OK I think I've learnt something here...

So a Slocombe, aka, centre drill is really intended to create holes for a shaft centre support for further machining on a lathe, but because of their narrow tip often get used instead of...

spotting drills... which have a precisely ground 90degree tip and are used to create a pilot hole for a bigger jobbing or twist drill.

Stub drills are conventional but shorter stiffer drills that are less likely to wobble or mistrack and can be used as spotting drills to start a twist or jobbing drill for deeper holes...

Did I get that right? Stub drills seem to be cheap (Dormer ones are £1 - £5), while spotting drills seem to be expensive (£10 - £40)

So what is the best practice, say, to:

drill a 6mm dia 15mm deep hole centrally in a 20mm steel shaft on the lathe?

drill a 6mm dia 30mm deep hole in a 50mm piece of ali in the mill?

assuming ownership of a complete set of 1 - 10mm in .1mm HSS twist drills...