Why Do Key Copies Stop Working?

It is quite common, and very frustrating to have a key duplicated only to find that the key does not work. Why does a duplicate key that is made from another previously duplicated key sometimes fail? This can be inconvenient, expensive and might even leave a person stranded.
What is the cause of this phenomenon, and how can it be prevented?

When a quality key duplication machine is used to make a duplicate key from the original factory-manufactured key the result should be a good copy. But, what happens when a copy is made from a copy, and then that copy is used to make another copy and so on? After successive copies are made the last key copy will cease to work because each successive copy loses a slight degree of accuracy, even if a quality key machine is used. The depths of the cuts of keys are measured in thousandths of an inch so, there exists a very slim margin of error. Due to these fine increments of measure, a slight variance in depth can result in a key’s failure because the tumblers inside the lock cylinder will not properly align on the shearline. What can be done to correct this problem?

The preferred solution to this problem is to make key copies only from the “Original” key. The original key is the key that came with the lock,or, that the lock was re keyed to. The original manufacturer’s key is sometimes identifiable by having a brand name,and/or, a series of numbers etched on the key. What if the original key is not available?

A new key can be originated on a code cutting machine, after the key’s proper spacing and cut depths have been determined. Other solutions may involve modifying the depths of the existing key,or re-pinning the lock cylinder that the key operates.