How to remove broken knob grub screw?

[matthewhouse]

Hello, I am nearing the end of the restoration of my Coomber schools radio, but I am stuck as what to do with one of the knobs where the grub screw has broken leaving just one side of the head. To make matters worse i think it has been cross threaded too. The previous owner removed the chassis for photographing and broke the head off on reassembly, I was able to wiggle the knob off the shaft but now need to refit it. The grub screw screws into a threaded brass hole.

Drilling it out looks rather difficult on the sloping surface, and I have tried undoing it by gripping what remains with a pair of tiny pointy pliers but got nowhere. Does anybody have any better ideas, a quick search here just gives ideas on removing jammed but intact screws.

Cheers,
Matty

[OErjan_S]

drilling a small hole, say 2 sizes down, partly trough it, and using a tap with right size for that hole, with lefthand thread to back it out usually works, if it is not stuck hard enough to break the tap.
same thing with a normal tap IF there is room for the screw to fall out on the other side.

a third way would be to drill the entire thing out as you said.

a more involved and even harder way is to grind a new slot in the head with a thin diamond burr (0.15mm or so), but that takes steady hands and patience.

even just drilling a hole straight trough the screw may loosen it some and allow it to be removed.

oh, almost forgot, i have had to grind the stub away on the inside and drill/tap a new hole on two knobs with badly stuck screws, easier than getting them out.

[Mike Phelan]

Matty
A couple of questions:

• Is the screw going into a metal insert or just into the knob?
• What is the knob made from - bakelite or other plastic?
• Any chance of a close-up of the knob showing the screw?

I can think of a few ideas depending on the answers.

[XTC]

Drilling into screws successfully is quite a difficult business I've found.

It might be an idea to practice whatever technique you decide on, on a spare knob or two first.

Pete.

[tvden]

Originally Posted by XTC

Drilling into screws successfully is quite a difficult business I've found.

It might be an idea to practice whatever technique you decide on, on a spare knob or two first.

Pete.

Drilling into grub screws freehand tends to make the drill go off center, its best if you have a small Pedestal Drill quite cheap to buy, as its more easy to keep the drill centered and also stops it breaking, where possible try and centre punch the screwhead

[Mike Phelan]

Originally Posted by XTC

Drilling into screws successfully is quite a difficult business I've found.
Pete.

Pete/Matty
One reason for that is that they are often hardened, and if the knob doesn't have a metal insert for the screw, it will end up in tears.

Another reason is that small drills are quite flexible; just drilling into the screw freehand is very difficult, and you'll probably destroy the knob.

As Den says, you really need a pillar drill, but also you need a centre drill; a small drill will just wander all over the place, even if you centre-pop the screw (difficult if it only has half a head!).
Centre (Slocombe) drills are available in any machine tool shop or model engineers' supplier.

If you are totally stuck, Matty, PM me and pop the knob in the post.

HTH

[OErjan_S]

hmm, the screws are normally fairly easy to drill with good sharp high speed drill.
I admit i forgot the centerpunching and grinding the area to be drilled smoot though, too much standard practice for me (instrumentmaker).

[matthewhouse]

Thanks everyone for the quick replies:

Originally Posted by OErjan_S

drilling a small hole, say 2 sizes down, partly trough it, and using a tap with right size for that hole, with left hand thread to back it out usually works, if it is not stuck hard enough to break the tap.
same thing with a normal tap IF there is room for the screw to fall out on the other side.

I did look at left hand thread taps and have used screw extractors on larger things, I think the hardest bit is drilling a hole without destroying the knob further, it has been repaired at some point in the past.

Originally Posted by Mike Phelan

Matty
A couple of questions:

• Is the screw going into a metal insert or just into the knob?
• What is the knob made from - bakelite or other plastic?
• Any chance of a close-up of the knob showing the screw?

I can think of a few ideas depending on the answers.

1. See pic
2. I'm fairly sure it's bakelite, it doesn't look like modern plastic.
3. See other pic (sorry it's not very clear)

I doubt a replacement knob would be available, it’s quite an odd radio, even Coomber don’t have any records of this model. It's a shame this knob is in such a bad way though it looks a lot better in real life.

[valveaudio]

Matty

Having seen the photo I would suggest drilling through the knob from the other side with a small drill this would centre into the recess in the grub screw from the inside, may even turn it out, once you have drilled through the screw it would give you a pilot hole to drill it out from the screwdriver side.
Save the material removed when drilling the initial hole, mix it with adhesive and fill the hole when you are finished.
It worked for me.

Trevor

[Silicon]

Do you know any friendly dentists?
Your average power drill has too much power for this problem. It could destroy the knob if it 'catches'

Dremel type drills have small 'grit' covered ball style drill bits which might be able to grind away the top of the damaged screw. If you used reverse rotation and a slow speed it might even grip the screw and unscrew it. You would probably need a rigid drill stand to attempt this method.

I have never done this. Its just optimistic theory.

Good luck.

[Mike Phelan]

I would not attempt drilling it from the outside. Trevor's idea is the best one - go for that!

[Skywave]

Would spark erosion solve the problem?

Al / Skywave.

[Radio_Dave]

Hi Matty,

The knob is off the shaft (so there's no pressure on the threads) and the screw doesn't seem, that, rusty so I'm surprised that it can't be removed with less dramatic methods

Have you tried a good soak in penetrating oil and/or heating up the screw with a soldering iron?

[Mike Phelan]

Originally Posted by Skywave

Would spark erosion solve the problem?

Al / Skywave.

Would possibly burn the knob, Al

[OErjan_S]

EDM is nearly cold process o surounding material, a high voltage discharge between material to be removed and a graphite electrode, trough a electrolyte (bakingsoda and water is comon homebrew), sadly it removes little material every discharge, a thin wire can cut a die "fast" but to do so requires computer controlled feeding of the wire and material to be cut, not to mention PSU capable of the voltage/amps...
method is frequently used to make dies for industry from slabs of steel already heatreated, and that is hardened steel that would become soft if exposed to heat above 250-350*C, the HAZ (heat afected zone) is few hundreds of a milimeter for the old equipment i was trained on (made in early 70's, so much rougher than modern stuff).

perhaps i misunderstood what he meant by "spark erosion".

[Nickthedentist]

Originally Posted by Silicon

Do you know any friendly dentists?
Your average power drill has too much power for this problem. It could destroy the knob if it 'catches'

We often have to dislodge threaded "pins" from teeth, which are essentially little headless screws about 0.5mm (or less) in diameter. The best thing for these is vibration from an ultrasonic scaler, often followed by running the periphery of a bur in the high-speed handpiece (~300,000 rpm with very little torque) against it to help unspin it.

I'm not optimistic about how well it would work for this though, looks rather tough.

Nick.

[Skywave]

Originally Posted by OErjan_S

EDM is nearly cold process . . . . etc.
Perhaps I misunderstood what he meant by "spark erosion".

I'll try again - and be a little more explicit.


Many years ago at a certain place of employment, I occasionally encountered the problem of taps (the types for making small machine threads - e.g. 6 BA) snapping off in the work-piece and close to the surface of the piece, too. The mechanical workshop in this organisation was always able to extract the broken tap by a process that I was subsequently told was called "spark erosion". The work-piece was always saved - unlike the broken tap .

I never did find out just what the electro-mechanical processes were in accomplishing this task; I was simply grateful for the end result. I have always assumed a high-voltage, medium-current source and some sort of electrode that contacts (nearly) the broken tap, and that akin to a spark-plug in a car, the metal gets eroded away by the sparking.

That's it: I simply do not know how realistic such a process would be w.r.t. this grub screw problem. It was simply an idea 'off-the-cuff'. I was hoping someone might come back with a response such as: "that's a good idea", etc. or: "Daft! It won't work because . . ."

Al / Skywave.

[Mike Phelan]

Originally Posted by Silicon

Dremel type drills have small 'grit' covered ball style drill bits which might be able to grind away the top of the damaged screw. If you used reverse rotation and a slow speed it might even grip the screw and unscrew it.
Good luck.

Hmmm - drill bits like these only work in one direction ....

I think we are trying to crack this particular nut with some expensive sledgehammers - back to Trevor's post!

[SWB 18]

Hi Matty,

I have sorted out a knob that looks similar to yours. It is 1.5" in diameter and approx 0.6" tall. Unfortunately it is marked 'TONE' but you may be able to polish that out. I have attached some photos.
If you would like it, just let me know.

Regards,

Phil.

[XTC]

How about attacking the brass insert? The brass is quite soft and that grub screw looks hard.

Drill two or three lines of holes through the brass and collapse the insert. Yes, it needs a pillar drill - I do have one I use for PCBs - I also have a healthy respect for drilling operations that go wrong.

With the brass insert out, the grub screw comes out too.

Then it becomes a different problem which is finding another knob, this time expendable, and removing its insert, possibly having a friendly model maker or machinist cut it to the right size, or having someone make up a new insert from scratch.

I can see a lot to go wrong with this scheme, and as I said before, it might be an idea to try it out on one that doesn't matter first.


Pete.