July 12, 2001
Press- and servo-driven roll feeds and gripper feeds are almost as common to the stamping industry as sheet metal and dies. Knowing how to use them effectively may not be so common.
In most cases, a press feed must meet three criteria to be successful. First, it must be flexible in terms of setup. Second, it must deliver the material with sufficient precision into the tool, and third, it must feed at the proper time. Other important considerations that determine the effectiveness of a feed are the time and skill required for setup, the cost of energy to operate it, and how the feed interfaces with the system as a whole.
A feed must be flexible enough in its setup adjustment to accommodate the full range of applications that will be run on a press line with respect to feed length, material width and gauge, feed and pilot release timing, and die heights. If the feed is for a dedicated system, these variables are fairly limited, but more often than not, the feed must be suitable for a wider range of applications.
A feed must not only move the desired amount of material into the tool, it must also place it precisely in the die front to back, side to side, and square with the tool. Misalignment results in binding and short feeds caused by slippage and strip buckling. Short feeding can result in broken dies and poor part production.
Nearly any new feed that is properly installed and set up can deliver a length accuracy that is acceptable for most applications. A press feed generally retains that accuracy if it is properly maintained, but the amount of maintenance and setup time required varies from one type of feed to another.
Regardless of the feed chosen, delivering material correctly without binding and misfeeding requires that a press feed be positioned on center and square to the tool and mounted rigidly so that no movement can occur between the tool and the feed.
In addition to proper feed installation, the tooling must be placed precisely on each setup. Using a registration device, such as positive stops on the bolster, is recommended to ensure consistent placement of the tooling. Without good-quality material, proper straightening, and precise alignment, problems will occur regardless of what feed is used.
A press feed must also be capable of keeping up with the speed of the operation. The amount of time that a feed has to deliver material is determined by the amount of time required to complete one entire press or shear cycle, minus the time that the tooling is engaged and the time required to detect a misfeed and stop the press. The longer the die engagement or the faster the speed of the operation, the less time there is for feeding material.
No discussion of feeds would be complete without mentioning pilot release. Pilot release is the momentary release of a strip to allow the pilot pins in a progressive die to align the material. The pins in the die correct slight misfeeds by pulling the material back into position.
This feature is used primarily with progressive dies, but it can be beneficial in other applications as well. Even if no pilot pins are used, a small amount of misalignment or camber can be corrected by releasing the material while it is held by the tool at the bottom of the press stroke. This momentary release can alleviate walking problems and help relieve built-up stress and binding of a strip through the feed caused by misalignment or camber.
Timing is critical to a successful pilot release, so the mechanism should be easily adjustable for each tool. Mechanically actuated pilot release ensures synchronization with the press at almost any speed, but the adjustment procedure can be somewhat cumbersome. Air-operated pilot release is easier to adjust if a programmable cam is available. However, air-operated pilot release generally has limited speed capability, although some units are capable of speeds as high as 400 strokes per minute (SPM). A servo-driven pilot release is completely programmable and capable of very high speeds, but it is a costly option.
The two basic feed types are roll feeds and gripper feeds. A roll feed can be powered either by a press or by its own self-contained drive system. Press-driven roll feeds such as rack-and-pinion or cam feeds are always synchronized to the rotation of the press. These feeds always begin their motion at some predetermined point in the press cycle and finish it at another predetermined point, regardless of press speed or die engagement.
Although index speeds must increase or decrease to keep pace with the press, the feeds can draw as much power from a press as they need to accomplish this within the limitations of the mechanical coupling to the press.
This differs from a self-powered unit, which begins its motion in response to a signal from the press but has a finite, minimum amount of time in which it can index based on the amount of power it can deliver and the load that it encounters. The point at which it finishes can therefore vary with the press speed. The faster the press runs, the later in the stroke a self-powered unit finishes.
This synchronization feature makes press-driven feeds suitable for high-speed indexing, feeding in-die transfers, or for use with unloaders and other applications that require that feed motion be tied to press rotation to prevent collisions.
Because of the lack of adjustment in timing, the feed motion for all dies cannot begin until a point in the stroke at which the deepest draw die disengages, which limits the feed window. On the other hand, air- and servo-powered feeds operate independently of the press and can be adjusted to begin feeding as soon as a die opens.
Inherent in their design, press-driven feeds exhibit a smooth motion called an s-curve move profile instead of the trapezoidal move profile provided by most other feeds. With an s-curve motion profile, the rate of acceleration varies throughout the index, eliminating sharp transitions in velocity that can cause slippage.
Most self-powered feeds move from a stationary condition directly into a fixed rate of acceleration, resulting in a sharp velocity transition called a jerk point. These occur at the beginning, middle, and end of each move. Press-driven feeds, on the other hand, make gradual transitions in velocity with high acceleration and deceleration in the interim. This eliminates jerk points while retaining the ability to make very high-speed indexes with good accuracy.
Drawbacks to most press-driven feeds include their difficulty of adjustment, feed length limitations, lack of inching capability, and absence of controls interface. Most require that gear sets, rollers, or mechanical linkages be changed to adjust feed length. They are also somewhat limited in their range of feed length adjustment. Because they are coupled directly to the press rotation, they cannot jog the strip for threading. They do not use electronic controls, mechanical feeds cannot accept setup information from or provide feedback to press control or automation systems.
Servo-driven roll feeds have been used in press feeding for a number of years. The concept involves using a closed-loop positioning drive—usually a servo but sometimes a stepper—to control the index position of the feed rolls.
Servo-driven roll feeds share many advantages with the press-driven variety, including minimal space requirements, low maintenance, and high speeds. Using a microprocessor-based control gives them an added dimension, however. Features such as programmable move patterns, self-diagnostics, auto correction, and the ability to communicate with automation devices differentiate them from other types of feeds.
Servo-driven roll feeds are available in a variety of configurations, including conventional two- and four-roll units, feeder/straighteners, unwinder/feeder/ straighteners, and zig-zag units. Feed control packages range in sophistication from simple-to-use, single-setup controls with thumbwheels or keypads to systems that allow programming of elaborate multiaxis move patterns and control of auxiliary functions and devices, as well as varying levels of memory and communications capability.
Most servo feeds use a trapezoidal move profile, but some are also available with controls that can execute s-curve move profiles. Systems that are electronically synchronized to press rotation are available as well. These units require a special controls package and feedback device, either a resolver or encoder, that is attached to the press crank to track press rotation. Their top speeds are still limited by the available drive power.
Gripper feeds move a strip with a linear motion rather than the rotary action of roll feeds. They are available in a variety of sizes ranging from simple, compact, press-mounted units to large cabinet-mounted models that include pull-through straighteners.
Gripper feeds use a pair of clamps. One clamp, called the retainer, remains stationary, while the other, called the gripper, moves during the feed and return strokes. During the feed stroke, the retainer releases the strip as the gripper clamps onto it and moves it forward during 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 during the bottom half of the press cycle while the tool is closed.
Because the return stroke requires about as much time as does 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 allow the strip to roll freely in one direction while preventing it from rolling in the other. 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 fall back, resulting in short feeds. Clamping is actuated by solenoid valves or air-logic valves. Timing can be controlled either electronically or through valve porting.
The pulling force for the gripper can be provided by an air or hydraulic cylinder or by a hydraulic motor or servomotor. The gripper is usually supported by guide bars or rails and is driven by cylinder rods, a 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 uses a cushion 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 can result in longer setup times.
Like electronic roll feeds, servo-driven gripper feeds offer programmability and the ability to interface with press automation systems. Servo-powered gripper feeds use a closed-loop servo drive coupled to a ball screw and nut instead of stops or cushions to position the gripper. Feed setup information is programmed into the control unit via an operator interface, or it can be downloaded serially from another device. The control unit then commands the servo drive to position the gripper accordingly.
Gripper feeds are limited to a specific maximum feed length based on the model selected, so the longest feed length required must be anticipated at the time of purchase. Each additional increment of length costs more, and a longer feed length capability dictates that the machine becomes longer and therefore requires more floor space.
Most stampers buy the shortest machine that will fulfill most of their needs. If a longer feed length than that for which the machine was designed is required, the feed must perform multiple cycles on each press stroke in a process commonly referred to as multistroking. This capability requires an optional and more costly controls package, and, because of the time required for the return stroke, the press usually must be operated in the single-cycle mode.
Air-powered grip feeds are generally inexpensive. They are commonly used in conjunction with pull-through straighteners to provide a cost-effective alternative to roll feeds with powered straighteners for applications requiring low to moderate speeds and limited feed lengths. Their low purchase price may be offset over time by higher setup, maintenance and energy costs.
Press feeding methods are limited only by imagination. Many important considerations in the purchase, setup, and operation of this equipment will determine how productive it will be. If the system is to work at maximum efficiency, each component must complement the others.