Page 131 - CITS - ED - Mechanical

ENGINEERING DRAWING - CITS

Designation of screw threads: eg Metric threads M20- meaning nominal diameter 20 mm coarse pitch (equal
to 2.5 mm Ref: IS: 4218-part II) or M20 x 1.5 means nominal diameter 20 mm pitch 1.5 mm.
M 20 x 1.5 “Triple start” where pitch is 1.5 mm and lead is 4.5 mm.
Threads are designated by their nominal major diameter and type of thread. Apart from the nominal dia and type
of thread, pitch; L.H (left hand) if it is not right hand thread and class of thread are indicated.

Eg. M24 - Metric thread nominal dia 24 m (Coarse pitch 3 mm)
M24 x 1.25 fine pitch 1.25 mm.
M24 x 2 5H Tolerance grade of internal thread.
M24 x 2 6g Tolerance grade of external thread.
M 24 x 2-7H8g fit of the nut and bolt.

For multi-start threads, Double Start, Triple start etc are mentioned.
Eg. M45 x Triple Start

BSW Thread: BSW - nominal diameter TPI 6 (Threads per inch)

BSF Threads: BSF
Example
– SQ 30 x 6 IS:4964 A square thread of nominal dia 30 mm and pitch 6 mm.
– Tr40 x 7 A single start ISO metric trapezoidal screw thread of nominal dia 40 mm and pitch 7 mm.

– Tr 40 x 14 (P7) LH
A multiple start left hand (double start) ISO metric trapezoidal screw thread of nominal dia 40 mm, lead 14
mm and pitch 7 mm.

Fig 5

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CITS :Engineering Drawing (Mechanical) - Exercise 8