Copy of a Copy: How Small Key Cutting Errors Stack Up

A copy of a copy can stop working because a key opens a lock through the depth and spacing of its cuts, and each duplicate traced from another copy inherits that copy's small errors and adds its own. Usually no single copy is far off. The drift builds up across generations until the cuts fall outside the narrow range the lock accepts, even though the key still looks right.

Drift builds one generation at a time

The original key is the reference. A copy traced from it lands close. A copy traced from that copy starts from a key that was already slightly off, and nothing has to go badly wrong at any single step for later copies to wander.

A drifted copy can look identical to the original and still behave differently in the door. The difference is spread thinly across several cuts, too small to spot at a glance, but the lock reacts to all of those cuts at once.

What the cuts on a key tell the lock

Most everyday door locks are pin tumbler locks. Inside the cylinder sits a row of spring-loaded pin stacks, each split into two parts. The plug, the part that turns, only rotates when the split in every stack lines up with the shear line, the seam between the plug and the outer housing.

The cuts on a key do the lifting. Each cut sits at a spot along the blade that matches one pin stack, and its depth decides how high that pin rises. Too shallow pushes the pin too high; too deep leaves it too low. Either way, a pin bridges the seam and the plug stays put.

A key, then, is a set of measurements: where each cut sits and how deep it goes. The lock allows a small window around each one because its parts have a little slack. Inside the window, the key turns. Near the edge, it catches. Past it, the key will not turn at all.

Is a copy as good as the key it came from?

A copy is not quite as good as its source, and the gap compounds. A common duplicating method traces the source key with a guide while a cutter shapes the blank to match, so the copy follows whatever it traces, errors included. If one copy comes out a touch shallow at a cut, the next copy traced from it starts shallow there and can end up shallower. Errors do not always run the same way, but when they do, they stack.

Spacing drifts the same way. A key clamped slightly off during tracing shifts every cut along the blade, and later copies inherit the shift.

Wear gets copied too. A key that has spent years on a ring and in a lock has rounded peaks and softened edges. Tracing reproduces that worn shape on a fresh blank, so the new key starts out already worn.

A fresh copy can also feel rougher than the original because cutting leaves small burrs. That roughness alone does not prove the cuts are wrong; a copy that still catches after some use points to the cut shape.

Picking a source key and handling a copy that drifts

The habit that heads off most of this: keep one original key, or one copy made directly from it, out of daily use and set aside only for duplicating. Label it so it never lands on a ring. Every new copy then starts from the same clean reference.

A drifting copy usually gives warning before it quits:

• It sticks partway in or needs to be eased back slightly before it turns.
• It turns only after a jiggle or extra pressure.
• It works in one door but not in another lock the original opens.

To check a suspect copy, line it up beside the original at the shoulders, the small stops just below the head, and look at both against a light. Rounded peaks, deeper or shallower valleys, or shifted cuts all point to drift. Do not force a copy that will not turn.

When nobody can say which key came from which, or too many untracked copies are in circulation, rekeying is a reasonable alternative to chasing fits. A rekey changes the pins inside the lock to suit a new key. Amsterdam Locksmith on the Upper West Side does lock change and rekey work; call (917) 200-2978 if you want to talk through which option suits your door.

Frequently asked questions

Is it better to copy from the original key or from a recent copy?

Copying from the original key beats copying from a recent copy, provided the original is in decent shape. A recent copy already carries its own small errors. If the original is badly worn, a lightly used copy made from it while it was newer can be the cleaner source.

Can a key that is slightly off damage the lock over time?

A key that is slightly off can add wear inside a lock, mostly because a key that catches gets jiggled and twisted harder. The bigger risk is the key itself: forced against a binding pin, a copy can bend or snap and leave a piece in the cylinder.

If a copy barely works, should I keep using it?

A copy that barely works is better retired than kept in rotation. Drifting copies tend to get harder to use, not easier. Replace it with a new copy cut from the original, and never use the barely working key as a source for more copies.

When does it make more sense to rekey than to keep making copies?

Rekeying makes more sense than making more copies when the original key is lost or too worn to copy from, or when nobody can account for how many copies exist, for example after staff or tenant turnover. A rekey sets the lock to a new key, so old copies stop working.