Filing cabinets are designed to keep documents safe and are therefore easy to lock. When you lose the key or inherit a filing cabinet with no keys, you may find it frustrating to break into. Some people try to drill out the lock, or bang at it with a screwdriver and hammer but there is no need to get so radical. You can get a lock to open with a few tools and a little patience.
Analyse the face of the lock itself. See if you can find the manufacturers name and the number of the lock etched into the metal. Jot these down on a piece of paper in case you scratch up the face of the lock too much to read it later.
Slide a tension wrench or an Allen wrench into the lock and apply medium but steady pressure in a clockwise motion. Use a paper clip doubled back on itself if you don't have lock-picking tools.
Slide a small flathead screwdriver into the hole of the lock and move the first point of resistance--the first wafer--up and down slightly until you are able to move the tension wrench a little.
Move the second wafer up and down slightly until it gets into the right position to move the tension wrench a little more. Continue with each of the wafers until the tension wrench can twist far enough to open the lock.