-mounted models that include pull-through straighteners.
There are many ways to feed a press, but many important considerations regarding the purchase, setup, and operation of this equipment will help determine how productive it will be. If the system is to work at maximum efficiency, then each component of the system must complement the others. An in-depth discussion of each potential application with production schedulers, managers, engineers, and equipment suppliers will net the most advantages.
Advancements in Press Feed Technologies
Pilot Release. Pilot release is the act of momentarily releasing the strip to allow it to be aligned by pilot pins in a progressive or blanking die. The pins in the die correct for slight misfeeding or misalignment by moving the material back into position. A small degree of misalignment or camber can be tolerated by releasing the material at the bottom of the press stroke while it is held by the tool even if there are no pilot pins. This momentary release helps relieve built-up stress and binding of the strip through the feed caused by misalignment or camber and alleviates walking problems.
Timing is critical to a successful pilot release, so it is best if it is easily adjusted for each tool. A mechanically actuated pilot release ensures press synchronization at almost any speed, but the adjustment procedure is somewhat cumbersome. An air-operated pilot release is easier to adjust if a programmable cam is available, but it has a limited speed capability (although some units are capable of speeds as high as 400 strokes per minute). A servo-driven pilot release is available as an option on some feeding equipment. It has the advantage of being completely programmable and is capable of very high speeds but is an expensive option.
Space-saving Line Configurations. Floor space utilization continues to be a major concern for stampers. Because coil feed lines require slack loops for light-gauge and medium-gauge processing, a feed line length of 40 to 50 feet can be reduced by as much as half by specifying compact coil lines that combine the three functions of unwinding, straightening, and feeding into one machine (seeFigure 1). For heavier-gauge applications, cradle/feeder/straightener designs also help save valuable floor space. These machines uncoil material while holding the coil's OD, rather than feeding from a reel holding the coil from the ID, and feed the material with a combination feeder/straightener.
Transfer/Progressive Operation. Some stampers require the ability to run transfer press operations and progressive-die work while using the same piece of capital equipment (see Figure 2). Recent technology advances in this area have empowered conventional coil feed lines to run progressive-die operations in large-bed presses. These lines are equipped with advanced features such as oscillating shear presses, indexing conveyors, and rotate stations so the stamper also can process square-cut and irregular-shaped blanks into the same large-bed press running in transfer mode.
Scratch-free Processing. Applications with critical cosmetic requirements such as prepainted and other specialty materials such as those with chrome finishes can be fed with equipment specifically built for the application. Feed rolls can be chrome-coated or otherwise finished to aid scratch-free performance. Other press line components can be designed for this requirement too. For example, threading tables and other contact surfaces can be lined with nylon materials to avoid damaging the strip (see Figure 3). Processing speeds typically are not affected in the effort to obtain productive scratch-free processing.
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