Author Topic: Grub Screw Extraction  (Read 224 times)

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Offline Peaky 556

Grub Screw Extraction
« on: Jan 13 2021 19:32 »
Good evening all,
Can I ask if anyone has been successful with removing something as small as a M3 socket head grub screw that is jammed in a brass hub?  I think I’ve had a bad batch of screws, as two have failed in quick succession, characterised by the key (1.5mm) rounding off the hole as it was being tightened onto a shaft.  I’ve managed to loosen them slightly to be able to remove them from the shaft, but even with now no preload on them they seem to have jammed well and truly.  Maybe the screw has fragmented and the action of turning the hex key has driven the fragments into the brass.
Thank you,

Offline Andy B

Re: Grub Screw Extraction
« Reply #1 on: Jan 13 2021 21:30 »
Is the object with the hub (a gear?) now separated such that you can drop it in a container of liquid on its own?
And are the grub screws mild steel (not stainless?)
If so, I'd try Alum. I've used in successfully to remove the broken off end of a drill in brass.
Plenty of references on how to do it on t'interweb. Alum is easily available on ebay, etc..
You'd be welcome to a few grams from the bag that I have, but Derbyshire police may not consider it an essential journey for you to pick it up!


Offline John Branch

Re: Grub Screw Extraction
« Reply #2 on: Jan 13 2021 22:33 »
I have every sympathy with you over this event.  I have been drilling, tapping holes and (ahem!) screwing bolts and grubscrews at a great rate over the past couple of weeks, and have experienced a larger than usual series of broken drills and taps and smoothed off socket heads, and hex keys with rounded edges.  Some of this is due to my natural and increasing clumsiness, but I think that the quality of such items from the new "workshop of the world" is sometimes questionable.  With grubscrews I now go into the shaft with a clearance drill, and get metal to metal in shear rather than rely on the clamping force to transfer the drive.  Roll or split pins can also sometimes be used.  I did not get this trouble with UK sourced BA sizes.

 How can we be sure of decent quality when buying these items?


Offline IanT

Re: Grub Screw Extraction
« Reply #3 on: Jan 14 2021 08:54 »
With small (non-ferrous) item like this - Andy's Alum method is probably the easiest & best.

Larger items can sometimes be drilled out but it becomes very fiddly as they get smaller (at least without spoiling the thread). A small CNC machine could probably do it - I've got an article on using one to repair very small watch parts and I don't think the work involved could be done manually (at least not by me).

Btw - if the grub screw is 'soft' and being driven onto a hard shaft, it may deform the (threaded) end and make it very hard to then unscrew. Turning a 'pip' on the end (or using screws that already have one) avoids this problem but takes time and needs a small accessory. It's just a short piece of brass rod, drilled and tapped through. The grub screw is screwed in (from the back) to protrude from the front and then a second screw is put in to lock it in place. It's easy to hold & modify it then and although this takes time, it can be a good investment sometimes. I've had this kind of problem on some of my machine repairs and it can be a real pain.

Broken taps/drills in cast iron or steel really need an EDM machine to remove but I've used a 6mm diamond tipped 'core' drill recently (in cast iron) to remove a broken tap, then plugged the hole and re-drilled & tapped it. It's VERY annoying when this happens!!       


Nothing's ever Easy - At least the first time around.

Offline Chris_P

Re: Grub Screw Extraction
« Reply #4 on: Jan 14 2021 09:04 »
"How can we be sure of decent quality when buying these items?"

Whilst it's very hard to be assured of quality when we buy it might be comparatively easy to test each screw in a suitable blind hole in a piece of appropriate scrap material before actually using it in a critical component.  If it fails then little is lost. We could get into a debate about potential plastic deformation and weakness in second use following over stressing in first test but I doubt this is a real issue for modeller's purposes where an if in doubt use a different screw (after test) approach should be fine.

Offline 753

Re: Grub Screw Extraction
« Reply #5 on: Jan 14 2021 10:05 »
I have fitted a couple of Slater’s motor / gearbox units to locos recently. The drive spur gear comes a small grub screw and key which I think is to small to get any decent torque without the risk of striping the flats of the grub screw.
I drilled the gear out to take a 6BA grub screw, it also helps if you can spot drill the axle to take the screw tip thus less torque required.


Offline Peaky 556

Re: Grub Screw Extraction
« Reply #6 on: Jan 14 2021 12:36 »
Thanks all, it’s obvious there has been a lot of frustration out there (and probably swearing) from similar calamities.  For my problem I will add that I also tried the next size up of Imperial Allen key (1/16”) but that wouldn’t grip either, nor would the depth of socket allow my screw extractors to grip.  The screw was black, so I took it to be a high tensile steel like a grade 12.9 “Unbrako”, but I’m probably misled there.  The remainder of the batch has gone into my rejects tub (rapidly filling with dodgy ballraces, motors, eccentric shafts, non-threaded nuts etc!).  I have a batch of stainless grub screws to try in future, from Boltbase whose products I generally find pretty good.

I will look into your Alum suggestion, thanks Andy.  I probably won’t call though, nor arrange to meet you and swap small packets of white powder for bundles of notes, or other suspicions may be aroused 😂.

On the matter of design features when using grub screws, I always file a flat on the shaft.  Not quite so good as the ‘pocketed hole’ method but easier on small diameter shafts.

I’ll continue having fun 🤔
Thank you,