The wedge
An axe is a good example of a wedge, as is any cutting or chopping
edge, such as a knife, a chisel, a pin or a nail. In these cases and
in all wedges, there are two or more inclined planes tapering to an
edge or a point.
When using an axe the sharp edge penetrates the surface of a log of
wood, then the sides of the wedge split it apart by applying a large
sideways force. Friction reduces the efficiency of a wedge so sometimes
extra force has to be applied by a heavy hammer.
A blow with a hammer forces the point of a nail into wood. If you
examine the point of a nail carefully you will see that it is really
four wedges, enabling it to pierce wood easily, then push the fibres
apart. This last action increases friction between the wood and the
nail, holding the nail firmly in place. Sometimes a nail splits the
wood when the sideways force is more than the wood grain can withstand.
A pre-drilled hole helps prevent this from happening.
More uses for wedges
Other examples of wedges in action are scissors slicing through
paper and pushing the separated halves apart, a chisel slicing a thin
layer from wood, and a guillotine slicing through paper or metal.
A wedge can be used to keep a door open. Friction holds the wedge to
the floor and the door is prevented from rising further up the wedge
by its hinges. This is a simple application of the wedge and the reverse
can apply. By tapping a wedge into a gap a heavy object can be raised
and secured.
What about wedge angle?
The wedge of a sharp knife has a very small angle between
the sides of the wedge. This means that it can cut easily but is not
very good at splitting. An axe has a larger angle between the wedges
and can cut and split wood fairly effectively. A blockbuster on the
other hand has a high wedge angle so is not good for cutting but excellent
for splitting logs for example where there is already a fine split.
Sometimes a combination of angles is used, for example in a chisel.
The first wedge has an angle for cutting, then the angle changes to
push the shaving away from the work.
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