Technical Information British Standard Threads
Basic Profile for Whitworth (BSW, BSF and WHIT.) Thread forms
Figure 5
British Standard Whitworth Form
H
—
6
The sides of the thread form an angle of 55° with one another, and
the top and bottom of the full triangle are truncated one-sixth of
the height. The actual depth of the thread is equal to two-thirds of
the height of the generating triangle and is equal to 0.6403 times
the pitch. The crests and roots are rounded to a radius of 0.137329
times the pitch. Figure 5.
r
H
h
27.5º
r
55º
H
—
6
Pitch P
H = 0.160082P
H = 0.96049P
—
6
Fig 5
2 H = 0.640327P
h = —
3
r = 0.137329P
The Whitworth Screw Thread Tolerance System
Figure 6
nut medium class
basic pitch 100
diameter 66 2 / 3
normal class
150
150
100
150
3/4" dia. and under
bolt
normal class
over 3/4" dia.
150
150
3/4" dia. and under
over 3/4" dia.
fundamental deviation (allowance)
medium class
free class
Pitch diameter tolerance zones of recommended combinations of classes of bolts and nuts having Whitworth screw threads. Figure 6.
British Tap Size Zone Limits
Figure 7
British Standard Zone 3 and Zone 4 limits are normally applied to
Whitworth and BA taps.
The values for position and tolerances are formulated and must be
obtained from the standard’s tables.
The accompanying chart shows the zone limits relationship for ground
threads. Figure 7.
Zone
4
Zone
2
Zone
1
basic
pitch diameter
476
Zone
3
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