Screw-cutting lathe
Encyclopedia
A screw-cutting lathe is a machine (specifically, a lathe
) capable of cutting very accurate screw thread
s via single-point screw-cutting, which is the process of guiding the linear motion of the tool bit
in a precisely known ratio to the rotating
motion of the workpiece. This is accomplished by gear
ing the leadscrew
(which drives the tool bit's movement) to the spindle
with a certain gear ratio for each thread pitch. Every degree
of spindle rotation is matched by a certain distance of linear tool travel, depending on the desired thread pitch (English or metric, fine or coarse, etc.).
devised the water screw, a system for raising water. Screw
s as mechanical fasteners date to the first century BCE. Although screws were tremendously useful, the difficulty in making them prevented any widespread adoption.
by hand, with or without the help of turning on a lathe with hand-controlled turning tools (chisels, knives, gouges), as accurately as the whittler could manage. It is likely that sometimes the wood blanks that they started from were tree branches that had been shaped by a vine
wrapped helically
around them while they grew. (In fact, various Romance
words for "screw" come from the word root referring to vines.)
s used to cut the threads. One method for making fairly accurate threads was to score a rod using an inclined knife with a wrap half way around the rod, the knife being precisely angled for the proper pitch. (This was the method Maudslay used to make his lead screws.) This made the screw slow and expensive to make and the quality 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.
s 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 leadscrew
s 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..
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
. 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).
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 distinction between "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).
s. These machines, although they are lathes specialized for making screws, are not screw-cutting lathes in the sense of employing single-point screw-cutting.
Lathe
A lathe is a machine tool which rotates the workpiece on its axis to perform various operations such as cutting, sanding, knurling, drilling, or deformation with tools that are applied to the workpiece to create an object which has symmetry about an axis of rotation.Lathes are used in woodturning,...
) capable of cutting very accurate screw thread
Screw thread
A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread...
s via single-point screw-cutting, which is the process of guiding the linear motion of the tool bit
Tool bit
The term tool bit generally refers to a non-rotary cutting tool used in metal lathes, shapers, and planers. Such cutters are also often referred to by the set-phrase name of single-point cutting tool. The cutting edge is ground to suit a particular machining operation and may be resharpened or...
in a precisely known ratio to the rotating
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...
motion of the workpiece. This is accomplished by gear
Gear
A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine....
ing the leadscrew
Leadscrew
A leadscrew , also known as a power screw or translation screw, is a screw designed to translate turning motion into linear motion...
(which drives the tool bit's movement) to the spindle
Spindle (tool)
In machine tools, a spindle is a rotating axis of the machine, which often has a shaft at its heart. The shaft itself is called a spindle, but also, in shop-floor practice, the word often is used metonymically to refer to the entire rotary unit, including not only the shaft itself, but its bearings...
with a certain gear ratio for each thread pitch. Every degree
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...
of spindle rotation is matched by a certain distance of linear tool travel, depending on the desired thread pitch (English or metric, fine or coarse, etc.).
History
The screw has been known for millennia. ArchimedesArchimedes
Archimedes of Syracuse was a Greek mathematician, physicist, engineer, inventor, and astronomer. Although few details of his life are known, he is regarded as one of the leading scientists in classical antiquity. Among his advances in physics are the foundations of hydrostatics, statics and an...
devised the water screw, a system for raising water. Screw
Screw
A screw, or bolt, is a type of fastener characterized by a helical ridge, known as an external thread or just thread, wrapped around a cylinder. Some screw threads are designed to mate with a complementary thread, known as an internal thread, often in the form of a nut or an object that has the...
s as mechanical fasteners date to the first century BCE. Although screws were tremendously useful, the difficulty in making them prevented any widespread adoption.
Wooden screws
The earliest screws tended to be made of wood, and they were whittledWhittling
Whittling is the art of carving shapes typically out of raw wood or bone with a knife.Whittling is typically performed with a light, small-bladed knife, usually a pocket knife. Specialised whittling knives are available as well...
by hand, with or without the help of turning on a lathe with hand-controlled turning tools (chisels, knives, gouges), as accurately as the whittler could manage. It is likely that sometimes the wood blanks that they started from were tree branches that had been shaped by a vine
Vine
A vine in the narrowest sense is the grapevine , but more generally it can refer to any plant with a growth habit of trailing or scandent, that is to say climbing, stems or runners...
wrapped helically
Helix
A helix is a type of smooth space curve, i.e. a curve in three-dimensional space. It has the property that the tangent line at any point makes a constant angle with a fixed line called the axis. Examples of helixes are coil springs and the handrails of spiral staircases. A "filled-in" helix – for...
around them while they grew. (In fact, various Romance
Romance languages
The Romance languages are a branch of the Indo-European language family, more precisely of the Italic languages subfamily, comprising all the languages that descend from Vulgar Latin, the language of ancient Rome...
words for "screw" come from the word root referring to vines.)
Metal screws
Early machine screws of metal, and early wood screws [screws made of metal for use in wood], were made by hand, with fileFile (tool)
A file is a metalworking and woodworking tool used to cut fine amounts of material from a workpiece. It most commonly refers to the hand tool style, which takes the form of a steel bar with a case hardened surface and a series of sharp, parallel teeth. Most files have a narrow, pointed tang at one...
s used to cut the threads. One method for making fairly accurate threads was to score a rod using an inclined knife with a wrap half way around the rod, the knife being precisely angled for the proper pitch. (This was the method Maudslay used to make his lead screws.) This made the screw slow and expensive to make and the quality 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
LatheLathe
A lathe is a machine tool which rotates the workpiece on its axis to perform various operations such as cutting, sanding, knurling, drilling, or deformation with tools that are applied to the workpiece to create an object which has symmetry about an axis of rotation.Lathes are used in woodturning,...
s 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
Leonardo da Vinci
Leonardo di ser Piero da Vinci was an Italian Renaissance polymath: painter, sculptor, architect, musician, scientist, mathematician, engineer, inventor, anatomist, geologist, cartographer, botanist and writer whose genius, perhaps more than that of any other figure, epitomized the Renaissance...
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
Flywheel
A flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels have a significant moment of inertia, and thus resist changes in rotational speed. The amount of energy stored in a flywheel is proportional to the square of its rotational speed...
to keep the rotation consistent. His design also used two leadscrew
Leadscrew
A leadscrew , also known as a power screw or translation screw, is a screw designed to translate turning motion into linear motion...
s 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
Henry Hindley
Henry Hindley was an 18th century clockmaker and maker of scientific instruments. He was the inventor of a screw-cutting lathe. He built a clock for York Minster, England, where he apparently lived for much of his life, in 1752....
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..
Modern lathes
The first truly modern screw-cutting lathe was likely constructed by Jesse RamsdenJesse Ramsden
Jesse Ramsden FRSE was an English astronomical and scientific instrument maker.Ramsden was born at Salterhebble, Halifax, West Riding of Yorkshire, England. After serving his apprenticeship with a cloth-worker in Halifax, he went in 1755 to London, where in 1758 he was apprenticed to a...
in 1775. He appears to have been the first person to put a leadscrew
Leadscrew
A leadscrew , also known as a power screw or translation screw, is a screw designed to translate turning motion into linear motion...
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
Diamond turning
Diamond turning is a process of mechanical machining of precision elements using lathes or derivative machine tools equipped with natural or synthetic diamond-tipped tool bits...
. 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
Dividing engine
A dividing engine is a device specifically employed to mark graduations on measuring instruments.-History:There has always been a need for accurate measuring instruments...
and in turn, some of the finest astronomical, surveying
Surveying
See Also: Public Land Survey SystemSurveying or land surveying is the technique, profession, and science of accurately determining the terrestrial or three-dimensional position of points and the distances and angles between them...
, and navigational instruments of the 18th century.
Others followed. Senot, in 1795, created a screw-cutting lathe capable of industrial-level production. David Wilkinson
David Wilkinson (machinist)
David Wilkinson was a U.S. mechanical engineer who invented a lathe for cutting screw threads, which was an extremely important development in the development of the machine tool industry in the early 19th century....
of Vermont employed a slide-rest in 1798. However, these inventors were soon overshadowed by Henry Maudslay
Henry Maudslay
Henry Maudslay was a British machine tool innovator, tool and die maker, and inventor. He is considered a founding father of machine tool technology.-Early life:...
, 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
Interchangeable parts
Interchangeable parts are parts that are, for practical purposes, identical. They are made to specifications that ensure that they are so nearly identical that they will fit into any device of the same type. One such part can freely replace another, without any custom fitting...
. 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).
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
Lathe (metal)
A metal lathe or metalworking lathe is a large class of lathes designed for precisely machining relatively hard materials. They were originally designed to machine metals; however, with the advent of plastics and other materials, and with their inherent versatility, they are used in a wide range of...
; thus, the distinction between "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).
Present day
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 thread forming and rolling and cutting with die heads. The latter process is the one employed in modern screw machineScrew machine
A screw machine may refer to a:* Screw machine , a small- to medium-sized automatic lathe that is mechanically automated via cams...
s. These machines, although they are lathes specialized for making screws, are not screw-cutting lathes in the sense of employing single-point screw-cutting.