View Full Version : Centre Drills

04-13-2008, 07:54 AM
Can any one explain what is meant by


All the best..markj

04-13-2008, 08:06 AM
I BS at least 5 times a day myself. Doctor told me to, said that it was good for the funnybone.

I think that the BS is for Brittish Standard (nothing to do the the above mentioned humor). Must be some signed engineering print with the queens approval on it somewhere showing exactly what size and angle the drill is to be made to.

edit - Lookie here toward the bottom -


04-13-2008, 08:21 AM
Thanks rockrat

Didnt mean for you to go searching for the answer ...thought someone here would just rattle it off out of their head ..

anyway thanks for bothering

here's the results for centre drill sizes FROM WIKI...maybe usefull to someone else

especially when the like of js sells them on ebay .,..and expects everyone to have this knowlege stored in their noggin

BS1 1/8"
BS2 3/16"
BS3 1/4"
BS4 5/16"
BS5 7/16"
BS5A 1/2"
BS6 5/8"
BS7 3/4"

all the best...markj

04-14-2008, 10:43 AM
OK - now that we know that.

How do you decide which size to use?

04-14-2008, 10:50 AM
There might be some shop rule for that, but if so, I've missed it. I use some judgement on how much support a piece needs or how small a hole I want in a part. For a 5/16" dia long shaft I might use a very small center just because esthetically I don't want the remaining hole to dominate the looks of end of the part. Similarly, if the part is held in the chuck but I'm just supporting the outboard end to reduce vibration, I might use a small one. If it's a fairly heavy part supported between centers, I would use one of my larger centers.

04-14-2008, 10:55 AM
I think that the BS is for Brittish Standard (nothing to do the the above mentioned humor).

I never saw the "BS" prefix before, so that may be a British thing. In the 'states it's just a number 2 center drill.

The Bell centers add a 10 prefix: 12 is a number 2 Bell center, 15 is a number 5 Bell center...

04-14-2008, 01:24 PM
OK - now that we know that.

How do you decide which size to use?

I use the center drill whose pilot is just smaller than the next drill I'm going to put in the hole - so the drill is doing the least ammount of work for the first 1/8" (or so) of hole depth so it can get itself lined up and going straight before it has to cut the full diameter.

04-14-2008, 07:14 PM
This is as good as I've seen in the one list - for drills and centre-drills:





This is worth a browse as well: