Done that and works without resistor.
What it does do

Both the leds on sensor and BOB light up when the switch senses.
Mach3 says limit reached and stops motor.
All good

What it does not do is show led on mach3 screen, only a quick flash and the led on the screen does not stay on, is this due to the resistor?

I am getting 0 volt and then 12 volt when switch is activated, will a resistor reduce this voltage to 5 volt and then show in Mach3 screen?