MANUFACTURING:
T p Tips for Taps — Part 2
● by Nick Martin
In Part 1 (SERVO May ‘09) of this guide, we examined the theory
of cutting threads and the different
types of taps. This month, it’s time
to get practical and learn some
tricks and techniques for tapping!
to give greater thread depth.
bottoming tap, the hole depth can
be reduced.
Hole Depth
Tapping Tools
Safety
For blind holes, you need to
work out the minimum depth
of the hole to drill. This is the
combination of the selected screw
Tapping can cause
injuries, particularly to your
eyes; wear safety glasses as a
bare minimum. You will often
need to get your face up close
to the part you are tapping.
When a tap fractures violently
or spray oil splashes back, you
will be very thankful for the
eye protection.
To cut threads with a tap, you
need some accessories; in time,
you will probably build up quite a
collection. Following is a list of
the main items you’ll need.
Wrenches
Drilling the Right
Size Hole
Photo 1.
Calculating
drilling depth.
Every thread size has an
ideal hole size. There is a
complicated formula to work
this out and the ideal size is
often not a standard drill size.
The easy way to find the right
drill size is to look it up;
the web has many good
references including www.
nswrfc.org/downloads
/Tapping_drill_chart.pdf.
Charts such as this one
often have several drill choices
for each tap. The larger size
drills produce a lower height
thread that has less engagement
with the screw. The up-side of this
is that the thread will be easier to
cut and the tap is less likely to
break in hardened steel or
titanium. For softer metals and
plastics, choose the smaller drill
Photo 2. T-handle and straight tap wrenches.
length, the tapered section of the
tap, plus a safety margin where
chips can accumulate. That sounds
more complicated than it really is;
Photo 1 explains it easily.
This picture is based on using
just an intermediate taper tap.
If you plan to follow up with a
Tap wrenches come in
many sizes and two basic
forms. The important thing to
remember is that one size
definitely does not fit all; using
an over-sized wrench is a sure
way to break a tap. For taps up
to around 1/4”, a T-handle
wrench is best; its shorter
handle prevents you from
applying too much torque and
the shape makes it easier to
align with the hole. There are
several varieties of this wrench;
some have a ratchet like a
screwdriver, while others have
guides to help align the tap
with your material. For taps
over 1/4”, a straight handle
wrench allows you to apply
more torque to cut threads.
I recommend the
combined T-wrench and guide
(top left in Photo 2). It is by
far the most used tapping tool in
my shop and makes aligning the
tap over the hole a breeze. It’s
available from McMaster (part#
2548A12). The other wrench
worth a special mention is the
machine mounted T-wrench (top
right in Photo 2). This can work as
22 SERVO 08.2009
