Adding homing-sensor to 4th axis on CNC


I have a 4th axis on (both) my CNC milling machine(s).
I want a home switch that indicates a perfect 0°.
So I can stop a job, switch off the machine and later home all 4 axis to their homing switches, jog to the last position and continue the program.

Later I'd like to do the same for a planned 5th axis.


Ordered the sensor. Not started doing it yet. Stay tuned... (this blog posting will be updated)

Hardware choices

This is my 4th axis. (50:1 harmonic drive gearbox with a K11-100mm 3 way chuck attached.)

M8 2mm sensing DC 5 V NPN LJ8A3-2-Z/BX-5V zylinder induktive näherungsschalter sensor arbeitsspannung 5VDC spezielle für MCU

This is the sensor "Finglei Electric LJ8A3-2-Z/BX-5V" (5V NPN induction sensor) I'd like to mount as a home switch.

Why inductive?

The sensor needs to not block the movement of the 4th axis past 360°.
For a simple, mechanical switch that can be solved by attaching a ramp to the disc.
However it also needs to allow movement in the opposite direction. That doesn't work well with mechanical switches.

Optical switches can get confused by dust and shavings.

So I have chosen an inductive sensor as the most reliable and simple option.


I have taken my information from this tutorial and this discussion.

  • connect blue to GND
  • connect brown to +5V supply voltage for the sensor (I could have used a 48V sensor but I wanted to play it safe)
  • connect the remaining black wire to +5V via a 100KOhm pull-up resistor
  • and also connect the black wire to the input.
My PLCM-E3P CNC (used with the PLCM-B1 breakout board) provides me with 15 inputs to use here. They can work with 5V signals.

You can't use the +5V from the stepper-enable outputs to drive the sensor inputs.
(e.g. for a tool-height probe).
I had to add an extra 5V supply just for that existing tool-height probe.


My plan is to
3D print a mount for the sensor on the side of the gear box.
(Possibly using the slots that already mount the gear box to the table to not accidentally drill into the gears inside)

Then attach a modified wascher to one of the bolts securing the chuck to the plate to detect when it passes the sensor once every 360°

The sensor must have a gap of no more then 2mm to it's metal target.
Update: In  MK2 I forgot to leave clearance for the heads of the M6x30 machine bolts. So I had to change the design slightly for MK3.

Here is the 3d printable design of the sensor holder:

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