Removing Stubborn Knobs

[cemelec]

Trying to bring an HP 3312A function generator back to spec, but, and this must be a common problem, I can't remove one of the knobs. There maybe more, this is the first one I've tried to remove.

See photo - the Vernier knob removed OK, but the 4 position switch knob is absolutely stuck.

I've got the right hex key, it fits the grub/set screw correctly, but I cannot turn it. I'm worried I might strip the internal hex. 

Any ideas?  Maybe gentle heating, some sort of lubricant or releasing stuff?

Thanks
Charles

[ebastler]

The grub screws on the switches are probably fastened quite a bit tighter, since they (presumably) meet a metal shaft, while the vernier axis seems to be plastic?

I would try a lubricant as you mentioned -- MoS₂ would be my personal preference. Let it seep in for an hour or so. I always feel slightly queasy when using it in the vicinity of plastics, but can't pinpoint any MoS₂-induced crumbling yet.

[Runco990]

I have found that heating the grub screw with a conical soldering iron tip works well.  (If you have enough room to poke a tip into the screw)  But be careful, don't cook it to the point of melting the plastic.

This has worked for me in almost all instances.  A friend of mine who works on a lot of old HP gear suggested it.
I'm guessing the expansion and contraction is just enough to loosen the "bite" between metals.

Another method suggested was to insert a proper fit hex bit and tap on it....  (percussive maintenance) to try to loosen the metal to metal bond, THEN turning it.  This has worked, but not as reliably.

Of course, on deeply recessed grub screws that's the only chance.  I have yet to have success with lubricants....

[AmnevaR]

I have found that heating the grub screw with a conical soldering iron tip works well. 

That is a nice tip, thank you! I'll add it to my bag of tricks 

Earlier I used the smallest nozzle from my hot air rework station for such work.

[highpower]

Rather than heating the grub screw and expanding it further I've had good luck using a penetrating oil delivered via a freezing spray. The cold spray contracts the screw instead of expanding it and gives the lube a better chance of getting down into the threads.
https://www.amazon.co.uk/MotoLodge-Loctite-Freeze-Release-Spray/dp/B00E2SR3M4/ref=sr_1_30?dchild=1&keywords=Freeze+and+Release+Penetrating+Oil&qid=1626745648&sr=8-30

[Runco990]

Rather than heating the grub screw and expanding it further I've had good luck using a penetrating oil delivered via a freezing spray. The cold spray contracts the screw instead of expanding it and gives the lube a better chance of getting down into the threads.
https://www.amazon.co.uk/MotoLodge-Loctite-Freeze-Release-Spray/dp/B00E2SR3M4/ref=sr_1_30?dchild=1&keywords=Freeze+and+Release+Penetrating+Oil&qid=1626745648&sr=8-30

Oooo.... I'll have to try THAT one!  HP knobs can be such fun!   
Although it isn't the threads that seize, it's the contact between the screw and the shaft.  Worth a shot!
And perhaps BOTH heating and cooling. 

I tried a shaped C-4 charge, but it left a mess.   

[bill_c]

Once you get past grub screw and still have a problem, these may help.
Automotive trim clip removal tool, can be found at just about any parts store.

[highpower]

I tried a shaped C-4 charge, but it left a mess.   

Probably didn't leave much of anything else though I'm guessing. 

I've dealt with a number of steel set screws that were hard to turn and squeaked and squawked ALL the way out of a collar even after they 'break loose' from a shaft. Especially in brass and aluminum (aluminium) since they tend to oxidize rapidly. I run a tap through every one of them to clean out the threads before re-installing a set screw. _{Your mileage may vary.}

[ebastler]

Although it isn't the threads that seize, it's the contact between the screw and the shaft. 

That seems counter-intuitive to me (which makes it even more interesting ). How do you know where the seizing occurs?

[Berni]

I had a similar problem on some other testgear knob.

The grub screw either was some weird imperial hex size or it was already rounded out by someone before me. Either way i could not get the thing to loosen. Eventualy figured out that i can forcefully jam a small thin point tapered triangle file into the head of the screw and then use pliers to rotate the file and the screw with it.

Tho you might want to be careful with this trick since if the screw is really really stuck in there its possible the file might break before the screw lets go.

So far i haven't seen these grub screw type knobs seize on a shaft between the knob and shaft. They always pulled right off, but the grub screws are sometimes ridiculously tight.

[David Hess]

I would take the next size up in hex key and grind down the flats to make an interference fit with the screw so that higher torque can be applied without stripping.  Alternatively I have read of people adding a drop of cyanoacrylate glue before inserting the hex key to do the same thing.

[Runco990]

Although it isn't the threads that seize, it's the contact between the screw and the shaft. 

That seems counter-intuitive to me (which makes it even more interesting ). How do you know where the seizing occurs?

A friend of mine who specializes in HP Agilent repair has told me this, I honestly have no reason to question him, as he has a solid background in metallurgy among other things.  He claims that the 2 metals partially fuse at the point of greatest pressure, which is the contact between the tip of the screw and the shaft.

What I DO know, it doesn't matter if you grease the threads, it'll be stuck tight again.  So I tend to trust his judgement.

However... no one is all knowing, so your mileage may vary.

Doesn't change the fact that the knobs are a bastard to get off. 

[richnormand]

More suggestions in this thread.
https://www.eevblog.com/forum/repair/remove-stripped-set-screw-on-front-panel-knob/
Good luck with it.

[highpower]

See photo -

Thanks
Charles

On that note, I would add one other suggestion that you grind off that rounded over tip on your hex wrench and get a good sharp flat end on it. Rounded ends like that and not getting it fully inserted into the head of the screw are what cause the majority of problems ~ stripped out screw heads.

[basinstreetdesign]

See this post.  These wrenches worked for me.   Suggested by Bushougoma.  Worked like a charm.

https://www.eevblog.com/forum/repair/small-allen-screw-too-tight-to-get-out/msg1357286/#msg1357286

[cemelec]

Thanks to all for the help. I've tried most of these suggestions:
Freezer spray
Heating with a soldering iron
Grinding my hex key to the correct geometry
Some slight mechanical shock to break any cold formed joint

Nothing works!
I'll try to fix the unit with the knobs in place.

One last possibility - if I can remove the plastic knob from it's inner brass sleeve, I can turn a new sleeve on my lathe - but then I need to remove the set/grub screws first, like catch 22

[factory]

The brass section is knurled, very unlikely the plastic outer could be removed without it breaking into pieces.

I've been trying a different method, I actually tighten the grub screw first before trying to unscrew it, I tend to use the long end of the Allen key and use pliers for more force if necessary, been having good success so far (YMMV), only failures have been on equipment that looks to have spent time at the bottom of a lake. 

The sizes commonly used by HP for the control knob grub screws are imperial 1/16" & 0.05" (50 thou), older equipment may also need 5/64". I have some good quality sets of Allen branded keys in the imperial sizes required for old HP, plus some loose smaller sized ones for those tiny ones.

David

[BrokenYugo]

If the key fits the hex well and the end isn't already all rounded off (grind down to good hex) don't be afraid to torque on it a bit. The springy feel of an Allen wrench gives an inflated sense of how hard you're turning it, and being springy doesn't allow you to put much shock load in to crack a tight screw loose.

[HighVoltage]

I had many of these problems before and always solved the problem with a torx bit.
Because the torx bits are conical they will always grip really good

If the set screws are really tight, do not use a torx screw driver, because you need to bring
axial force in to the bit. So, I use short torx bit and a 1/4" wrench for rotational turning and
some soft pliers to press the bit axial in to the screw.

Even if they are corroded, this will work.

Also, keep in mind that earlier HP gear has imperial sizes for the hex screws and out typical
metric tools can not work.

[cemelec]

Success!

Brilliant, I tried the method suggested by HighVoltage and managed to remove both grub screws.

I didn't use Torx, because the hex size is somewhere between a T4 (too small) and a T5 (too big)

I used a metric 1.3mm hex bit , clamping it gently but firmly against the knob with a small adjustable wrench.

I turned the bit using a 4BA spanner (a vintage British screw size) which is about 4mm, which also would have worked. When one of the screws came loose there was a slight cracking noise, suggesting the breaking of a cold weld. (not the plastic knob which is in perfect condition)

Thanks again to all

[HighVoltage]

Nice that it worked.

For using a size in between, I usually have a larger size and grind it down a little until it fits.

Here are some really nice bits I bought a few years ago.
And together with this little ratchet, it works really well.
 

[tooki]

If the key fits the hex well and the end isn't already all rounded off (grind down to good hex) don't be afraid to torque on it a bit. The springy feel of an Allen wrench gives an inflated sense of how hard you're turning it, and being springy doesn't allow you to put much shock load in to crack a tight screw loose.

Or buy a top quality hex key like PB Swiss that doesn’t wear out like most brands do.