Have you ever given someone a key to something but then later on decided it wasn't such a good idea and you fear that they have copied it, I recently had that problem. There are plenty of lock smiths that will re-key a lock for a fee or you could go out and buy a new door knob. I was running low on cash and decided to take the knob apart and see if there was a way to change the pins (and my key) in a way that the original key wouldn't work. This is my first instructable and is based on what I did. But first, I am NOT responsible for any damage you do to your door knob.
Step 1: Remove the Knob
My door knob has a small hole between the part that rotates and the part that attaches to the door (refer to picture). All door knobs can be taken apart, yours will probably be different than the picture. When the door knob is turned the correct way there is a button that can be pushed to release the knob. Remove the knob but remember how it came off.
Step 2: Remove and Examine the Lock
On my particular brand of door knob the lock mechanism is only held in by a decorative ring and is easily slid off to remove the lock. Also remember how it came out although it should only fit one way. Once its out, test it and see how it works. Basically the key goes in and each tooth on the key pushes a different pin in the lock to a specified height so that they all align and the barrel can turn freely. there are plenty of sites that explain this much better than I can, How Lock Picking Works. If you open the link for this step keep it open for the next step too.
Step 3: Decide
Now comes the more confusing part. At least one of the pins has to be changed in order for the original key not to work, since you cant add metal to the key you have to add metal to a pin, there are two options; make a longer pin to go into the lock mechanism or "steal" a longer pin from a different part of the lock. I gave it a lot of thought and finally decided to steal the longest pin in the mechanism and use it to replace the shortest pin. Its really difficult to explain, first remove the metal cap that holds all of the springs in the mechanism, the springs fly so do this carefully. Before you go and remove any of the pins take a look at the key you plan on modifying. There will be a series of ridges, one for each hole. Pick the highest and the lowest ridge on the key and figure out which pins sit in those two ridges. Remove the pins and the spring in the hole that corresponds with the highest ridge on the key, put the contents of it off to the side. Now replace everything you just took out with the contents of the hole that corresponds with lowest ridge on the key. So you want to end up with one completely empty hole and one hole that has a different set of pins in it, in addition to all of the pins you didn't mess with. Once you have done this make sure the pins don't fall out and put it to the side. If it is still unclear there is a diagram on the next page.
Step 4: Modify the Key
All you have to do to the key is make it work in the lock. If you followed the last step only one ridge on the key has to be modified, I drew an example. In the picture the red vertical on the key is the highest ridge and the green vertical is one of the lowest ridges. The green pin is moved to the red ridge which is then filed down to the same height as the green ridge on the key. Your key will most likely be different. When filing down your "red ridge" make sure is is exactly the same height as your "green ridge", also make sure the transitions to the ridges around your "red ridge" are slanted enough so that the pins don't get stuck. I used a nail file to file down the key so I wouldn't mess up with my dremel tool, but feel free to use what ever you have on hand. Once you have ground down the key you can dispose of everything that came out of the now empty hole in the lock mechanism, those pieces wont work anymore.
Step 5: Put It Back Together
Now you can test it and put it back together. If you can't seem to get the key to turn, move the pins back to their original places except the the contents of the hole that you disposed of. If it works congratulations you have successfully re-keyed your lock. Just put it all back together and your all set.
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