An Adjustable Watch Case Wrench

From The American Horologist and Jeweler magazine, January, 1946

An Adjustable Watch Case Wrench
By Ronald L. Ives

The relatively recent adoption of the so-called waterproof watch, with a "wrench back" has led to the adoption of a number of special wrenches.

Some of these are thoroughly practical devices; others. although workable, are quite complex in their operation; still others are poor makeshifts, hard to use and likely to scar up the back of the watch.
Study of a large number of these "wrench back" watches brought in for repair disclosed that there are three general types of backs. One type is composed of equilateral polygons, with an even number of sides.

A second is composed of slotted types, with an even number of rim slots. The third type consists of backs drilled or punched. The number of holes is commonly two, three, or four. In addition, a few odd types, having an uneven number of sides or rim slots, are occasionally seen.

After a number of experiments, the wrench shown in Fig. 1 was designed to open all standard wrench back watches. This wrench consists of a toolmakers' clamp, modified to take twin feed screws, which are coupled together by gears. The tips of the jaws are milled down to slightly less than 1/16" in width, and fit into the slots of type two cases. On the flats, just beyond the milled section, a 1/16" pin, mounted in a screw boss, is inserted into the jaw. These (one in each jaw) fit the pin holes of type three cases. The milled portions of the jaws, opposite the pins (flats, Fig. 2) can be adjusted to fit most type one backs. The slight concavity of these flats is intentional, and enables the jaws to grip backs that are convex in the centers of the faces.

A schematic diagram of the wrench, showing the parts, and the types of watch back into which they fit, comprises Fig. 2.

Utility of this wrench depends upon the care with which it is made. The adjustments should work smoothly, but with a minimum of shake. If the assembly "weaves" under stress, as it will with loose threads, it is likely to slip off at inopportune moments, scratching the watch back. 

Exact dimensions of the wrench are not important, and can be modified to suit materials and conditions, but exact parallelism of the jaws, and tight fit of the feed screw threads, are very important. Threading is best done in a machinist's lathe, to insure exact parallelism.

With this wrench, more than 95 per cent of all wrench back watches seen can be opened provided the back has not been cemented on with fingernail polish. If this has been done, a thorough soaking in acetone or ether may be necessary before the watch can be opened. As a last resort, the back can be heated to loosen it, by the momentary application of a bunsen flame. This is definitely a last resort, and cannot be used on "pot metal" watch backs, as they will melt at quite low temperatures.