A copy is only as good as the key it was traced from
A duplicating machine does not know what your lock needs. It follows the shape of whatever key you hand over, using a guide that traces the pattern while a cutting wheel shapes a blank at the same pace. If the pattern key is worn, the machine reproduces the worn shape faithfully. The result is a brand new key that carries all of the old key's flaws, and sometimes a few extra ones.
This is why people are sometimes surprised that a copy fails when the original still works. The original has been wearing in with the lock for years. The pins and the key have rubbed against each other until they fit together, tired but compatible. A crisp new copy cut from that worn pattern does not match the lock's pins in the same way, so a combination that worked by habit now fails by a small margin.
Think of it like a photocopy of a photocopy. Each generation loses a little sharpness. A key cut from an original is usually fine. A key cut from a copy is a little less accurate. A key cut from a copy of a copy of a copy may no longer be reliable at all. If your key was copied several times in a row, the error has probably accumulated across generations.
Tolerances: the tiny numbers behind a working key
Pin tumbler locks use pins of different lengths, and the key has cuts at specific depths that line up each pin precisely. Manufacturers define those depths in steps, often a few thousandths of an inch apart. That means a cut that is off by even one step can leave a pin sitting just above or below the shear line, and the cylinder will refuse to turn.
Several small errors can combine. The pattern key may be worn by one step in some positions. The blank may sit slightly off in the machine jaw. The cutter wheel may be dull, rounding the bottom of a cut. The machine's guide may be set a fraction wrong. Each is small, and any one might be forgiven by a loose lock. Together they can push a copy beyond what the lock will accept.
Quality blanks matter too. Keys are cut from brass or nickel silver blanks that must match the keyway profile of the lock. A blank from a different manufacturer that looks identical can have a slightly different shoulder position or groove shape. Cuts are measured from the shoulder or tip stop, so a blank that is a hair off will put every cut a hair off. Matching the correct blank to the correct keyway is a basic part of doing duplication properly.
- Worn pattern key transferring worn cuts to the copy
- Blank placed slightly off in the machine clamp
- A dull or worn cutting wheel rounding the cut bottoms
- A blank that is the wrong brand or keyway for the lock
- Burrs left on the key after cutting
Burrs, rough edges and unfinished keys
A freshly cut key often has tiny burrs of brass along the cuts. A well-finished copy has those burrs removed with a brush or a deburring wheel, but a rushed one may not. A burr can catch a pin and stop it from sliding smoothly, and the key will feel gritty or stiff. In some cases the key works for a few turns and then runs smoother as the burrs wear down, which is why a copy sometimes starts working after you have forced it a few times.
Do not rely on that. Pushing a rough key into a good lock can scratch the pins and the keyway and shorten the life of the cylinder. If a new key feels gritty, take it back and ask for it to be cleaned up or recut. If you try to deburr it yourself, only use a soft brass brush, because filing changes the key's cuts.
Finish matters on the key surface as well. A key with a rough, uneven blade can drag against the keyway walls. That friction adds up in tight locks, especially those with restricted keyways or high-security profiles. A smooth, even blade glides in and out and helps the pins do their job.
Worn originals and why re-cutting by key number works
Most locks are pinned to a specification: a series of numbers that describes the exact depth of each cut. A locksmith can often determine that specification from the lock itself or from a record, and cut a key by number instead of by tracing a worn pattern. We call this cutting by key number. Because the numbers describe the ideal key, the result does not carry any wear from your old one.
Where the specification is unknown, a technician can read the pins directly. With the cylinder out, they can measure the pin heights and work backward to the cuts that would lift them correctly. A key made this way matches the lock as it is today, not as an old key remembered it. It is a more careful route than copying, and it is why a visit often fixes keys that several copies could not.
If your lock was keyed at the factory or at a hardware store, there may be a number stamped on the key or on the cylinder face. The guide on how to find your key number explains where to look, though a technician may still need to verify the number against the lock before cutting. Vehicles often have their own record systems, and not every dealer or locksmith has access. When that route is open, it is the cleanest way to a fresh key.
When the lock is the problem, not the copy
Sometimes the copy is actually fine. If your new key is accurate but the lock has worn out, the old key may work only because years of wear have shaped the lock around it. A fresh and accurate key might not fit a worn cylinder as well as the old one did. That seems backward, but it is a real effect, especially on older residential locks with soft brass pins.
A quick test helps. Try the new key in a different lock of the same make, if you have one. Try the new key with the door open to rule out a binding bolt. Try the old key a few times to see whether it works every time or only on some tries. If the old key is also unreliable, the lock is probably worn, and a rekey or replacement is the sensible route. Key works only sometimes covers that case in more detail.
If the lock is the weak link, putting more money into copies is a waste. Rekeying fits new pins and produces new keys to match, restoring tolerances in a single visit. Our lock rekeying page explains how it works and what to expect.
Key types that are harder to copy
Some keys are not ordinary flat brass blanks. Laser-cut or sidewinder keys have a cut milled into the side of the blade rather than along the edge. They need a different kind of machine, and a standard duplicator cannot cut them. Dimple keys have round pits on the face of the blade and need yet another type of cutter. A copy shop with the wrong equipment may attempt to cut a poor substitute, and it will fail.
Restricted or high-security keys carry markings that say do not duplicate, and many are controlled by patent or by authorization cards. A locksmith has to verify authorization before cutting such a key, and in some cases the factory or a registered dealer must supply it. If your key has that marking, expect a different process, one that protects you from unauthorized duplication. High security locks explains the difference.
Vehicle keys add another factor. Many have a transponder chip that must be programmed to the car's computer as well as being cut correctly. A key blank can be perfect and still fail to start the engine because the chip has not been paired. The transponder key programming page explains the second half of that job.
What to do first when a fresh copy fails
Start with a calm check. Does the key go all the way into the keyway? Does it turn easily with the door open? Does it behave the same way every time? Try the key in the lock it was copied for and not a similar-looking one, since many keys share a shape but not a specification. If you have the shop's receipt, note where and when it was cut.
If the copy is clearly wrong, take it back quickly. Many shops will recut a key that does not work, particularly when you bring in the original. Bring the original key and the failed copy together so the shop can compare. Do not keep forcing a key that feels wrong, since a stuck or broken copy in the lock adds a second problem to the first. If the key does jam, see key stuck in door lock.
If you can no longer find a good original, or the original is badly worn, a mobile locksmith can come to the lock, read it and cut keys on the spot. That is often faster than shuttling between shops with a stack of failed copies. The page about mobile key cutting describes how that visit works.
How a locksmith fixes a copied key problem
The technician will examine the original, the copy and the lock. They check the key blank for the correct keyway and brand, look at the cuts for wear and compare the depths against the manufacturer's chart. They test how the lock turns with a known good key if one is available. From there, they pick the cleanest route: cut by number if the specification is known, read the lock if it is not, or rekey the lock if it has worn out.
When you ask for several keys, cut them all from the same accurate source, not one from another. A second cut from a first cut is a copy of a copy, and it brings back the same problem on a smaller scale. A good technician cuts every spare directly from the reference and tests each key in the lock before leaving.
Quotes are given by phone. If the job needs reading or rekeying several locks, an on-site visit may come first. A service call charge applies for dispatch, and it is explained on the phone before anyone comes to you. The phone quote guide walks through what to expect.
If your copy does not work, do not give up and do not keep copying the same flawed key. Call, tell us what happened, and we will explain how to get a key that fits the first time.
Keeping spare keys accurate over time
The best protection against bad copies is a good master pattern. Keep one key in a safe place, rarely used and not carried on the ring, and use it only to cut new keys. Because it never wears, it stays accurate. Mark it clearly so it is not mixed up with daily keys. This simple habit prevents the generational drift that makes copies fail.
Replace daily keys before they are completely worn. A key that has started to feel rough or tight is due for a fresh cut. Waiting until it breaks makes the replacement harder, since you may have only a damaged key to copy. The guide on spare key storage ideas offers practical advice about where to keep a spare safely.
Test every new key right away. Insert it, turn it several times with the door open, then closed, and confirm it unlocks and locks without catching. A few minutes at the door beats discovering a failed spare when you are locked out. If a copy does not pass, take it back the same day with the receipt and the original in hand.
Questions people ask
Why does my new copy fail when the original works?
The original has worn in with the lock over years, while a new copy is cut to tight tolerances traced from a worn pattern. Small errors in the blank, the machine or the pattern key can push a copy slightly off. Recutting from a more accurate reference or from the lock's key number usually fixes it.
Can a bad copy damage my lock?
It can. Forcing a rough or inaccurate key can scratch pins and the keyway, and a key that jams or breaks leaves a fragment inside. If a copy feels gritty or does not turn easily, stop using it and have it recut or checked before it causes a second problem.
Is a copy of a copy really worse?
Yes, because each generation carries the previous key's wear and adds its own small errors. After several generations the cuts can drift beyond what the lock tolerates. Cutting every spare directly from an accurate source avoids the stacking of errors.
Can any shop copy any key?
No. Laser-cut, dimple, restricted and transponder keys need different equipment or authorization. A shop without the right machine may cut a poor substitute. A mobile locksmith can identify the key type and bring the correct equipment to cut, and program when needed.
Should I rekey the lock instead of copying again?
If your original is badly worn or the lock itself has become unreliable, rekeying is often the better route. It fits new pins and new keys that match, restoring tolerances at once. If the original is in good condition, a clean cut from it or from the key number may be enough.
How do I get a quote for a recut?
Call and describe the key, the lock and what happened with the failed copy. Quotes are given by phone, and complicated jobs may need a visit first. A service call charge applies and is explained before dispatch.
Describe the situation by phone and we will tell you what the job involves. Quotes are given by phone. Jobs that cannot be quoted without seeing them need a technician on site, and the service call charge is explained on the phone before anyone is dispatched.
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Written by Claude (AI) (AI) for 24/7 Locksmith Orlando.