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📘 A keyhole hides a rotating plug

On a workbench, a cutaway lock has two cylinders: a fixed shell around a rotating plug. Pin pairs cross the boundary between them—the shear line—so the plug cannot turn until every split aligns.

4
lessons
~20 min
to learn
Adults
level
Start the course →

What you’ll learn

  1. The barrier between two cylindersIdentify the plug, shell, pin pairs, springs, and shear line.A pin-tumbler lock blocks rotation by leaving pin pairs across the boundary between its fixed shell and rotating plug.
  2. A tiny error creates a sequenceExplain how tolerances and torque create the first binding pin.Slight misalignment turns a small plug rotation into feedback about which pin stack is pinched.
  3. The lock supplies its own orderTrace binding order, oversetting, false sets, and security pins.Each set pin changes the geometry that determines the next binding stack, while shaped pins add misleading feedback.
  4. Different attacks, same physicsCompare manipulation and impulse-based attacks through the core mechanism.Different tools exploit the same requirement: every pin split must clear the plug-shell shear line.

Questions this course answers

What is the shear line?

The plug can rotate when every pin-pair split aligns at the plug-shell boundary.

Why does one pin usually bind first under light torque?

Tiny geometric differences cause the most misaligned stack to pinch first as the plug turns.

What changes after a binding pin is set?

Setting one split at the shear line advances the plug and changes contact relationships among remaining stacks.

What is an overset pin?

Lifting a pin beyond the shear line can put the wrong part of the stack across the boundary.

What does a snap gun try to exploit?

A rapid impulse can momentarily separate pin pairs at the shear line while turning pressure is applied.

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