Why duplicate keys sometimes fail

Anyone who has copied keys has met the disappointing duplicate. It slides into the lock, it looks like the original, and it refuses to turn, or it turns only with coaxing. The problem is that a pin tumbler lock is very picky. Each pin has to rise to exactly the right height, and an error far smaller than the thickness of a fingernail can stop the plug from rotating.

Because the tolerance is so small, a duplicate can fail in many ways. The cut depth may be off, the spacing may drift, the blank may be a different profile, or a rough edge may snag a pin. Most of these errors are invisible without careful measurement, which is why a copy that seems perfect can still fail at the door.

Picture a pin sitting on a small spring inside the lock. The key has to push it up until the gap between two pieces lines up exactly with the edge of the plug. If the pin ends up a hair too high it blocks the turn just as surely as a hair too low. Failure is not a sign that you did something wrong. It is a normal part of the process. A good key cutter expects to test and adjust copies, and will usually try the new key in a lock before handing it over, because a trial on the spot catches problems that no visual check can. What matters is knowing the likely causes, so you can spot them and ask for the right fix.

Keys wear down. Every turn in the lock rubs the cuts against the pins, and over time the sharp peaks round off and the valleys get shallower. A key that has been used daily for years has lost some of its shape, even if it still works.

A duplicate traces the worn shape, not the original design. The machine follows the old key and cuts a new blank to match. The new key therefore inherits the wear and has the same slightly rounded cuts. Sometimes the lock itself has worn to match the old key, so the worn key works and the fresh copy, which is cut crisply, may not. The pins and the plug settle over time into a small groove of habit. A worn key follows that groove, and a sharp new key asks the lock to do something it has forgotten how to do.

The reverse happens too. A worn original can work in a worn lock, while a new copy with sharper cuts may bind. If a copy fails and the original is well used, this is a likely cause. The guide on what happens when a key wears down explains how to tell.

Every copy loses a little accuracy. The tracing process has a tolerance, so each new key is slightly off from the one it came from. When you copy a copy, the small errors add up. After two or three generations the key can drift enough to stop working.

This is why a locksmith will often ask for the original, or the cleanest key you have. It also explains why a key that your neighbor copied for you from a copy may not work at all. If you have a family of copies made over many years, the sensible move is to retire the oldest and make fresh ones from a trusted source, then mark the new set so you can tell it from the old. Throwing away the tired copies, or keeping them only as tools for a lock you do not mind, prevents the mix up that makes people carry a key that almost works.

Where the lock has a code, cutting by code gives a key that does not depend on any previous copy. The locksmith uses the depth and spacing numbers to make a key that matches the design of the lock. This avoids the drift. Our key programming and automotive locksmith pages describe code based work for vehicles.

Copies fail when the blank does not match the original. Two blanks can look alike but differ in width, groove depth or length. A blank that is a little too thin can wobble and fail to lift the pins uniformly. One that is too thick may not enter at all.

Length matters, too. Some keys stop in the lock against the shoulder, and some against the tip. If the blank has the wrong stop, the key will sit in a different place and every cut will be misaligned. The cuts may be perfect for the blank but wrong for the lock. Imagine a key blank that is a few thousandths shorter than the original. Every cut ends up shifted along the blade, and the key looks just like the original at a glance while lifting every pin to the wrong place.

A careful cutter compares the new blank with the original before cutting, matching the profile and the length. If a copy fails, ask whether the blank type was verified. See keyways and key blanks explained for how profiles and blanks relate.

• Compare the new blank to the original before cutting.
• Check whether the key stops on the shoulder or the tip.
• Match the width and groove profile to the original.
• Ask the cutter to test the copy in the lock.
• Bring the lock's key if it is unusual or old.

Key cutting machines are precise, but they need care. The cutter wheel wears, the guide that traces the original can get dirty, and the clamps that hold the keys can shift. A machine that is not correctly aligned will cut every key a little off.

Even a good machine depends on the operator. If the original key is not seated squarely in the vise, the copy will be cut at a slight angle. If the blank is not clamped against the same reference as the original, the depths will be wrong. Small errors in setup become real errors in the key.

In busy shops, where keys are cut in large numbers, mistakes are more common. A locksmith working at your door has the lock to test the key on the spot, which closes the loop. If the key fails, adjustments can be made and the key recut. That ability to test is one of the advantages of a mobile service. Our van carries the cutting equipment, so a key can be made, tried in the cylinder it is meant for, and adjusted until it turns the way the original did when it was new. Rural customers in Shasta and Siskiyou counties often value that, since a second trip to town for a recut key is a real cost in time.

Cutting leaves a rough edge, called a burr, along the new cuts. Burrs are tiny curls of metal that cling to the edge of the key. A burr can catch on a pin, jam in the keyway or leave filings in the lock. A copy with a burr may refuse to enter or may feel gritty.

A good cutter removes burrs with a wire brush or a deburring wheel after cutting. If the key is not cleaned, you may notice metal dust on your fingers or a rough feel along the blade. Running a fingernail along the edge can tell you if it is smooth. A rough edge is easy to fix, so do not accept a gritty key.

Burrs can also harm the lock. Metal bits can travel into the cylinder and cause wear. If you suspect a burr, ask for the key to be cleaned before you use it. Do not force a key that feels rough. A light pass with a brass bristle brush clears most burrs, and the cleaning takes seconds. Avoid using sandpaper or a file on the cuts yourself, because even a small stroke changes the depth and turns a rough key into a wrong key. Forcing can bend the key or jam it in the lock.

Keys are made from different metals. Brass is common, but some blanks are nickel silver, steel or zinc alloy, and they vary in hardness. A soft blank wears down sooner, while a very hard blank can be hard to cut precisely. A low grade blank of poor quality may bend or break.

A bent key looks fine but will not align the pins. Keys bend from being used as pry bars, from being forced in a stiff lock or from being sat on in a pocket. If the original is slightly bent, the copy will not copy the bend, so the copy may work while the original does not, or the other way round.

Check your keys by laying them flat on a table. A bent key will rock. If your original is bent, straightening it can be risky because it may snap. It is often safer to have a new key cut by code or from a good original. Our team can advise you at the door. If a key shows signs of a bend, we will recommend cutting a new one from the code or from a good original, rather than trying to bend the old one back. A bent and straightened key is weaker and more likely to snap in the lock.

Sometimes the copy is fine, and the lock is the problem. A dirty or worn cylinder can reject a new key that is cut cleanly, while accepting a worn key that has adapted to it. Dust, old grease and worn pins change how the key feels. A lock in a dusty barn or a shop with sawdust in the air collects grit that a clean house lock never sees. Lock cylinders on outbuildings and gates fail this way often, which is one reason copies seem to work in some doors and not others.

Test the new key in more than one lock if you can. If it works in another lock of the same type, the key is probably fine, and the problem lies with the cylinder. A little cleaning and a light dry lubricant, as described in lock lubricants, what helps and what hurts, may help.

If the lock is worn and the keys do not turn smoothly, the cylinder may need to be rebuilt or replaced. Rekeying the cylinder to a fresh key set can also settle things. Our rekeying and lock repair work cover these situations.

Vehicle keys fail for extra reasons. A traditional cut can be perfect and the car still will not start, because the transponder chip in the key is not programmed to the vehicle. The ignition may turn, but the immobilizer will not release the engine without the right signal. The car quietly asks the key to identify itself, and if the chip stays silent or gives the wrong answer, the engine will crank without starting or will not crank at all.

Some people copy a car key at a hardware store and are surprised that it opens the door but does not start the car. The cut part works and the electronic part does not. Others have a fob that works for the remote functions but cannot start the engine. These are different systems that must be matched.

For vehicles, the safest route is a locksmith who can cut and program. Read transponder keys explained and a spare key now versus later to see why waiting until every key is lost makes the job harder. Our car key replacement page describes what we do.

First, do not force it. A key that resists can bend or break, and a broken key in a lock is a bigger problem. Take the key out gently and compare it with the original. Look at the length, the width and the pattern of cuts. Check for burrs. Hold the two keys side by side against a light, with the shoulders level, and see whether every peak and valley lines up. If one cut sits visibly deeper or shallower, you have found the fault, and you can point to it when you talk to the cutter.

Second, return to the cutter and explain what happened. Ask them to test the key in the lock, adjust it or cut a new one. A reliable cutter will want to make it right. If the original is worn, consider having a key cut by code or rekeying the lock so that new keys fit well.

Third, if you are in a hurry or the lock is a security concern, contact a locksmith. Our team brings key cutting equipment to your door, so we can test each key in your lock on the spot. Use the request form to describe the lock and the key. The phone line is coming soon. For household work see the residential locksmith page.