I don't know that there is an exact formula for what size centre drill to use for what size job. Although, I suppose some busy bee somewhere has sat down and worked it all out. Mostly it's a matter of common sense, which of course is usually far more common than sense. These days with revolving centres that don't need a large bearing suface, it's probably even less critical than ever. Whatever works will work. One thing you want to look out for is if you are building models or tools or stuff for display, then honking great centre holes left in the ends of small pins or shafts etc look a bit off. So keep them small for that kind of thing. Or turn the end off afterwards. Otherwise use a good size centre drill so there is less chance of snapping the "tit" off the end, but don't drill it all the way in on the tapered section. Halfway in or a shade more will do the job. I tend to use the centre drill with about a 5/16" or 1/4" shank for most jobs. Smaller for tiny stuff. Bigger on the industrial stuff.

Edited By Hopper on 25/05/2022 11:07:46

Edited By Hopper on 25/05/2022 11:08:45

Recommended in one of my reference books, is a minimum 1/32" (0.8mm) depth of the coned part of the drill; irrespective of drill diameter.

(I happened to find that while in this session, when replying to another question!)

I tend to go further than that. It does no harm, except perhaps to appearance if in a visible component on a scale model.