Centre drilling with lathes...

[Robin Hewitt]

Also, I've noticed that standard drills 'wobble' when drilling holes from the tailstock - so does a center drill solve all the potential problems associated with this wobble

A Slocombe centre will do it darnedest to align itself with the centre of rotation, even if your tailstock is a tadge out of alignment. Don't know how they do that but I'm sure it's very clever.

To get a larger drill to start on centre (no wobble) make sure you open the hole out a tadge bigger than required diameter using the wider part of the Slocombe.

There are a lot of crap Slocombes out there and they can be a booger to clear when they snap off. I try not to use anything except Dormer but I am superstitious. Once bitten, twice shy.

[irving2008]

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...

[Robin Hewitt]

I know I'm a bit fuddy-duddy and old-hat with my Slocombes but I do like them for starting a Jobber.

You get that 60 degree cone to align the bit PLUS a perfectly concentric pilot hole to clear the awkward drill centre.

The Jobber goes in real easy to the bottom of the pilot by which time you have a much better hold on centre.

I'm not going to argue the why's and wherefore's, but I think they are neat