Hey Martin,
Generally it is not recommendend to cut power to a VFD while spindle runs.
But if your risk assessment requires the immediate cut off from electricity of the whole system (e.g. for cases of fire), you should ensure that the whole installation is completely cut off by an Emergency Switch. This would be Stop category 0 according to EN 60204-1 - STO (Safe torque off). In this mode the spindle would not be decelerated (it would be left coasting), but it is ensured that it produces no more torque. Also it is important that the switch that cuts off power is of a type that stays off after a power failure when power comes back.
one way is to use the safety function of the VFD. Every VFD has Input Terminals which can either be configured as ISO 13849-1 Safety Inputs or can be programmed to trigger the spindle stop / trip event.
There are two ways to trigger such an input. The most usual way is to use a Safety Relay which also has an Emergency Stop Switch connected to it. When using this in emergency case, the spindle is stopped and the installation also has a reset button which has to be pressed before work can be continued. Such a safety configuration is shown here. You also have the possibility to connect an Emergency Switch directly to such an Input on the VFD, without using a safety relay.
This would be Stop category 1 according to EN 60204-1 - SS1 (Safe stop 1). The machine is brought to a controlled standstill, then the STO safety function is activated. This however only applies to the devices connected to the Safety Relay, so if you want it to take care of other devices than VFD/spindle (e.g. also stop the program on your CNC Controller), you have to wire the safety relay in a way that it triggers safety functions of other devices too.
The other way is to use the “Estopped” functionality of the Onefinity/Buildbotics Controller. Unfortunately, the Onefinity Controller (in contrast to Buildbotics Controller) makes use of this functionality only through the software emergency stop button shown on the screen.
The Hardware Emergency Button on top of the Onefinity Controller Case does NOT trigger this “Estopped” mode, and therefore does NOT take care of stopping the spindle or router if pressed!
But such a Hardware Emergency Button however can easily be wired to the Onefinity controller with two wires as described here. When such an externally wired Emergency Stop button triggers the controller’s “Estopped” functionality, not only the stepper motors of the CNC are disabled, but also the “Stop” command is sent to the spindle using G-code / M command over the ModBus/RS-485 communication, and even a trim router could also be stopped then if it is connected over a SSR or relay to the “tool-enable” functionality / pin on DB-25 I/O connector (see here for this).
Note you could also trigger the safety inputs of the VFD this way in case you don’t have ModBus/RS-485 communication between Controller and VFD.
All this, about connecting and using an Emergency Switch, is explained here and here.