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

(i) EXTERNAL THREAD
MACHINE DRAWING
It is a continuous helical groove or ridge cut along the external surface of the
cylinder, e.g. threads on bolts studs, screws etc. FIG 2.2(a) shows an external
thread.
(ii) INTERNAL THREAD
It is a thread on the internal surface of a hollow cylinder. FIG 2.2(b) shows the
internal threads, e.g. threads of a nut.
(iii) SCREW PAIR
The bolt and nut together called as screw pair. One or more such pairs are used to
join two parts.
(iv) PARALLEL AND TAPER THREAD
A thread formed on the surface of a cylinder is called as parallel or straight thread.
Refer Fig 2.3(a)
(A) PARALLEL THREAD
(B) TAPER THREAD
Fig 2.3
A thread formed on the surface of a cone called as taper thread. Refer FIG 2.3(b)
42 ENGINEERING GRAPHICS