Page 130 - CITS - ED - Mechanical

ENGINEERING DRAWING - CITS

Different types of threads

Square threads (Fig 1): In this thread the flanks are perpendicular to the axis of the thread.
Square threads are used for transmitting motion or power. Eg. Screw jack, vice handles, cross-slide and compound
slide, activating screwed shafts.
Designation
A square thread of nominal dia. 60mm and pitch 9 mm shall be designated as Sq.60 x 9 IS: 4694 -1968.
Trapezoidal threads: These threads have a profile which is neither square nor ‘V’ thread form and have a form
of trapezoid. They are used to transmit motion or power. The different forms of trapezoidal threads are:
– acme thread
– metric acme thread
– buttress thread etc.
Acme thread (Fig 2): This thread is a modification of the square thread. It has an included angle of 29 . It is
o
preferred for many jobs because it is fairly easy to machine.Acme threads are used in lathe lead screws. This
form of thread enables the easy engagement of the half nut. The metric acme thread has an included angle of 30 .
o

Fig 1 Fig 2

Buttress thread: In Buttress thread one flank is perpendicular to the axis of the thread and the other flank is at
45 . These threads are used on the parts where pressure acts at one flank of the thread during transmission.
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Fig 3 shows the various elements of a buttress thread. These threads are used in power press, carpentry vices,
ratchets, etc.
Knuckle (round) thread (Fig 4): it has a semi circular form, different from Vee and trapezoidal thread. It is not
sensitive against damage as it is rounded. It is used for valve spindles, railway carriage couplings, hose connec-
tions, etc.

Fig 3 Fig 4

Convention of threads: Since drawing the profile of threads is cumbersome and does not serve any special
purpose, the thread forms are conventionally represented by thin line. Fig 5 shows the convention of threads on
the screw and end view.

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CITS :Engineering Drawing (Mechanical) - Exercise 8