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Mechanical combination safe locks have been used for over a century. The example for this article will be a Sargent and Greenleaf (S&G) safe lock, but most other brands will have similar basic construction.
Most safe locks usually consist of three or four wheels (called a wheel pack) which each rotate on a single rigid post in the lock case. Each wheel contains projections which form the connection between each wheel. Each wheel also contains a notch on its outer edge (figure 1).
The safe dial is connected to a spindle. The spindle extends into the lock case through the hollow post. A drive cam is attached to the end of the spindle. The drive cam also contains a drive pin projection. During one complete rotation of the safe dial, the drive cam pin projection will contact the projection of the adjoining wheel and that wheel will begin to rotate in unison with the drive cam. During each succeeding revolution of the safe dial, the projection of the next adjoining wheel will be contacted and that wheel will also will begin to rotate.
Most safe locks contain a lock bolt. When the lock bolt is extended, the safe cabinet boltworks are prevented from moving. A spring-loaded lever is connected to the lock bolt which controls lock bolt movement. The lever contains a narrow bar called a fence. The fence rests on the outer edges of the wheel pack. When the correct sequence of numbers is dialed, notches in each wheel are aligned at the fence location (figure 2). The spring-loaded fence will then enter the groove made by the notches and continued rotation of the drive cam will move the lock bolt to the unlocked position (figure 3).
Manipulation depends on the extra tolerances machined into every safe lock because of mass production. Instead of having to hand-fit each part during assembly, parts such as the shaft screw retaining the lever are made a few thousands smaller than needed to assure that any chosen lever and screw will easily operate together. During manipulation, this added looseness allows the lever to tilt slightly which provides indicators which can be used to determine the safe combination.
A second factor in manipulation is the outside operating diameter of individual wheels. Variations in the outer diameter of each wheel or variations in the wheel hole diameter can help the manipulation process.
If the outside operating diameter of one wheel is slightly higher than the remaining wheels, the notch in that wheel will be more easily identified during manipulation.
Preparing for Manipulation
You can either mount a safe lock on a board and remove the lock cover while practicing or you can order a cutaway practice lock and mount as shown in our photos. Lockmasters sells an S&G cutaway lock, part number SG6730CUT, and a LaGard cutaway lock, part number LAG3330CUT. Call Lockmasters at 800-654-0637 for further information.
First, determine the two contact points for the drive cam. These two points are where left and right edges of the drive cam notch touch the lever. Figure 4 shows the contact point when turning the dial counter-clockwise (left) and figure 5 shows the contact point when turning the dial clockwise (right).
When the dial is turned to the left, the contact dial reading for this example lock is 13 (figure 6). When the dial is turned right, the contact point is between a 6 and 7. The right turn reading is approximately 6 1/4.
The 1/4 distinction is important because in order to manipulate a safe lock, you must note the small changes between marks such as 1/4, 1/2 and 3/4. Contact points will vary on individual safe locks depending on what quadrant the drive cam was attached to on the spindle.
In order to expand readings over a wider area, a homemade pointer tool was developed (figure 8). It was made from a suitable plastic plumbing cap, a two-inch hose clamp and a bent length of coat hanger wire. The plumbing cap was enlarged on the inside slightly to fit over the dial ring. Slits were sawed into the end cap to allow the clamp to compress and tighten both the pointer and the end cap around the dial ring (figure 9). A sheet metal plate was fastened under the dial ring to hold paper for marking (figure 10).