Screw thread

[1] In most applications, the lead of a screw thread is chosen so that friction is sufficient to prevent linear motion being converted to rotary, that is so the screw does not slip even when linear force is applied, as long as no external rotational force is present.

The tightening of a fastener's screw thread is comparable to driving a wedge into a gap until it sticks fast through friction and slight elastic deformation.

The cross section to measure this angle lies on a plane which includes the axis of the cylinder or cone on which the thread is produced.

The terms when used in reference to screw thread pitch have nothing to do with the tolerances used (degree of precision) or the amount of craftsmanship, quality, or cost.

Fine threads are less likely to vibrate loose as they have a smaller helix angle and allow finer adjustment.

The reduced material condition, due to the unused spaces between the threads, must be minimized so as not to overly weaken the fasteners.

For example, tables of caliper measurements show 0.69 female ID and 0.75 male OD for the standards of "3/4 SAE J512" threads and "3/4-14 UNF JIS SAE-J514 ISO 8434-2".

However, this ideal condition would in practice only be approximated and would generally require wrench-assisted assembly, possibly causing the galling of the threads.

For this reason, some allowance, or minimum difference, between the PDs of the internal and external threads has to generally be provided for, to eliminate the possibility of deviations from the ideal thread form causing interference and to expedite hand assembly up to the length of engagement.

Such allowances, or fundamental deviations, as ISO standards call them, are provided for in various degrees in corresponding classes of fit for ranges of thread sizes.

Achieving a certain class of fit requires the ability to work within tolerance ranges for dimension (size) and surface finish.

For imperial, H or L limits are used which designate how many units of 0.0005 inch over or undersized the pitch diameter is from its basic value, respectively.

A perfectly sharp 60° V-thread will have a depth of thread ("height" from root to crest) equal to 0.866 of the pitch.

This fact is intrinsic to the geometry of an equilateral triangle — a direct result of the basic trigonometric functions.

The resulting flats on the crests of the male thread are theoretically one eighth of the pitch wide (expressed with the notation 1⁄8p or 0.125p), although the actual geometry definition has more variables than that.

The result is that tap and die wear is reduced, the likelihood of breakage is lessened and higher cutting speeds can often be employed.

Leonardo da Vinci understood the screw principle, and left drawings showing how threads could be cut by machine.

In 1569, Besson invented the screw-cutting lathe, but the method did not gain traction and screws continued to be made largely by hand for another 150 years.

[8] Standardization of screw threads has evolved since the early nineteenth century to facilitate compatibility between different manufacturers and users.

[16][17] The 60° angle was already in common use in America,[18] but Sellers's system promised to make it and all other details of threadform consistent.

Meanwhile, in Britain, the British Association screw threads were also developed and refined for small instrumentation and electrical equipment.

During this era, in continental Europe, the British and American threadforms were well known, but also various metric thread standards were evolving, which usually employed 60° profiles.

Efforts were made in the early 20th century to convince the governments of the U.S., UK, and Canada to adopt these international thread standards and the metric system in general, but they were defeated with arguments that the capital cost of the necessary retooling would drive some firms from profit to loss and hamper the economy.

It was during this era that more complicated analyses made clear the importance of variables such as pitch diameter and surface finish.

A tremendous amount of engineering work was done throughout World War I and the following interwar period in pursuit of reliable interchangeability.

Classes of fit were standardized, and new ways of generating and inspecting screw threads were developed (such as production thread-grinding machines and optical comparators).

Therefore, in theory, one might expect that by the start of World War II, the problem of screw thread interchangeability would have already been completely solved.

Problems with lack of interchangeability among American, Canadian, and British parts during World War II led to an effort to unify the inch-based standards among these closely allied nations, and the Unified Thread Standard was adopted by the Screw Thread Standardization Committees of Canada, the United Kingdom, and the United States on November 18, 1949, in Washington, D.C., with the hope that they would be adopted universally.

With continental Europe and much of the rest of the world turning to SI and ISO metric screw thread, the UK gradually leaned in the same direction.

The UK has completely abandoned its commitment to UTS in favour of ISO metric threads, and Canada is in between.