Hey Martin,
do you remember when we discussed the different outputs of the Hitachi WJ200 (aka Omron MX2)?
Since the AL0..2 outputs are solid state relays which generally do not hold their state when you use them to trigger an external relay with minimal trigger current, and also are not dimensionated strong enough to switch a dust collector by themselves, I assume you mean the two open collector outputs on pins 11 and 12.
What do you mean with postprocessor?
Usually the two outputs on the VFD are to switch the spindle coolant pump and the dust collector on and off (this way you can also make that the spindle coolant pump runs a little bit longer after the spindle is stopped).
Regarding workpiece coolant, regardless of flood, mist or air cooling, you have the M7 and M8 commands. You would switch these coolants on by the CNC controller usually, since these commands that are made for this.
If I understand you right, when milling aluminium and plastic, you don’t want the dust extraction to run? This could be achieved in different ways. You could wire the CNC controller output that gets activated when you start one of the workpiece coolants (load-1 or load-2) to an input of the VFD and use the internal logic to make the output for the dust extraction only switch on when this workpiece coolant is off. This is the advantage of a good VFD: The internal logic. Would the solution that when one of the workpiece coolants is activated (with M7 of M8 commands), this makes the dust collection staying off be satisfying?
The other way would be an external logic. Load-1 or Load-2 on Onefinity Controller are XORed with the output 11 or 12 that switches dust extraction on.
As far as I remember the ‘tool-enable’ pin (usually M3 command on PWM spindles/router) does not work when a VFD is selected as tool, but I got to look if this is true. I remember there was a topic dealing with this.