ahead of the tap. Even with this
design, I recommend clearing
the chips out of the hole regularly
for cleaner threads and fewer
breakages.
bottom of a blind hole.
Intermediate or Plug
This variation has only 3 to 5
reduced threads and is the most
commonly stocked type. It is a little
harder to align in a hole and cuts
threads almost to the bottom of
blind holes. Don’t worry about
alignment problems; we will
eliminate them forever in Part 2!
or hardened steel, coated taps are
preferred or even mandatory to
avoid breakage.
Spiral
The spiral provides good
transport of chips back out of blind
holes. Low spiral taps work for
materials that produce short chips,
while high spiral taps are meant for
long, stringy chips and deeper holes.
I find spiral taps are more fragile
and would only recommend them
for shallow blind holes.
Bottoming
This last variation has just one
or two taper threads and is only
used to finish threading down to
the bottom of a blind hole or to
clean up damaged threads; you
would never start tapping a new
hole with a bottoming tap! You can
avoid buying bottoming taps simply
by drilling holes a bit deeper than
required and only partly threading
them with a plug tap.
Oxide
Oxide is a good and inexpensive
general purpose coating if you mainly
tap steel parts; it helps to reduce
friction and prevent material sticking
to the tap. It is not recommended
for non-ferrous metals, however, I
have never had a problem with
oxide coated taps in aluminum.
Thread Rolling
This type of tap does not cut
metal but forms it under pressure so
no chips are produced, making it
ideal for blind holes. Only soft and
ductile metals work with roll
forming taps and fortunately
aluminum works very well. These
taps have almost no flutes and are
very strong; if handled correctly,
they are not likely to break.
Lubrication is very important with
this type of tap; it is a specialty item
and is not worth the expense for
most builders.
Nitride
Nitride is another cheap and
versatile coating, mainly for softer
and abrasive materials such as
aluminum, bronze, copper, and
fiberglass. It increases surface
hardness and extends tap life.
Oxide over Nitride
This is my personal favorite,
particularly for titanium. It is a
little more expensive than either
coating alone, but worth it in
harder materials.
Recommendation
I recommend using spiral point
taps where possible; they produce a
very clean thread and are less likely
to break than most other types. If
spiral points are not available, use
straight taps and save a few cents.
Recommendation
The ‘classic’ method of tapping
a hole is to start with a tapered tap,
move to an intermediate tap once
the threads are well started, then
finish with a bottoming tap if you
are tapping a blind hole. This
method sells many sets of taps but is
seldom required; most professionals
would start with an intermediate
tap and only use a bottoming tap if
absolutely required.
Another trick is to grind off the
point on an intermediate tap so it
will cut closer to the bottom of a
blind hole (as shown in Photo 2).
I hardly ever buy bottoming taps
nowadays.
TiN (Titanium Nitriding)
TiN is a general purpose coating
with a gold color. It reduces friction
and adds a little hardness. I notice
little difference to uncoated taps
and the higher cost is hard to justify.
I have had really bad results with
TiN coating in titanium.
Tap Taper
As well as the design categories
above, each type of tap can come in
a different end taper (also called the
end chamfer).
Coatings
TiCN (Titanium Carbonitriding)
TiCN is an improvement on TiN
with lower friction and better wear
resistance. It has a dark blue or
purple color and is mainly used for
machine tapping in very difficult
materials. It is the most expensive
coating and is seldom required
Full Taper
These taps have 7 to 10
reduced height threads at the end
of the tap. The reduced diameter
makes a tapered tap very easy to
align in a hole and start the thread
cutting process. By the same token,
they will not cut threads to the
Surface coatings on taps
improve performance, reduce
wear, and reduce breakage. For
hand tapping in easy machining
metals, uncoated taps work just
fine and the often high cost of
coating is hard to justify. In hard
to machine materials like titanium
PHOTO 2. The different end tapers.
Note that the lower two taps have
their ends ground back.
SERVO 05.2009
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