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

MACHINE DRAWING
2.10 INTRODUCTION
FREE HAND SKETCHES OF MACHINE PARTS
In freehand sketches of machine parts, the students must do the drawing without the use
of scale, instrument etc., Appropriate measurement is taken and correspondingly a table
for each figure must be made showing calculated values. The figure must show the
dimensions in terms of diameter 'd'.
2.11 CONVENTIONAL REPRESENTATION OF THREADS
In actual projection, the edges of threads would be represented by helical curves. It takes a lot of
time to draw helical curves. So, for convenience sake threads are generally shown by
conventional methods recommended by B.I.S
2.11.1CONVENTIONAL REPRESENTATION OF EXTERNAL V-THREADS
The Bureau of Indian standards has recommended a very simple method of representing Vthreads.
Fig 2.35 shows the simplified representation of external V-threads. According to
this convention, two continuous thick lines and two continuous thin lines are drawn to
represent crest and roots of the thread respectively. The limit of useful length of the
thread is indicated by a thick line perpendicular to the axis.
CONVENTIONAL REPRESENTATION OF EXTERNAL V-THREADS
Fig 2.35
The other way of representing external V-thread is as follows.
(i) Draw a rectangle (see fig 2.36) representing a cylinder with diameter equal to the
nominal diameter of the bolt.
(ii) Draw a line AB perpendicular to the bolt.
(iii) Make a point B' such that BB' = 0.5xpitch. BB is called as slope = 0.5P for a single
start thread. B' is located on the lower line for a right hand thread (RH thread)
A P
B B'
SLOPE = 0.5 P
RIGHT HAND V-THREAD
Fig 2.36
ENGINEERING GRAPHICS 71