Consider these options before deciding what is right for your application
June 13, 2012
As more metal formers find themselves taking on more jobs with lower quantities, they have developed higher interest in quick die change. Many want to automate the entire clamping process: Hit a button and the clamps quickly engage or disengage. Hydraulic, magnetic, or combination clamping systems are options for stampers, but they shouldn't think one is necessarily better than the other without a thorough investigation.
It’s in the way that you move it.
When it comes to single-minute exchange of dies, stampers should focus first on moving the die in and out of the press. That’s where they make the biggest impact in the search for reduced die-change times. A set of ball or roller-die lifters can do wonders when it comes to turning an hourlong changeout process that used to rely on a lift truck to a 10-minute exercise—unclamping and rolling out one die, rolling in the next die set, and clamping it in place.
But keep in mind that it’s also in the way that you clamp it.
Many stampers still rely on manually clamping their dies. This can prove troublesome as different setup personnel may use different numbers and types of clamps to secure the dies. Obviously, not applying enough torque to the clamping bolts can affect the integrity of the setup, but applying too much torque can cause bolt failure.
As a result, some stampers are looking to remove the human element from the clamping process altogether. They want to press a button and have the clamping completed. The two best ways to accomplish this are with hydraulic or magnetic clamping systems.
Other stampers, however, prefer a manually positioned and torqued clamp, with an assurance that it is properly torqued. In this case, a hydromechanical clamp with a preload indicator pin may be the answer.
Before deciding what type of clamping system makes the most sense, a metal stamper will want to figure out just how much clamping force is necessary to match the die weight and the forces exerted on the tooling during the stamping process. If the clamping system is unable to overcome the forces working against it, the die can move during stamping, causing possible damage to the die and bringing production to a halt.
Generally, the total clamping force should be about 20 percent of the press’s capacity. For example, a forming job in a 200-ton press would require 40 tons of clamping force.
A clamping system also needs to ensure that the clamping forces are applied consistently over the surface of the die.
Hydraulic clamping systems have been in use for more than 50 years in the metal forming industry. They are still heavily used because the technology is flexible and reliable.
The stamper, however, can maximize time savings associated with quicker die change only if it is also committed to standardization of die designs. This consistency in the dies keeps clamps in the same position and at the same height, which saves valuable time during changeover. Having to adjust a screw or make adjustments to the clamps defeats the purpose of installing one of these systems.
Of course, most stampers don’t have standardized dies. Even the smallest of shops might have 100 to 200 different dies. The problem can be overcome with the use of standardized subplates. For instance, by relying on a subplate of standard dimensions, a stamper can use one size of side rails or rollers for guiding the dies and the same positive stops and locator pins for final die positioning. Additionally, the subplates standardize clamping height for all dies.
From a material handling perspective, the subplate’s surface allows for the die rollers to make contact with a smooth surface rather than the holes and cutouts often found on the bottom of die sets. This reduces rolling resistance and simplifies the die setter’s job.
Die parallels are also part of the standardization process. They deliver standardized shut and pass heights and also leave room for a conveyor to catch falling scrap and remove it from the press.
For those stampers that don’t want to get into making subplates for all of their dies, they may elect to create subplates on the dies they use 80 percent of the time. For the other ones, they can rely on separate plates, where dies can be prestaged and bolted to them as needed.
With the discussion of standardization complete, it’s time to focus on the variety of clamping options associated with hydraulic systems. Hydraulic clamping systems involve a full range of clamps, lifters, pumps, and control valves. Plenty of options exist for the metal former looking for alternative approaches to tackle its quick die change project.
The positioning of the clamps is a good example. External clamps that slide into T-slots and clamp on the edges of the die or subplate are more adaptable and are often a better choice for retrofitting existing presses with multiple die sizes. Fixed-mounted clamps that are bolted directly on the bolster or slide are a good choice if the die plates are standardized.
Internal clamps, integrated into the bed or slide, are located close to the forces that can cause die deflection. These are the clamps most often specified for high-speed applications, deep-draw dies, progressive dies, and lamination dies. Typical internal clamps include:
To ensure safety, a clamping system should be designed so that it will maintain clamp pressure and the die will not move, even if power is lost or a hydraulic line is broken. Zero-leakage directional control valves and remote pilot-operated check valves are used to accomplish this.
Typically, electrical controls are included to provide constant monitoring of the clamping pressure at the clamps. Sensors elevate safety monitoring by keeping track of such things as clamp positioning, in the case of a traveling clamp, or whether a clamping piston is in the proper clamp or unclamp position.
Magnetic clamping in the metal forming industry hasn’t been as widely adopted as hydraulic clamping, but it has been around for several years. It actually first was used in the clamping of tooling in the injection molding industry before being accepted in metal forming applications.
Standardization of dies is usually not required if a magnetic clamping system is in place. Upper and lower plates (see Figure 2) have to be installed and bolted to the bed and slide. The control system for the magnetic plates initiates the clamping and unclamping of the die and constantly monitors the safety of the magnetic system.
This type of clamping process relies on electropermanent magnets, which remain in the clamped or unclamped state even in the event of a power failure. The magnetic plate will change from unclamp to clamp with about two seconds of electrical power. In that short amount of time, clamping is complete, and the press is ready to run.
The magnetic force is built up in a highly concentrated magnetic field that is distributed across the surface of the magnetic plate. This provides uniform, distortion-free clamping and helps to improve product quality, reduce scrap, and lessen die wear. The field penetrates the die base plate about 0.625 in., and because of the limited penetration, has no effect on the die’s or workpiece’s material makeup.
Magnetic clamping’s versatility is really evident during the design phase. All aspects of the dies and press are taken into account. The shape and size, magnet placement, and clamp force of each plate are custom to each application. Additional slots for die lifters and holes for die locator pins and guides can be incorporated into the magnetic plate design.
Any stamper that wants to use a magnetic clamping system must review its dies to ensure that they can be clamped safely. Different die sizes and footprints mean different clamp forces. For example, smaller dies mean reduced clamp forces. For maximum clamp force, the die surface must be clean, flat, free of large pockets, and constructed of 1020 steel.
As stated previously, once activated, the magnetic plates will maintain their clamp force on the die until the system is demagnetized. To ensure that the press and die are operated in a safe environment, the press control system is interlocked to the magnetic clamping controls. The system monitors the magnetic force, die position, and temperature during the forming process. If a die movement occurs, the control stops the machine.
If the die is being hand-fed, more T-slots are added to the upper magnetic plate so that they can be used with secondary mechanical die retainers. If the load on the die is greater than the clamping force of the subplate and the die were to break away, the mechanical retainer would still hold it in place.
Stampers wanting to go the manual route but still looking for the peace of mind associated with properly torqued clamps may find their answer with hydromechanical clamps.
When this clamping nut and T-bolt are in place and tightened properly, the integral piston is pressurized, preloading the bolt and extending a preload indicator pin. This pin provides the operator with immediate feedback that the clamp is pressurized and the die is safely clamped.
The clamping torque for this clamp is very low for the clamping force that is created. A clamp and a 1-in. T-bolt create 22,000 lbs. of clamp force with only 22 ft.-lbs. of torque.
Any decision on a clamping system should involve an investigation into the stamping application and a detailed discussion with a person that deals with quick die change issues on a day-to-day basis. Budget, project goals, die size, and tooling makeup all influence what type of clamping system is best-suited for the application.
The result of such discussions may prove surprising to the stamper. It is not uncommon to mix the different types of systems, such as hydraulic clamps on the slide where it may be difficult to properly torque the T- bolts, or hydromechanical clamps on the rolling bolster where hoses may interfere with the operation of the press during die change. Then again, nothing is standard when it comes to designing an efficient quick die change system suited to a specific application.