for hand tapping.
Uncoated taps work perfectly
fine in softer materials; if that is all
you work with, then save some
money. If there is some chance you
will need to tap harder or abrasive
materials, then the fairly small extra
cost of oxide over nitride is worth it.
Many people buy TiN coated taps
purely for the bling value. If the
extra cost is small, then go for it,
but I don’t generally see much
Taps are made from various
steel alloys; as always, higher
performance costs more! If you
only need to tap a few holes in
softer material, then the cheapest
HSS steel is all you need. For longer
tool life and for tough materials,
the more expensive alloys are
sometimes necessary. For
cheapskates like me, part two will
cover sharpening taps to get the
maximum life from HSS.
This is your cheapest option —
too cheap in my opinion. If you
need a few holes tapped in a soft
material, this is the most economical
option; just remember the taps will
go blunt quickly and are often less
This is the best choice for most
applications. Taps will last well in
aluminum and mild steel but will
blunt quickly in alloy steels and
are risky to use in hardened steel
Cobalt and Vanadium Alloys
Taps with cobalt and vanadium
are only required for tapping
hardened steel, titanium, or very
abrasive materials like fiberglass.
Some brands are quite brittle and
require great care to use.
Thick Threads vs.
The many standards for thread
geometry could fill a book; however,
readers of SERVO are only likely to
be interested in three of them. They
are UNC (Unified National Coarse),
UNF (Unified National Fine), and
metric. Most thread standards have
different pitches. As the pitch
becomes finer, the threads become
smaller, shallower, and closer.
Unified National threads are
specified in TPI (threads per inch),
while metric is specified in thread
pitch (the distance between
For most applications, coarse
threads have these advantages:
• Easier and faster assembly, with a
reliable start and less chance of
• Nicks and burrs from handling are
less likely to affect assembly.
• They are less likely to seize in
extreme temperatures and in
joints where corrosion will form.
• They are less likely to strip when
threaded into lower strength
• More easily tapped in brittle
materials and materials that
In some areas, fine threads are
• Screws with fine threads are
about 10% stronger that coarse
threads due to their greater cross-section area. This is a win when
the tapped material is hard, but is
not so important in soft materials
where fine threads are more likely
to pull out.
• In very hard materials, fine
threads are easier to tap. You are
much less likely to break a fine
tap in hard steel and titanium.
• Fine threaded screws can be
adjusted more precisely because
of their smaller helix angle.
• Where the length of engagement is
limited, they provide greater strength.
A fine thread is usually best when
tapping thin sheet metal.
• Thinner wall thickness can be
used because of their smaller
thread cross section.
As a rule of thumb, always
select a coarse thread unless there is
a specific reason from the above list
to use a fine thread. I stick to coarse
threads where possible simply to
reduce the number of taps and
screws I have to buy.
Next time, I will move on to the
practical side of cutting threads, with
plenty of techniques and tips. SV
Event Results and Upcoming Events
Completed Events Feb
10 to Mar 15, 2009
March 14 & 15, Newcastle, UK.
2009 Fighting Robots UK
2009: Peoria, IL,
24 SERVO 05.2009