Screw-cutting lathe

A screw-cutting lathe is a machine (specifically, a lathe) capable of cutting very accurate screw threads via single-point screw-cutting (which is the process of precisely guiding the motion of the tool bit in relation to the rotating motion of the workpiece). Until the early 19th century, the notion of a screw-cutting lathe stood in contrast to the notion of a regular lathe, which lacked the parts needed to guide the cutting tool in the precise path needed to produce an accurate thread. Since the early 19th century, it has been common practice to build these parts into any general-purpose metalworking lathe; thus, the dichotomy of "regular lathe" and "screw-cutting lathe" does not apply to the classification of modern lathes. Instead, there are other categories, some of which bundle single-point screw-cutting capability among other capabilities (for example, regular lathes, toolroom lathes, and CNC lathes), and some of which omit single-point screw-cutting capability as irrelevant to the machines' intended purposes (for example, speed lathes and turret lathes).

Today the threads of threaded fasteners (such as machine screws, wood screws, wallboard screws, and sheetmetal screws) are not cut via single-point screw-cutting; instead they are generated by other, faster processes, such as deformation between forming dies and cutting within thread dies. The latter process is the one employed in modern screw machines. These machines, although they are lathes specialized for making screws, are not screw-cutting lathes in the sense of employing single-point screw-cutting.

History

The screw has been known for millennia. Archimedes devised the water screw, a system for raising water. Screws as mechanical fasteners date to the first century BCE. While tremendously useful, the difficulty in making screws prevented their adoption.

Early machine screws were made by hand, with files used to cut the threads. This made the screw slow and expensive to make and the quality is highly dependent on the skill of the maker. A process for automating the manufacture of screws and improving the accuracy and consistency of the thread was needed.

Lathes have been around since ancient times. Adapting them to screw-cutting is an obvious choice, but the problem of how to guide the cutting tool through the correct path was an obstacle for many centuries. Very old lathes used a mechanism that provided for back-and-forth motion, which rotated the workpiece first one way and then the other. Leonardo da Vinci created drawings showing screw-cutting machines that did away with this back-and-forth system and replaced it with a system that maintained rotation in one direction. He also added a flywheel to keep the rotation consistent. His design also used two leadscrews to guide the tool, perhaps to average out the error in the leadscrew construction. It is unknown whether this machine was ever built, but it is an example of Leonardo's genius.

Dozens of designs followed but few were significantly accurate. Henry Hindley designed and constructed a screw-cutting lathe "circa" 1739. It featured a plate guiding the tool and power supplied by a hand-cranked series of gears. By changing the gears, he could cut screws with different pitch. Removing a gear permitted him to make left-handed threads.Daumas, Maurice, "Scientific Instruments of the Seventeenth and Eighteenth Centuries and Their Makers", Portman Books, London 1989 ISBN 978-0713407273] .

The first truly modern screw-cutting lathe was likely constructed by Jesse Ramsden in 1775. He appears to have been the first person to put a leadscrew into actual use (although, as Leonardo's drawings show, he was not the first person ever to think of the idea), and he was the first to use diamond-tipped cutting tools. [Rybczynski, Witold, "One Good Turn, A Natural History of the Screwdriver and the Screw", Scribner, 2000 ISBN 0-684-86729-X] His device also included a slide rest and change gear mechanism. These form the elements of a modern (non-CNC) lathe and are in use to this day. Ramsden was able to use his first screw-cutting lathe to make even more accurate lathes. With these, he was able to make an exceptionally accurate dividing engine and in turn, some of the finest astronomical, surveying and Navigational instruments of the 18th century.

Others followed. Senot, in 1795, created a screw-cutting lathe capable of industrial-level production. David Wilkinson of Vermont employed a slide-rest in 1798. However, these inventors were soon overshadowed by Henry Maudslay, who in 1800 created a screw-cutting lathe that is frequently cited as the first. Clearly, his was not the first; however, his did become the best known, spreading to the rest of the world the winning combination of leadscrew, slide-rest, and change gears. These late-18th-century screw-cutting lathes represented the breakthrough development of the technology. They permitted the large-scale, industrial production of screws that were interchangeable. Standardization of threadforms (including thread angle, pitches, major diameters, pitch diameters, etc.) began immediately on the intra-company level, and by the end of the 19th century, it had been carried to the international level (although pluralities of standards still exist).

ee also

*Lathe
*Lathe (metal)
*Screw
*Screw thread
*Screw machine

References