Disc tumbler lock

A disc tumbler lock or wafer lock is a type of lock that utilizes a set of flat discs to prevent the lock from opening unless the correct key is inserted.

Without a key in the lock, the discs (red) are pushed down by springs. The discs nestle into a groove in the lower part of the outer cylinder (green) preventing the plug (yellow) from rotating.

When the correct key is inserted, the discs (red) are raised up out of the lower groove in the outer cylinder, but not so high that they enter the upper groove in that cylinder.

With the discs unobstructed, the key, discs, and plug (yellow) can rotate.

Design

In a cylindrical disc tumbler lock, a series of flat discs or wafers holds a cylindrical plug in place. The discs are fitted into vertical slots in the plug, and are spring-loaded, causing them to protrude into diametrically opposed wide grooves in the outer casing of the lock. As long as any of the wafers protrude into one of the wide grooves, rotation of the plug is blocked, as would be the case if there was no key, or if an improperly bitted key were inserted.

A rectangular hole is cut into the center of each disc; the vertical position of the holes in the discs vary, so a key must have notches corresponding to the height of the hole in each disc, so that each disc is pulled in to the point where the disc edges are flush with the plug, clearing the way for the plug to rotate in order to open the lock. If any disk is insufficiently raised, or raised too high, the disc edge will be in one of the grooves, blocking rotation.

Types and Wafer Arrangements

Disc tumbler locks configurations vary with manufacture. The most common is the single-bitted, five-wafer configuration most commonly found on desk and cabinet locks and some key switches. The above illustration represents this type.

Some disc tumbler locks utilize a stack of closely-spaced wafers designed to fit a specific contour of a double-sided key and works on the principle of a carpenter's contour gauge.

Disc tumbler locks can utilize single-bitted or double-bitted keys. Though disc arrangements within the plug may vary, such as automotive locks, where the discs are arranged in opposed sets, requiring a double-bitted key, the operating principle remains the same.