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Quick-change die handling.

Die changing should take minutes, not hours or days. Here are some ideas on how to cut the time with improved diehandling equipment.

According to Daniel R Leighton of Atlas Automation Div, the stamping industry has accepted rather basic manual methods of changing tooling. "We have pushed, pulled, skidded, jacked, dragged, pried, and winched these dies in and out of our presses, because no other methods existed."

What, you're still doing it that way! Yes, and it takes up to 12 hours to change over a press-waiting for cranes and forklift trucks, tugging at chains, adjusting die locations, readjusting by trial and error, and making first-part quality from a die that you just dropped.

Naturally, because changeover takes so long, you're going to run a big lot, then pay for inventory costs. And, when the forklift hits the press, bolster, or dies, you can expect extra maintenance costs. Finally, operators may resort to unsafe practices to manipulate dies.

Die systems for the working press

As Dan Leighton told members of SME and FMA, "Instead of dragging dies out of presses like a caveman would pull his load, modern die-change systems have reapplied the same caveman's invention of the wheel. These systems roll the die out of the press rather than skidding it. They use hydraulic or pneumatic T-slot lifters, or other types that require bolster modifications.

"The lifters raise the die (or die and subplate combo) off the press bolster, placing it on wheels. This cuts the force required to move the die to about one percent of the die's weight. Skidding takes 70 percent."

Most quick-change systems use subplates the same size as the press bed and 2 to 3 thick. Tapped holes mount many different dies, and the holes locate the components accurately on the plate. The plate, in turn, mounts in a known position on the bolster. The setup is similar to that of a toolholder in a machining center, except that, in some cases, dies may mount permanently to the subplate. Unlike the bottom of a die, a subplate offers a smooth surface for T-slot lifters to roll against.

Types of die changers

Systems to change dies range from simple T-slot lifters and bolster extensions on a single press to rolling bolsters and automatic diecart systems for large multipress stamping lines. No one system is best for all plants.

Bolster extensions are fixed, pivoted, mobile, or removable. They mount on the front or back of the press and support a new die, ready to install, or the old die when removed. Hydraulic lifters in the bolster's T slots reduce the force required to move the die in and out of the press. Manual power is sufficient for dies that weigh less than 5000 lb. Side guides or keyways locate the dies accurately.

Although cost of extensions is low, you can't prestage dies without interfering with press operation. The die change can't start until the press has stopped and the access area is clear.

According to Leighton, the next step is the die table. The simplest is a fixed-height table that stays in place on one side or on the end of the press. Automation can be built around this table as required, and hydraulic T-slot lifters can reduce forces needed to assist accurate locating of subplate or dies.

An adjustable-height die table can service two adjacent presses of different heights, or a portable table can service several presses with varying heights. A forklift or crane can transport portable tables from place to place.

Die tables share the same disadvantage as bolster extensions: Prestaging can't be done before production shutdown. They serve best where the bolster-to-floor height is below 12" or where there isn't enough room to move carts out into the aisle.

A variation, the T table, uses T-slot lifters in the bolster plus side guides or a keyway to control leftto-right location. Again, dies mount on subplates so that all sizes appear to be the same size to the die-change system. A powered-roller conveyor table mounted behind and parallel to the press bed can solve the access problem. Pivoted or fixed bolster extensions with rollers mount on the table and pin to the press bed when the die is about to be moved, allowing access to the press.

A push-pull module rolls out heavier dies, and a manual system with two-station linear actuator takes care of lighter tools. Although the T-table system takes up a fixed area behind the press, it is an effective, low-cost, quick-change system that works without forklift trucks or cranes. It can serve two presses back to back with another set of bolster extensions on the other end of the table.

More mobility

If space is a problem, a mobile selfpropelled die table may be the answer. It's a battery-powered walkbehind vehicle with attached die table. It inserts and removes dies or subplates from presses or storage racks in the range of 2011 to 80" from floor level. Typical units handle up to 12,000 lb. Front corners of the platform attach to the press bolster or die rack, and a power unit handles leveling and die insertion.

Unfortunately, use of self-propelled die tables requires several steps, adding to die-change time. Furthermore, the tables need skilled operators who can hook up the table to the press, level the components, and make the die changes smoothly.

Die carts allow faster die changes with less effort. They appear in several configurations, usually employing subplates. A double-station diecart system works well where a press is congested with automation equipment. The cart moves on flush floor tracks leading to one side of the press. A new die (or subplate) is prestaged on one side of the cart while production runs in the press.

When the run is completed, press automation moves out of the way, and the die cart powers into position to roll out the old die into an empty station. Further cart movement brings a new die in line with the press for insertion. After the exchange, the cart goes back to its storage area away from the press. The automation rolls back into position, and production restarts-all in a few minutes.

Smaller single-station die carts can work with storage racks that hold dies next to the press at bolster height. The die cart moves between the rack and the front of the press to exchange dies. It takes a few minutes longer than the double-station cart, but is low cost and expandable. Just add more racks for additional storage. Forklift trucks are not needed once the racks are loaded.

Variations include a system with two die carts-one on the front of the press, the other at the back. Floor tracks are installed on both sides of the press. This setup handles very large dies, and provides the fastest possible change, according to Leighton.

Multiple press lines can use die carts, and, for minimum downtime, the rule is: One more cart than the number of presses. During production, operators prestage die carts with new dies on subplates. When the existing job is completed, automation moves out of the way, perhaps on the same track system as used for die carts.

In a three-press cell, for example, dies exchange on the following schedule: The top cart, which is empty, pulls the old die out of the first press. The next cart pulls the old die out of the second press, while pushing a new die into the first press. The third and fourth carts perform the same routine. When the exchange is completed, the carts roll to the storage area, the automation rolls in, and production begins again. Only a few minutes are required to change dies in the three presses.

In this system, all dies for the next run can be stored or prestaged on carts. Storage racks will be necessary only if desired for a third shift. This setup costs a lot and takes a lot of space, but one die-cart system can serve a second or third line of presses with very little additional cost.

Rolling bolsters

You can achieve quick die change with a rolling bolster, and you don't have to use subplates or hydraulic T-slot lifters. However, there are some problems. First, a bolster should be rigid-a tough challenge here because it has to be hollowed out to make room for the mechanism that lifts the bolster off the press bed and allows it to roll.

Second, a rolling-bolster mechanism is subject to the strain of press blows, with possible loosening of fasteners and damage to components. Third, it's difficult to get all four wheels to raise in unison, with the result that one of the wheels may carry more than its share of the load. The lift mechanism can thus bind. Also, debris under the bolster can be a problem. Chips and dirt must be removed to ensure that the bolster is level when it lowers to the press bed.

Fourth and finally, if the raising mechanism does fail, it usually occurs in the press, making it tough to get the bolster and die out for repair-thus delaying production rather than improving it. Justification

Is quick change worth it? Many US plants have press uptime of only 35 to 50 percent. Competitors in other countries, however, get 80-percent uptime. Mr Leighton notes that if we cut die-change time from a range of 8 to 12 hours down to 10 to 15 minutes, we can expect to double our uptime.

With quick-change systems in place, we're ready for just-in-time (JIT) operation. Then we can expect a 90-percent cut in work-in-process inventory and manufacturing lead time, plus an 80-percent reduction in start-up scrap and rework, and a 50-percent cut in required plant and warehouse space. Also, safety will improve because dies are under control and move smoothly in and out of presses and storage areas-with minimum use of cranes and forklift trucks.

Quick-change equipment offers easier die maintenance (work in the open, not under the press ram), faster automation setups because dies locate accurately, and simplified scheduling. Furthermore, part quality improves, largely because it's easy to stop a questionable run and switch to some other part while engineers study the erring die set.

For more information about diechange systems (DCS), contact Atlas Automation Div, Automated Manufacturing Systems Inc, 201 S Alloy Drive, Fenton, MI 48430 or circle 472.
COPYRIGHT 1989 Nelson Publishing
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Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Author:Miller, Paul C.
Publication:Tooling & Production
Date:Jan 1, 1989
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