A Combination Box Lock

1 Lock dial from front

by Richard Phillips

Recently I acquired an uncommon single-dial box lock. It is a straight lock, (1)which would mean an unusual placement of the linkplate (missing in this lock) if used on a wooden box. Or perhaps, it was intended for the double-wall of a metal cashbox? Such a linkplate location would not be unusual for a metal box.

 1 Lock dial from front
1 Lock dial from front

The lock has a brass case, lacquered, and the dial and bezel are nickel-plated brass. There are two wheels with a row of holes around the periphery(2), looking like a hand-change combination lock. However, the drive pins are factory fixed, and the wheels are not removable.

2 Cover plate showing wheels aligned with the viewing hole

The wheels are mounted on a tube which can rotate, and the dial spindle is connected to this tube by a splined shaft coupling(3), which enables the combination to be changed (see below).

3 Drive cam with splined coupling to dial spindle

The bolt is pivoted in the middle(4), and spring-impelled to pivot its hooked end away from the linkplate. But it can only do so when the correct combination has been dialled.

4 Inside of lock showing bolt

As the dial makes its final turn a small leaf spring on the bolt impels it to pivot so the end can enter the gates in the wheels: it is a direct-entry lock(5). Despite tweaking the spring on the bolt, unless the combination has been dialled very accurately, the spring is barely strong enough to push the bolt tail into the gates. A little wiggling of the dial can be needed.

5 Inside lock with bolttail engaged in drive cam

Only when the correct combination has been dialled and the bolt withdrawn can the box lid be fully shut. Turning the dial then throws out the probe end and the hook end grips the linkplate, so the box is locked shut.

If the lock were fitted to a closed box, lifting pressure on the lid would press the probe against the wheel peripheries, and the gates might be felt. Difficult to do without the linkplate. The gates are narrow, about 1½ numbers wide, so dialling must be accurate. There are 50 numbers around the dial, so quite an impressive number of differs in the key-space.

The combination may easily be changed, though not chosen. A small screw on the back holds a sliding plate, with a keyhole slot to hold the dial spindle(6). It this plate is slid up, the dial can be pulled forward, rotated somewhat, and re-inserted. The splined shaft coupling and the drive cam are now at different numbers on the dial. There is a window in the back cover to allow the gates to be seen as the dial is turned, and hence to find the new combination.

6 Back of lock showing sling plate which secures the dial spindle

Compared with the ‘key’ security, however, the physical security is modest, with only a single link hooked. Although this lock is comparatively expensive, its physical security is no better than a common lever or cylinder box lock. To judge from the wear on the plating, it has been much-used.

The maker and age of the lock are unknown, probably US inter-war period. The only marking is a stamped ‘3’ on the lockcase and cover plate.



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