When selecting standard taps for your tapping application, there are
three basic factors for you to consider:
- The Material to be Threaded
For optimum results, select taps that
are recommended for the material to be threaded; two flute taps are
recommended for some materials in certain sizes and pitches and three,
four or six flute taps for other materials, sizes and pitches.
- The Type of Hole to be Threaded
An open or through hole - passes through the workpiece. Plug chamfered
taps are usually recommended for this type of hole. When the material
to be threaded produces a continuous chip such as steel, aluminum,
magnesium, or a die casting, plug chamfered spiral pointed taps are
highly recommended for open or through holes for diameters through
A blind hole - does not pass through the workpiece. If this type of hole
is required to be threaded close to the bottom, a bottoming chamfered
tap is necessary. If the blind hole is sufficiently deep to allow a longer
chamfer, plug is recommended. In either case, tapping problems are minimized
if space is provided at the bottom of the hole for chip collection.
Select a chamfer according to the type of hole you are going to tap.
- The Class of Thread Required
The Unified Thread Form, designated
UNC and UNF for coarse and fine pitch series respectively, was adopted
as standard by Canada, Great Britain and the United States. It is mechanically
interchangeable with the American National Thread Form, NC and NF,
but there is a difference in the pitch diameter tolerances. In the
America National, pitch diameter tolerances on both the nut and screw
are equal with the nut above and the screw below basic, but in the
United Series, the tapped hole has 1.3 times the tolerance of the screw
for the corresponding class. The United Thread classifications 2B and
3B, signify internal thread and 2A and 3A, external thread.
NOTE: The above recommendation table is to be used for general recommendations.
A wide selection of optional standard taps is available with variations
in flutes and chamfers to meet specific requirements.