Unified threads

The Unified system of screw threads 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. Both of these systems use a similar form of thread profile which is illustrated basically in Fig. 15.4.

Pitch P

Pitch P

Unified Thread Form

Fig. 15.4 Unified screw thread H = 0.86603P, Rc = 0.108P and Rr = thread

0.144P where P is the pitch of the

Fig. 15.4 Unified screw thread H = 0.86603P, Rc = 0.108P and Rr = thread

0.144P where P is the pitch of the

Unified threads are covered by BS 1580. Types in common use include the following;

(a) UNC is a Unified coarse pitch thread, with progressive pitch sizes (i.e. the pitch varies with the diameter).

(b) UNF is a Unified fine pitch thread, also with progressive pitch sizes.

(c) UN is a Unified thread with a constant pitch (e.g. an 8 UN thread has 8 threads to the inch regardless of the diameter).

Different classes of fit are obtainable by manufacture within alternative tolerance ranges and these are specified in BS 1580 Normally the same class of internal and external thread are used together. Sellers or American thread (Fig. 15.5). This type was the American National thread in common use before the introduction of the Unified National thread, as it is described in the USA and Canada, or the Unified screw-thread in Great Britain.

Unified National Coarse Series

Bolt

Fig 15.5 Sellers or American thread H = 0.866P, H' = 0.6495P, F = 0.1083P = H/8 = H/6

Bolt

11 m

Fig 15.5 Sellers or American thread H = 0.866P, H' = 0.6495P, F = 0.1083P = H/8 = H/6

Whitworth thread (Fig. 15.6). The general shape of the thread shown has been used in a standard BSW thread, in fine form as the BSF thread, and as a pipe thread in the BSP thread.

Pitch P

Pitch P

Unified Whitworth
Fig 15.6 Basic Whitworth form H = 960491P, h = 0.640327P, r = 0.137329P

The British Standard Whitworth thread was the first standardized British screw-thread.

The British Standard Fine thread is of Whitworth section but of finer pitch. The reduction in pitch increases the core diameter; also, small adjustments of the nut can easily be made.

The British Standard Pipe threads are used internally and externally on the walls of pipes and tubes. The thread pitch is relatively fine, so that the tube thickness is not unduly weakened.

Pipe threads are covered by BS 21, which was adopted as the basis for ISO 7/1 where the metric values were conversions of the inch values, to obtain interchangeability.

The basic forms of the British Standard taper and parallel pipe threads are based on that of the British Standard Whitworth thread.

The Whitworth thread form is shown in Fig. 15.6. The thread angle of 55° is measured in an axial plane section, also the vee-section is truncated, at top and bottom, by one sixth, with crest and root rounded by equal circular arcs. The theoretical thread depth is shown as h = 0.640 327 P where P is the thread pitch. In the taper pipe thread (Fig. 15.7), a taper of 1 in 16 is used, measured on diameter.

British Association thread (Fig. 15.8). Generally used in sizes of less than 1/4 inch on small mechanisms. This range of threads extends down to a thread size of 0.25 mm and is covered by BS 93.

Note. BS 93 is an 'obsolescent' standard. The standard is not recommended for the design of new equipment, but is retained to provide a standard for servicing

Standard For Engineering DrawingTaper Engineering Drawings
Fig. 15.8 British Association (BA) thread H = 1.1363P, H = 0.6P (approx.), R = 0.18P, S = 0.268P

equipment in use which is expected to have a long working life.

BS 4827 specifies the requirements for ISO miniature screw threads from 0.30 mm to 1.4 mm diameter with 60° form and are used in delicate instruments and watch making.

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Responses

  • JESSIKA
    How to draw unified national thread?
    8 years ago
  • vickie
    What emgineering thread type is common to america?
    8 years ago

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