The most common cause of this issue is the spindle’s mains wiring diverting the signal from the touch plate to earth, rather than back to the origin point in the Nighthawk.
The Nighthawk has pull up resistors, capacitors, and related circuitry on its PCB to protect itself from power surges as well as filtering the signals coming from peripherals such as limit switches and probes to avoid false triggers. While this provides protection, it comes at a cost of some very slight electrical resistance.
As your spindle is earthed with a very low electrical resistance, and electricity will favour the path of least resistance, the low voltage probe signal from the Nighthawk can leak to earth through the spindle.
A simple solution which works for a good percentage of users is simply swapping your wires around on the probe connector. As the probe is simply closing a circuit there is no polarisation, however having the positive connection going to the touch plate and the negative attached to the spindle often allows enough of the signal to return to the Nighthawk to trigger the probe.
Another common solution is to unplug the spindle cable from the top of the spindle while probing, then reconnecting it after probing. This process disconnects your spindle from the mains wiring while probing so the path to earth isn’t as accessible, making the path to the Nighthawk the more favourable one for the low voltage electrical signals.
For a permanent solution you may need to consider a non-conductive probe such as a mechanical indexing probe. These types of probes do not depend on an electrical path closing between the spindle and a touch plate, instead they activate when the probe’s centre pin deflects upon being pressed to a surface.
PLEASE NOTE:
If you do some searching online, you may notice this is a common issue with conductive probes and is not an issue unique to our Nighthawk family of controllers.
We have had users approach us with information they have found on various websites and forums that suggested ways to temporarily disconnect the spindle earth while probing, using methods that do not also isolate the power.
DO NOT ADD ANY DEVICE IN YOUR EARTH WIRING TO FACILITATE EASY DISCONNECTION.
Disconnecting the earth wire while leaving the active wires connected is incredibly dangerous, risks causing electrocution, and depending on your location may also be ILLEGAL.
With the earth wire disconnected, your electrical safety switch (RCD) in your switchboard WILL NOT function in the case of a fault that causes the spindle body or machine frame to become energised at mains voltage.