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Centre drill sizes

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Anthony Knights25/05/2022 10:56:36
622 forum posts
243 photos

I am about to do some turning between centres and was curious as to the relationship between the centre hole size on the work piece and it's diameter. I have tried various searches on the internet to see if a table of dimensions exists, but without success. Of course, trawling the internet is an art, which means devising the correct search terms to enter to get a sensible result. Some people are better at this than others. Can anyone point me in the right direction?

Emgee25/05/2022 11:01:04
2426 forum posts
290 photos

**LINK**

Emgee

Hopper25/05/2022 11:05:47
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6393 forum posts
334 photos

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

Chris Evans 625/05/2022 11:54:50
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2057 forum posts

I always shorten the "Tit" on a centre drill to lessen the chance of breakage. My understanding of sizes is Number 1 is 1/8" 2 is 3/16" 3 is 1/4" 4 is 5/16" Rarely have the need for anything bigger.

Nigel Graham 225/05/2022 12:00:06
2133 forum posts
29 photos

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.

not done it yet25/05/2022 23:19:30
6809 forum posts
20 photos

I would just use the largest that I consider practical. We (mostly) don’t often make really large items on our machines - if we did, we might use a fixed steady to help matters?

The small pilot is there to both prevent the centre bottoming in the hole and also to contain some lubricant, for when running at the tailstock end with a solid centre (which is likely regarded as better practice) unless a good quality revolving centre is available.

It is likely that cutter interference with the centre may be a problem, so small tool-holders/cutters may need to be selected, extra length left for turning off later (as per Hopper, above), or half-centres used. Don't need any centre holes left unless the part is expected to be reconditioned in the future.

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