Screw threads and conventional representations

This chapter focuses on screw threads and their conventional representations. Screw threads may either be left or right handed. The left-hand thread is used for special applications and the right-hand thread is the one normally used on nuts and bolts. The Unified system of screw threads are covered by BS 1580, which was introduced by the United Kingdom, Canada, and the United States to provide a common standard thread for use by the three countries. The International Standards Organisation (ISO) recommends the system as an international system of screw threads in inch units, in parallel with a similar system in metric units. The square thread is used to transmit force and motion since it offers less resistance to motion than “V” thread forms. This thread is widely used on lathes and is sometimes slightly modified by adding a small taper of about 5° to the sides as an aid to production. Acme threads are more easily produced than the square thread, which are often used in conjunction with split nuts for engagement purposes and are applied in valve operating spindles. Buttress thread is used for transmitting power in one direction only and a common application of this thread can be found in workshop vices.