2. Not Enough Time on Short Feed to Stop

The trouble. The job runs so fast on a short feed that not enough time is available to stop the press to prevent die damage.

In this case, a progressive die runs at 150 strokes/min. The feeder finishes at TDC; the die closes at 140 deg., and the press stopping time is 180 msec. A quick calculation of the critical angle (the last point in crankshaft rotation to signal an emergency stop without die damage) puts it at 338 deg., or 22 deg. before the feed can finish.

The resolution. Install a partial-feed detection sensor.

If you have a short feed on a high-speed operation, you don’t necessarily have to accept that you will crash the die on every short feed. If fact, an easy way to detect some short feeds in this type of operation is to check the strip progression during the feed while enough time exists to stop the press.

If a die runs at such a high speed that there is insufficient time to stop the press by the time the feed completes, install two proximity sensors to detect feed progression.

The sensor in the background detects full feed progression and is set to top-stop the press in the event of an error. The sensor in the foreground will detect the strip about halfway through the feed progression. When the die runs normally, the partial-feed sensor detects the strip during the progression before the critical angle. This sensor is set to emergency- stop the press.

This arrangement effectively prevents die damage due to misfeeds because short-feeds are much more common than over-feeds, and most causes of short-feeds (such as roll slippage, improper setup and partially pulled slugs) occur at the beginning of the feed cycle.

3. Sensor Locks “On,” Causing Nuisance Stops

The trouble. You’re ejecting a part down a chute and using an infrared diffuse-reflective photosensor to detect the part. You can get it running on the bench, but when you try to run it in the press, the part-ejection sensor occasionally locks “on” without reason, causing a nuisance stop.

The resolution. Change the sensor type.

Detecting a part as it moves near a background is a difficult sensing application. This often occurs if you use a photosensor to detect a part exiting down a chute or passing on a conveyor. Ideally you would detect the object from the side, but space constraints may force you to sense the object from above. This can be challenging. Proper sensor selection is the difference between success and failure.

As the name implies, a diffuse-reflective photosensor operates on the principle of reflectivity. The emitter sends out a cone-shaped field of light perpendicular to the face of the sensor; the sensor actuates when the receiver detects some of the emitted light reflected back. It is one of the more popular sensors for detecting air-ejected parts. For this reason, many stampers attempt to use it to detect objects. Unfortunately, it is ill-suited when parts move near a background.  

A diffuse-reflective photosensor will actuate when the receiver ‘sees’ only a small amount of the emitted light reflected back to it. This enables this sensor to detect small, fast-moving, oddly shaped targets, making it suitable for detecting air-ejected parts. Unfortunately, this attribute also causes the sensor to have difficulty detecting an object near a background because it also picks up the background itself.

With painstaking adjustment, it may be possible to succeed with such an application on the bench, but when installed on a machine, the results usually are disappointing. Any small change in the mounting angle, distance or reflectivity of the background (such as when it becomes coated with oil) can cause the sensor to lock onto the background periodically, causing sensor failures and missed actuations.

An optical-convergent sensor (sometimes called a distance-settable reflective sensor) is a better choice. Like the diffuse-reflective photosensor, an optical-convergent sensor is a reflective photosensor, but instead of transmitting light at 90 deg. to the face of the sensor, the emitter angles toward the center axis of the sensor. The receiver is angled toward the same spot but from the opposite direction, so that the two beams converge at a spot in front of the sensor. This sets up a V-shaped sensing beam that allows the sensor to detect objects in the area where the emitter beam and receiver viewing area intersect.

This beam arrangement also enables the sensor to ignore anything outside of the area where the beams cross, because when the emitter beam reflects an object outside of the convergent area, the reflected light will miss the receiver.

Install the sensor so that the background is beyond the convergent area, while the part to be detected will pass through the point where the emitter beam and receiver viewing area intersect.

These three applications, with their unforeseen problems, represent the types of real-world failures that usually are caught rather than taught. When implementing a die-protection program, your learning curve never ends. MF

 Press Controls and Sensors Series, Part 1: Five Factors in Selecting a Resolver-Based Die-Protection Controller 

Press Controls and Sensors Series, Part 2: Pairing Sensor Types with the Problems They Address

Press Controls and Sensors Series, Part 3: Best Practices for Die Protection

See also: Wintriss Controls Group LLC

Technologies: Sensing/Electronics/IOT

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