A look at the basics and equipment alternatives
July 12, 2005
Reducing die changeover times is mandatory to become a world-class stamping operation. The most practical way to accomplish it is through careful evaluation of pressroom needs, and the implementation of cost-effective die change equipment and procedures. In today's manufacturing environment, lean budgets are a way of life, which makes retrofitting die change equipment a cost-effective approach.
Most stamping operations must reduce costs, which requires a productivity boost as well as improved part quality. Efficient die handling and quick changeover procedures can help.
The most obvious quick die change benefit is increased productivity. This productivity gain is accomplished by reducing downtime associated with changeovers. In some cases, the number of presses needed to produce a part is reduced, and operator overtime can be eliminated.
Longer tooling life and better part quality also are possible. When dies are handled in a controlled and safe manner, there is less chance for damage. When dies are placed consistently in the same press location, part quality also becomes more consistent.
Employee safety also can be improved with consistent quick die change procedures and equipment designed specifically for the task. The improved work environment can lead to better attitudes in general—an intangible factor, but an important one.
Die changeover consists of internal and external activities. An internal activity is any adjustment or action made while the die is in the press. During this time, the press is not running. An external activity is any adjustment or action made to a die while another die is in the press running production.
Skidding requires a force of 60 percent to 70 percent of a die's weight; rolling requires 1 percent to 3 percent of its weight.
Anything to minimize internal activities will help reduce die change time and increase press productivity. The following three areas need to be addressed to minimize internal die changeover activities:
Most roller rails are mounted into existing T slots or pockets milled into the bolster.
To implement an effective program, all levels in the organization—from top management to the shop floor—must share a common goal and vision. A cross-functional team should be established with a well-defined mission.
Next, a thorough investigation of your stamping operation is needed to obtain a good understanding of current conditions, what and where the problems are, the magnitude of each problem, and root causes. This allows the team to develop plans, prioritize efforts, and analyze where to make changes. With this information, the team can establish a proposed action plan with a schedule and budget for each action item.
Because of time and cost constraints, most programs are implemented over a period of time. The team should address this requirement and design actions so they can be implemented in phases as time and capital expenditures allow.
Skidding Dies. In the past skidding dies in and out of the press was the most common method of changeover. This method applied to small dies (a few hundred pounds) up to large automotive dies (100,000 lbs. or more). Skidding often results in damage to presses, material handling equipment, and the dies. It is still viable with small dies, but any die over a few hundred pounds can be handled safer and easier with rollers. When you consider the force required to skid a die (60 percent to 70 percent of the die's weight) versus the force required to roll a die (1 percent to 3 percent of the weight), it's easy to justify the rolling concept (see Figure 1).
Press-mounted Roller Rails. With this method rollers are mounted on rails in sections located in the bolster (see Figure 2). These rails usually are installed in existing T slots or pockets milled into the bolster. Because T slots are common, no major costs are associated with mounting the roller rails in the existing slots. If T slots do not exist, it may be more cost-effective to mill two rectangular slots in the bolster and mount fewer heavy-duty roller units.
Die tables have support rollers and a push-pull mechanism mounted on top to assist in removing and inserting dies.
Subplates(the common denominator). A common problem associated with retrofitting die change equipment is interfacing existing dies with the new system. These existing dies can be all sizes, mounted with or without parallels, and require different clamping methods. A painless way to overcome these varying parameters is to add a subplate. The subplate acts as a common toolholder for system uniformity. Subplates normally are the same size as the bolster, so die size variation is not important as long as it doesn't exceed the bolster's size. The press "sees" only the subplate, so locating it in the press can be simple, and clamping it to the bolster is consistent and fast because it's the same each time.
When the subplate is located accurately in the press and the die is located accurately on the subplate, repeatable die location is ensured every time. This saves start-up time with any associated automation such as coil feeders, pick-and-place units, and transfers. The subplate provides a flat surface that contacts rollers in the press and other equipment that handles the die during the changeover process. This minimizes or eliminates the cost of having to alter dies so that they interface with your die change system. An added bonus is that the subplate covers the bolster, which keeps it free of slugs and debris.
If you are starting a new program with new dies, die shoes can be designed to interface directly with the quick die change system. Die shoes provide similar benefits without having to add a subplate.
Many systems installed today are automatically controlled with a computer or programmable logic controller.
Bolster Extensions. These units are structural frames with rollers on top that support the die and subplate outside the press. Normally they are attached to the bolster and become an integral part during changeover. Bolster extensions can be fixed, removable via quick-release details, mounted on a pivot to swing out when not in use, or mobile (mounted on casters so they can be rolled in and out of position).
During changeovers, the old die is manually pulled onto bolster extensions. It's removed via fork truck or crane, and the new die is placed on the extensions. The new die is manually pushed into the press, clamped in place, and production restarts. This system normally is used with dies weighing less than 5,000 lbs. Bolster extensions are easy to install on the front and rear of a press, and a die can be prestaged and not interfere with the die being removed.
Die Tables. The die table is a stand-alone framework located adjacent the press. It normally has support rollers and a push-pull mechanism mounted on top to assist in removing and inserting dies. These units can be fixed in place or portable. Tables can be single-stationed (similar to bolster extensions) and require a crane or fork truck to move dies. However, the most common configuration allows for a new die to be prestaged ahead of time, and the changeover is performed without the aid of any other piece of material handling equipment.
The T table uses a conveyor oriented parallel to the press bed; a set of bolster extensions links them together (see Figure 3). The new die is prestaged at one end of the conveyor during production. At changeover, the old die is pulled (via a powered mechanism) onto the conveyor's center section, which moves the old die to one end while moving the new die to the center section. The new die then is pushed into the press, secured, and production resumes.
Die Carts. Powered die carts are commonly used when dies are more than a few thousand pounds or an awkward size. They also are used to keep the press clear of physical obstructions like tables and bolster extensions. A die cart can be dedicated to one press or used for multiple presses. Carts normally have steel frameworks with these three basic components:
Carts can be single-stationed to handle only one die at a time or multiple-stationed, which allows dies to be prestaged on the cart.
Automatic Changeover. The tables and carts described previously require an operator to control the process via a control panel. Many systems installed today are automatically controlled with a computer or programmable logic controller (see Figure 4). A one-button changeover is now a reality.
Figure 5shows a range of costs for different types of quick die change equipment. The large variance is due to different die sizes and weights, as well as complexity.
Reducing die changeover time is mandatory to becoming a world-class stamping operation. The most practical ways to accomplish it are through careful evaluation of pressroom needs and the implementation of cost-effective die change equipment and procedures. In today's manufacturing environment, lean budgets are a way of life, which makes retrofitting presses for quick die change a cost-effective approach.