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Articles about masterkeying have appeared dozens of times in Locksmith Ledger. Masterkeying has been the topic of books by well-known people in our industry. Full one or two day classes on masterkeying are regularly presented at almost every locksmith convention. With this background, what could possibly be new or different about masterkeying that has not already been discussed?
What will be presented here is what might be called “plug-in” masterkeying. It is designed to streamline the dull repetitive process of generating numbers and quickly move you into the profitable part of your job which is key cutting and rekeying.
First, a short primer on key bitting arrays. Yale set the general size for pin tumbler keys 150 years ago. Given this size, it was discovered that the Yale key size allows approximately 10 steps, or increments of depth from the highest possible cut on the key to the lowest possible cut. Lock companies may differ on what the dimension of each depth increment may be (usually between .0125” and .019”) but regardless of the dimension being used, the amount of different possible increments for most popular lock companies is 10.
Most popular lock systems use five pin chambers for keying. Some commercial systems may use six or seven pin chambers, but this article will concentrate on a five chamber key system.
Variables such as parts wear and mass production can provide unplanned tolerances in a lock cylinder. With enough tolerance, key cuts such as 13131 and 14141 may unintentionally operate the same cylinder. This is called key interchange and must be avoided. In order to avoid interchanges, masterkey systems are designed to use a larger dimension of two increments between key cuts. Therefore in a masterkey system using key cuts such as 13131 and 15151, cuts of 14141 would not be used at all in order to prevent interchanges.
Most popular keying systems use a depth numbering system of either 0-9 or 1-10. Since single increment steps should be avoided when masterkeying most lock brands, cut choices in a given chamber will be either 02468 or 13579. Note that the two choice groups are all even numbers or all odd numbers.
One of the key cuts in each chamber will be set aside for the master key cut. This leaves four choices of cut numbers per chamber for individual key combinations in the system which are called change key combinations. Due to four cut choices per chamber, this masterkeying system has become known as the quadrant system.
In order to use this plug-in masterkey system, you must know the cut numbering used by the lock system you are rekeying. As example, Schlage uses a numbering system of 0-9. “0” is the shallowest cut and “9” is the deepest cut. Sargent sectional keyways use a 1-10 numbering system. “1” is the shallowest cut and “0” (10) is the deepest.
Knowing the numbering system is important because not all cut combinations can be used due to maximum adjacent cut (MAC) problems. When a very deep cut is made next to a very shallow cut, the “V” shaped cutter will remove so much material on each side of the deep cut that no key blank material is left for making an adjacent very shallow cut. Each lock company has their own MAC rule.
Before using the plug-in system you must develop the master key cuts. A good master key combination should contain one of the highest cuts in at least one space. This prevents someone from recutting another key in the system to produce a master key. The best master key cuts will have a combination of odd and even adjacent numbers. Master key cuts which use all even or all odd numbers will produce change keys with many repetitive cuts such as 44446 or 53333. These key combinations do not look secure, may allow the key to be removed in the wrong position and may be easier to pick open.
With the introduction of electronic locks many years ago, some locksmiths predicted that by this time all metal keys would be extinct. Nothing could be further from the truth.
Two powerful techniques can assist locksmiths when assessing existing master key systems. These techniques work on all types of pin-tumbler locks, regardless of chamber counts, or progressions under...