Cast Epoxy Thread Repair

This article describes a process which I used to re-build several rows of broken threads on a boxwood screw arm of a Casey Clark & Co. plough plane. The process involves casting replacement threads out of epoxy cement, which is tinted with earth pigments, in order to match the color of the original threads. The results I obtained were very good from both a mechanical and visibility standpoint.

I began collecting old tools several years ago, and find great satisfaction in restoring them to working order, then using them in my shop. The plane in question was found at an outdoor farm auction in the fall. It was a drizzly rainy day, and all of the non-furniture items were layed out on hay wagons in the rain. There were several tools, and when I spotted a screw arm plough plane, my heart began racing. This type of plane had been on my wanted list for some time. I gave it a quick but thorough going over, and everything was perfect. One of the locking nuts was seized, and there was a small chip in one thread near the tip of one arm. The thought of this tool sitting out in the rain was very disconcerting, and I casually mentioned to the owner that all of his nice wooden tools were getting wet. "Oh, they'll be O.K. once they dry" was all I got, so I left it at that, and kept my eye on the plane. My wife and I wandered around some more, grabbed a coffee, then moved back in once the auction was about to start. I took another quick look at the "my" plane, and sure enough, some bungler had forced the seized locking nut, stripping three rows of threads. My heart sank, I kicked myself for not insisting that it be taken out of the rain, and eventually bought it for $75. Once at home, I decided that I wanted this tool to be usable, and that I had to figure out a way to re-build the missing threads. The fresh break stood out like a beacon from across the room. Here's what I did.

I purchased some "Rubbertex Molding Compound" and mould release spray from the Lewiscraft craft store, some plasticine, some two part epoxy cement (the squeeze tube kind, avoid the fast setting 15 minute epoxy, you may need the open time), and a selection of earth pigments from Lee Valley. Using the good screw arm, I selected an area of threads that matched up with those that I wanted to re-build. These threads were given several coats of paste wax. A "dam" was then built, using plasticine, around the area from which the mould would be lifted (note: you're working with the good screw arm, not the one needing repair). The exposed area within the "dam" was at least 2 threads wider than the area needing repair, and no more than half the circumference of the screw.

The area within the dam was lightly sprayed with mould release. Using a soft artist's brush, a thin layer of the Rubbertex compound was painted onto the threads within the dam. Once dry, another layer was painted on, and so on, until the compound was about 1/8 to 3/16" thick. This took 24 to 36 hours, as approximately 2 hours drying in was required between coats. Once dry, the thread mould that was created peeled away smoothly, without leaving any traces. The plasticine was trickier to get off. I used a dental pick to clean it all out.

Next, I drilled three anchor holes (1/16" bit) in the area needing repair. Because this was a large area I felt the repair would need the extra grip, and yes, I cringed at having to do this. For smaller areas or single threads this would not be necessary. I then mixed up some test batches of epoxy, mixing in various combinations a earth pigments to try and match the color of the boxwood threaded arm. In the end, the yellow ochre pigment alone gave the best match.

The inside of the mould was sprayed with mould release. A small amount of tinted epoxy was mixed and applied (I used a toothpick as an applicator) to the area needing repair, as well as to the mating surface inside the mould. I found it easier to repair a small section at a time (90 degrees of circumference). The mould is pliable enough that you can "roll" it on, beginning off of the repair, and rolling towards the repair. This will push the excess epoxy out of the end of the mould. The excess squeeze out was allowed to set for 15 minutes or so, then picked off with a dental pick. Allow everything to set 24 hours, then peel away the mould. Voila! You have casted replacement threads. In my case, three castings were required to repair damage that was three threads wide and 270 degrees of circumference. The match is so good that it is only noticeable if you are looking for it.

When filling the mould, try to eliminate all air bubbles by running the toothpick right down into the threads. Any air bubbles in the moulded threads can be disguised as small chips, by shaping with an x-acto knife. A word of warning regarding the rubbertex compound. It will slightly darken the wood of the area where the mould is created. Next time I will give a third or fourth coat of wax before making the mould. You may also wish to try dental putty, if you have access to it. It is faster setting and is likely to be less reactive with the wood. I could not find a supplier nearby, but would have tried it had it been available.

The best part about this re-build is that the plough is now completely usable, and I don't have to worry about the damage spreading. Now if I could only find a set of plough plane irons...

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