Machine Design

Sensor Sense: Proximity sensors for stamping dies

Difficulties that plague die-stamping operations include overfeeds, underfeeds, slugs, and missed hits.

This flat-pack proximity sensor tells a controller that the die has closed properly. A spacer plate under the sensor raises it to the correct detection height for this die.

Proximity sensors head off these and other related conditions by detecting speed, accuracy, target orientation and position, part ejection, and hole placement.

The use of proximity sensors within a die depends on the complexity and sophistication of the die as well as sensor environmental conditions. As most industrial sensors detect metal objects using electromagnetic induction, the material being detected, size of the target, and the physical conditions near the die all help determine sensor types. Physical conditions include the presence of weld fields, high or low-temperature extremes, possibility of RFI, and the type of connected device. For example, sensors must supply more output current to a relay than to a PLC input.

Proximity sensors offer numerous advantages in stamping applications. They are solid state so there are no moving parts to wear out. Sealed devices are impervious to oil, coolant, or other fluids. And the noncontact nature of the device reduces the chance for physical damage.

Sensors have numerous form factors useful in die stamping including miniature, rectangular, low profile, and ring. Flat-pack sensors embedded in dies monitor stripper plates to determine if slugs are pulled into the dies. Cylindrical sensors in spring-loaded lifters make sure materials are in proper position before dies close.

With some die operations approaching 1,500 strokes/min, it is even more critical to ensure proper sensor placement and operation. Sensors monitor critical points in progressive dies for bends, short feeds, long feeds, and slugs. Other sensors mounted in the grippers of transfer dies verify panels are in place before they are transferred to the next station.

Sensor specialists exist to help stamping companies determine the best locations and types of sensors to use in dies to help cut downtime, part defects, and maintenance. According to Turck Inc. in Plymouth, Minn., only a few stamping companies use this service.

Turck Inc. ( provided information for this column.

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