THREADING
TOOLS
S O M T A P 1 2 C A T A L O G U E
182
Tap Terminology
TAP THREAD TOLERANCE
Basic sizes and tolerance classes
To allow for clearance between mating internal and external threads, taps are manufactured with oversize allowances
added to the basic diameters.
These basic diameters plus the oversize allowances establish:
a) the minimum effective diameter; and
b) the minimum major diameter.
NUT
BOLT
Limits of Tolerance
Effective Diameter - The tolerance is the amount of variation allowed in the manufacture of the tap. This tolerance is
added to the minimum effective diameter to establish the maximum effective diameter.
It follows that:
Basic Effective + Oversize = Minimum Effective
Basic Effective + Oversize + tolerance = Maximum Effective.
The effective diameter can only be measured with special tap measuring equipment.
Major Diameter - The minimum major diameter is established by adding the oversize allowance to the basic major
diameter (the nominal thread size). Therefore, on measurement, the major diameterof the tap is larger than the nominal
thread size, and must not be used to judge the size of the tap.
The maximum major diameter of the tap is governed by the thread form and is therefore not subject to a tolerance.
Tap Tolerance Classes
Relationships of Tap Classes to Nut Tolerances.
Metric Threads
Unified Threads
ROOT
THREAD
1/2
WIDTH OF
CREST
MINOR
EFFECTIVE DIA.
MAJOR DIA.
TRIANGULAR
CREST
BASIC
WIDTH OF
ROOT
MINOR
EFFECTIVE DIA.
MAJOR DIA.
1/2
PITCH
6G
5G
4G
MINIMUM
NUTS
8H
7H
6H
5H
4H
MIN.
BASIC
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
TOLERANCE BAND
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
CLASS 1
CLASS 2
CLASS 3
TOLERANCE BAND
NUTS
1B
2B
3B
MINIMUM ALL NUT BASIC
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
CLASS 1
CLASS 2
CLASS 3
Class 1 Tap
This is the closest to basic, having little oversize allowance, and is normally specified for “close” fit threads, eg. Unified
3B, Metric 4H, 5H.
Class 2 Tap
This is normally specified for “medium” fit threads, eg. Unified 2B, Metric 6H, 4G, 5G.
Class 3 Tap
This is furthermost above basic size and used for “free” fit threads, eg. Unified 1B, Metric7H, 8H, 6G.
