Roll reversal: From air feeds to servo roll feeds
The metal forming industry is witnessing the emergence of a new standard in coil feeding
The air-powered gripper feed was once the standard press feeding device in most stamping operations. It was simple and cost- effective. Servo-powered roll feeds have emerged as an attractive alternative, especially as the technology has proven itself over the years and the cost has fallen with its wider adoption.
To say that it was a long process to get the stamping industry to recognize the benefits associated with the servo-driven roll feeder is a bit of an understatement. It was more like pulling teeth with a pair of pliers.
What was the reason for the reluctance to embrace servo roll feed technology? The higher cost associated with the technology turned off many stampers, and others feared their press manufacturing maintenance people wouldn’t be accepting of a coil-feeding device that wasn’t primarily mechanical.
Times have changed, however. Advances in servo and control technology have helped to win the industry over and have actually transformed the servo roll feed from a high-priced, intimidating technology into a cost-effective and user-friendly means to feed coil into a stamping press (see Figure 1). Whereas metal formers used to rely heavily on traditional air feeds for press feeding applications, they have an attractive alternative.
Stampers are now able to produce very close-tolerance parts at higher operating speeds more consistently than they have ever done before. Servo press feed technology now can accommodate speeds as fast as 2,000 strokes per minute with accuracy as tight as ±0.0005 in. for special applications. However, most standard servo feeds allow operators to run at 300 to 400 SPM with an accuracy of ±0.003 in.
Press Feeding Tradition
The traditional air-powered hitch feed, or gripper feed (see Figure 2), has served as a valuable and reliable means of press feeding for years. The technology provides fairly accurate and dependable feeding of stock, mostly for light- and medium-duty applications.
The basic design features an air cylinder that powers the back-and-forth linear motion of a gripper to feed the stock. A stationary retainer clamp also plays a key role.
During a typical feed stroke, the retainer clamp releases the coil strip as the gripper clamps and moves it forward through the top half of the press cycle—while the press window is completely open. On the return stroke, the gripper releases the strip and the retainer holds it while the gripper retracts through the bottom half of the stroke.
The clamp-and-release cycle is critical to accurate feeding with this type of traditional press feeding approach. However, because the clamps are air-powered, they are subject to timing errors, especially for the metal former pushing for higher feed rates. If the timing is off, the strip can remain free and fall back, leading to the coil being processed at the incorrect incremental length. This can lead to extensive downtime, because getting the line back up and running requires mechanical trial and error to dial in the correct feed length again. This also occurs when a job is complete, and the press has to be prepared for another run.
Stampers using air-powered feeds have had to be aware of two possible shortcomings:
- The feed grips only on one side so the coil may be subject to binding and erratic motion.
- Adding an unpowered roll straightener can contribute to the unbalanced pull on the pneumatic grippers, creating scrap and possible die damage.
For many years the air feed was very attractive because of its low cost. However, the technology lacked flexibility, particularly as high-volume jobs disappeared and were replaced by more medium and short runs, which require more setups. Stampers realized that excessive downtime quickly outweighed any initial cost savings achieved with the initial purchase price.
Additionally, because air-powered gripper feeders had so many moving parts, the potential for wear on the feeders’ components was high. For example, the constant slamming back and forth of the gripper feeds to a positive stop induces wear over time.
Press Feeding in Transition
Original servo roll feed designs, which relied on DC-based motors, were not user-friendly; were expensive; and required a lot of maintenance, which drove up the cost of ownership even further. All of these factors played a part in inhibiting the technology’s widespread adoption.
The introduction of AC brushless servomotors changed that. Relying on that new motor design, servo roll feeds made large strides in terms of acceptance in the metal forming industry in the early 1980s.
Today the servo feed has proven its ability to perform in challenging environments with high reliability, speed, and performance.
Servo roll feeds make use of a closed-loop positioning drive that precisely controls the feeding action of the rolls. This precision results in very accurate positioning, even in high-speed scenarios. Additionally, the technology requires less maintenance because it does not have as many mechanical parts as air feed rolls; for instance, timing belts have replaced gearboxes.
The technology’s microprocessor-based controls can be used to program move patterns and to perform self-diagnostics and autocorrection. The control technology also can be preprogrammed to store up to 500 jobs.
The advanced nature of the control technology allows it to interface easily with press controls or other outside sources to receive job information.
The precise control associated with servo roll feeds is especially helpful when working with surface-sensitive material. Prefinished, marking-sensitive coil stock can be fed into a press without fear of marring. The servo-driven feed rolls provide a smooth transition of the material through the feed into the die. The electrical technology associated with servo feeds replaces the abrupt stop-and-go motion once provided by mechanical functions.
Current servo roll feed designs also can handle very thin materials, 0.002 in. thick, to vey thick materials, 0.75 in. thick, in widths up to 84 in.
Whereas the adoption of servo roll feeds was not widespread in the 1980s, that is not the case today. Industries currently using servo feed technology include Tier 1 automotive suppliers, truck chassis, agricultural equipment, and lighting and electrical product manufacturers.
A Case Study: Battery Manufacturer Switches to Servo Roll Feeds
The switch to modern servo roll feed technology can be pretty dramatic. For example, as part of its continuing improvement activities, a leading alkaline battery manufacturer was looking for a way to upgrade its press feeding performance. It needed something more than its older pneumatic roll feeds were delivering.
Specifically, the manufacturer wanted to improve feed length accuracy and maintain the surface quality of the steel being roll-fed to the battery can processing lines. The company’s manufacturing engineers searched the Internet to research advancements in servo roll feed technology that could meet its performance and cost criteria (see Figure 3).
Upon deciding which technology made sense, the manufacturer then began an extensive testing phase. Test runs on the battery can materials were conducted for the customer to review. Encouraged by those results, the battery manufacturer then had one of the servo roll feed units installed at its facility. The engineers collected data over a three-month period to prove out the ability of the servo feed to improve speed and maintain the high level of surface quality required.
The servo technology passed the test. The battery manufacturer has plans to add 10 more of the press feeds on other processing lines for other battery can sizes.
Unlike those companies using the early 1980s servo devices, this battery manufacturer and others are getting more durable and user-friendly features. In this particular instance, the mechanical design has a three-roll entrance material support section, solid matte chrome-finished rolls, an adjustable-stroke pneumatic cylinder for both roll pressure and pilot release, a planetary reducer, cluster gear power transfer, and a pivoting upper roll assembly for full gear mesh (not a slide block). It also has C-framed, heavy-duty, 1.25-in. edge guides and an entrance curve section or chute support bracket. A 3-in. touchscreen displays length and speed inputs. The control, which stores up to 200 jobs, also has an on-the-fly microadjustment feature.
These types of improvements in controls and software will advance roll feed and straightener technology. However, in terms of Star Wars-type advancements, manufacturers need look no further than the motors themselves. The next generation probably will involve the application of linear motors.
STAMPING Journal is the only industrial publication dedicated solely to serving the needs of the metal stamping market. In 1987 the American Metal Stamping Association broadened its horizons and renamed itself and its publication, known then as Metal Stamping.