Template Grinding to Make Watch and Clock Parts

Introduction
Template grinding is a technique for fabricating the "flat work" parts of watches and clocks. The method was popularized by William O. Smith in his book "Twenty First Century Watchmaking", where he presents the method in exhaustive, meticulous, painstaking, and repetitive detail. This article is simply a brief photographic overview. Readers of normal intellegence can work out numerous refinements.
The method
The idea is to mark out a pattern on a strip of sheet metal and then grind to the outline using a slotted saw table and small abrasive wheel driven by the watchmaker's lathe. With appropriate magnification, amazingly complex parts can be rapidly fabricated.
Materials
Pre-hardened shim stock is ideal for this purpose. It can be obtained in precision ground thickness. The metal is usually somewhat harder than spring-hard, but this is often ideal for watch parts. Because the process forms the outline by grinding, there is no need for heat treatment before or after cutting.
Templates
The key difficulty is getting a legible outline on the shim stock. W. O. Smith suggests coating the polished stock with beeswax, laying the original part on top and heating from below in a blueing pan until the metal discolors. I've tried this method several times and only succeeded in setting myself on fire.
Smith also suggests soft-soldering the original part to the flat stock. This method works very well, but in the example I present here, the orignal part was not flat until I beat on it with a big hammer. Then it was broken into so many pieces that I had to make a new template from scratch.
I've also tried making high-resolution prints on paper with the computer and glueing the pattern to the stock. This sort-of works, but the paper and glue build up residue during grinding that makes the outline hard to see.
Using silk screen photo-emulsion might give excellent results. In that case, you would laser print the part in black on acetate and then burn the outline on the emulsion-coated shim stock. I haven't tried this method, but it sounds good, doesn't it?
For the example presented here, I used a brass template. Although this method takes a bit longer, it makes a clear and stable outline to follow.
Tools
I use inexpensive Dremel-tool arbors and small grinding disks. For very fine work, wheels called "separating disks" can be obtained in very small widths. I have some disks that are 0.004 inches wide. In general, it is a good idea to use the widest wheel suited to the part to avoid rapid wear and breakage.
The wheel is driven in the watchmaker's lathe at fairly high speed. The part and template are supported on a saw table. The table should be precisely on-center and the slot should closely fit the wheel. For fine work, I attach a fresh sheet metal plate to the saw table and saw a new slot using the wheel.
For finishing, a set of molded-abrasive wheels work well. Cratex is one brand that comes in a nice set with the necessary arbors.
Hole location
Frequently, flat parts have multiple holes that must be precisely located. When making a replacement part, the orginal part may be clamped to the shim stock to use as a drilling template.
I use a refinement with this technique that gives very precise center locations. First, I clamp or solder the original part to the stock. For each hole, I turn up a simple steel transfer punch: The outer diameter of the punch just fits the hole in the orignal part. At the center of the punch, I form a very short point. Because the punch fills the hole, the center is very accurately marked. Spoting with a micro-sized center drill keeps the twist drill from wandering. I usually drill the hole under size and broach to the final diameter.
So far, I've always been able to drill feeler gage stock using ordinary high-speed steel twist drills. For an application where a very hard part is required, carbide circuit board drills will cut through nearly anything if you can avoid breaking them.
Using a brass template
I clamp the broken original part to a thin sheet of brass shim stock. This can be roughed out with scissors and then rapidly filed to the final outline.
I use an optical comparator to examine the template and compare it to the original part. The template can also be measured with the micrometer and touched up with fine escapement files.
When the template is finished, I attach it to the shim stock with soft solder. Holes are then located and finished as described above. Finally, the outline is ground on the lathe using a stereo microscope.

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