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Remove that knob

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6.3volts
(@6-3volts)
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This is probably on page one of the beginner's manual but I just worked out how to loosen rusted grubscrews from bakelite knobs.

I was trying to remove a very rare knob recently and even with a two day soak in oil the slot in the screw was threatening to twist off in the hole.

In my other life as an engineer I often use heat to break the hold of rust so I left a hot soldering iron in gravity contact with the screw for about 45 mins. When it had cooled down I was amazed just how easily it unscrewed and left an undamaged thread in the knob. Hope this is helpful

Jonathan

 
Posted : 09/09/2017 10:17 am
PYE625
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Another good tip there, I guess the combination of WD40 and use of heat to expand and then allow to cool to contract the stuck items is usually useful in freeing up. Nothing worse than having to drill out a grub screw.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 09/09/2017 1:11 pm
occiput
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It is a useful method, but be aware that it can sometimes result in the moulding in the knob shattering as the grub screw expands.  I regard it as an "if all else fails..." method and, to be fair, the OP does seem to have tried the most usual "kind" way.

I have also had some success with gently warming the knob rather than the grub screw, using a hot air gun.  The key word here is "gently", and you need to be sure that the knob really is bakelite.  It's a good idea to make a shield to fit behind the knob, so that the heat doesn't get at the cabinet.  Whatever you do, don't try this with a thermoplastic knob, or it will all end in tears.

709379

 
Posted : 09/09/2017 2:04 pm
sideband
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For those who have never tried this, it's a good tip so thanks for posting it here.

I had a similar problem on a Murphy radio a few months back when one knob (there's always one....) positively refused to budge and likewise I was afraid the grub-screw slot would break in the bakelite knob. WD40, oil, switch cleaner....nothing worked. Kill or cure....I decided to try heat so the soldering iron was brought into use, the tip of which was inserted into the appropriate screw hole and left to heat for around 10 minutes. It was surprisingly easy to remove the screw after that.

 
Posted : 09/09/2017 2:13 pm
occiput
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To add to my last: in over 40 years of trying to dismantle rusty things of all descriptions, if you are using a penetrating oil, my experience is that Plus-Gas Formula is head-and-shoulders above the competition.  It's not the cheapest by a long shot, but as with a lot of things, you get what you pay for.  If you can find it in a tin with a spout, rather than an aerosol, it is a good deal easier to control both the quantity applied and where it is applied to.  The usual disclaimer applies, of course.

I'm not a huge fan of WD-40 for anything other than its original purpose as a moisture repellent.  I find is "fairly indifferent" as a penetrating oil.

If you are reduced to drilling a grub screw out, and I agree with the general view that this is very much the last-line method, you may find that a set of left-handed twist drills is a useful investment.  In an ideal world, you'll want to get the workpiece under a drill press, and it's a shame that not many of these sold for the hobbyist market are capable of running in reverse.  In practice, you may be left with no alternative than to use a hand drill, and now that these are sold to double up as power screwdrivers, most of them, especially cordless, will run backwards. If you apply just enough pressure to get the drill flutes to bite, you will, unless you are very unlucky indeed, find that, at some point in the process, the flutes will "pick up" on the grub screw and wind it out of the hole for you.  At worse, you will drill all the way through the screw, but you're resigned to doing that anyway.

Incidentally, at the risk of teaching Granny and all that, if you are drilling a grub screw out, the correct approach is to start with a drill which is just smaller than the root diameter of the thread.  With careful work it is often possible to either cause the grub screw to break up or to weaken it sufficiently that will lose its grip on the thread walls; however the thread in the knob will often be sufficiently undamaged by this approach that it will take a new screw without the need to sleeve and re-tap.

The way to stop grub screws sticking in the first place is, of course, to put a tiny smear of Vaseline on the threads before running them in, but everyone knows this, don't they?

709379

 
Posted : 09/09/2017 2:39 pm
Cathovisor
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Oddly enough, I just took delivery of a 400ml aerosol of PlusGas Formula A which, like Occiput, experience has taught me to be far superior to similar products that claim to free rusted parts. I note that, apart from the tin no longer being blue, is that it also comes with a 'straw' - one of those sadly, all-too-easily-lost plastic tubes.

 
Posted : 09/09/2017 3:47 pm
Katie Bush
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Not very practical with a knob that's stuck on a chassis, but one way I found to loosen stubborn grubscrews in knobs is the liberal application of of hot water! Even boiling water straight from the kettle, then just repeatedly and gently working the screw back and forth. I find often found that doing this with the offending screw immersed in the water has spectacular results.

As I say, not a lot of good for a knob still attached to a radio, TV or record player etc. but very effective on knobs out of the spares box.

 
Posted : 09/09/2017 10:37 pm
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