Proximity switches are the most frequently replaced sensor on any CNC machine tool — and the most frequently ordered wrong. The part number on the sensor body may have worn off, or the machine builder may have used an obscure OEM-branded variant that's no longer available. If you order the wrong switching logic, the machine won't reference properly or will throw persistent alarm codes.
Here's how to identify what you need without a part number.
The Four Things You Need to Know
Every proximity switch has four critical specifications:
- Sensing type: Inductive (metal only), capacitive (any material), or magnetic (requires a magnet target)
- Output logic: PNP (sourcing) or NPN (sinking)
- Output state: Normally Open (NO) or Normally Closed (NC)
- Physical dimensions: Thread size (M8, M12, M18, M30), body length, and connector type
PNP vs NPN — The Critical Difference
This is where most ordering mistakes happen. PNP switches source current to the load — when triggered, the output wire goes to +24V. NPN switches sink current from the load — when triggered, the output wire goes to 0V.
Most Japanese CNC controls (Fanuc, Mitsubishi, Mazak) use NPN inputs.
Most European controls (Siemens, Heidenhain) use PNP inputs.
Many modern PLC-based machines accept either with configuration.
If you're unsure, check the wiring diagram. If the sensor's output wire connects to a PLC input card with a common terminal connected to 0V, you need PNP. If the common terminal connects to +24V, you need NPN.
A quick field test: with 24V applied to the sensor, trigger it and measure the output wire. PNP will show +24V when triggered. NPN will show 0V when triggered.
Normally Open vs Normally Closed
Normally Open (NO): Output is off when the sensor is NOT detecting a target. Output turns on when metal is detected. This is the most common configuration for general position sensing.
Normally Closed (NC): Output is on when the sensor is NOT detecting a target. Output turns off when metal is detected. Used for safety circuits and overtravel limits — if the cable is cut or the sensor fails, the output drops, which the control interprets as an overtravel condition (fail-safe).
Home/reference switches are almost always NO.
Overtravel limit switches are almost always NC.
Common CNC Proximity Switch Applications
| Application | Typical Type | Logic | Output |
|---|---|---|---|
| Axis home/reference | Inductive M12 or M18 | NPN (Fanuc) / PNP (Siemens) | NO |
| Overtravel limit | Inductive M12 or M18 | NPN (Fanuc) / PNP (Siemens) | NC |
| Tool clamp confirm | Inductive M8 or M12 | NPN | NO |
| Door interlock | Magnetic | NPN or PNP | NC |
| Coolant level | Capacitive M30 | NPN or PNP | NO |
Browse our full range of proximity switches — we stock the most common M8, M12, M18, and M30 sizes in both PNP and NPN outputs, suitable for all major CNC control systems.