Pivot steel turned down with a graver I've nearly had a French crystal regulator done for a couple of weeks, but the rate adjusting assembly has been acting quirky because I lacked a single screw of the right type. I decide to make a screw from scratch to solve the problem. I had an old broken screw that fit the threads, so I knew some of the critical dimensions.
The process started with finding some hardened steel rod of the appropriate outer diameter. In this case, the screw head will be the largest diameter of the part. I chose some steel rod just a bit bigger than the size of the finished screw head. I chucked the rod into a collet in my lathe. I used an antique American Watch Tool Co. watchmaker's lathe, but any small precision lathe for metal would work. Using a carbide graver, I turned the piece down where the screw threads were to be. Using a graver is something like turning wood on a lathe, but with metal on a very small scale.
The process started with finding some hardened steel rod of the appropriate outer diameter. In this case, the screw head will be the largest diameter of the part. I chose some steel rod just a bit bigger than the size of the finished screw head. I chucked the rod into a collet in my lathe. I used an antique American Watch Tool Co. watchmaker's lathe, but any small precision lathe for metal would work. Using a carbide graver, I turned the piece down where the screw threads were to be. Using a graver is something like turning wood on a lathe, but with metal on a very small scale.
Threading the end of the screw Next, I used the screw I had and experimented with screwing it into different dies until I found one in my metric die set that seemed right. I cut threads on a short section of scrap metal to test within the clock. Once I found the size that fit, I installed the die into a handle and cut the threads with the rod still in the lathe. The handle was held stationary, while the I turned the lathe spindle by hand. No power was used during this operation, and I probably should have removed the green urethane belt from the spindle pulley.
Using a knife-edge file to part off the screw Once the threads where cut, I used the graver to start a parting cut. I made the cut in from the end to leave room for what would become the screw's head. Once the placement of the cut was established, I used a Swiss knife-edge file to part away the piece. The file was held with a hand at each end, the end with the handle resting on the lathe tool rest. I did use power to the lathe for this operation.
A screw slotting file on the filing rest The screw was then removed from the collet. I found a new collet that fit the turned-down section under the screw head. The screw was flipped around (head out) and chucked into the lathe once again. I filed the head smooth, and added a slight chamfer to the head. I then burnished the head so that it was shiny. This not only looks good, but is important to the bluing process to come. I swapped the lathe tool rest for the filing rest. I placed a screw slotting file on the rest, and adjusted the height until it was in the center of the head. With the headstock spindle locked, I filed the slot in the head by hand.
Before and after bluing the machine screw The the screw complete, all that remained was to blue it. This is the appropriate look for a French clock. The blued surface also prevents oxidation. The screw was dropped into my homemade bluing pan -- and old metal spatula with turned up edges and a hole drilled in the bottom. I held the screw over an alcohol lamp until it reached the desired color. It leaves a lovely blue-black color that's unmistakable.
The completed screw on a penny for scale The finished screw has a rounded tip by design. It screws through a solid block of brass into a drilled space through which a threaded rod passes. The rod has a rounded notch in it where the end of the new screw enters the drilled hole. The tip of the screw fits into this notch. This allows the threaded rod to turn freely, but it cannot slide out of the drilled hole. It's a simple and clever arrangement, but it only works if you have a screw of the right size, length, and shape.
I did almost everything while wearing a magnifying eye loupe. I didn't realize how small the screw was until I sought out a penny to use in a photograph for scale. The screw works perfectly and cannot be distinguished from the 100 year old screw right beside it on the clock.
I did almost everything while wearing a magnifying eye loupe. I didn't realize how small the screw was until I sought out a penny to use in a photograph for scale. The screw works perfectly and cannot be distinguished from the 100 year old screw right beside it on the clock.
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