Reducing unprofitable die changeover time
November 29, 2001
Reducing the time it takes to change dies it important to all stampers, especially for custom stampers that run small-quantity jobs. This article summarizes the ways in which automation has helped in this process and then covers two die change methods that are used in a quick change system: the standardized clamping system and the V-notch, or key, system.
The pressures of increased competition and the need for stampers to stay competitive affect every aspect of their operations. One of the key ways stampers can create an efficient and cost-effective operation is to reduce downtime on the shop floor.
One of the principal causes of downtime in a stamping operation is die changeover.
Before the development of quick-change methods, die changes were trial and error. Many operators simply used a tape measure to measure from the edge of the bolster to the flame-cut edge of the die set. After that they ran coil across the top of the die and visually checked for straightness. If it wasn't straight (and it usually wasn't), the operator adjusted the position and then test-fed the coil. If the coil bound up, more adjustments were made until the die and the feeder lined up exactly.
In other shops, operators lined up the edge of the bolster (or press bed) with the feed equipment and machined the front face of the die set so it was in line with the feed line. They took a measurement to make sure the die was positioned correctly, and then made smaller adjustments to ensure exact feed equipment alignment.
If the clamped die was skewed by even a fraction of an inch, the feeder would never align because the die was not straight, an error that could take significant time to discover and correct. An experienced operator could correct this problem, depending on its parameters, in 15 to 30 minutes. With an inexperienced operator, it could take hours.
The main justification for reducing the time it takes to change from one product to another is that it also reduces the per-piece cost. This is a major concern for companies that change jobs frequently, particularly for custom stampers that run small-quantity jobs on high-speed presses.
To create an efficient job changeover process, stampers must consider all the parameters of the job, such as passline height, coil width and thickness, feed progression, line speed, die measurements, and die protection schemes. Modern automation simplifies much of the process, particularly for feeder lines and coil handling.
Stampers can use automation to speed up die changeover with such technologies as die carts, which are automated die changers that allow for die prestaging and domino die changes, and moving bolsters that roll out the old die and roll in the new one.
Also available are automated press controls that adjust shut height and passline height, as well as automated die protection devices such as programmable limit switches, proximity switches, load monitor alarms, counters, and even preventive maintenance counters on the tool. Most automated solutions are costly, and stampers will realize savings only when they choose automation that is compatible with their machinery.
Developing a quick-change system requires advanced planning. It's best to determine up-front how much the system is going to cost and if the return on investment justifies the expenditure.
Shops that change jobs at least three times a week will realize a faster ROI than those that change over once a week or less. Two cost-effective methods of die changeover help in any quick-change system. One is a standardized clamping system and the other is the use of a V-notch, or key, system.
Standardized clamping systems ensure that operators don't waste time finding and fashioning scrap steel into a wedge to hold the die in position with an ill-fitting clamp. A standard clamp height lets stampers use the same clamps all the time. Most shops that use this method select a standard clamp height, even for older mechanical clamps that are tightened by hand.
Different types of clamps and clamping methods also exist. Some newer-style clamps slide under the clamp bar and cinch up hydraulically; others slide right into a T slot and are cinched hydraulically. Some presses have a T slot where a T nut slides in and the operator pulls down on the nut to clamp the die to the bolster or the ram to the die set.
An even faster alternative is a standard plate size. No matter what size the die is, the plate underneath is the same size, and the clamps never move. This method speeds up the changeover process because the die simply slips into place. The downside of this method is increased costs — the cost of the plate and the cost for the die shop to put the plate on must be added to every die. But for stampers that change jobs frequently, the investment probably is worth it.
The second method is a V-notch, or key, system. The V-notch method is used in conjunction with a mounting plate, located on the bottom of the die. (An alternative to the mounting plate is to mount two small plates on the sides of the parallels, flush with the bottom.)A V-shaped groove, or key, is cut to the back side of the mounting plate to use with a locator pin in the bolster. These locator pins can be of various diameters.
The opposite edge of the plate has a straight, machined edge. When the straight edge of the plate abuts a second locator pin in the bolster, the two pins then are parallel to the feed line.
The operator measures the distance from the center of the feed line to the center of the locator pins and uses that measurement to establish the distance from the locator pins in the bolster to the center of the feed line.
The die is set in and slides over until the V-groove centers the die, left to right. The die then is pushed up against the pins to align the center of the feed line in the die with the center of the feed line equipment.
Although many methods can reduce job changeover downtime, these methods will go a long way toward reducing changeover time at a cost stampers can live with.