January 27, 2014
Excessive press downtime between jobs is stamping’s perennial albatross around the neck. To expose the culprits in your shop, try scrutinizing every step in a coil handling system, from the coil itself to the material pilot guides in the tool.
Excessive press downtime between jobs is stamping’s perennial albatross around the neck. Maybe upkeep is being performed on the die, or perhaps personnel are performing reactive maintenance. Downtime culprits abound, but some of the most common can be found not in the press itself, but in the system that handles the feeding of material into the die.
To expose the culprits in your shop, try scrutinizing every step in a coil handling system, from the coil itself to the material pilot guides in the tool (see Figure 1).
At many shops, waiting for a coil is the No. 1 culprit of excessive downtime. No matter how organized and labeled a press area is, you can’t start stamping without material. More often than not, an operator waits for material because forklifts or cranes are busy in other areas of the shop.
This usually occurs because the stamping line uses a single-coil payoff system that leaves the press operator “coil-dependent.” Single-coil payoff systems require coils to be at the ready the moment the previous job ends, but quite often crane operators or forklift drivers are busy doing something else.
To tackle this problem, first consider the floor plan. A press line located next to a wall or in a similar confined space may force you to access the payoff device from only one side. In these instances, single-reel payoff equipment may be your only practical option. The unfortunate result is “coil dependency.”
In these cases, you can consider using a dedicated coil car that can be staged with a coil and always ready when the coil changeover occurs (see Figures 2 and 3). This gives the crane or forklift operator a significant amount of time to retrieve and prepare a load-side coil. In some instances, one car may be able to service two adjacent press lines.
Compared to the run time, press downtime takes place in a small time window—at least, that’s the ideal. This is why it’s best to load coils while the press is running, and double-reel payoff systems make this possible (see Figure 4). Because these reels have a load side and a run side, they require that you have access to both sides of the line.
If the press runs narrow, thin strips, you also can consider a horizontal pallet reel, which can hold multiple coils. A rule of thumb is that these reels are typically limited to strip no more than 6 in. wide and 0.060 in. thick. Modifications can be made, but it’s best to consult your coil handling vendor for applications outside this window. Wider and thicker materials need a larger loop radius and may kink if forced into a tight arc as they twist from vertical to horizontal.
If you hear a violent, hideous helicopter noise coming from the shop, you may be experiencing one of the most dangerous mishaps that can occur in a pressroom. Someone loads mislabeled or misapplied coil onto a payoff mandrel and seems to take the necessary precautions. The coil keepers and hold-down arm are in place. Then the band is cut, and the coil’s energy explodes violently, possibly telescoping, potentially even hurling the leading edge of the coil strip with enough force to penetrate a concrete wall.
When choosing a press feed system, be sure to consider the material types and thicknesses, including their yield and tensile strengths, and ensure all feed-system components can handle them. This will allow your vendor to recommend the correct options, including the type of hold-down arm, coil keepers, threading, peeler/debender, and guiding.
Material like advanced high-strength, low-alloy (HSLA) steel can have yield strengths exceeding 200,000 PSI. To contain all that energy requires a heavy-duty, usually hydraulic hold-down arm. Without it, you open the door to a time-consuming, extraordinarily dangerous situation.
When it comes to straightening, first make sure that you have the right equipment for the material grade and thickness at hand. A straightener for HSLA steel should not be the same as a straightener for cold-rolled steel. The roll and journal diameters, bearing ratings, and even the motor horsepower must be sized correctly to ensure acceptable machine performance and longevity.
When you have the right straightener for the material, and proper setup documentation of straightener roll settings, eliminating coil set shouldn’t take long. But sometimes it does, especially if you run a tryout part followed by production parts, and never document the roll-penetration setting. If the setup person documents this, he can provide a valuable tool to eliminate downtime and waste—all while keeping the operators honest.
If you look at the side edge of a strip, you’ll notice that it bows in the middle. That’s coil set, and it’s there because the middle of the strip has a greater “memory” of being coiled than the sides. The stronger (higher yield strength) and thicker the metal is, the more memory it has. As the coil depletes, the strip comes off a tighter inside diameter, which increases its coil set.
Straighteners, be they the motor-driven or pull-through variety, help eliminate coil set. In a typical application, a 7-roll straightener may be set so that the first rolls penetrate 50 percent of the material thickness, subsequent rolls penetrate 25 percent of the material thickness, and the last rolls contact the material just enough to put a slight up-bow or down-bow into the metal, depending on what works best for the die in the press. The rolls also may need to be adjusted as the coil depletes to account for the greater coil set.
The exact setup depends on the number of rolls, their diameter, and how close together they are—and again, this depends on the material and required straightness for the application. Regardless, their position for each job should be documented on a setup sheet. This doesn’t eliminate downtime and waste altogether, but it does give the operator a good starting point and minimizes the number of scrap pieces he’ll need to run.
Today these setups can be automated in the feed-line controls. But even if a line doesn’t have such “go to position” automation (described later), simply documenting setups for all jobs reduces scrap and saves a lot of adjustment and tryout time.
Slack loops between the straightener and servo feed require that at least two feed lengths be in storage. Applications producing large components have long feed lengths and, hence, longer, deeper slack loops, usually requiring a pit. This makes it impractical for technicians to thread the coil material manually, which is why these setups almost always have a threading table. The table allows new coil to be threaded across it and into the servo feed; the table then drops to allow the material to fill the looping pit.
Thinner, narrower coil operations may not require a looping pit, often because it’s possible for technicians to thread material without one. But just because it’s possible doesn’t mean it’s efficient. To thread a new coil manually may take two operators to guide the strip into the feed, and this can take considerable time. Threading tables can shorten this time and, not least, make life easier for the operators.
Of course, excessive downtime also can result from setup issues, and this can relate to problems in the slack loop. As a rule of thumb, multiply the material thickness by 360 to get the radius you need in the slack loop. As material thickness increases, so does the horizontal length of the slack loop. Multiply this result by 4 to obtain the distance you need in the loop area to allow the strip to feed in a smooth U shape between the straightener and servo feed.
If you force material into a loop that is too short for the material thickness, you will straighten the material and then reintroduce coil set. If the loop pulls flat, it means the straightener is sized incorrectly and is not keeping up with the press feed, which can cause material misfeeds and, ultimately, a crashed die.
The distance requirements in the loop can be different for a coil cradle, especially if it’s arranged such that the strip comes off the back of the coil, loops up and over, then goes down into the looping area. But no matter how the slack loop is arranged, its radius must be maintained.
The servo feed advances the strip into the press area itself, and you need to adjust the edge guides to match the width of the strip. You also need to adjust the servo-feed height to account for dies with different passline heights. You don’t want the servo feed to be feeding material significantly uphill or too far downhill. This results in rubbing or binding against the die guides. Up to 12 in. of adjustment may be needed, depending on your die height variations.
It takes time to adjust this feed manually; the operator must loosen a bolt and adjust a screw or jack to move the servo feed up or down. Other feeds can be adjusted manually with a hydraulic pump.
Still others are adjusted automatically. The go-to-position option is becoming more popular with stampers trying to reduce their coil-to-coil time, especially those aiming for changeover times of 5 minutes and less.
The go-to-position control system stores recipes of all the setup elements needed to start a stamping job. These include feed length; strip acceleration and deceleration; and various signaling, including electronic signals to the servo feed, auxiliary elements like air blowoff values, as well as a pilot release signal. (As the pilot pin in the die enters the material, a signal is sent from the press to open the feed rolls and make final material positioning adjustments.)
For every job these controls can automatically set the passline height and edge guides in the servo feed and straightener, the threading table height, the straightener roll positions, and sometimes even the keepers that hold the coil onto the reel. Some final tweaking may be necessary—one coil’s forming characteristics can differ from another coil’s—but those adjustments usually are minor.
Taken individually, performing adjustments manually may not seem to take long. But taken together, they add a lot of time to your changeovers. And unlike coil retrieval and loading onto a dual-reel system, these press feed adjustments can’t take place when the press is running. As operators turn bolts here and adjust straightener rolls there, the minutes are ticking by—and in most stamping operations, these minutes are very expensive.
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. Print subscriptions are free to qualified stamping professionals in North America.