Protecting dies against press system faults

Mechanical switches and grounding switches were among the first devices to be used for automated die protection. Early sensing systems based on mechanical switches simply opened the top stop circuit of a press when a fault was detected. These systems, however, offered no protection from faulty wiring or failed switches, and they did not offer the ability to handle complex sensing needs.

Increases in stamping press speed have limited the application of these switches, as well as relay logic devices. The advent of solid-state electronics in the 1980s and their maturation in the 1990s resulted in the development of compact sensors designed to provide die protection in harsh stamping press environments.

Applying this technology requires an understanding of how to operate and locate commonly used sensors. Connectivity, part-to-sensor relationships, fault annunciation, and logic control also must be understood to ensure a successful installation—one that permits unattended press operation and minimizes die downtime.

Two primary die-sensing objectives are misfeed and double-thickness detection. A feed that slips or distorts a strip causes misfeeds, as do excessive camber and erratic stock width. Within the die, slug pulling can stall a strip or result in marked parts. The strip can fail to lift or be inherently weak, resulting in the strip folding over in the die.

Detecting a misfed strip protects a die when the strip does not advance to the proper point in the die. Double-thickness sensors help prevent die damage by detecting slug pulling, strip folding, and failure to eject a part.

Quality assurance functions can also be performed by the sensors. Load cells or sensors can be mounted under individual die progressions to monitor die forces exerted for forming or piercing. Sensors can also be used to monitor part feature presence or absence and feature size. In such cases, the sensors monitor part quality rather than die faults.

Types of Sensors

The two primary modes of operation for electronic sensors are digital and analog. A digital sensor is either on or off, similar to the operation of a light switch. An analog sensor changes state gradually, from a minimum to a maximum voltage (or vice versa), as a function of the distance from the detected object or area of the object.

Several types of sensors are available in either digital or analog models. These include:

1. Proximity sensors, which change state as a conductive material is moved into their sensing range.

2. Photoelectric sensors, which respond to the presence or interruption of a light beam. They are available in reflective and through-beam configurations and can be used with fiber-optic cables to save die space.

3. Piezo devices, which generate a voltage in response to a change of stress. More rapid fluctuations in stress and/or higher stresses result in higher voltage; a constant stress results in zero voltage.

4. Strain gauges, which change output as a function of the strain.

Basic Parameters for Applying Sensors

Reaction Time. A prime consideration for any sensing system is reaction time, the time it takes for a sensor to change state when it senses an object. While electronics are considered to be inherently fast, stamping operations that require monitoring can also occur very quickly.

For example, a press operating at 300 strokes per minute (SPM) has total stroke time of 200 milliseconds (0.200 seconds), an eternity by electronics standards. However, most events in a die are monitored at the bottom of the press stroke, which, at 300 SPM, can leave as little as 4 milliseconds (0.004 seconds) to capture an event. Press speeds of 1,200 SPM can require sensor rise times in the single-digit-microsecond (0.000001-second) range.

Position in the Die. The proper placement of sensors is determined by analyzing potential faults in the die operation. The first step in designing a die is always a strip layout, which provides a die designer with the basis upon which the tool is to be designed. Once that is completed, the analysis of the potential die faults should be made, detectable events identified, and the sensor locations determined.

For example, a strip layout may be determined to present a weak, easily disrupted feeding condition in the last stations of a progressive die. Because the strip is likely to fold over in the die as a result of this condition, detecting the progression of the carrier at the exit end of the die can provide the needed protection. Placing a misfeed pilot at the beginning of the die, however, may not detect this condition soon enough to prevent die damage. The strip would move into the die normally but would not exit normally, and the fault would not be detected.

Position Relative to the Strip or Die Component. As the die design develops, the proper sensor position relative to the strip can be determined. Because die real estate can be in short supply, activating a sensor indirectly may be the best option. Spring-loaded misfeed pilots are a good example because their displacement can be sensed. If space permits or function requires, the sensor can act directly on the strip. Some examples are sensing a pilot hole or a finished part before it is cut free from the strip.

The strip position must be repeatable on every press stroke if a sensor is to work reliably. Sensing stations, especially those that sense product features, require accurate part placement and control which is equal to that required by forming stations. Otherwise, a false reading is likely to result.

Sensor Selection. The appropriate sensor should be selected based on the material being stamped, sensor speed requirements, the event to be detected, the sensing distance, and the available space. As with any design decision, compromises may be needed to provide the desired protection and reliability. Verifying that the sensor output circuits properly interface with the target controller is also important.

Sensor Protection. A sensor should not be placed in a position where the fault that it is designed to detect can also destroy it. For example, rods meant to activate a remotely located sensor can impact the sensor if rod motion is not restricted.

To protect them from the faults they are intended to detect, sensors should be embedded in a steel block, with only the sensor face exposed. Other good defensive mounting tactics are positioning the sensor so that the fault motion occurs away from the unit and limiting the motion of a remote activator to only the amount required to trigger the sensor.

Connections. Die handling for maintenance purposes should also be considered when mounting sensors.

Sensor wires should terminate within the die so that all of their connecting wires can be removed. Connecting wires that are allowed to trail from a die when it is removed from the press are likely to be damaged. Wires within a die should be recessed into holes or channels to prevent pinching by die hooks and during operating and maintenance activities. Sensor wires should terminate in blocks attached to the die to isolate any strain on a connecting wire from the sensor.

If a tool is designed to be changed over to run other similar parts, the sensors must be considered when the new components are being designed. Placing duplicate sensors in these components may save time during changeovers and improve reliability when the new components are used. When production is sufficiently high to justify two tools, complex sensing stations can be designed to be interchanged between them to save the costs of purchasing duplicate sensors.

Because electronic sensors cannot directly interface with press circuits, a controller must be incorporated. The controller converts sensor signals to die faults through programmed algorithms, enabling sensors to turn on and off and detect or see events that occur during one press cycle.

While the controller may be permanently wired into the press setup, the die must disconnect easily for maintenance purposes. The two approaches to this both have advantages and drawbacks.

One method is to provide a connecting cable for each sensor. That way, the wire from the sensors must be run only to the nearest outside edge of the die. Obstacles can arise, however, when a die has many sensors requiring numerous connections.

A second method, single-point connection, reduces connecting time, eliminates the possibility of connecting errors, and presents a cleaner, neater appearance around the press. Routing the wires from all of the sensors to a common point on the die may prove to be difficult, however. A combination of the two methods—installing a connector on each side of the die—is also a common practice.

Verification. A procedure to verify that sensors have not failed or been bypassed should be included in the controller logic program. This can be accomplished by programming the controller to check for the opposite state at some point or for a change of state in a sensor.

For example, a double-thickness proximity sensor should see the spring-loaded stripper at the bottom of every stroke. A controller can be programmed to look for a change of state rather than simply a high or low voltage from a double-thickness sensor. Then, the sensor must be activated on each stroke. This way, for example, a piece of metal that has become attached to the proximity face with grease will not satisfy the logic program and will show a sensor fault condition.

Defining Each Sensor's Purpose

Although an in-depth discussion of controllers and programming logic is beyond the scope of this article, two major points must be understood by a die designer.

First, a die sensor control must have an annunciator to alert the operator to which sensor detected a fault and stopped the system. The more sensors in the die, the more important this becomes. Second, unless a company has been using sensors and programmable logic controls with its dies for some time, the person programming a control may not understand die operations. It is imperative, therefore, that the die designer describe in detail:

The purpose of each sensor.What constitutes a normal condition.What constitutes a fault condition.When the sensor state changes with respect to press rotation.How fast the press runs and what action is expected when a fault condition occurs.

If these requirements are not communicated clearly, the system may not work properly, resulting in reworking parts or a decision to abandon the system altogether.

Conclusion

Any die fault condition that diminishes the productivity of a die deserves a careful analysis for detection by a sensor. Double-thickness detection can sense stripper misplacement of 0.001 inch. Misfeed detectors can signal a fault condition instantly and prevent a press from closing. Logic controllers can be permanently mounted directly to a die, eliminating any reprogramming.

Solid-state sensors designed into dies can reduce die maintenance and increase production time. Stopping a press before a die is damaged allows operators to diagnose a problem without being distracted by badly damaged tooling. Metal stamping is a challenging and difficult business. A well-conceived and applied die protection system can make it a little less difficult.