Page 1932 - Master Catalog 2017, Inch

Technical Information
Tolerances Over Basic Pitch Diameter

Internal Unified Thread
For the tap user’s convenience, new
generation WIDIA-GTD ™ taps are sized for
the intended class of fit of the internal
screw thread. ISO standard 2857
specifies the manufacturing tolerances
of the tap pitch diameters for ISO metric
threads of tolerances classes 4H to 8H
and 4G to 6G.

Examples:
4H tap corresponds to ISO 1
6H tap corresponds to ISO 2
6G tap corresponds to ISO 3

Similarly, ASME B1.1 recommends tap
pitch diameter tolerances for 1B, 2B, and
3B classes of fit.
1B However, it is recognized in many tapping
applications that taps should have a pitch
diameter that is larger than industry
standards. These applications include
tapping abrasive materials or CNC
applications where machine and tool
2B
concentricity is excellent. Taps
manufactured to WIDIA ™ standards are
designated with an X after the class of fit
3B
(example 6HX or 3BX). These graphs
illustrate the differences.

nominal pitch diameter (basis)

Internal Metric Thread

ISO 3
Technical Information ISO 2 7G

ISO 1 6H 6G

4H

nominal pitch diameter (basis)

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