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TAPS ENGINEERING DATA
TAP CLASSES AND SIZE
CLASS OF FIT / CLASSES OF THREADS / TAP SIZE
DRILLS CLASSES OF THREADS AND TAP SIZE: TAP SIZE: Due to material variability and
There is a direct relationship between machining conditions, taps rarely cut
END MILLS the size of a tap and the size of the their own size. The thread size produced
thread that it cuts. Size refers to pitch is usually larger, but can be smaller
ROUTERS diameter and its relationship to the class due to shrinkage. Tap manufacturers
of fit required. If two threaded parts are realized that to tap a specified class of
THREAD MILLS assembled, the looseness or tightness of thread, several different ground thread
& TAPS the fit is determined by contact on the tap limits would be required. These
flanks of the threads only. This contact is limits represent small, defined variations
ENGRAVERS controlled by the pitch diameters of each in tap size. A numbering system was
part. developed to designate each series of
BORING BARS limits, but these limit numbers are not to
CLASSES OF THREAD: When threaded be confused with the classes of threads.
REAMERS parts are mated, the two parts must Ground thread tap limits are designated
assemble with a degree of tightness by the letter H (high) above basic pitch
dictated by the use of the fastener. In diameter, or L (low) below basic pitch
addition, the internal thread must be diameter, and these numbers establish
large enough to allow the external thread to enter it for the the tolerance range in relation to basic pitch diameter. As an
required length of engagement. A system of thread classes, example, in sizes 1” and smaller, an H1 tap has a tolerance
each representing a comparative degree of tightness, has been range of from basic to .0005” over basic; an H2 tap from
established and universally adopted, to provide manufacturers .0005” over basic to .001” over basic, (see Chart 1A on
and users of threaded products with a common language this page). In addition, metric threads are also designated in
of specification. The thread classes designate minimum and much the same way. The thread tap limits are designated by
maximum pitch diameters for internal and external threads. the letter D (ground, high) above basic pitch diameter, or U
It is important to remember that classes of thread actually (ground, low) below basic pitch diameter. As an example, in
represents manufacturing tolerances. The closer the tolerance sizes M25 and smaller, a D1 tap has a size of .0005” over basic
required, the higher the cost involved in producing the parts. to tap max. P.D.; a D2 tap has a size of .001” over basic to tap
Therefore, designers and engineers should always try to select max. P.D., (see Chart 1B). The Tables on pages 191-193 list
the class of thread with the widest permissible tolerance. recommended limit numbers for different classes of thread.
Several different limit numbers are available for each diameter
SAWS and pitch combination. Consequently, it is possible to select
the “H” or “L” limit, or the “D” or “U” limit most suitable for
the required tapping operation. Please contact our Customer
Service Dept. for questions regarding tap limits and their
relation to classes of fit.
TECHNICAL CHART 1A CHART 1B
Pitch Diameter Limits for taps to 1” diameter inclusive: Pitch Diameter Limits for taps to 1” diameter inclusive:
INDEX (Metric taps generally have more manufacturing tolerance
L1 = Basic to Basic minus 0.0005 than 0.0005 to the minus side.)
H1 = Basic to Basic plus 0.0005
H2 = Basic plus 0.0005 to Basic plus 0.0010 U1 = Basic minus 0.0005 = min. tap P.D.
H3 = Basic plus 0.0010 to Basic plus 0.0015 D1 = Basic plus 0.0005 = max. tap P.D.
H4 = Basic plus 0.0015 to Basic plus 0.0020 D2 = Basic plus 0.0010 = max. tap P.D.
H5 = Basic plus 0.0020 to Basic plus 0.0025 D3 = Basic plus 0.0015 = max. tap P.D.
H6 = Basic plus 0.0025 to Basic plus 0.0030 D4 = Basic plus 0.0020 = max. tap P.D.
Taps larger than 1” dia. are ground to a 0.0010” tolerance D5 = Basic plus 0.0025 = max. tap P.D.
on the pitch diameter and are, for example, D6 = Basic plus 0.0030 = max. tap P.D.
H4 (Basic plus 0.0010” to Basic plus 0.0020” ).
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