HP 8640 schematic

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.