Casting a Replacement Gear
for the
HP 8640B RF Signal Generator
This process resulted in excellent quality replacement gears for my 8640B. Understand,
however, that this is the first silicone molding I've ever done. Many decisions were made
to increase precision and reduce the risk of failure, at the expense of more work in the
form of more finishing operations.
The first thing one should do is a brief survey of the information on the web. Google the
terms silicone, RTV, RTV1000, "TASK 9", molding, casting and urethane in various
combinations. Learn about the different ways of making molds, as you will almost
certainly modify this procedure, depending on your resources.
Here is my materials and equipment list. Don't give up if you don’t have everything:
1 pint RTV1000 (from Eager Plastics)
2 pints TASK 9™ (from Eager Plastics)
Scale (0.1g resolution or better)
Solo™ 3 oz. plastic cups
Plastic teaspoons
Stirring sticks (stiff, round, not drink mixers, maybe some 1/8" rod)
Delrin rod, turned to 0.925" diameter (many other materials acceptable)
Elmer's Glue Stick
Vacuum pump and small chamber with window (4" PVC pipe & ½" Lexan suitable)
Oven (a small metal box with a light bulb would suffice)
Thermometer
Small lathe
Mill/drill, mill, or X-Y table for drill press
I chose to use RTV1000 from Eager Plastics as my mold compound. It's a 2-part RTV
with a 10:1 mix ratio, so you'll need a small scale to weigh it. A double pan balance is
handy because you can place a small plastic Solo on each side and weigh directly. This
material is, frankly, amazing in its ability to capture details of a surface, stretch without
tearing, and release the part without sticking. Because I've chosen to do the mold with an
overhang, this material contributes greatly to success.
For the casting compound I used Smooth-On TASK 9, also purchased through Eager
Plastics. This is a high tensile strength urethane with a 1:1 mix ratio (by volume, not
weight). You should understand some things about these urethane casting compounds.
They are not good for high temperature, and TASK 9 will soften at about 131F/55C.
That's not very high, but seems ok for the 8640B. I wouldn't use it in tube equipment or
next to heatsinks. I was very skeptical of this material when it came out of the mold, as it
seemed neither rigid or strong. Urethanes seem to crosslink for a very long time. Even
though the "demold" time is listed as an hour, the part would be subject to warpage. Do
not demold that quickly. Instead, cure for the specified 1 hour at room temperature, then
transfer the mold to an oven. Bake it at 150F for 4-6 hours. The part must be in the mold,
or it could sag or warp. This will improve almost all the mechanical properties
tremendously. Note that the data sheet properties are measured after a full week of
curing, and the material is quite impressive at that point.
The cast part can only be as good as the part you use to make the mold. For a gear to
work correctly, there must be as little variation in the teeth as possible. When the 8640B
gears age, they shrink. When they shrink around the brass bushing, the hoop strength is
exceeded and the gear cracks, popping open in one or more places. You must eliminate
this crack as completely as possible to use the part as a master. In my case, the rear-most
gear (without the set screws) was the best candidate, as it had a slight crack, but not full
separation. Pressing the bushing out would surely break it. I gently chucked up the gear
in the lathe and bored out most of the bushing. When the bushing was starting to break
through, I simply rolled the foil thin metal out with needle nosed pliers. This allowed the
gear to relax and the crack to shrink to near zero. My other gear (with the set screws) was
split. It was an easy matter to put a small rod in the drill press and push out the bushing
towards the flat side of the gear. The gear was then placed over the needle nosed pliers
and the crack pushed open to about .010". A drop of cyanoacrylate glue was injected, and
the gear immediately clamped closed for an hour. This gave me two very good gear
masters, but I decided to use the first one so as not to complicate the mold with the set
screw holes. If you have to use a gear with set screws, plug the holes with molding clay.
Sand the back of the gear smooth and flat on 220 grit carbide paper.
There are many ways to make molds which Google will reveal. I wanted an easy to fill
simple cavity mold. This would allow degassing of the urethane, and would insure
perfect alignment, since there was only one section. On the downside, the back of the
gear has to be finished on the lathe.
I cut a piece of Delrin to just under the root diameter of the outside spur gear (0.925"),
and about 1.25" long. In hindsight I should have made it 1.75" long. It was cross drilled
at one end so it could be supported, and drilled through the center with a 1/16" drill for
venting. This was glued to the back of the gear with "stick glue" from the office supply
store. I like this for temporary bonds, as it dissolves in water. The Delrin rod was
carefully centered by examining the distance to the root of the gear teeth on each side.
Let it dry for a good few hours.
You should now have an assembly that can be hung from the top of the 3 oz. plastic cup
by placing a wire or rod through the cross drilled hole. The gear should end up suspended
about 0.5" from the bottom of the cup.
Using the scale, mix about 1 oz of RTV1000 in another plastic cup. Mix thoroughly and
put the cup in the vacuum chamber. Pump it down to degas the mixture. It will foam up,
and can easily overflow the cup, so bring the pressure down at a rate that keeps the
material in the cup. A note about my vacuum chamber- I use a short length of 4 or 5"
diameter schedule 40 plastic pipe. This is turned flat and smooth on each end. The base
and lid are ½" thick Lexan, with the lid having a small brass fitting near one edge. For
strength, don't put the fitting in the center! A simple flat ring gasket was cut from a sheet
of rubber and is used both top and bottom. At this size, the stresses are very reasonable
for the materials involved. Do not attempt a larger chamber unless you are experienced
with vacuum design. My pump is an old Edwards vane pump.
Pour the degassed RTV1000 into the mold cup, filling about 0.35" over the top of the
gear. Put the mold cup in the vacuum chamber and repeat the degassing. Don't worry
about the material wetting high up on the Delrin rod, as the thin film will be cut away
later. You may never get all the bubbles to stop, but after a few minutes the remaining air
will be negligible. Bring it back to atmospheric and allow at least 4 hours for curing.
More is better.
Cut the cup in a few places, peel it back, and remove the mold and gear. Run a sharp
knife around the Delrin rod at the surface of the mold and remove the thin skin from the
rod. Now, the big event. Using your thumbs, carefully start pulling the mold away from
the rod. Keep working it until you can see down to the top of the gear. Now, push the rod
to one side and keep working around to get the gear over the step and moving up the
bore. Walk it around until it's free of the mold. Remember this step because it's exactly
the same maneuver you'll use to remove the finished parts. Inspect your mold. Look for
voids, tearing, or other imperfections. With luck, it will be a perfect replica of the master
gear.
Now we cast! Mix up a couple teaspoons (maximum) of TASK 9. The viscosities of part
A and B are quite different, so you won't get equal quantities using spoon measures. Use
a permanent marker on the plastic cup for the fill heights, and then fill to those. Mix
quickly and degas. You only have 7 minutes! I cast in the cold unheated garage, so the
pot life is a bit longer. Bring it back to atmospheric.
Pour the urethane into the mold, filling about 0.2" past the gear area. Hint- I now place a
tight fitting paper tube in the top of the mold so the urethane can bubble up without
spilling. Put the mold in the vacuum chamber and degas. It will seemingly degas forever,
but don't go beyond the time when the remaining urethane in the other cup starts to
thicken slightly. Whatever air remains will be negligible. Bring it back to atmospheric
and let it sit for 1 hour to cure. As described earlier, do not demold the part. Put the mold
and part in a low temperature oven and bake at 150F for about 4-6 hours. This will
greatly improve the properties of the part.
Now, using the same type of maneuver as before, pull the top of the mold sideways and
separate the top of the part. Work your way around the mold until the gear is exposed and
walk it up out of the mold using your thumb to gently pull the center of the part to one
side.
Admire it!
I turn a simple brass bushing on the lathe. 0.423" OD, 0.2505" ID, 0.25" long. Rough up
the outside with a very light knurl, a file, a grinder, or the method of your choice. Mix
and put a thin coat of J.B. Weld epoxy on the inside of the gear, and the outside of the
bushing. Slip it into place and cure. Warm it if you like to speed things up, but don't heat
the gear excessively.
Your finishing method will likely vary, but here's mine: I chuck up the gear on the round
back area left by the top of the mold- remember I carefully centered the delrin rod on the
master gear for a reason. I drill the urethane through the bushing with a slightly
undersized drill, then ream with the same reamer I used to make the bushing. Now the
gear has a well aligned hole all the way through.
In the old days of machining, taper mandrels were common. I jam the gear on a suitable
tapered mandrel (these taper only a thou per inch or so), then chuck it up and face the
back side to be flush with the bushing. I then pull it off the mandrel and touch up the back
side on the 220 grit paper. The free rotating (rear) gear is now done.
Most people would make a one time expanding mandrel to hold the gear by turning a
short piece of brass to 0.250", tapping the end, and slitting it with a saw. A flat head
screw would then expand the mandrel to hold the gear for machining. If you don't remove
the mandrel from the lathe, it will run perfectly true.
I prepare the next gear exactly the same way, then drill and tap the set screw holes. The
gear is clamped in a mill vise with a short plug or ball in the front of the bushing, to
prevent crushing the bevel gear portion. A 0.111" drill (#34) is deeply chucked so only a
short piece is exposed, and a hole is drilled through the urethane only, at 0.157" from the
back surface of the gear. This will cut into the spur portion of the gear very slightly,
though less than my factory gears. A #43 drill is then chucked up and the hole continued
through the brass. It gets hand tapped with a standard #4-40 tap. There are two set screws
at 120 degrees from each other.
Rebuilding the range mechanism is beyond the scope of this write-up, so hopefully you
took good notes and photos when taking it apart. A few hints. The small bevel gears that
will mate with the new cast gears should be checked and adjusted. If they're tight on their
axle, they should be opened up ever so slightly with a suitable reamer. If you don't have
one, don't mess with them. There should be no significant slop, but excess friction in an
already complex assembly, won't help things any. Mine had shrunk enough to be quite
stiff on their axles, and required about 0.0005" of material removal to turn freely. The
clips that restrain the gears should be adjusted so the teeth line up with the new gears and
run smoothly. The last gear to engage and lock is the freshly cast gear with the set
screws. Be sure you have good tooth engagement without excess friction. Use a
magnifying glass. Poor engagement could result in stripped gears or other problems. Be
sure to check the output frequency of your 8640B with a counter, as the front panel
counter will be wrong if you don’t get the knob/gear mechanism properly synced to the
actual range switch module. The first time I reassembled mine, the front panel indicated
1Mhz, but the actual output was 2Mhz. I was off by one position. This was fixed by
simply loosening the universal joint opposite the knob/gear mechanism, changing the
setting by one range, then retightening the joint.
That's how I did it, but there's lots of room for improvement. I took certain paths because
I have the equipment to deal with those choices, and valued precision over convenience. I
also wanted to do this once, and then move on to other projects, like using the 8640B!
For example, it may take experimentation and several trials to get a more complicated
mold working. If you lack a vacuum pump, lathe, mill/drill, or some other item, there are
still ways to do this. I'm guessing, but if the materials weren't degassed at all, the gear
would probably still be serviceable. It might take rolling the mold at 45 degrees or so to
distribute the urethane into all the teeth. It's probably reasonable to cast the brass bushing
right into the urethane in one operation. Without a mill/drill, one could eyeball the set
screw holes at a slight angle, avoiding the spur gear. The mold could be set up as a two
part mold, avoiding machining of the back of the gear. I would recommend making new
bushings as reusing the old ones limits one to very few new gears.
Good luck! I'll certainly entertain any questions, but I've tried to put just about everything
I learned into this one document.
Conrad Hoffman
choffman@rpa.net
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