Have you ever thought, “Just what is it that actually allows us to pick a lock?” It is the inability of the manufacturer of any product to machine parts to an almost flawless level of tolerance. Even if they could reasonably approach their goals, the expense alone would be astronomical.
Therefore, we, as locksmiths, are able to pick a lock, so to speak, due to the reality of this situation. To see specifically what is involved, we must look at a typical cylinder.
The tolerance inadequacies to which I refer can be categorized for easy reference. The first is the difference between the plug and the shell. An acceptable amount of difference is approximately .005 or about .0025 all around the plug (see above). The process by which the keyway is “cut” into the plug is called broaching. This process is easily observed when a blank or cut key is inserted in the keyway and “play” is felt due to a significant tolerance differential.
Probably the most significant problem of this sort is the drilling of the chambers. This takes three forms: Plug diameter differential, off-center chambers, and concentricity. This is caused by the cost effective but necessarily imperfect process used to manufacture these cylinders, namely gang drilling-a process by which you drill all the chambers at once, and sequential drilling where you drill one chamber after the other. (See illustrations.) In either case, both methods are imperfect because the drill bit itself changes a microscopic amount each time it is used to drill a chamber. It is no surprise then, that after a hundred or a thousand holes the diameter and the centering functions based on its original diameter are no longer accurate. However, in deference to the manufacturer, he could not possibly stay in business and change the bit for each hole of set of holes.
We are therefore left with a necessary evil. but one which we can use to great advantage. When turning tension is applied to the core (without the proper key inserted) again tolerance plays a large role in the next operation … Not all of the pins will bind at the same time. Locate those pins, lift them to the correct position (shear line). Follow by doing the same to the next pins to reach the cylinder housing. The only objects which keep the lock from opening are the pins.