Remove botched screw from tuning dial knob

[langwadt]

Sheding some light in the hole, revealed what I was afraid of.

?? But it looks like just a plain slotted grub screw, intact. Or is one side of the slot broken off? It's hard to tell from the pic.
Is it possible your flat bladed screwdriver was just too fat to fit in the slot?
If so, sacrifice some cheap (but tough)  screwdriver by grinding it down till it just fits. Don't let the metal get hot while grinding. Veeery gently, lots of water for cooling.

Anyway, it also looks like the knob can be rested on a bit of soft wood on the opposite side to the screw, while you...
Get a small pin punch that fits in the hole. Put it against the screw top. With a small tack hammer, strike the punch firmly. The idea is to do two things: shock-break any oxides/corrosion/metal contact welds between the screw and the knob, and also drive the screw point into the shaft a little. Thus leaving it a bit loose.
Then try the screwdriver again.


I have a similar but worse problem atm. A toothed-belt drive cog, with a small hex head grub screw. Except the hex has been well rounded off.  The shaft has a mating hole, so I can't just forcibly drag the cog off with a puller.
Fortunately I don't have to preserve the cog, so as a last resort can just angle grind it down to near the shaft, then split it with a chisel.

if the screw is steel and the cog something else like aluminium you could try etching it out with alum

https://hackaday.com/2015/08/03/dissolve-steel-drill-bits-in-alum-from-the-grocery/

[frozenfrogz]

Slightly OT:

This thread is driving me nuts! The tension is killing me!

 

Will he succeed? Will he be beaten by a set screw?
Why does it take for forever for the OP to get it done?
Am I impatient? Is it much more complicated then usual?
How can we help this guy to just get it done already?

Oh the horrors of being damned to merely watch and wait for someone to just do it. xD

[Ian.M]

if the screw is steel and the cog something else like aluminium you could try etching it out with alum

https://hackaday.com/2015/08/03/dissolve-steel-drill-bits-in-alum-from-the-grocery/

Yes, but I'd hesitate to try that if the shaft is also steel.

Maybe in the O.P's situation I'd consider it as a last resort short of cutting the knob away.  I do have a few ounces of alum sitting on the shelf . . .   Also, if you do use alum,  pack behind the knob with well greased string to keep the alum out of its bearings, or you'll probably FUBAR it.

[frozenfrogz]

I have a similar but worse problem atm. A toothed-belt drive cog, with a small hex head grub screw. Except the hex has been well rounded off.  The shaft has a mating hole, so I can't just forcibly drag the cog off with a puller.
Fortunately I don't have to preserve the cog, so as a last resort can just angle grind it down to near the shaft, then split it with a chisel.

Try force-feeding it a slightly oversized TX or slotted head driver.
This has helped me out several times.

The driver/bit should be decent quality though!

[6PTsocket]

There are bits By Alden calked Grabit. One end drills just a dimple and the other end grabs the screw. With a hex or torx  drive setscrew you probabably wouldn't even have to drill. Use a hex bit extension to get the drill away from the front panel. The pilot dimple (I wouldn't even call it a hole) and the extraction are both done with the drill in reverse. They come in small sets. It was hard to tell in the picture but I hope the setscrew is not too deep in the knob.

Sent from my SM-G900V using Tapatalk

[michalism]

Hello!

thank you all for your comments and advice...this is a little bit overwelming with so many options to try :p

Slightly OT:

This thread is driving me nuts! The tension is killing me!

 

Will he succeed? Will he be beaten by a set screw?
Why does it take for forever for the OP to get it done?
Am I impatient? Is it much more complicated then usual?
How can we help this guy to just get it done already?

Oh the horrors of being damned to merely watch and wait for someone to just do it. xD

LOL!!!
Sorry for creating so much stress to you! Not in my intentions at all!

As said, I am working full time and these days I finish late at nights. I also have a 3y.o. son, that is taking a lot of my attention

I hope that I will have some results during the weekend for you!

Thanks again!

[frozenfrogz]

No worries!
I might be over-exaggerating a bit. 

[SilverSolder]

I guess we all feel your pain...   stuck screws,  and 3 year old kids, are not always quickly dealt with! 

[michalism]

OK then!!

Up for the great Finale! 

The screw is out!
Much easier than expected, although this thing is sitting on my shelf for more than two months now, and I had several previously failed attempts.
What I did now: I lightly heated the knob, and then use a light hammer to tap it. A couple of days ago I had sprayed the hole with penetrating oil. After it colled down, I used 4-5 different flat headed screw-drivers trying to "feel" if the screw was moving. Then as I was getting ready to go to the epoxy-drop solution, the screw come out as if it was never stuck! 

As you can see from the pictures, this is not a botched screw, it's just a piece of screw that a technician or ex-owner put in place to keep the knob on the shaft, because he had lost the original piece. That's my assumption.

Now that I see the extracted part, I consider myself extremely lucky that this turned out like it did...

I am also including pictures from the inside, as promised. I would be terrible to mess that up...

Anyway, thanks for all your help, and sharing my agony!!!

I promise to come back with more questions

[tggzzz]

The screw is out!

It is good to see:

• you've successfully extracted the culprit
• someone that listens and thinks

[Chris56000]

Hi!

Well d;ne!

You should ne able to locate a replacement no problem –an M1.5 x 8 or M2 x 8 grub–screw should do to replace it and you can get these in packs of 10 from fleabay!


Chris Williams

[Ian.M]

I wouldn't trust a fleabay grubscrew.  If it isn't properly hardened it wont bite on a steel shaft and if it even slightly over-hardened, it will be too brittle and if it jams again the head is likely to break.  You already know its a PITA to remove so unless you are planning to flip the item for cash as a private seller as soon as you've fixed it, cheapening  out on small parts is a false economy.

Incidentally, if you ever have to fabricate a grubscrew from a bolt, (Not recommended.  Least evil is if you shape it to a point to match a drilled dimple in the shaft) don't cut the slot too deep.  Otherwise the leverage from the screwdriveer will break it

[michalism]

I already have a small set of such screws, which I bought from a reputable local hardware store.
These are so cheap, it makes no sense to get them from China, IMHO.

[TerraHertz]

Mine is out too. Turned out the reason the grub screw hex socket rounded off, was that the grub screw was not hardened at all. And some form of thread-locking compound had been used.

As a last despairing attempt before destroying the pulley with an angle grinder, I finally tried drilling the screw. Expecting to get just a blunted drill. Surprisingly the drill cut in and went down the center of the screw easily. After going right through it cut off enough of the screw's end to free the pulley from the shaft.
The hollow remnant of grub screw was _still_ solidly glued in the pulley. Using a larger drill bit, it finally came free and screwed inwards, into the empty shaft hole. And was short enough to come completely free and drop out in the center.

Edit to add: speaking of grub screw sets, they are very cheap from Aliexpress. Pic: a couple I bought in 2017.
Store name: Son Day Day Up Store
200pcs Stainless Steel Hex Socket Set Screw Grub Screws Cup Point Assortment Kit M3-M8 With Plastic Box
$ 4.99 per box.

[MathWizard]

Few except service trade old-timers and antique radio restorers know that cord drives are notorious for re-stringing problems if you don't have the diagram from the service manual or an exact record of which way round, how many turns and whether it was wound top to bottom or bottom to top on each drum or spindle it went round.  e.g. if you get the top to bottom or visa-versa winding direction wrong on one spindle or drum even though the clockwise/anticlockwise winding direction is correct and all the rest of the cord route is correct, the cord will typically pile up on itself and jam. 

Any broken cord's exact placement and route should be documented before removal to aid correct re-stringing.  Back in the days of valve radios, if the owner had removed or disturbed the remains of a simple broken cord, it would typically add hours to the repair if you didn't have the official diagram from the service manual + a lot of re-stringing experience.  However at least those valve radios had a metal chassis and robust mechanicals in the drive system, so permanent damage not resulting from violent frustration was unlikely.

Newer radios with cord drives and too much ageing plastic are an even worse problem.  When the little lug the end of the tensioner spring goes over snaps off a plastic drum, and manufacturer spares support for the model is long gone, or the post on the plastic frame supporting a pulley at one end of the tuning scale cracks, days of heroic bodgery and hand-crafted parts are your only hope.

So how did they calculate it, or what principles should let someone figure out the best way, for a given system ?

I have a rare radio that I took apart, and by accident took apart the dial cord assembly. It has a main wheel, w/ a string, a tuning shaft, and 2 bobbins with a larger upper, and smaller lower section. IDK if it even used both halves.

So what book would describe how to design such a pulley system ? I could come up with some stuff, but measuring or including string friction and stretching, which is essential, well forget it.

I'm sure it's really some partial differential equations, and some Lagrangian/Hamiltonian system would be what describes it best.