You don't fool around with home security. The front door locks automatically when you leave. And there's no way you'd be a big enough sucker to hide a spare key beneath the welcome mat. But what will you do if you lock yourself out? Smash your own window? Not a chance.


Instead, plan to pop the lock like a pro—with a bump key. The filed-down key can open the lock, but using it correctly takes some skill. Stash a bump key by the front door, and you've covered yourself without creating an open invitation to intruders. Here's how to make the best tool you can use for a DIY B&E.

You Should Do This If:

You tend to forget the "Phone, Keys, Wallet" mantra before you walk outside.

Moment of Satisfaction:

Having enough finesse to bump open a lock faster than using the actual key.

Biggest Pain in the Ass:

Not smashing your fingers when hammering the key home.

Materials and Tools Required

(1) key blank matched to the model of your lock

(1) anchored vise

(1) round file

(1) triangular file

(1) fine Sharpie

(1) ruler

(1) sheet of 300-grit sandpaper

(1) bump hammer, rubber mallet, or screwdriver

Difficulty and Cost

Easier than picking the lock and cheaper than calling a locksmith.

Building It

1. Figure out your key type and lock model

Check the original key's face to find the alpha-numeric model—the SC1, WK2, and KW1 are popular key types in North America. Head to the hardware store and pick up a blank (uncut) key of the same type.


2. Sketch the template

Line the original key up even with the blank. Find the lowest valley among the teeth and mark it with the Sharpie. Using the ruler and Sharpie, draw a line from that point, along the length of the blank. This will be the stopping point for the file. Then, sketch the original key's teeth on the blank as a series of 5mm-tall triangular humps.

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3. Start filing

Insert the blank into the vise and start filing. Use the round and triangular files to make the blank's teeth match the outline—compared to the original, the new teeth should have the same height and spacing, with the first tooth sitting at the tip of the key. Smooth the file's burrs along the key's edges with a bit of 300-grit sandpaper.


4. Put it to work

Proper bumping technique takes a little while to get down. But once you do, it becomes second nature. Insert the key roughly 3/4 of the way into the lock—the rearmost tooth should be just peeking out of the tumbler. Apply gentle lateral pressure to the key, as if you were turning it normally in the lock. Then strike the key, using the bump hammer, rubber mallet, or the butt of a screwdriver.


The impact shoves the bump key into the lock. Done correctly, the teeth cause the individual pins to jump free of the shear line and release the tumbler.

Troubleshooting

If the bump key doesn't slide smoothly into the lock, don't force it. Use a bit of dry graphite lubricant to reduce the friction and try again. If it still doesn't fit, you may need to get back to work with the files and sandpaper. Too much excessive force can destroy the lock, requiring it to be drilled out of the door and completely replaced.


If you give up completely, you can also order a set of bump keys, but you better be a licensed and bonded locksmith if you're thinking about doing that.

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