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Drilling Small holes in Gunmetal

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RMA31/01/2019 21:48:45
313 forum posts
4 photos


Good evening. I thought this thread might be the best place for my question. I'm not ready to do this yet but I thought I'd ask for some opinions as I haven't drilled anything like this before. Nothing like planning ahead!

I need to drill the gunmetal cylinder castings on my Britannia to bolt on the cladding. I'll be using either 10 or 12 BA bolts, so the tapping drill will be small. I'd be glad to hear from anyone who has done this and the technique they used to avoid drill breakage. I'll be drilling the holes on my milling machine.

Thank you



Edited By JasonB on 01/02/2019 06:56:33

J Hancock01/02/2019 08:53:25
832 forum posts

Whatever else, make sure the chuck is holding the drill dead centre, no 'wobble'.

JasonB01/02/2019 09:00:55
22574 forum posts
2634 photos
1 articles

As it is only cladding you can go for a lower percentage thread depth so the tapping drill size can be increased slightly which will help the tap more than the drill..

Drill the holes and then without disturbing the work on the mill tap them using the spindle to guide the top of the tap so it stays true to the hole. Small spotting drill dimple to locate the hole then a decent split point drill not a 10p cheapie.

If you do go down to 10BA then think about making a small knurled ring to go around the tap that can be turned with your fingers to give better feel than a big tap wrench.

norm norton01/02/2019 09:35:19
183 forum posts
9 photos

It is essential to have sharp drills for bronze with no lip wear whatsoever, or they will drill undersize and then bind.

Best to buy new ones for the job and get a decent quality brand. Cheap ones will jamb. This link is a good ME supplier and is happy to post just a few drills. **LINK**

Have you got a nice small chuck to fit the mill? Best not to use silly high speeds, just 600-800 RPM keeps it controllable. Feed down slowly, just fast enough to produce a slow output of swarf. Apply a spindle brake slightly to stop it descending under its own weight. Lift the drill out every 1-2mm to clear the swarf.

Have fun!

RMA01/02/2019 10:46:46
313 forum posts
4 photos

Thank you all for some great advice. I will be buying new drills for this, do you advise drilling dry? I won't be going in that deep as it's only to hold the cladding on, the holes of which will have to line up. Any tips for doing this, other than clamping?

norm norton03/02/2019 10:29:14
183 forum posts
9 photos

For small drills of this size I have never found that adding anything wet helps. Lubricant starts to work when things are getting hot.

As Jason has said, once the hole is drilled then use the chuck to help line up the tap. For 10 BA and smaller grip the tap with a small Pin Vice, not a Tee Holder, and just use finger and thumb to rotate and sense the resistance. Don't grip the Pin Vice in the chuck, just use a pointed rod to visually align things. One finger on top of the Pin Vice to get it to bite and descend. Take it out every turn and brush off the swarf.

For bigger jobs a spring centering tool can be held in the chuck, but I don't find one helpful with tiny taps, and the tops don't have locating holes anyway. If you have not got a Pin Vice then look for some genuine used Eclipse ones on eBay. Don't buy cheapie £10 new sets as they won't grip and will be off-centre.

Phil P03/02/2019 11:25:24
802 forum posts
194 photos

I use a totally different method which completely removes the risk of breaking a tap in the main casting.

I machine some short brass bushes with a tapped hole in the centre to suit the screws used, in my case they were Metric 1.2mm ones, and made in a batch on my watchmaking lathe.

Then simply drill a hole for the bush to fit where you need the screws to go in the casting, and either press fit or loctite them in position.

Worst that can happen is you break a drill or a tap in one of the bushes, but just throw that one away and make another.

From memory I think it might have been Graham Meek who put me onto this method.


Russell Eberhardt03/02/2019 11:39:32
2728 forum posts
86 photos

I broke a number of 12 BA taps until I discovered Rocol RTD compound. I've not broken one since.


Richard S203/02/2019 11:58:13
229 forum posts
134 photos

I survived all the drilling and tapping of my gunmetal cylinder work without any drill or tap breakage. Sizes 7,8,10,12 and 14ba. Much of the drilling was using an old 1980s Black and Decker 2 speed drill on a stand. I'm an average metalwork hobbyist, so if I can do it, anyone can with patience and care following the prescribed tapping drill sizes and following all the advice mentioned above.


Additionally, I used a small vacuum with a clear tube to ensure as much cut swarf was removed from the holes regularly between each thread cutting action. Enjoy doing it.


norm norton03/02/2019 16:55:26
183 forum posts
9 photos
Posted by Russell Eberhardt on 03/02/2019 11:39:32:

I broke a number of 12 BA taps until I discovered Rocol RTD compound. I've not broken one since.


Well said Russell. I completely forgot that. I use the same all the time - essential on the taps.

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