How to remove broken knob grub screw? - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

matthewhouse · 26th Mar 2008, 10:26 pm

Hello again. Trevor what an excellent idea! I never ever would have thought of that. I will certainly be giving this a go.

Phil, I will send you a PM if I end up destroying the knob, thanks for digging that one out.

Thanks for the other suggestions as well, I will report back shortly.

Cheers
Matty

Phil G4SPZ · 26th Mar 2008, 10:32 pm

A technique I have used in similar circumstances is to find an old screwdriver blade or a piece of metal rod slightly smaller in diameter than the grub screw, then file a profile into one end, aiming to be as close a fit as possible to the broken end of the grub screw. This is a trial-and-error process. Then apply a small amount of araldite to the filed tip of the rod, push the rod on the grub screw ensuring that the adhesive doesn't spread beyond the grub screw, hold in position and leave to cure fully. Finally, and very carefully, unscrew the rod using a Mole wrench or suitable tool, and unless it is siezed solid into the brass bush the grub screw should come out.

Sean Williams · 26th Mar 2008, 11:16 pm

Just a last suggestion - Drop the whole thing in a cup of coca-cola (the real stuff - cheap alternatives are not corrosive enough)

Leave it for a day, this will loosen any corrosion, and hopefully the grubscrew will just come undone without effort - Might be easier to screw it in rather than out.....

Nickthedentist · 26th Mar 2008, 11:29 pm

Quote:

Originally Posted by Sean Williams

Might be easier to screw it in rather than out.....

That's a very good point that's often overlooked

chipp1968 · 28th Mar 2008, 10:44 pm

If you hav a dremel type tool you may be able to grind a central mark for the drill to start .

Tom_I · 29th Mar 2008, 12:01 am

Yes, I'd try that. I've had a couple of similar problems. In both cases I've ground the head of the grub screw flat with a Dremel tool, then drilled into it with a very fine bit. In both cases the vibration has loosened the grub screw in its thread.

radiolukey · 6th Apr 2008, 5:02 pm

Matty how did it go? I have a radio with a similar problem in the house.

Dillen_de · 10th Apr 2008, 12:03 pm

Is the screw brass ? The way it broke makes me think so. If brass, can't a piece of thick copper wire or similar be soldered to it ?
I think a slim solder tip can heat up the screw from the inside. Maybe the heat alone will losen it a bit.
Or would the heat damage the knob ?

Martin

Station X · 10th Apr 2008, 12:11 pm

This can be done, but you have to be extremely careful not to get solder in to the threads. Brass is better than copper, as it can take more torque without twisting. Clean/flux everything and tin the end of the brass rod. Then place it in contact with the screw and heat it with a flame WELL AWAY FROM THE KNOB. Heat will be conducted along the rod and melt the solder. Allow to cool and then hopefully undo the screw.

Darren-UK · 10th Apr 2008, 2:12 pm

I would point to the words of Mike Phelan in post#3 concerning what material the knobs are made of. On any radio, ensure the knobs are indeed bakelite before attempting the solder trick. Find out using the hot pin trick. I knows there's radios around upon which the knobs appear to be bakelite but aren't.

Station X · 10th Apr 2008, 3:35 pm

If you or the grub screw are still stuck Matthew, then you are welcome to send the knob to me. I can mount it on an angle plate on the milling machine, mill off the remaining half of the head, drill out the screw and retap the thread in the hole. I'd be very careful, but there is always some risk of damaging the knob.

**********************

Edit. I see that Mike Phelan has made a similar offer.

matthewhouse · 10th Apr 2008, 7:17 pm

Hello everyone, apologies for the late reply I've been busy with work.

Anyway, in the end I decided to try to grind the head of the screw with a Dremel style tool, with a small cylindrical diamond grinding bit fitted, this quickly flattened the top of the grub screw. Then I fitted a standard 2mm HSS drill bit into the Dremel and this quickly drilled into the screw. I inserted a very small screw extractor into the hole but it wouldn't budge. I turned it a bit harder but the grub screw just split. So I drilled through from the other way, but the tip of the grub screw was very hard (much much harder than the top 3/4) The friction just heated the brass insert up and softened the inner ring of plastic behind the knob. So i pulled the brass insert out of the soft plastic. I intend to enlarge the new hole in the brass and thread + fit new grub screw. And then fill the old damaged hole with filler and inks etc. to match the surrounding. The tip of the old screw is still jammed into the brass tight.

Thanks for the help, I will let you know how how it goes.

matthewhouse · 10th Apr 2008, 7:22 pm

Quote:

Originally Posted by Dillen_de

Is the screw brass ? I think a slim solder tip can heat up the screw from the inside. Maybe the heat alone will losen it a bit.
Or would the heat damage the knob ?

Martin

Hello,
The screw was standard steel not brass, and as I found, the knob is not bakelite as it softened.

Oldtimer74 · 14th Apr 2008, 3:43 pm

I have two methods for getting out grub screws. I almost always find they are so far down the hole you stand very little chance of drilling/grinding etc. They are almost always hardened, steel.
1) If the knob is plastic, old ones are usually bakelite, and here is no brass or metal inner then you can dissolve the steel away with acid. I find nitric acid the best but you do have to be careful what you are doing. Don`t immerse the knob in the acid. just put a couple of drops in the hole and when no more gas is evolved wash out and repeat. If you can get the knob off the shaft then you can get a bit more acid in down the shaft hole. Keep the acid off the polished surface as there may be very slight attack.
2) This does make the hole a little larger than original.
Make or get someone with a lathe to make you a trepanning tool. This is a hollow tube just big enough to go over the screw (most are 4 BA but some may be 6 or even 2 BA) with a thin wall. Only a few thousands of an inch are required. Cut teeth on the one end of tube which you can then turn by hand or slow speed electric drill (bench drill with firm hold of the knob is best but if you cannot get at it to do that then you can do it by hand). First drill out to a small depth the hole with a clearance drill for the screw size to enable you to get the trepanning tool centrally aligned, if there is not enough of the original screw to act as the guide.
After you have got all the metal out then you can fill the hole with epoxy, coloured with some of the trepannings and then redrill and retap or simply tap a size larger.
Pat

radiolukey · 6th Jul 2008, 12:29 pm

http://www.btronics.co.uk/items/trx2743.jpg

matthewhouse · 6th Jul 2008, 3:07 pm

Thanks for the advise, (and the very useful tips Pat) and sorry for the long delay, in the end I could not remove the old remains of the grub screw, so I drilled a new hole in the bottom of the knob, threaded it and fitted a new hex head grub screw. I have filled the old hole and just have to decorate it to make it blend in a little more.

All the best,

26th Mar 2008, 10:26 pm	#21
matthewhouse Octode Join Date: Jul 2006 Location: Willand, Devon, UK. Posts: 1,023	Re: How to remove broken knob grub screw? Hello again. Trevor what an excellent idea! I never ever would have thought of that. I will certainly be giving this a go. Phil, I will send you a PM if I end up destroying the knob, thanks for digging that one out. Thanks for the other suggestions as well, I will report back shortly. Cheers Matty

26th Mar 2008, 10:32 pm	#22
Phil G4SPZ Dekatron Join Date: Apr 2005 Location: Bewdley, Worcestershire, UK. Posts: 4,748	Re: How to remove broken knob grub screw? A technique I have used in similar circumstances is to find an old screwdriver blade or a piece of metal rod slightly smaller in diameter than the grub screw, then file a profile into one end, aiming to be as close a fit as possible to the broken end of the grub screw. This is a trial-and-error process. Then apply a small amount of araldite to the filed tip of the rod, push the rod on the grub screw ensuring that the adhesive doesn't spread beyond the grub screw, hold in position and leave to cure fully. Finally, and very carefully, unscrew the rod using a Mole wrench or suitable tool, and unless it is siezed solid into the brass bush the grub screw should come out. __________________ Phil Optimist [n]: One who is not in possession of the full facts

26th Mar 2008, 11:16 pm	#23
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,518	Re: How to remove broken knob grub screw? Just a last suggestion - Drop the whole thing in a cup of coca-cola (the real stuff - cheap alternatives are not corrosive enough) Leave it for a day, this will loosen any corrosion, and hopefully the grubscrew will just come undone without effort - Might be easier to screw it in rather than out..... __________________ Engineers make things work and have spare bits when finished

28th Mar 2008, 10:44 pm	#25
chipp1968 Rest in Peace Join Date: Jul 2006 Location: Somerset, UK. Posts: 2,356	Re: How to remove broken knob grub screw? If you hav a dremel type tool you may be able to grind a central mark for the drill to start .

29th Mar 2008, 12:01 am	#26
Tom_I Heptode Join Date: Jul 2006 Location: North Herts. UK. Posts: 549	Re: How to remove broken knob grub screw? Yes, I'd try that. I've had a couple of similar problems. In both cases I've ground the head of the grub screw flat with a Dremel tool, then drilled into it with a very fine bit. In both cases the vibration has loosened the grub screw in its thread.

6th Apr 2008, 5:02 pm	#27
radiolukey Retired Dormant Member Join Date: Apr 2008 Location: Exeter, UK Posts: 2	Re: How to remove broken knob grub screw? Matty how did it go? I have a radio with a similar problem in the house.

10th Apr 2008, 12:03 pm	#28
Dillen_de Tetrode Join Date: Jun 2007 Location: Copenhagen, Denmark Posts: 62	Re: How to remove broken knob grub screw? Is the screw brass ? The way it broke makes me think so. If brass, can't a piece of thick copper wire or similar be soldered to it ? I think a slim solder tip can heat up the screw from the inside. Maybe the heat alone will losen it a bit. Or would the heat damage the knob ? Martin

10th Apr 2008, 12:11 pm	#29
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: How to remove broken knob grub screw? This can be done, but you have to be extremely careful not to get solder in to the threads. Brass is better than copper, as it can take more torque without twisting. Clean/flux everything and tin the end of the brass rod. Then place it in contact with the screw and heat it with a flame WELL AWAY FROM THE KNOB. Heat will be conducted along the rod and melt the solder. Allow to cool and then hopefully undo the screw. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

10th Apr 2008, 2:12 pm	#30
Darren-UK Retired Dormant Member Join Date: Apr 2003 Location: Blackpool, Lancashire, UK. Posts: 4,061	Re: How to remove broken knob grub screw? I would point to the words of Mike Phelan in post#3 concerning what material the knobs are made of. On any radio, ensure the knobs are indeed bakelite before attempting the solder trick. Find out using the hot pin trick. I knows there's radios around upon which the knobs appear to be bakelite but aren't.

10th Apr 2008, 3:35 pm	#31
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: How to remove broken knob grub screw? If you or the grub screw are still stuck Matthew, then you are welcome to send the knob to me. I can mount it on an angle plate on the milling machine, mill off the remaining half of the head, drill out the screw and retap the thread in the hole. I'd be very careful, but there is always some risk of damaging the knob. ********************** Edit. I see that Mike Phelan has made a similar offer. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

10th Apr 2008, 7:17 pm	#32
matthewhouse Octode Join Date: Jul 2006 Location: Willand, Devon, UK. Posts: 1,023	Re: How to remove broken knob grub screw? Hello everyone, apologies for the late reply I've been busy with work. Anyway, in the end I decided to try to grind the head of the screw with a Dremel style tool, with a small cylindrical diamond grinding bit fitted, this quickly flattened the top of the grub screw. Then I fitted a standard 2mm HSS drill bit into the Dremel and this quickly drilled into the screw. I inserted a very small screw extractor into the hole but it wouldn't budge. I turned it a bit harder but the grub screw just split. So I drilled through from the other way, but the tip of the grub screw was very hard (much much harder than the top 3/4) The friction just heated the brass insert up and softened the inner ring of plastic behind the knob. So i pulled the brass insert out of the soft plastic. I intend to enlarge the new hole in the brass and thread + fit new grub screw. And then fill the old damaged hole with filler and inks etc. to match the surrounding. The tip of the old screw is still jammed into the brass tight. Thanks for the help, I will let you know how how it goes.

14th Apr 2008, 3:43 pm	#34
Oldtimer74 Rest in Peace Join Date: Apr 2008 Location: Worcester, Worcestershire, UK. Posts: 330	Re: How to remove broken knob grub screw? I have two methods for getting out grub screws. I almost always find they are so far down the hole you stand very little chance of drilling/grinding etc. They are almost always hardened, steel. 1) If the knob is plastic, old ones are usually bakelite, and here is no brass or metal inner then you can dissolve the steel away with acid. I find nitric acid the best but you do have to be careful what you are doing. Don`t immerse the knob in the acid. just put a couple of drops in the hole and when no more gas is evolved wash out and repeat. If you can get the knob off the shaft then you can get a bit more acid in down the shaft hole. Keep the acid off the polished surface as there may be very slight attack. 2) This does make the hole a little larger than original. Make or get someone with a lathe to make you a trepanning tool. This is a hollow tube just big enough to go over the screw (most are 4 BA but some may be 6 or even 2 BA) with a thin wall. Only a few thousands of an inch are required. Cut teeth on the one end of tube which you can then turn by hand or slow speed electric drill (bench drill with firm hold of the knob is best but if you cannot get at it to do that then you can do it by hand). First drill out to a small depth the hole with a clearance drill for the screw size to enable you to get the trepanning tool centrally aligned, if there is not enough of the original screw to act as the guide. After you have got all the metal out then you can fill the hole with epoxy, coloured with some of the trepannings and then redrill and retap or simply tap a size larger. Pat

6th Jul 2008, 12:29 pm	#35
radiolukey Retired Dormant Member Join Date: Apr 2008 Location: Exeter, UK Posts: 2	Re: How to remove broken knob grub screw? http://www.btronics.co.uk/items/trx2743.jpg

6th Jul 2008, 3:07 pm	#36
matthewhouse Octode Join Date: Jul 2006 Location: Willand, Devon, UK. Posts: 1,023	Re: How to remove broken knob grub screw? Thanks for the advise, (and the very useful tips Pat) and sorry for the long delay, in the end I could not remove the old remains of the grub screw, so I drilled a new hole in the bottom of the knob, threaded it and fitted a new hex head grub screw. I have filled the old hole and just have to decorate it to make it blend in a little more. All the best,