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Drilling out Grubscrews?

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Howard Lewis10/07/2012 23:17:09
2460 forum posts
2 photos

I have made a horrible boo boo and need to remove a 6BA grubscrew, which is thoroughly jammed in a part that I cannot scrap and replace..

The only thing that has made any progress was a left handed drill, and even that seems to have run out of steam at the bottom of the socket.

Any ideas Please?

Nicholas Farr11/07/2012 10:13:45
1998 forum posts
958 photos

Hi Howard, grubscrews a normally quite hard, so you may need a carbide drill or find someone with a spark erosion machine.

Regards Nick.

colin hawes11/07/2012 10:37:17
502 forum posts
18 photos

Drilling HT screws with a HSS drill requires a VERY sharp drill run slowly and must not be allowed to rub. It must be resharpened immediately if even slightly loses its edge. Colin

David Littlewood11/07/2012 12:14:30
533 forum posts


What is the part made of? If it is brass or bronze you should be able to dissolve it out with acid (10-20% sulphuric acid, for preference) or even alum. May take a couple of days, and keep replenishing as the acid down the hole will deplete rapidly. Remove any oil first. If the part is aluminium alloy, it might work, but the alloy may be stained or even damaged, depends on the alloy. In either case, test first on a piece of the same material if you can. Steel? - tough luck.


Edited By David Littlewood on 11/07/2012 12:15:53

Terryd11/07/2012 12:40:11
1926 forum posts
179 photos

Hi all,

I have considered carbide drills for this sort of problem before but have been put off solid carbide drills due to their relative brittleness. My question is, would carbide tipped masonry drills work if they were sharpened correctly on a green grit wheel?



Ian S C11/07/2012 12:57:30
7447 forum posts
230 photos

I'v used a masonry drill, resharpend on the green grit wheel, run at a slow speed with plenty of weight on it, you must keep the feed up. Ian S C

Chris Trice11/07/2012 12:59:43
1362 forum posts
9 photos

It's a shame you've already drilled it. Sometimes, you can get these out by hammering in an old (emphasis on old) screwdriver of a suitable slightly larger size, or a torx style bit, which gives you a better purchase on the grub screw hex than a rounded allen key.

David Clark 111/07/2012 14:48:04
3357 forum posts
112 photos
10 articles

Hi There

Masonry bits would be a waste of time.

You need a carbide drill.

Set the component to be drilled in the milling machine vice.

Line the hole up with the drill.

Raise the table very slowly rather than trying to drill with the quill.

regards david

Trevorh11/07/2012 14:56:10
290 forum posts
84 photos

try contacting Cromwell tools and getting either a stellite drill bit or better is the set of screw drills

I bought my engineerss a set each and they work great on any broken screw even taps

not to highly priced but well worth it

Clive Foster11/07/2012 15:09:39
1896 forum posts
62 photos

A good quality brand new or carefully sharpened masonry is known to work well on hard steel given sufficient pressure at very low rotational speed. Drilling a bearing race in that manner was a known salesman / party trick with the old screw feed direct hand ratchet turned Coles and (presumably) similar agricultural drills. I doubt if any normal power drill will run slow enough or be able to apply sufficient pressure to reliably replicate the trick. I'm none too sure that a 6BA thread would stand the strain of that much pressure anyway.

If you have access to a good milling machine a centre cutting carbide end mill may well be a better bet than a carbide drill. I have successfully drilled out 2.5 mm taps using one. Last one of 5 killed one flute but it got the job done.


Clive Hartland11/07/2012 15:41:41
2481 forum posts
40 photos

We are talking about a 6BA grubscrew here, I do a lot of these in Alu castings where a Stainless steel screw has broken off flush.

My method is similar to Davids but I use a small drill first right through the center of the screw.

Then I open it out a 0.2mm at a time and eventually I reach the tapping size and the threads then disintegrate and I wind them out.

If its really badly damaged by ham fistedness, drill out oversize and make a threaded plug 2 sizes up and loctite in place. Further, use a helicoil if possible.

Drill and tap before or after if you have a drilling guide like a cover to work with.


Gordon W11/07/2012 16:30:14
2011 forum posts

If all else fails try grinding a masonery drill into a spade point, but left-handed. Run the drill in reverse as slow as possible. Has worked but might break the bit ! Left handed - use the lathe or twist the belt on the drill.

Falco11/07/2012 23:40:25
60 forum posts
7 photos

I have had to remove a throrughly rusted 4mm grubscrew in a brass doorhandle yesterday. I used a carbide endmill and positioned the handle very carefully on the mill table.

Two methods come to mind that you might try.

Run an end mill of the core size of the grubscrew down the exact centre of the grub screw. I have found that this can be done on the mill with careful centring and using an endmill. I have never been able to do it successfully with any kind of pointed drill, carbide or otherwise. A pointed drill will invariably go off centre no matter how careful you are.

Another method is to use a carbide end mill that is slightly smaller than the core size of the grubscrew to drill out the bulk of the centre of the grubscrew. This will not remove or damage the existing thread. Then use a smaller endmill, positioned off-centre to JUST cut through the threads on one side of the screw. This will usually allow you to prise out the remainder of the offending screw but will leave the threads intact except where the small endmill cut them on one side. It will usually leave enough thread to allow you to use the same size grubscrew in the hole again.

Gentle feed is essential as is clearing out the swarf as soon as it appears. Use a blower or a magnet near the cutter to grab the swarf out of the hole. Otherwise it can jam the small end mill and cause a breakage.

Boiler Bri12/07/2012 13:19:01
812 forum posts
287 photos

Sorry 'Moderator' i do not agree with you.


I sharpen a Masonary drill and cut it out with that. slow speed and plenty of cutting paste.

never failed



Edited By Boiler Bri on 12/07/2012 13:20:20

David Clark 112/07/2012 14:24:16
3357 forum posts
112 photos
10 articles

Hi There Boiler Bri

Where can you get a 6BA core size masonry drill?

regards Moderator.

Richard Parsons12/07/2012 17:13:18
645 forum posts
33 photos

I would do a ‘Google search’ for a “A Simple Self-Acting Spark Erosion Machine” by Derek Lynas. It is in the form of a pdf file and is a simple bit of kit. I am waiting for the bits to come from R.S.  The link I added did not work  but the Google search did.

No it does not use microprocessors and will live quit happily on the shelf until you need it



Edited By Richard Parsons on 12/07/2012 17:17:35

Tony Pratt 112/07/2012 17:45:58
935 forum posts
3 photos

You will not be able to get a 2.3mm dia. masonry drill which is needed for this size screw! Give a good quality HSS drill a go, slow speed and plenty of lube should work. If not a solid carbide drill is guaranteed to work if you are careful.


Boiler Bri12/07/2012 17:50:02
812 forum posts
287 photos

I wan not talking about a specific size just what the experience i have had

David Clark 112/07/2012 18:03:19
3357 forum posts
112 photos
10 articles

Hi Boiler Bri

And I was talking about the original question, a 6BA core size drill.

regards david

Howard Lewis12/07/2012 20:36:42
2460 forum posts
2 photos

Many Thanks to all who have replied.

I have now solved the problem, after a fashion.

The method was use a 3mm hard drill (ie TC tipped like a masonry drill, but ground for metal) run at the max speed of the mill/drill, with plenty of pressure, as recommended.

This drilled out the grubscrew, so the brass flywheel is now secured with an M4 grubscrew.

Again, Many Thanks for all the advice.

Buying some Stellite drills may be a good idea, against future mishaps, (which are QUITE likely in my case!



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