Recasting a broken small nylon gear

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:

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:

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

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Hint for repairing broken plastic cogs / worn cogs For the situation where a section of teeth only on a plastic cogwheel are worn or missing. Wrap a piece of thin polythene sheet around a section of the undamaged cog teeth.Wrap copper wire diametrically around the cog pulling the wire into the root of each tooth over an "arc" 2 teeth wider each side than the broken section. Coat with epoxy to form a mould and allow to cure. Undercut the edges around the broken section,demount the mould and reposition over the broken section. Bind on and make good the cog with epoxy in 2 or 3 stages.Finally fettle with a file.

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Cool. When plastic parts of my toys broke when I was a kid, my dad (a patternmaker) would make molds from the originals with a little hands- on and bring back a polyurethane replacement.

Non-functioning Exabyte 8505's must be a dime a dozen on the surplus market, or well they were in the past decade, don't know if they're still cluttering up storage rooms or landfills today. But once they become truly rare, this is the wave of the future:

It's incredible, complete working mechansims can be made this way!

Tim.

msg wrote in news:_-OdnWwSE53cBSPUnZ2dnUVZ snipped-for-privacy@posted.cpinternet:

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

msg wrote in news:_-OdnWwSE53cBSPUnZ2dnUVZ snipped-for-privacy@posted.cpinternet:

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).

--
bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an 
infinite set.

bz+ser@ch100-5.chem.lsu.edu   remove ch100-5 to avoid spam trap

...or a CAD drawing. With that, there are a multitude of options. There are companies which can take your CAD file and deliver a finished gear in less than a week.

jak

Our local SAGE building, which once housed the mighty AN/FSQ-7 tandem CPU, 100 CRT stations and several floors of peripherals and realtime film projection systems, now hosts the Natural Resources Research Institute (sort of swords to plowshares); the NRRI has a rapid prototyping system that could have made my gear from a CAD drawing in a few minutes, but alas I don't have access...

Michael

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

These guys make getting custom parts pretty easy by providing you with simple/free CAD software and taking you all the way through to getting a part. No need to be a fully set up mech eng.

The simple example posted on their site is actually a gear...

Russ

I recycled two 8505XL drives about two years ago. Too old and not very reliable. I also dumped my pile of DDS-2 and DDS-3 drives. I'm still using some DLT but prefer AIT for anything important.

1. Search the net for a replacement gear. There were plenty of vendors that appeared when searching for "Exabyte gears" or "Tandberg gears". For example:

Some of the tape driver repair shops might sell you a gear from a dead parts drive. Unfortunately, if the gear died from nylon shrinkage, the replacement gear may have the same problem. Be careful here.

1. Find a stock gear from some vendor that will fit or work. It doesn't have to be nylon and might be aluminum or brass.
2. Glue the pieces back together with epoxy. This may be difficult with a 3/8" dia gear as there's little surface area to support the glue. I've done this for the chronically broken gears in the HP8640B signal generators. The trick was to remove the brass hub. Enlarge the hole in the center with a round file so the brass hub fits better. Glue the gear pieces together without the brass hub. Use a jewelers file to clean up the epoxy slop on the gear teeth. If your gear doesn't have a hub, use a drill bit smeared with grease as a form. If any of the teeth are missing, epoxy is NOT strong enough to act as a replacement. You'll need a replacement gear.
3. Buy a dead Exabyte 8505 on eBay and cannibalize it for parts.

My shop time is worth $75/hr. If your gear rebuilding exercise takes more than about 90 mins, it's cheaper to buy the replacement drive.

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

That's where I got this drive but ya takes yer chances on eBay I guess...

It's not intended for production use, just to recover tapes made on my EXB8500 (full height) which has increasing read error issues and chokes on the last tape of a five tape set awaiting a restore.

I'm considering getting an IBM 3590 fibre channel library (if I can stomach the shipping costs) since I have a large box of media and the cost of media is the limiting factor for me in newer tape technology.

Michael

One of the places I was thinking of when I mentioned many resources available...if you have the drawing. IMO, it would be a little difficult to draw a gear with their software (at least the version I looked at a couple of years ago); but not impossible.

Otherwise, their site, software and their business is pretty amazing. They'll build you *anything*--out of almost any material--as long as you can spec' it accurately enough.

Well worth downloading their application just to play with....

jak

Jeff Liebermann wrote in news: snipped-for-privacy@4ax.com:

does epoxy bond to nylon??? I'd not bet on it.

I think I'd glue it back together as best as possible,then use the casting materials one can find at hobby/craft stores to make a mold and cast a new gear.

--
Jim Yanik
jyanik
at
kua.net

I certainly discovered that under the right conditions, the epoxy-nylon bond is very good, however I suspect that the bond I observed is just the molten nylon conforming to the irregularities of the epoxy mold in a very complete fashion. However I don't expect to be able to simply 'epoxy' broken nylon parts together, but perhaps there may be a way to liquefy the (set and cured) epoxy-nylon joint with a hot air pencil to create a permanent bond (I will need to try this next time). Thinking a bit more, perhaps hot air welding of a cyanoacrylate-bonded nylon repair would work also.

I don't know how I would do this on the tiny gear that is the subject of this thread however without distorting it beyond usefulness, without using a mold, at which point I am back to my original approach of recasting it.

Michael

Michael

It would indeed by nice to have a variety of standard nylon gear stock on hand; has anyone found a cheap source for such kits?

Michael

has some but there's such a wide possible variety that hitting for a specific gear (or two) might be useful.

--
Rich Webb     Norfolk, VA

8mm drives are selling for $30-80 on eBay, some new in box.

Better yet, move up to much faster, cheaper, and more reliable storage media, like thumb drives or usb disks.

Sometimes is better to ust start over than to try to patch something so ancient.

Yes:

I didn't use any manner of special epoxy. Just the cheapo hardware store 2 hr epoxy. I've also done it successfully with JB Weld. Surface prep was degreasing with some alcohol and deglazeing with sandpaper. However, the 8640B gear in the photo is much larger than the 3/8" gear in the Exabyte. Unless there's sufficient surface area, it's not going to hold.

Incidentally, some construction "structural" adhesive is an epoxy and nylon mix. I'm wondering if I should have used some of that instead.

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

Caveat Emptor or You lose. Maybe another drive, this time from a different vendor? (Two risky wrongs might just make one right). Another used drive is still cheaper than the time needed to do the repair job (which has a limited chance of success and survivability).

Worn head. It's history. I've had extremely bad luck reading tapes that were created on a different drive, especially if there was a difference in head wear and age. I once called a large number of vendors trying to find a replacement with matching firmware for a DDS-2 drive because the new and improved firmware would not read the old firmware.

I'm finding it cheaper to use NAS with gigabit ethernet for daily backups. Terabyte drives and servers are about $250 from Maxtorch, Western Dismal, Buffalo, and Seagate.

Or, you can built your own with FreeNAS:

I threw one together out of junk. It works nicely.

However, my customers want something they can store in the fire vault and isn't sensitive to handling, so I'm still using tape (AIT and DLT) and some DVD backups.

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