Forstner bits for sure

depending on what size and which mechanisms you use ,will determine what sizes you will need , I am no expert and have made about a dozen 12" pepper/salt mills

My original cutters were el cheapo ,but have since switched to Carbide cutters and find them much superior ,not that expensive on Ebay either . So far I have stuck with the crush grinders with a 25yr manufacturers guarantee

A few of our Kit suppliers and stockists have them available . I started making them as a break from pens and enjoy the challenge (most times) lol.
Have fun with them ,cheers ~ John