Exercise 2: Nail Clippers

Description Case 2:  Nail Clippers

     Description                                  Analysis

Nail clippers combine two steel          FEATURE:
levers to make a strong, stable          WHY:
tool that clips off the end of a
finger nail with little applied
force and much control.

Clippers consist of three steel          FEATURE:
strips about 1 cm wide, 5 cm             WHY:
long, and 1 mm thick.
A steel post (3 mm in diameter
and 1 cm long) connects all
these strips (Fig. 1).
The bottom strip is riveted
to the post at right angles;
the other two strips fit over
the post through a circular
hole in each that lets them
move freely along its length.
                                         FEATURE:
The top strip forms the handle           WHY:
of the clippers.  It bends
upward at a 45-degree angle
about one fifth of the way
from the end that passes over
the post, against which the
the handle's short end pivots.

The bottom strip is straight,            FEATURE:
with a short 90-degree bend and          WHY:
beveled cutting edge on the end
nearest the post.

The middle strip gently
bends upward about 10 degrees
near the end away from the post.
It is welded at that end to the
bottom strip (below it).
At the other end, which is free
to move, it has a short vertical
section (bent toward the lower
strip),  also with a beveled
cutting edge.

The handle (top strip) forms a           FEATURE:
second-class lever, with its             WHY:
fulcrum at the post (F in
Fig. 2).  Gentle force moves
the long end through a long
distance, applying high force
(at the bend) to the middle
of the strip below it.

The middle strip forms a                 FEATURE(S):
third-class lever, with its
fulcrum (F) at the welded                WHY:
end.  High force applied to
its middle by the handle bend
(above it) moves the cutting
edge gently through enough
distance to meet the facing
edge below it, carefully
cutting any finger nail
inserted between the beveled
edges.