msg wrote in
:

I needed to repair an Exabyte 8505 8mm tape drive which had a broken
14 tooth approx 3/8 inch diameter nylon gear in the cartridge handling
mechanism, and lacking suitable spares, I decided to attempt to reform
the gear; this post describes the method and results.

A suggestion was made in a previous Usenet post to assemble the
fragments of a broken nylon gear and immerse them in a pot of epoxy
heated to a thin consistency, allow it to set, and then heat the works
until the nylon reformed. No mention was made of actual results or how
the gear would be removed from the epoxy mold:

http://groups.google.com/group/sci.e.../msg/93ebc61f0
893eb4f?dmode=source

I wired together the two pieces of the broken gear with a bit of small
gauge nickel wire around the arbor portion of the gear, mixed up
"steel" JB-Weld into a small metal bottle cap just a bit larger than
the gear diameter, heated the epoxy with a heat gun until thinned and
immersed the gear to the level of the top of the arbor section. The
assembly was allowed to cure overnight. The following day I heated
the assembly from beneath using a heat gun set to a low setting while
observing the bit of nylon visible from the top of the pot. After a
few minutes, the nylon began to expand and extrude from the assembly.
I stopped heating and using a flat tool, pushed the nylon back down
flush with the rest of the epoxy mold. I repeated this heating and
pressing procedure another time and then allowed the assembly to cool.

Using a small drill bit in a Dremel tool, I milled a groove around the
perimeter of the epoxy mold and popped-out the slug containing the
gear. Sanding the underside smooth revealed the pattern of gear with
the teeth clearly visible, but also revealed that the epoxy had
disappeared from the center hole of the gear. Grinding the epoxy mold
material away from a gear tooth using an emery wheel in the Dremel
tool also revealed that the epoxy had fused with the nylon and was
inseparable. I wound up "carving" the gear out of the mold with the
emery wheel.

The center hole was restored by milling it out with a number 60 drill
bit in the Dremel tool, working from both sides to preserve centering
(under magnification parallax can become distorted) and to cut a D
shaped hole to accommodate the drive shaft.

The key points to be made are that using this process will produce a
solid gear but it will be fused with the epoxy mold material and
cannot be simply separated from it.

The results are pictured in this photograph which shows the gear
installed in the tape drive mechanism:

http://www.cybertheque.org:81/ext/gear/gear1.jpg

It works as intended but long-term reliability is as yet unknown.

I did not apply any grease to (or even degrease) the original gear
parts before immersing them in the epoxy; I don't expect that using
grease or mold release would have altered the results or permitted
removal from the mold. Perhaps a multistage casting process starting
with a latex mold, followed by a casting of a slug of the gear which
then could be used to cast a mold from a pot metal which then could be
used for casting fresh gears is another solution, especially if
quantities of the part were useful to have (since this failure mode is
common in this tape drive, it may make sense), but for me, reforming
the original gear was adequate.

Michael


go to a hobby/crafts store,and you can buy rubber casting material.

BTW,PAM no-stick spray works for a release agent.

--
Jim Yanik
jyanik
at
kua.net