A Text Book on Engineering Graphics - Central Board of Secondary ...

MACHINE DRAWING
(v) RIGHT HAND AND LEFT HAND THREADS
Consider any nut and bolt. Hold the bolt firmly in left hand and rotate the nut
clockwise by the right hand, the nut will screw on the bolt of the threads are right
handed. It is abbreviated as RH thread. A left hand screws thread when assembled
with a stationary mating bolt, screws off the bolt for clockwise rotation. It is
abbreviated as LH thread.
Observe that mostly the bolts and nuts that we use in daily life have RH thread.
Also we can observe that all the jewellery mating pieces have LH thread.
(vi) PITCH, P
It is "the distance between the corresponding points on the adjacent threads,
measured parallel to the axis". Refer FIG2.2 (a)
(vii) LEAD,L
It is "the distance moved by a nut or bolt in the axial direction in one complete
rotation".
(viii) SINGLE START AND MULTI START THREADS
(ix) CREST
(x) ROOT
(xi) FLANK
When only one helix, forming the thread runs on a cylinder, it is called as single
start thread. If more then one helices run on a cylinder, it is called as multi start
threads.
i.e. L=P in the case of single start
L=2P in the case of double start
L=3P for triple start and so on.
It is the edge of the thread surface farthest from the axis, in case of external
thread and nearest to the axis, in case of internal thread
It is the edge of the thread surface nearest to the axis in case of external thread
and farthest from the axis, in case of internal thread.
The surface connecting crest and root is called as flank.
(xii) THREAD ANGLE
It is "the angle between the flanks measured in an axial plane".
ENGINEERING GRAPHICS 43