A copy of a copy can work poorly because every duplicate reproduces the cut depths and spacing of its source key with a small margin of error. When that copy becomes the source for the next one, its errors carry forward and new ones are added on top. The drift builds with each generation, so the copying chain, not bad luck, is usually the variable to look at.
Why copying errors build up instead of cancelling out
A duplicate key is cut by tracing a source key and shaping a blank to match it. The tracing follows whatever shape the source key has now. If one cut on the source sits slightly deep, or a peak has worn round, the new key inherits that shape. It also picks up any small slip from its own cutting.
That is why the drift is cumulative. A first copy carries one round of error, and a copy of that copy carries two. Wear only removes metal, and the machine has no way of knowing what the original looked like, so errors rarely undo each other. Over a few generations the cuts move away from the depths the lock was built around.
What the cuts along a key blade actually set
Most door keys operate a pin tumbler lock. Inside the cylinder is a row of small spring-loaded pins, and each pin is split into stacked pieces. The cuts along the key blade are a series of depths at set spacing, and each cut lifts one pin stack to a certain height. The plug turns when every split lines up at the shear line, which is the seam between the turning plug and the fixed housing around it.
Locks have tolerance, meaning a small amount of slack in how high each pin has to sit. A pin that is slightly off can still clear. Past that slack, the pin catches across the shear line, and you feel drag or the key will not turn at all.
That slack differs from lock to lock. A worn cylinder usually gives more play than a newer one. A copy near the edge of tolerance can turn easily in one door and grind in another door set to the same key. The difference points at the key first, not necessarily at the second lock.
A copy that looks identical is not always one that fits
Held side by side, a copy and its source often look the same, so people assume a duplicate is a clone. The eye cannot judge the small depth differences a pin responds to. Looking identical and being within tolerance are separate questions. A key can pass the first test and still sit right at the edge of the second.
The small errors in a duplicate key come from a few general sources:
- Wear on the source key. Rounded peaks, shallower valleys, and a softened tip or shoulder all get copied as if they were the intended shape.
- How the blank sits in the machine. The key is lined up by its shoulder or tip, and a slight offset shifts the cuts along the whole blade.
- The blank itself. Blanks vary a little in thickness and in how snugly they fit the keyway, the shaped slot the key slides into.
- The tracing and cutting. The guide follows the source key while the cutter shapes the blank, and small misalignment between them shows up as a depth or spacing error.
None of these usually ruins a key on its own. Stacked across several generations, they can.
Choosing a source key and spotting one that has drifted
Copy from the key with the fewest copies between it and the original and the least wear. Often that is a key that came with the lock and has stayed off a daily ring. A spare of unknown history, passed along by a former roommate or tenant, is a risky place to start.
A drifted key tells you through your hand. It turns stiffly, or only after you wiggle it, lift it, or ease it back slightly in the keyway. It works in one lock but not another that should match. Two copies of the same key behave differently in the same door. Any of these makes the key a poor source for more copies, whatever its history. Forcing a binding key also adds wear to the pins, so treat the binding as a reason to replace the key.
One habit keeps future copies close to the original. Set aside your least-copied key as a reference, label it, keep it off your daily ring, and copy from that key when you need more.
If you cannot tell which key is the cleanest source, or the whole set has become unreliable, call Nostrand Locksmith at (516) 644-8976 about key cutting and duplication, or about a rekey for a fresh set.
Frequently asked questions
Should I throw away a key that is several copies from the original?
A key several copies removed from the original can stay in use if it turns smoothly in every lock it is meant for. It is a poor choice as the source for more copies, because its accumulated errors would pass to the next key. If it binds, needs jiggling, or fails in one of its locks, retire it and get a replacement cut from a cleaner key.
Does a worn original key make its copies worse?
A worn original key does pass its wear on. Duplicating copies the shape the key has now, not the shape it had when new, so rounded peaks and shallow cuts become part of every copy. An original that has spent years on a busy ring can be a weaker source than a little-used first copy, so judge a source key by its wear as well as its generation.
Is it worth rekeying instead of cutting more copies from a questionable key?
Rekeying is often the better choice when no clean source key is left, when several copies behave differently, or when you do not know how many copies are out there. Rekeying changes the pins inside the lock so old keys stop working, and new keys are cut to match the new pins. That gives you a fresh first generation instead of another step down a worn chain.