How to Avoid Nuisance Stops

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An almost certain way to ensure the failure of a die protection sensor program is to allow nuisance stops to occur on your machines. A 'nuisance stop' is a machine stoppage that is initiated by the die protection system when no real problem exists.

Nuisance stops almost always result in eventual die damage - but not directly. Nuisance stops cause machine operators to lose confidence in the system to the point where they will ignore real errors, and in cases where the nuisance stops are frequent enough to adversely affect production, turn the entire system off.

Here are some common causes of nuisance stops, and how to avoid them.

Sensors that have to be installed at run time
The only two things a press operator should have to do in order to set up the die protection system is to select the correct program on the control, and plug in a single cable. If the operator is required to locate, install, and adjust sensors that you move from die-to-die, the likelihood of a correct setup drops precipitously.

Rather than moving a sensor from die-to-die, permanently install a sensor on each die that needs it. Compared to die crashes, sensors are cheap. Even without a die crash, the additional setup time required to install and adjust sensors will quickly offset any 'savings' realized by not buying a sensor. To paraphrase Henry Ford, "If you need a machine sensor and don't buy it, then you will ultimately find that you have paid for it and don't have it."

Adjustable Sensors
A properly installed sensor will never have to be moved or adjusted. Problems arise when sensors are installed so that they're "in the ball park" with the expectation that the operator or setup person will do the final adjustment when the die is in the press. When the main goal is "Get this thing running so we can make parts", the final sensor adjustment often ends up being less than ideal, which can result in frequent nuisance stops or poorly protected dies.

Perhaps the most important step in the installation process is bench testing in order to find the exact right location for the sensor. The ideal place and time to do this is on the test bench where accurate measurements can be made, during a period of time where the pressure to produce parts is at a minimum. The operator gets the 'last crack' at the die. If your goal (protect the dies) is different than the operator's goal (get the press running by whatever means necessary) - die protection becomes secondary.

Poor Wiring Practices
Sensor setup should not include uncoiling a nest of individual sensor cables, and referring to a setup sheet to see where to plug them in. This is the case when die-mounted sensors are individually connected to a press-mounted control interface. This inefficient wiring scheme requires each sensor to have attached to it a length of cable sufficient to reach the press-mounted interface. When the die is not in use, these permanently attached sensor cables are hastily coiled up and stored with it. The most common reason for sensor failure is cable damage. This typically occurs while the die is being maintained or transported, so the problems are often discovered during or shortly after setup.

The most efficient wiring method is to connect the sensors to a die-mounted junction box, which is connected to the press-mounted control interface with a single, replaceable cable. The sensor wires on the die can be kept short and protected from physical damage. The sensors are then connected to the controller by plugging in a single prewired cable that ensures that all of the sensors are connected to the proper inputs on the control, eliminating the need to the operator to match the sensor with their corresponding inputs at run time.

Tolerances Too Tight
Modern electronic sensors are amazingly precise and accurate. When an inexpensive off-the-shelf proximity sensor has a repeatability of less than 100 millionths of an inch, it is easy to install sensors to detect events like misfeeds as small as 0.001". However, just because you can doesn't mean you should. If your feeder can hold a tolerance of +0.005" and you install your sensors to detect a misfeed of +0.003", you are going to get nuisance stops.

The whole premise of die protection is to detect die-threatening events and stop the press. If your die is designed so that the pilots can align the strip if it is misfed by +0.020", any feed progression within that 0.040" window is not die threatening, and should not be detected by the feed sensor(s). You should use up all of the 'slop' available to you when you install the sensors. This will make setups more forgiving, and nuisance stops less frequent.