[AMRadio] Stuck Set Screw


D. Chester k4kyv at charter.net
Fri Apr 27 14:15:05 EDT 2007


Here's my success story of a 75A-4 set screw.

I acquired the receiver in the early 80's, and never had any occasion to 
remove the front panel until one of the dial cables broke.  I disassembled 
the tuning knob and vernier mechanism without any problem, but once the 
panel was removed, the plastic disc kilocycle dial had to come off the shaft 
so that I could gain access to the pulley to attach the new dial cord.  Then 
I discovered that someone had rounded out the spline-head (Bristo) set 
screw.  I had the proper Bristo wrench, but the hole was so  rounded out 
that it wouldn't catch.  It would just rotate in the hole.  I suspect 
someone had attempted to use an allen wrench to turn it and broke off the 
splines inside the hole.

The receiver sat for months while I tried everything I could imagine with no 
success.  I was about to the point of drilling through the rivets and 
removing the plastic disc from the bushing to get better access so I could 
drill out the screw.  But one day, I probed the hole in the screw-head using 
some long drill bits, and discovered that a #49 drill would latch firmly 
into the hole.  But the problem was that this would tighten the screw, not 
loosen it.  I made some inquiries over the internet and found a mail order 
company that  sold left-hand (anti-clockwise) drill bits.  I ordered several 
#49 left-hand drills.  They were too short for the job, so I took a piece of 
1/4" metal shaft material and drilled a hole in the end of the shaft, a size 
just barely large enough to admit the shank end of the drill .  I filled the 
hole with JB Weld epoxy and shoved the drill bit shank inside, slowly 
forcing the epoxy to ooze out  the hole around the shank as I inserted it, 
until the shank bottomed out in the hold I had drilled.  Once the epoxy 
hardened, I tested the tool, and I believe the drill bit would have broken 
before the expoxy bond would have failed and allowed the drill shank to 
rotate inside the hole.

Using this tool, I was able to get the drill into the hole in the set-screw 
head.  It caught firmly, but the set screw would not budge.  Collins had 
used some kind of green  bonding agent similar to Loc-Tite to secure the 
screw.  I suspect that to be the reason the screw was rounded out in the 
first place.  I decided that I needed to heat the screw to soften the 
Loc-Tite, but it was located just behind the plastic dial disc, and how was 
I going to heat the screw without damaging the plastic dial?

I found a slim, miniature low-wattage soldering iron.  The barrel of the 
iron was no more than 5/16" in diameter, and the tip was threaded to screw 
into the end of the barrel. The threads were the same as a #8-32 machine 
screw.  I found about a 2 1/2" long 8-32 brass screw in my junk box.  I 
ground off the screw head, and ground the end down to a pointed tip just 
large enough that it could be inserted part way into the hole in the screw 
head.  That would heat up the set screw, but now the problem was to avoid 
damaging the plastic dial.  I made a heat shield using a piece of aluminium 
foil sandwiched next to a thin piece of insulating material - either wood or 
bakelite (don't remember).  That allowed the thin, hot screw and barrel of 
the soldering iron to remain for a considerable length of time in close 
proximity to the plastic disc without melting it.

I tried heating the screw with this arrangement.  I didn't damage the dial, 
but the screw still wouldn't turn.  I came to the conclusion that the 
problem was insufficient heat transferring between the end of the makeshift 
brass soldering iron tip and the steel screw.  Then it occurred to me that 
the perfect medium for improving the heat transfer would be heat-sink 
compound.  Alas, my tube of sink compound had completely dried up and was 
hard as a rock.  But I found in my junkbox the remains of an old solid state 
stereo amplifier.  I took the power transistors off and sure enough, there 
was enough soft heat sink compound left behind that I could scrape it up 
into a blob, then I applied the blob to the end of the soldering iron tip 
and inserted the tip into the hole.  I plugged in the soldering iron and let 
it cook for several minutes.  When I took it out I immediately inserted the 
tool that I had made from the #49 left-hand screw.  Applying just a little 
torque, the screw broke loose, and I was able it remove it, threads intact.

After I re-strung the dial cord, it was time to re-mount the kilocycle dial. 
That set screw has extra fine threads, and my junk box seemed to contain 
many small screws the proper diameter and length, but they were all coarse 
threaded.  But when I dumped the whole box of screws into a  sorting tray, I 
managed to fine ONE lone screw the proper length with the correct thread 
pitch.  It was a slotted screw instead of Allen or Bristo, but that didn't 
matter to me.  Using a jeweller's screwdriver, I tightened the new set screw 
just enough to make sure it was a snug fit, but I made a point not to over 
tighten it, and I didn't use any Loc-Tite.

That was several years ago, and I have used the receiver almost daily ever 
since.


Don k4kyv 




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