Technical Information ISO Metric Tap Class & Tolerance
The ISO metric system of tap tolerances comprises three classes of tap sizes which are calculated from the Grade 5 nut tolerance, irrespective of the nut grade to
be cut as follows:
ISO, Class 1 – Class 2 – Class 3
The tolerances of these three classes are determined in terms of a
tolerance unit t, the value of which is equal to the pitch tolerance value TD2
grade 5 of nut (extrapolated up to pitch 0.2mm):
t = TD 2 grade 5
The value of the tap pitch diameter tolerance is the same for all three classes
1, 2 and 3: it is equal to 20% of t.
The position of the tolerance of the tap with respect to the basic pitch
diameter results from the lower deviation the values of which are
(see Figure 3):
for tap class 1: + 0.1 t
for tap class 2: + 0.3 t
for tap class 3: + 0.5 t
Choice of tolerance class of the tap with respect to the class of thread
to be produced.
Unless otherwise specified, the taps of classes 1 to 3 will generally be
used for the manufacture of nuts of the following classes:
ISO, Class 1: for nuts of limits 4H and 5H
ISO, Class 2: for nuts of limits 6H and 5G
ISO, Class 3: f or nuts of limits 7H – 8H and 6G.
Figure 3
nut limits
8H
7H
6H
5H
ISO tap classes
4H
Class 3
t
Class 1
0.1 t
0.7 t
0.3 t
0.5 t
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pitch diameter of basic profile
Class 2
474