Taps and dies are cutting tools used
to create screw threads in solid substances including, but not
limited to, metal, wood, and plastic.
A
tap is a bolt-like tool for making female threads. The
process of cutting the threads in a hole is called "tapping"
the hole.
A die is a
nut-like tool for making male threads. The process of
cutting with a die is called "threading".
A tap cuts a
thread on the inside surface of a hole, creating a female surface
which functions like a nut. The die cuts a thread on a
preformed cylindrical rod, which creates a male threaded piece which
functions like a bolt.
Both tools have threads, but the threads on taps and dies have
cutting edges to cut threads in smooth materials. A tap screws into a
smooth hole and makes it into a threaded hole; a die puts threads
onto a formerly smooth shaft.
Taps come in three types: plug, taper,
and bottoming.
Plug taps are the most commonly seen. A plug tap has
a fairly steep taper at its point, so that it will cut full depth
threads after only a few turns. This is the most efficient type for
production use.
Bottoming taps have no taper at all.
As a result, bottoming taps cannot be used to start a new thread.
Bottoming taps are used only to bring an existing thread down to the
bottom of a "blind" hole (a hole that does not go all the way
through the material).
Taper taps are like plug taps, except that they
have a more gradual taper to the point. The gradual taper makes it
easier to tap a straight hole by hand. This is the one primarily
used in the lab.
Taps are metal cutting tools, and
therefore need to have very hard cutting edges. Because of the type
of steel and heat treatment required to enable them to cut metal,
they are extremely brittle. Considerable care is needed to use taps
without breaking them. This is especially so in the case of the
smaller size. If you break a tap off in a hole, it can be very
difficult or even impossible to remove the broken piece! A broken
tap cannot usually even be drilled out, because the tap is as hard
as or harder than the drill bit.
Taps are usually broken by bending,
not twisting.
Break Up Chips
As the tap cuts, it makes 'chips' of
metal that has been cut away. The tap has three or four "flutes"
(grooves) to allow the cutaway chips to escape from the hole. If the
flutes become clogged with chips, the tap will bind. If you try to
force it, it will break.
The chips tend to be formed in long
strips. Good tapping technique requires that you break up these
strips into smaller pieces so that they can escape along the flutes. This is
done by turning the tap backward.
The
procedure is to turn the tap
forward one-third to one-half turn, or until you feel increased
resistance. Then turn it backward until you feel it turning more
easily one-quarter to one-half turn, typically. Generally, you should use some sort of cutting fluid
with taps. There are special cutting oils made for the purpose, but
these are not usually needed except when using power tools.
Generally, hand tapping of steel, any convenient oil will do nicely.
Correct Hole Size
It is essential that you drill the
correct size hole for the size of tap you plan to use. If the hole is
too small, you are more likely to break the tap when trying to cut
the threads. If the hole is to large, the tap will go through
easily or make poorly defined threads that are shallow and easily striped.
Tap Drill Chart For Some Common Sizes
Drill bits come in four size systems: fractional
(AKA - imperial, SAE),
number, letter, and metric sizes (see drill bit set in lab).
There are two threading systems in common
use: SAE (American) and ISO (metric).
SAE threads are described in the form
(diameter)-(threads per inch)-for instance: 1/4-20 means that the
bolt is 1/4-inch in diameter and has 20 threads per inch. Diameters
smaller than 1/4-inch are specified by a number size. For example,
#10-32 is a very common size; the "#10" is equivalent to 3/16-inch
and there are 32 threads per inch.
Metric threads are specified in the
form (diameter)-(millimeters per thread). For example, 5 mm-0.8 means
that the diameter of the bolt is 5 millimeters and that the threads
are 0.8 millimeters apart. ISO metric threads are standardized to the
extent that the thread pitch is not usually mentioned, unless it is
non-standard for the diameter for example, 5 mm-0.8 is often just
called 5 mm, but if the thread pitch is other than 0.8, it would have
to be specified. The same for, 6 mm-l.0 is a standard ISO size, and
could be referred to as "6 mm."
Threading gauges,
pictured above, are also referred to as pitch gauges and are used to
measure the pitch or lead of screw threads. The uppermost gauge in
the image is an ISO metric pitch gauge, the larger gauge in
the center is for measuring the Acme Thread Form, and the
lower gauge is for imperial or (SAE) screws.
Thread pitch gauges are used as a reference tool in determining the
pitch of a thread that is on a screw or in a tapped hole. This tool
is not used as a precision measuring instrument. This device allows
the user to determine the profile of the given thread and quickly
categorize the thread by shape and pitch. This device also saves
time, in that it removes the need for the user to measure and
calculate the thread pitch of the threaded item.
