A new success method with jammed grubscrews?

[Billy T]

I had a bakelite instrument knob with a frozen grub screw, and over several months of unsuccessful efforts (following all the tips here except drilling out because that required equipment I do not have) I finally decided to try a phosphoric acid based rust converter/remover.

In this instance the knob was not on the equipment so I put it in a plastic container with sufficient slightly diluted rust remover and left it for a month or so. I left it a fair while because it was saturated with WD40 and other lubricants, so I figured it would take a few weeks to get going on the rust. However, when I took it out today, to my great surprise the grub screw had disappeared, dissolved away completely!

Had I returned to it after just a week or so, it seems very likely that the rust would have vanished and the grub screw come free.

This may be a worthwhile 'last-ditch' trick for really stubborn screws, or screws with a split head, even those still on the shafts. A blu-tack dam around the hole would contain any overfill, but you would have to be careful how you managed any subsequent leakage once the rust jamming the knob had gone.

Still, desperate situations sometimes require desperate measures!

Cheers

Billy

[Station X]

Ferric Chloride (PCB Etching Fluid) and dilute Sulphuric Acid also dissolve steel readily. I'd be inclined to check progress fairly often though because they'll dissolve the insert in the knob and the shaft too if left long enough.

[paulsherwin]

Interesting idea and certainly worth trying as a last resort. My concern would be that the acid might attack the knob material given such prolonged exposure, but other last ditch methods risk damaging the knob too and there's nothing to lose.

Good tip.

Paul

[Boom]

Interesting Billy. Did the knob have a brass insert?

[Billy T]

Originally Posted by David Tilley

Interesting Billy. Did the knob have a brass insert?

No it didn't, but it was a very long grub screw so it was extraordinary that it should vanish completely, and ordinarily such a dramatic result would not be necessary!

I recently purchased a few kilos of old knobs as a cheap source for grub screws so I plan to do some experiments to evaluate the risks. I'm cleaning and reselling the good knobs, but raiding the grub screws from the broken and battered ones. Getting to the point, there is one rare red-bakelite knob in the lot that I really do want to recover, but it too has a solidly frozen grub screw, so I'm going the repeat the experiment but with a Blu-Tack dam and some more Blu-Tack to hold the knob upright. If possible I'll insert an old pot-shaft as well to see if it comes to any harm from leakage etc.

At the same time as all that I'll put a brass insert with a broken grub screw into a container with the same solution and monitor progress. I don't think the solution will harm non-ferrous metals but there's only one way to find out. It is not necessary to dissolve the grubscrew completely, just taking the rust products off and perhaps etching the surface away by a 'thou' or so should be enough.

Watch this space!

Cheers

Billy

[Station X]

It is not necessary to dissolve the grubscrew completely, just taking the rust products off and perhaps etching the surface away by a 'thou' or so should be enough.

That's correct. If I manage to break off a threading tap in brass or gunmetal I use sulphuric acid on it. After an hour or two it's generally possible to shake out the remains of the tap.

I recently purchased a few kilos of old knobs as a cheap source for grub screws

I'm all for reusing stuff, but I prefer to buy new grubscrews which are available in all the common thread forms. The type with an internal hexagon are best. So long as you use the right key it's impossible to damage the hexagon. They don't rely on axial force to keep a screwdriver in place either. As far as I'm concerned the slotted grubscrew is a pretty useless product. There are better solutions now.

[dseymo1]

I've heard of people using Coca-Cola to free rusted fasteners, but have never tried it myself. Makes sense, though, as it contains a proportion of phosphoric acid, and is well-known for its cleaning ability on coins, etc. Very cheap, too, in the sort of quantities required. Pity it doesn't taste better (to me).

[Billy T]

Originally Posted by Station X

I'm all for reusing stuff, but I prefer to buy new grubscrews which are available in all the common thread forms. The type with an internal hexagon are best. So long as you use the right key it's impossible to damage the hexagon. They don't rely on axial force to keep a screwdriver in place either. As far as I'm concerned the slotted grubscrew is a pretty useless product. There are better solutions now.

I'd much prefer to use new hex-socket grubscrews as you describe, but haven't found a source here that doesn't want to sell me a either a smorgasbord of sizes rather than just the ones that I want, or a lifetime supply. Buying overseas is a costly exercise as well, so recycling old screws from knobs that are past redemption is the easiest option. A dab of suitable lubricant such as vaseline will stop them ever freezing again.

Cheers

Billy