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Flat bottom hole 3/16 o/d

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3404611/08/2019 10:20:24
695 forum posts
7 photos

Need to drill a hole 1 inch deep by 3/16 inch o/d in brass and end up with a flat botton - cylinder for a small mamod type engine.

I am thinking use a normal drill to start with and then use a milling cutter or a flat bottom drill to finish.

Just wondering which method is the norm ?



David Standing 111/08/2019 10:33:32
1266 forum posts
45 photos

I would use a slot drill for that. Clear the swarf regularly.

Brian Wood11/08/2019 10:34:16
1942 forum posts
37 photos


The usual approach is to drill the hole almost to full depth and then swap the drill for a specially ground flat bottomed drill to create the bottom of the hole.

The skill lies in getting the geometry of the second drill end correct but it can be done in drills of this diameter. In my professional life we used to call on the services of a particular man in the toolroom to create flat bottomed holes in all manner of exotic aero engine materials. These were 0.050 inch diameter [ actually 3/64 inch] and about 1/2 inch deep. Skilled work indeed with sensitive drilling machines.

Regards Brian

JasonB11/08/2019 10:51:17
16018 forum posts
1684 photos
1 articles

I doubt the bottom needs to be totally flat, so grind a drill to an almost flat end or make a D bit. Not worth trying to find an extra long series slot drill to reach that deep.

Kiwi Bloke11/08/2019 11:13:40
227 forum posts
1 photos

...or, if appropriate, you might consider a through hole with an inserted plug - secured by your favourite method.

Speedy Builder511/08/2019 11:28:35
1800 forum posts
127 photos

'D' drill anyone ??

Nicholas Wheeler 111/08/2019 11:54:35
268 forum posts
16 photos

I normally use a milling cutter for flat bottom holes, but none of my 3/16 cutters are any where near long enough for a 1" deep hole

3404611/08/2019 12:56:53
695 forum posts
7 photos

Thanks all


3404611/08/2019 12:56:54
695 forum posts
7 photos

Thanks all


old mart11/08/2019 18:52:28
442 forum posts
42 photos

If you look closely at a slot drill, you will see that it will not produce a flat bottomed hole. Nearly flat may be within your tolerances, however.

Ian P11/08/2019 21:12:04
2144 forum posts
89 photos

I have just realised what it was about this thread that had unsettled my tiny brain. I have followed it from the start hoping to find out what engine had a piston 3/16" diameter (I thought a Mamod would be larger than that).

The unsettling bit was trying to get my head round a hole with an 'outside' diameter of 3/16". All the holes I have come across have an inside diameter!wink 2

Ian P

(As to the hole in question, for me its a long series slot drill to clean up after normal drill, or investigate why the hole needs a flat bottom and whether it can be eliminated.)

Michael Gilligan11/08/2019 21:24:03
13800 forum posts
599 photos
Posted by Ian P on 11/08/2019 21:12:04:


The unsettling bit was trying to get my head round a hole with an 'outside' diameter of 3/16".


If you succeeded in getting your head round it, Ian ... I think you have effectively demonstrated that it must have an outside diameter.

angel MichaelG.

Edited By Michael Gilligan on 11/08/2019 21:25:25

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