Iso standard threads

The most common threads in use are the ISO metric screw threads (M) and BSP threads also called G threads for pipes.

These were standardized by the International Organization for Standardization in 1947. Before that, there were separate metric thread standards used in France, Germany, and Japan, and the Swiss had a set of threads for watches.

Generating screw threads

Page 23 of Colvin FH, Stanley FA (eds) (1914): American Machinists' Handbook, 2nd ed. New York and London: McGraw-Hill. Summarizes screw thread rolling practice as of 1914.

There are various methods for generating screw threads. The method chosen for any one application is chosen based on constraints—time; money; degree of precision needed (or not needed); what equipment is already available; what equipment purchases could be justified based on resulting unit price of the threaded part (which depends on how many parts are planned); etc.

In general, certain thread-generating processes tend to fall along certain portions of the spectrum from toolroom-made parts to mass-produced parts, although there can be considerable overlap. For example, thread lapping following thread grinding would fall only on the extreme toolroom end of the spectrum, while thread rolling is a large and diverse area of practice that is used for everything from microlathe leadscrews (somewhat pricey and very precise) to the cheapest deck screws (very affordable and with precision to spare).

The various methods are summarized below.

Thread cutting

The excess material is cut away, with taps and dies for most smaller diameters, or with single-point thread-cutting on a lathe for larger ones (or smaller ones needing very high concentricity).

Thread rolling

The material is extruded into a male thread through mechanical pressure as the screw blank is rolled between a matched pair of flat dies. (See Cold forming.) Thread rolling is more common for high-volume production, and produces threads of diameters typically smaller than one inch. Also, materials with good deformation characteristics are better used with rolling; these materials include softer (more ductile) metals and exclude brittle materials, such as cast iron. A rolled thread can often be easily recognized because the thread has a larger diameter than the blank rod from which it has been made. (However, necks and shoulders can be cut or rolled to different diameters, so this in itself is not a forensic give-away.) Also, the end of the screw usually looks a bit different from the end of a cut-thread screw. Rolled male threads tend to be slightly stronger than cut male threads. Thread rolling is a very economical way of producing large quantities with good dimensional accuracy. The cost of thread rolling depends on the quantity; the more parts made, the cheaper the unit cost.

Thread forming

This is the female-thread analogue of the male-thread-rolling process described above. The material is extruded into a thread through mechanical pressure by a tap that is similar to a cutting tap except that it has no flutes. Instead of cutting, the tap squeezes the material out of its way. Formed female threads tend to be slightly stronger than cut female threads.

This process is more often employed in soft, ductile metals (such as aluminum) than in hard, brittle metals (such as cast iron).