Step 4 of 6
Pin tumbler lock step 4 of 6: Hitting the shear line
All the way in, the key’s shoulder against the plug, each key pin sits in its own cut. The right key’s cuts are exactly as deep as its key pins are long, so every key pin’s top lands right on the shear line and every driver pin sits wholly in the housing: nothing crosses the gap any more.
Tools: none, just your hands
Common mistake: a wrong key, or a badly cut tooth. A key cut wrong at one place, or a different key that nearly matches, lifts that pin too far: here the third cut is 1.2 mm shallower, so the third key pin pokes 1.2 mm up into the housing. Four pins line up, the fifth crosses the shear line, and the plug jams however hard it is turned. Forcing it only bends the key: use the right key, or have a copy cut again from the original. The switch in the player shows what goes wrong.
All 6 steps
What is in it
- Housing · the fixed brass body with five pin chambers
- Plug · the part that turns, with the keyway
- 5 key pins · of different lengths
- 5 driver pins · all the same length
- 5 springs · on top of the stacks
- Key · five cuts, each a different depth
- Cam · at the plug’s back, throws the bolt
Questions
How does a pin tumbler lock work?
Stacks of pins cross the gap between the turning plug and the fixed housing, so the plug cannot turn. The right key lifts each stack by exactly the right amount, so the gap between every key pin and its driver pin lines up with the gap between the plug and the housing, the shear line, and the plug turns.
What is the shear line?
The thin gap where the plug meets the housing. The plug can only turn when no pin crosses it.
Why will a wrong key not turn?
Its cuts are not the right depths, so at least one pin stack stops too high or too low and a pin still crosses the shear line, blocking the plug.
What are the springs for?
They push each pin stack down: onto the key when it is in, and across the shear line when it is out, so the lock locks again as soon as the key is taken out.
How does turning the key move the bolt?
A cam at the back of the plug turns with it and, past a quarter turn, catches the bolt and slides it back into its case.
Safety
Taking a lock cylinder apart lets its small springs and pins spring out, and a lock put back together wrong can fail to lock: have a locksmith service or rekey a door lock.
About this guide
The cylinder here is a common 5-pin design of our own, cut away down its middle so the pins show. Its key follows Schlage’s standard spacing and depths: cuts 3.97 mm (0.156 in) apart, each depth step 0.381 mm (0.015 in), cut with a 100-degree cutter. The plug, the pins’ lengths and the key’s other sizes are our own, in proportion.
The torque readings are illustrative, to show when the plug is blocked, turning freely or pulling the bolt; they are not measurements of a particular lock.
Sources: Allegion: Schlage’s combinating rule (cut spacing, depth steps, cutter angle), LockWiki: the shear line.
The 3D models, drawings and text are our own.