Screw thread production to close limits

Compared with other simple mechanical devices, such as the lever and fulcrum, the gear and the wheel, which date back to unknown times, the screw thread may be termed of almost recent origin. Its discovery is credited to Archimedes, a native of the Greek colony of Syracuse in Sicily (about 256 B. C.). On a visit to Egypt, he undertook to improve upon the primitive method of dipping water out of the stream for irrigation. For this purpose, he wrapped a plane of sheet metal around a long rod or cylinder in a coarse spiral, producing a thread the pitch of which made it resemble the turns of a modern twist drill. When this was inserted in a close-fitting tube and one end of the latter placed under the surface of the river, revolving the spiral in the proper direction discharged a constant stream at the upper end. The Egyptians evidently did not take kindly to progress since the ancient method of using skin buckets and a windlass persists to this day.

Like many other basic discoveries of mechanics, the screw thread remained little more than a principle for centuries. Leonardo da Vinci developed it further and from it evolved the principle of the screw propeller, but as this did not receive any practical application until many hundreds of years later, he was, like many other advanced thinkers, away ahead of his time. Some of the early uses of the screw thread were its application to the instruments of torture employed by the Inquisition, to baling or compressing devices and to the earliest attempts at printing. A descendant of the latter still exists in the form of the old style letter-copying press.

So far as is known, the first method of threading a screw was to forge a blank, upset one end to form the head and then cut the thread by filing. Beson (France, 1569) invented a screw-cutting gauge for the lathe which was used in practically the form it originated until 1841, when an Englishman (Hindley, of York) improved upon it. Hindley's invention was used for many years by watch-makers for producing small screws.

A hand-operated machine for producing screws was invented in 1836 by an American and was superseded in 1848 by an automatic machine, but both of these machines were designed for the production of wood screws.


Nomenclature

A screw may be defined as a solid cylinder having a helicoidal ridge, rib or thread, projecting from its surface, while the technical definition of a thread is "an inclined plane wrapped around a cylinder in such a manner that the height of the plane is parallel to the cylinder." The application of this definition may be demonstrated by cutting a right-angled triangle out of paper and wrapping it around a rod the circumference of which is equal to the base of the triangle. The course of the thread is represented by the hypothenuse of the right-angled triangle coiled around the cylinder, the base of the triangle equalling the circumference of the cylinder while the height of the triangle is equivalent to the pitch of the thread. The rake of a thread is the amount the thread inclines from a right angle to the axis of the cylinder.

The elements of a thread are its form, its angle or slope, its pitch, its lead and its depth. The forms of threads in common use are the V and its modifications with flat or curved top and root, known as the U. S. and the Whitworth standards respectively: the square, in which, as the term indicates, the thread form is a square the sides of which are perpendicular to the axis of the screw: the acme thread, which is a modification of the square, adopted for greater ease of manufacture: and the buttress thread in which one side is perpendicular to the axis of the screw while the other is given a slope. In addition to these there are a number of other forms used for special purposes. The V-thread and its modified forms are generally used in machinery, for practically all small mechanical devices and for pipe connections. Square threads are used for lifting purposes or wherever power is to be applied in both directions, a common application being to the lead screw of the engine lathe. The buttress thread is employed where power is to be applied in one direction only, while the acme thread is generally employed on screws the threads of which must be capable of withstanding considerable pressure as well as wear from frequent use, as on valve stems.

The angle or slope of the thread ranges from 29 in the acme form to 90 in the square form, the more commonly employed V-thread having an angle of 60.

The pitch of a single-threaded screw is the distance between the centers of two adjacent threads measured on a line parallel to the axis of the screw, or, in any screw, whether single or multiple-threaded, it is the distance the nut is moved by one revolution of the screw. Pitch is more frequently used, however, to indicate the number of threads to the inch on a screw and this is equal to the number of revolutions the screw must make in order to advance the nut a distance of one inch. Consequently, the pitch is always equal to 1 divided by the number of revolutions that the screw must make in order to move the nut one inch. More or less confusion results from the varying use of the term pitch to define the same size thread, as for example, a four-pitch thread and a 1/4-inch pitch thread are the same, or four threads to the inch. The power of a screw to draw the parts it unites into contact depends upon its pitch, while its ability to withstand stress and its durability depend upon its shape and depth. The pitch diameter of a screw is its outside diameter less a single depth of the thread.

Further confusion also results from the interchangeable use of the terms "pitch" and "lead" to denote the same thing, as well as the common though erroneous employment of the term "pitch" to indicate the number of threads per inch. Screws are very generally spoken of as being "8-pitch" or "12-pitch" when what is meant is 8 or 12 threads per inch, equivalent to 1/8" or l/12"-pitch.

The "lead" of a screw thread is the distance the nut will move forward on the screw for each revolution of the screw. In a single-threaded screw, the pitch and lead are always equal, because the nut will move forward the distance from one thread to the next, or the equivalent of the pitch, if revolved once. In a double-threaded screw, the nut will move forward two threads for each revolution, or twice the pitch, so that in this case the lead is double the pitch: in a triple-threaded screw, it is three times the pitch. Multiple-threaded screws are employed where it is necessary to increase the lead in order to advance the nut more rapidly.


DOWNLOAD FREE BOOK:
Screw thread production to close limits