I'm going to explain how a pin tumbler lock works first to make things easier
to understand. These are the types of locks on most homes, padlocks and some
automotive locks. They have a tolerance of about 0.002 - 0.005 inches for the
key to work. A lot of key duplicating machines that you find in hardware
stores et cetera are calibrated to cut keys at 0.001 below the original. If
the height on the key cuts (the grooves cut by the machine) is higher on the
copy than the original, the key won't work, if it's lower you can usually
raise up and jiggle the key to make it work. I recommend going to a locksmith
for duplicates because their machines are typically in better tune. Ok, now
back to how pin tumbler locks work. Inside of the pin tumbler lock is a plug
(the small round part you stick your key in) with holes drilled through the
top of it. When your locksmith rekeys the lock he inserts the key that it
should fit into the plug and drops brass tumblers (thick round pins) into
these holes. He uses tumblers that come up even with the surface of the plug
with the key in place. Contained within the part the plug goes into are the
same number of holes drilled through the top and sealed at the top. There are
springs pushing pins downward (they are the same length as each other and the
same diameter as the ones in the cylinder). A plug follower (a half inch shaft
will work) is used to hold these in place when the plug is removed. Once the
locksmith puts the pins in the plug so they are even, he removes the key and
pushes the plug into the lock (with the tumblers in the plug NOT lining up
with the ones in the outer cylinder) displacing the plug follower. He then
turns the plug, the spring loaded pins push into place and he replaces
whatever part holds on the back of the plug to keep it from pulling out. When
the key is inserted, the tumblers line up evenly with the surface of the plug
and allows it to turn. There's a linkage attached to the back of the plug that
turns the actual mechanism that operates the thing the lock switches. Disc
tumbler locks work the same except the tumblers are rectangular discs with
rounded ends and have rectangular holes in the middle of them located at
different heights on each disc. The springs are also located in the plug and
come out with the tumbler. This makes them a little trickier to work on. I
hope this is understandable.
-Alan