Conventional representation of threads

The essentials

Drawing the true helical form of a thread is slow, so drawings use a simplified convention. The rule to remember is that the crests of a thread are drawn with thick continuous lines and the roots with thin continuous lines. For an external thread the crests are at the major diameter; for an internal thread the crests are at the minor diameter.

Key points

• External thread, side view: major diameter thick lines; minor (root) diameter thin lines; a thick line across marks the end of the useful thread.
• External thread, end view: a thick full circle for the major diameter and a thin circle of about three-quarters (an open arc) for the minor diameter.
• Internal thread in section: minor diameter (crest) thick lines, major diameter (root) thin lines; hatching is carried up to the thick crest lines.
• Internal thread, end view: a thick full circle for the minor diameter and a thin three-quarter arc for the major diameter.
• Internal thread not in section (hidden): both diameters shown with dashed lines.
• In an assembly of a bolt in a tapped hole, the external thread conceals the internal thread where they overlap; the external thread convention is shown there.

Exam tips

• Ask yourself "where are the crests?" for each thread; those lines are thick.
• The thin three-quarter circle in end views helps examiners instantly recognise a thread.

Common mistakes

• Drawing both diameters of an internal thread as thick lines.
• Stopping the hatching at the major diameter of an internal thread instead of the minor diameter.