bz wrote:
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...sg/93ebc61f089
3eb4f?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
There is a plastic 'shape lock' or 'friendly plastic' that comes as white
pellets.
It 'melts' at ~ 60 C, turning clear.
It can be melted in hot water.
When it is melted and cooled slightly, you can mold it with your finger,
like clay.
It can be pressed into a mold. It takes details well.
After it cools, it turns white. It is tough, like nylon. It can be
machined.
As long as your machine doesn't get close to 60 c inside, it seems to make
a very good substitute for nylon.
Oh, when it is hot, it will 'stick' to most other plastics.
I bought a container of it and have used it to fix several things,
including my wife's sewing machine(a plastic part on the threader broke), a
broken gear in a printer(I used shape lock to fuse the pieces together and
re-enforce the gear), a broken paper tray in a printer(I replaced a couple
of broken plastic pieces), and a broken paper shredder(replaced a broken
piece of plastic).
I used to buy "PlastiPair" (powder and solvent kit) at Radio Shack until it
was discontinued (apparently due to hazardous materials issues); it worked in
the same fashion and was particularly useful to copy knobs and of course was
not a thermoplastic so it could be used to make insulators and to reinforce
and repair standoffs and whatnot installed near power amp. tubes, etc.
I imagine that modelers still have a variety of malleable compounds for
custom shape making. I used to save the waste plastic from the PlastiPair
kits, grind it to a fine powder and reuse it with solvent. The issue
is determining the exact composition of the solvent, the last bottle of
which in my stash, sadly, has long since evaporated. It probably was
a mixture of xylene, toluene, TCE and who knows what else.
Michael
Michael