Background:

The combination lock was invented in China, although historical records provide little specific information about its development. Combination locks came into popular use in the United States in the mid-1800s to secure bank vaults. The locks, integrated into the vault doors, are a colorful footnote to the history of the old West as western films testify. In 1873 James Sargent foiled many a real bank robber by perfecting a time lock that, coupled with a combination lock, kept everyone out of the vault until the clock or clocks that regulated the lock reached the time at which it was set to open, usually once a day.

Combination locks come in two varieties: hand and key change. One kind of hand combination lock that does not use internal wheels is the pushbutton lock, commonly installed in office doors and affording some measure of security. Pushing three or four buttons in order or together releases a shaft or deadbolt, allowing the door to open. The internal mechanism operates similarly to conventional padlocks.

The Manufacturing Process

• Making the internal components

1 The lever, locking latch, and disc shaft are all made by injection molding, a process in which molten zamak is poured into a mold and subjected to heat and pressure until it solidifies into the shape of the mold. Although the post is shaped at room temperature, it is also formed under high pressure. The combination disc and the cam are made of cold-rolled—passed under huge rolls without being heated—flat strip steel; after being cold rolled, the steel is put in a blanking die, a sophisticated cookie cutter, which cuts (or blanks) out the properly shaped piece. The internal disc spring is made from stainless steel round wire and produced on a spring winder that automatically twists and turns the wire to form the traditional coil spring. The shackle collar, like the combination disc and cam, is made from cold-rolled flat strip steel that is blanked in a blanking die. The inner case is produced from flat steel strip and drawn to a cup configuration. This process requires great pressure to stretch and compress the material as it is pressed or drawn around a die, whose shape it takes.

• Making the outer parts

2 The outer case is manufactured similarly to the inner case but from stainless steel sheet instead of strip. The back cover, also stainless steel, is blanked in a blanking die. The durable shackle is made of round bar stock and machined on a screw machine, then formed to its U-shape and notched to accept the locking latch. It is finally annealed (heated to great temperature before being quenched in water) to make it resistant to hacksaws and bolt cutters. The combination dial, also zamak, is injection molded, then chromized. This process involves heating the part in a salt bath rich in chromium. The steel absorbs the chromium, which hardens on the surface as it cools rapidly. The dial is painted black and white wiped, which leaves the numbers highlighted against the black face.

• Plating the components

3 Several plating and finishing processes can be used to protect the components against corrosion. The lever, disc shaft, combination cam, and dial are chromized. The inner case, shackle collar, and lever post are all cadmium-plated. The shackle and locking latch are copper nickel-plated. The outer case, of stainless steel, is mechanically polished to enhance luster.

• Assembling the lock

4 Assembly of the components is precise with the back plate disc shaft, combination cam, and spacers forming one subassembly. The outer and inner cases are riveted together and then pierced at the point where the shackle is inserted. The combination dial, outer and inner case unit, and combination cam are then fastened together. Finally, these sub-assemblies and the remaining parts are fitted together. The lock case is closed and the edges folded over and sealed. Conventional hardware fasteners that can be released with the proper tool are not used.

• Labeling and packaging

5 The remaining operation is the application of a removable tag or label to the lock. On this tag is the combination, determined randomly by machine draw. Lock manufacturers today jealously guard their combination setting procedures. A typical combination lock is sold in blister pack, a rigid molded plastic with cardboard backing, although locks may also be individually boxed.