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Screw threads were in use in the late Middle Ages. Nuts and bolts were used to hold together the parts of armor suits.


A Tap is a tool to cut an internal screw thread.
    Who first discovered the use of screw thread is a mystery, but it was certainly many centuries ago. Probably it was suggested by Achimedes screw which consisted of a cylinder inside of which a continuous screw extending the full length, forms a spiral chamber. By placing the lower end in water and revolving the screw the water is raised to the top.
    In any case, we know that screw threads were in use in the late Middle Ages. Nuts and bolts were used to hold together the parts of armor suits.
    There are, of course, only three ways to join metal to metal, by welding, by riveting, or by screw threads.
    The great advantage of the latter method of joining metal to metal is that the two pieces can be taken apart and re-assembled, without damage to either.
Before 1800, screws were cut by hand, each screw was different and hard to make. As Lewis Mumford writes in his book Techniques and Civilization, "Every bolt and nut was a sort of specialty in itself".
A metal-working revolution occurred between 1775 and 1825 that changed the whole pattern of man's living and introduced the modern Machine Age.
Up until this period, the chief metal-working tools were the hammer, the cold chisel and the file.
Then came Watt and his steam engine which provided the power needed to run machinery, and Henry Maudslay with his screw-cutting lathe. Mumford calls this "one of the decisive pieces of standardization that made the modern machine possible".
For the first time it became possible to design and cut uniform, accurate screw threads.
The era of interchangeability and mass production was born. Eli Whitney was able to amaze the U.S. war department in 1799 by assembling 10 muskets from parts which were interchangeable.
All during the 19th Century the design and accuracy of metal-working tools were improved. The invention of the turret lathe about 1845 was an important milestone in manufacturing with metal.
The tap fast became a key tool in Manufacturing.

Its counterpart, the die for cutting external threads, went through some major changes in design. Until 1872, external threads were chiefly cut by "jamming" a rod into a threaded hole in a metal plate. Known as "jamb plates", these early thread-cutting tools were crude and inaccurate. In 1872 John J. Grant came to Greenfield, Massachusetts, with an idea for a die which actually cut metal, the two-piece adjustable die. The demand for this improved tool was so great that a major industry was born almost overnight.
While the changes in the basic design of taps through the years have been less spectacular than in dies, the improvement in precision has been no less remarkable, The generating of threads on a tap by grinding in place of cutting, and the development of better steels and methods of heat treatment, all have made it possible to product taps of greater and greater accuracy and longer wear life. The introduction of the "Gun" Tap was the most important tap design innovation the present century.
Nearly all taps being made today are precise to a degree impossible 50 years ago. They must be so to meet the demands of a civilization which literally "hangs by a thread" in its millions of machines, in its automobiles, in its planes, and in the factories that produce the comforts and conveniences as well as the weapons of our age.





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Last modified: January 16, 2002