How I Fixed A Broken Gear Tooth

Written in 1997 by:
   Grant Erwin  
   Seattle Metalheads Society
   Uncopyrighted, use as you wish .. editing suggestions welcome!


I recently acquired a vintage metalworking machine. Fired with enthusiasm, I tore it down completely, intending to do a full restoration. The entire effort almost derailed, however, when I discovered a broken tooth in the gear train. This document describes all of the various considerations in the repair.

First, I considered having another gear cut. There are a lot of gear manufacturers who can cut any gear there is. However, the cheapest quote I got was well over $300, and this was out of my price range, so I discontinued this search.

Next, I considered buying a stock gear. I called most of the local stocking distributors, and got the Boston Gear catalog, and found the Stock Drive Web page, and so on. The problem was that my gear was helical, and had 94 teeth, and had only a 10 degree helical angle. It turned out that there was no way at all anyone stocks this gear. I did, by the way, consider replacing the 4 gears in this particular gear train with spur gears. This would have cost about $200 plus some machining. Better than $300, and has the side effect of having all new gears, but still basically way too much money.

The next thing I considered was milling out a slot, silver- soldering in a blank tooth, and cutting the tooth to shape with a shaper. There are three problems with this approach from my perspective. First, I don't have a milling machine. Second, I don't have a shaper, even if I did know how to grind a tool to the perfect profile. Third, several gear companies told me no matter what kind of repair I did on a cast iron gear, to stay away from anything that required heat, like a brazing operation, on just one part of the gear. This made sense to me, so again I abandoned this train of thought.

Finally, I decided to take the advice of a couple of guys from the Internet who were most helpful. I decided to drill and tap 3 holes, put in set screws, rough grind the set screws to approximate shape but slightly smaller, and to cast in place epoxy putty to form a new gear tooth over the 3 pins made from set screws. I figured the epoxy putty would bind to the remaining threads of the set screws, and that the tough metal the set screws are made of would sufficiently strengthen the new tooth. I have been successful at this method, although I have not yet tried the gear under load. The remain- der of this text describes in detail how exactly to fix a gear tooth with a couple of bucks worth of JB Weld and about ten cents worth of set screws.

The first problem is how to hold the gear for machining. I found a solution that worked well for me. I mounted the gear on its shaft, which was 3/4" in diameter, and put the shaft into a 5C collet fixture that was quite solid and stable. I have seen these fixtures offered at very reasonable prices from a number of vendors.

I fixed the gear rigidly in the fixture, as described above, and then put the entire fixture in the bench vise. I took a very small fine file and filed the broken tooth off flat. The file I used had no teeth on either side, so it didn't mar the unbroken teeth.

After filing the tooth flat, I scribed a line down the middle of the tooth. This line was 10 degrees from the axis of the gear, which is the helix angle. Then, I center-popped 3 centers by eye. I then took the fixture over to the drill press, where it bolted down to the table. I located the holes with a wiggler, and drilled and tapped for (in my case) 4-40 set screws. This actually was the most difficult part. I find it difficult to get a very small drill bit to start where I want it, and this time was just the same way. Regardless, I got 3 usable holes, and they tapped just fine. I always use high quality taps - they cut better, and breaking a tap in this application would have been extremely difficult to recover from.

I then Loctited in the 3 set screws. I only tapped them about 3 threads deep (maybe 1 diameter) because I didn't want to weaken the gear body.

Then I cleaned some teeth on the opposite side from the break until they were as clean as I could get them. I cut a couple pieces of scrap black sheet metal, maybe 1" x 5", approximately 1/16" thick. Not too flexible. I cut out some rectangles of wax paper, and carefully folded them around the metal strips, taping them up the back side. Then I took the gear outside and lit my oxyacetylene torch, with just a little bit of oxygen, which generates a very yellow, sooty flame. I played the flame directly over the gear teeth and got a lovely velvety black soot layer. Then I took the gear back inside, and clamped the metal strips over the cleaned, sooted teeth, and mixed up some JB Weld, and gooped it in with a little piece of sheet brass cut just a little thinner than the gear. I covered about 5 teeth. The JB Weld says it sets up in 4-6 hours and cures overnight. After 6 hours, I put the gear in the vise, took a suitable block of metal, and gave the molding a robust tap from the side. It shattered!

Chastened, I cleaned up the teeth again, and tried again. What I figure I did wrong was to put on too much JB Weld, worked it in too much instead of letting it run in, and tried to remove it before it was strong enough. Putting on too much caused it to flow sideways over more teeth, getting a better grip. Working it in overmuch mechanically removes the soot, which acts as a releasing agent. Anyway, the second time around, I got a piece which spanned 4 teeth and came out perfect.

Then I got out the Dremel tool and a solid carbide burr, and went to work on the set screws. I didn't care very much about the final shape, I just wanted to leave as much meat as I could while allowing a reasonable amount of epoxy to cover. I used the mold to test the fit until I was satisfied that the pins were ground down enough.

Then, I took the same sheet metal strips I used when I made the mold, and again applied wax paper, this time to only one of them. I "sooted" the mold I'd made, again with the torch, and clamped the one side piece back onto the gear. I applied some JB Weld, then set the gear on its side (the 5C fixture has a 90 degree body, so you can work the piece either way) with the strip down. I had discovered that the JB Weld flows with gravity before setting up, and I wanted to use that. So, I put a "bottom" on with the wax-papered metal strip. Then I carefully pressed the sooted mold over the JB Weld, clamping it into place. It was gratifying to see the surplus putty run out the top, forming a nice little drop which would then tend to run down and fill any interstices before hardening. The next day, I took off all the clamps and the side piece, and carefully filed away the small excess amount of JB Weld. The advantage of letting it go out the side is that the side of the gear is a flat surface, which is very easy to machine. One of the guys who had given me advice on the Internet had suggested that you drill holes in the top of the mold, and put both sides on. I didn't want to fiddle around with cutting off any "sprues", or having them in any significant way impede me removing the mold.

Anyway, the next day, the mold came off cleanly, leaving a perfectly-formed tooth. It is actually not even very obvious, since the soot bonds with the epoxy to turn it black just like the gear. It sure looks fixed now! I plan to run the gear below its rated drive capacity, to attempt to make this repair last. The next gear failure on this machine will nec- essitate a major repair involving 4 new gears, and I hope to keep that from happening in my lifetime!

2008 Update: The gear has worked flawlessly now for over 11 years!