Page 276 - Aeronautical Engineer Data Book

222 Aeronautical Engineer’s Data Book
• Squareness: The formal term for this is
perpendicularity. Simplistically, it refers to
the squareness of the axis of a hole to the
datum surface of the material through
which the hole is made.

12.4.3 Screw threads
There is a well-established system of toleranc­
ing adopted by ANSI/ASME, International
Standard Organizations and manufacturing
industry. This system uses the two complemen­
tary elements of fundamental deviation and
tolerance range to define fully the tolerance of
a single component. It can be applied easily to
components, such as screw threads, which join
or mate together (see Figure 12.7).

For screw threads, the tolerance layout shown
applies to major, pitch, and minor diameters
(although the actual diameters differ).

Fundamental
deviation (FD)
(end of range nearest
ES
the basic size) T Tolerance 'range'
'Zero line' NUT FD El
(basic size) es
BOLT
ei T Tolerance 'range'

FD is designated by a letter code, e.g. g,H
Tolerance range (T) is designated by a number code,
e.g. 5, 6, 7
Commonly used symbols are:
EI – lower deviation (nut)
ES – upper deviation (nut)
ei – lower deviation (bolt)
es – upper deviation (bolt)
Fig. 12.7 Tolerancing: screw threads