[ARC5] Stuck set screw

[Michael Hanz]

These set screws are rarely easy to remove after so many years of being 
untouched.  The other problem is the short distance from the screw to 
the control panel, which limits what you can get in there to help.

The single most effective way I have found to break it loose is to 
emulate an impact wrench.  Sometimes you can use the L shaped Bristol 
wrench for this, hitting it with a small hammer while holding it with 
needle nose pliers applying CCW torque.  Protect the panel from wild 
rebounds with some sort of cover - I learned that the hard way.

If that doesn't work, by removing the plastic handle of a DS series 
Bristol screwdriver, you can adapt it to an extension bar and use it 
with one of the _small_ impact driver tools. (Yeah, I know, it pains me 
to modify a good tool like that, but if you get desperate enough...)   
You also have to cut the spline down in length to a very short section 
to reduce the torsional twist of the spline during the jolt (a long 
spline absorbs the torque like a torsion bar), but 9 times out of ten it 
will get the screw out with one light blow.  Use a _small_ hammer, not a 
5 pound sledge.  I think sometimes they used a drop of lacquer on the 
screw as a locking mechanism back in the day.




If all else fails, the screws _can_ be drilled with an appropriately 
sized _carbide_ drill.  The screws are usually hardened, but a carbide 
bit will do it without working up a sweat.  If you use two drills, one 
the outside diameter of the Bristol wrench followed by the second being 
the minor diameter of the set screw thread, it will usually stay 
centered.  Use medium speed, light pressure.  You still may have to make 
some sort of adapter to connect the drill to the rotating mechanism - 
unless the knob is close to the edge of the panel, conventional chucks 
are usually too big and require tilting of the drill to the point where 
it's not very helpful in getting to the contact point on the shaft.

If it's essential to get it out with minimal damage, there are firms 
that specialize removing these and broken taps with EDM (electrical 
discharge machines).  It will depend on how much you want to spend.

           73,
  - Mike  KC4TOS

On 11/17/2019 2:08 PM, Waldo Magnuson via ARC5 wrote:
> The set screw in the audio volume switch on a C-1254A control unit IS a Bristol set screw. The wrench fits tightly, I’ve used Blaster penetrating Oil but even with excessive torque the screw has not come loose. Any suggestions- heat/cold/drill?
> Skip W7WGM

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