On Wed, 20 Jun 2018 18:09:22 -0700, (Dave
Platt) wrote:

In article ,
Jeff Liebermann wrote:

There are a few potential mechanical problems. The big one is that
the bevel gears in the frequency range and modulation rotary switch
mess are cracked. Someone makes replacement gears using 3D printing,
but I haven't seen them. I would prefer brass.

There's a guy in India who has been making sets in brass... I'm in the
middle of installing a set now. He posts on the HP test-equipment
mailing list, and has some of the brass gears up on eBay.

https://www.ebay.com/itm/Brass-Gears-for-the-HP-8640B-Signal-Generator/153024403695

You should have taken my advice and glued the cracked gear back
together with epoxy. The trick is to temporarily put the gear back
together with a hose clamp or fixture. Then take a big rat tail file
and enlarge the hold in the plastic until the brass lock ring fits
easily in the hole. Or, you can use a proper reamer and a mill.
Remove the set screw and plug the hole with wax, or a longer screw
smeared with a little mold release (grease or Vaseline).

Leaving the gear halves clamped together, smear some 24 hr epoxy on
the knurled part of the brass lock ring, and let it dry overnight.
Remove the temporary clamp holding the gear together, remove the hole
plug, clean the threads, re-insert the set screw, and put it back into
the gear assembly.

I've done about 10 gears like this without experiencing any problems.
Well, not quite. The last one I did, I either got the mixture wrong
on the epoxy, or used some ancient tube of epoxy which refused to
harden. I had to scrape it off and glue it again, which worked.

Incidentally, while you have the gearbox apart, bend the other
non-broken gears a little to see if they are in any danger of breaking
in half. Most will break at the set screw with very little pressure.
Might as well have it break now, rather than while you're using the
generator.

If you still have the old gears, I'll be happy to glue them together
for you for the price of shipping and small donation to support my
decadent and lavish lifestyle.

The other issue I've run into, with that switch assembly, is that the
two white plastic parts (the shaft which holds the two small planetary
bevel gears, and the rectangular piece which clamps onto this shaft
and actuates the rear switch decks) are a bit dodgy. The tighten-it-
down clamp head on the rectangular piece was cracked, and even after
repair it isn't gripping the shaft well enough to prevent slipping.

I haven't seen that yet. I'll take a look at the two I have when the
bench is clear. That might explain some backlash in the shaft.

And, the new brass gear which _should_ rotate freely on this shaft, is
rubbing and squeaking... either its hole is a bit under-sized, or the
plastic shaft isn't dimensionally stable after all of these years.
So, I have a bit of adjustment to do.

I'm not thrilled with the machining quality shown in the photos. It's
adequate, but not great. If anything, the plastic shaft has SHRUNK
making a loose fit more likely than a tight fit. I can measure mine
if you need a comparison. I would bet the hole has burrs, shavings,
or is undersized.

I'd love to replace both of these plastic bits with something more
stable (maybe a metal shaft with metal axles for the planetary bevel
gears, and a composite or Delrin assembly in back) and might try
fabricating something if I can't get the existing plastic parts back
into service.

Can you generate the G code needed to make the part? I'm helping
setup a CNC mill that can possibly make it.
http://802.11junk.com/jeffl/pics/CNC-conversion/
The catch is that we don't have a rotary indexer or gear cutter, so
all the teeth will need to be shaped with a small end mill. That's
slow and not fun. It would almost be easier to make a silicone rubber
mold and cast the gears from resin.



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558