Proximity Sensors on machine tools are typically capacitive, voltages are typically very forgiving 12-35 VDC is the norm. They are sized by metric measurements, the most common being 12mm. They either have an attached or removable cable. As the name implies the target never touches the sensor, and is normally spaced .03-.05" from the tip. Strippit® used both round, and rectangular variations. Unfortunately they also used all different electrical types, Normally Open, Normally Closed, NPN (sinking), and PNP (sourcing). The most common mistake when ordering parts, has been these sensors. If you aren't sure of the type please ask I can help identify the proper part. Although the wrong switch usually won't damage the controller, it is real confusing if you get the wrong types installed.