Repairing threads can take several paths:
The Edison 100 thread per inch feedscrew found on the Class M, Spring Motor,
Triumph, and Home is an odd piece of tooled steel. When I first encountered
one that needed repair I surmised that it must be a hardened steel. I was very
wrong. Putting it on a Rockwell hardness tester showed that it was very soft
by most standards of todays steel and was very much like ledloy 12L14, a very
soft steel that is made that way to be easily machined. My first attempt for a
thread chaser was a simple one. I took a nice Edison halfnut and ground down
one side at an angle to make a thread chaser. It worked fairly well but had to
be firmly mounted on a halfnut arm of a reproducer carriage with an adjustment
for the downward pressure. Occasionally the tool would do more damage than
good and I would have to scrap the feedscrew or cut a new thread after
machining off the old one. I made an improved version with a twin halfnut
Triumph arm where one halfnut was original and the second one
was the chasing tool. It worked better but it still was not perfect. The
thread depth on a 5/8ths diameter 100 thread per inch feedscrew are around
0.0064" if memory serves. In soft metal it does not take much to damage the
threads beyond functional use.
I haven't done so lately; but, if the mandrel and shaft is a rare one that is
irreparably damaged, making a new shaft is an option for someone with machine
tool skills. My last effort for making a new feedscrew on the mandrel shaft
was to make a die and die holder that would cut a very nice thread on a well
prepared shaft of ledloy. That took me a week and several 'rethinks' about
what was not working correctly.
I have purchased a dinged up mandrel with a shaft that had a good feedscrew
thread. In that case you can remove the damaged mandrel and use the good
feedscrew with the old, hopefully good, mandrel. I am no big fan of certain
sellers on the Internet who take good machines and part them out but their
donor machines have helped on a couple of my restorations.
If your feedscrew is one of the very early ones with a buttress thread, you
have almost no choice but to have a new one made. It took me two days just to
make the cutting tool to do a buttress thread. The thread itself it could only
be cut in one pass at a precise depth of 0.007". The tool lost its razor sharp
edge quickly. Each use required sharpening on a surface grinder with a newly
honed fine wheel. After tackling such fine threads I marvel at the ability of
the Edison Phonograph Works to turn out these feedscrews in such huge numbers.
Having a grandfather who was a tool and die maker along with help from many old
timers in the missiles and space industry certainly helped me do the fine
machining. Sadly, most of those talented machinists have left this mortal coil
and the new generation I encounter today have trouble identifying which end of
a screwdriver to hold. The average age of a CNC machinist today is 54! One of
my mentors, Arthur Lutz, is still working at
90...
Merry Christmas, Happy Hanukkah, and Seasons Greetings,
Al
Phonographic Curmudgeon
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