Wired the Arduino to take power from the 12V supply for the LED light ring. No longer have a need for a separate wall-wart to power the Arduino.
Also upgraded to the latest version of grbl 0.9i. This has support for spindle control by default. That means that the function of pins 11 and 12 are swapped. I need to change the wiring on my screw shield to support the changed functionality. All working OK.
When my SuperPID arrives I’ll be able to control the spindle speed from the part program. I expect a minor problem with the run signal for the SuperPID. It may require a small circuit to go to 5V when pin 11 is not 0V. We’ll wait and see.