Connecting relay to digital outputs

[cropwell]

Are you talking about a 12v coil relay or a 12v relay module like one of these https://uk.banggood.com/12V-1-2-4-8-...r_warehouse=CN

I ran my Kress spindle for 5 years using a relay to switch it, now I have converted to a SuperPID speed controller and the difference in noise levels is amazing. It is much quieter.
One of the points to be aware of is that the control signals to the spindle relay can be all over the place until Mach3 takes control, so you could get the spindle starting and stopping as the computer boots up and before Mach3 takes control of the port.