machine tool

A machine tool is a machine for handling or machining metal or other rigid materials, usually by cutting, boring, grinding, shearing, or other forms of deformations. Machine tools employ some sort of tool that does the cutting or shaping. All machine tools have some means of constraining the work piece and provide a guided movement of the parts of the machine. Thus, the relative movement between the workpiece and the cutting tool (which is called the toolpath) is controlled or constrained by the machine to at least some extent, rather than being entirely "offhand" or "freehand". It is a power-driven metal cutting machine which assists in managing the needed relative motion between cutting tool and the job that changes the size and shape of the job material.

The precise definition of the term ''machine tool'' varies among users, as discussed below. While all machine tools are "machines that help people to make things", not all factory machines are machine tools.

Today machine tools are typically powered other than by the human muscle (e.g., electrically, hydraulically, or via line shaft), used to make manufactured parts (components) in various ways that include cutting or certain other kinds of deformation.

With their inherent precision, machine tools enabled the economical production of interchangeable parts.

Nomenclature and key concepts, interrelated

Many historians of technology consider that true machine tools were born when the toolpath first became guided by the machine itself in some way, at least to some extent, so that direct, freehand human guidance of the toolpath (with hands, feet, or mouth) was no longer the only guidance used in the cutting or forming process. In this view of the definition, the term, arising at a time when all tools up till then had been hand tools, simply provided a label for "tools that were machines instead of hand tools". Early lathes, those prior to the late medieval period, and modern woodworking lathes and potter's wheels may or may not fall under this definition, depending on how one views the headstock spindle itself; but the earliest historical records of a lathe with direct mechanical control ''of the cutting tool's path'' are of a screw-cutting lathe dating to about 1483.. This lathe "produced screw threads out of wood and employed a true compound slide rest".

The mechanical toolpath guidance grew out of various root concepts:
* First is the spindle concept itself, which constrains workpiece or tool movement to rotation around a fixed axis. This ancient concept predates machine tools per se; the earliest lathes and potter's wheels incorporated it for the workpiece, but the movement of the tool itself on these machines was entirely freehand.
* The machine slide ( tool way), which has many forms, such as dovetail ways, box ways, or cylindrical column ways. Machine slides constrain tool or workpiece movement linearly. If a stop is added, the ''length'' of the line can also be accurately controlled. (Machine slides are essentially a subset of linear bearings, although the language used to classify these various machine elements may be defined differently by some users in some contexts, and some elements may be distinguished by contrasting with others)
* Tracing, which involves following the contours of a model or template and transferring the resulting motion to the toolpath.
* Cam operation, which is related in principle to tracing but can be a step or two removed from the traced element's matching the reproduced element's final shape. For example, several cams, no one of which directly matches the desired output shape, can actuate a complex toolpath by creating component vectors that add up to a net toolpath.
* Van Der Waals Force between like materials is high; freehand manufacture of square plates, produces only square, flat, machine tool building reference components, accurate to millionths of an inch, but of nearly no variety. The process of feature replication allows the flatness and squareness of a milling machine cross slide assembly, or the roundness, lack of taper, and squareness of the two axes of a lathe machine to be transferred to a machined work piece with accuracy and precision better than a thousandth of an inch, not as fine as millionths of an inch. As the fit between sliding parts of a made product, machine, or machine tool approaches this critical thousandth of an inch measurement, lubrication and capillary action combine to prevent Van Der Waals force from welding like metals together, extending the lubricated life of sliding parts by a factor of thousands to millions; the disaster of oil depletion in the conventional automotive engine is an accessible demonstration of the need, and in aerospace design, like-to-unlike design is used along with solid lubricants to prevent Van Der Waals welding from destroying mating surfaces. Given the modulus of elasticity of metals, the range of fit tolerances near one thousandth of an inch correlates to the relevant range of constraint between at one extreme, permanent assembly of two mating parts and at the other, a free sliding fit of those same two parts.

Abstractly programmable toolpath guidance began with mechanical solutions, such as in musical box cams and Jacquard looms. The convergence of programmable mechanical control with machine tool toolpath control was delayed many decades, in part because the programmable control methods of musical boxes and looms lacked the rigidity for machine tool toolpaths. Later, electromechanical solutions (such as servos) and soon electronic solutions (including computers) were added, leading to numerical control and computer numerical control.

When considering the difference between freehand toolpaths and machine-constrained toolpaths, the concepts of accuracy and precision, efficiency, and productivity become important in understanding ''why'' the machine-constrained option adds value.

Matter-Additive, Matter-Preserving, and Matter-Subtractive "Manufacturing" can proceed in sixteen ways: Firstly, the work may be held either in a hand, or a clamp; secondly, the tool may be held either in a hand, or a clamp; thirdly, the energy can come from either the hand(s) holding the tool and/or the work, or from some external source, including for examples a foot treadle by the same worker, or a motor, without limitation; and finally, the control can come from either the hand(s) holding the tool and/or the work, or from some other source, including computer numerical control. With two choices for each of four parameters, the types are enumerated to sixteen types of Manufacturing, where Matter-Additive might mean painting on canvas as readily as it might mean 3D printing under computer control, Matter-Preserving might mean forging at the coal fire as readily as stamping license plates, and Matter-Subtracting might mean casually whittling a pencil point as readily as it might mean precision grinding the final form of a laser deposited turbine blade.

Humans are generally quite talented in their freehand movements; the drawings, paintings, and sculptures of artists such as Michelangelo or Leonardo da Vinci, and of countless other talented people, show that human freehand toolpath has great potential. The value that machine tools added to these human talents is in the areas of rigidity (constraining the toolpath despite thousands of newtons ( pounds) of force fighting against the constraint), accuracy and precision, efficiency, and productivity. With a machine tool, toolpaths that no human muscle could constrain can be constrained; and toolpaths that are technically possible with freehand methods, but would require tremendous time and skill to execute, can instead be executed quickly and easily, even by people with little freehand talent (because the machine takes care of it). The latter aspect of machine tools is often referred to by historians of technology as "building the skill into the tool", in contrast to the toolpath-constraining skill being in the ''person'' who wields the tool. As an example, it is ''physically possible'' to make interchangeable screws, bolts, and nuts entirely with freehand toolpaths. But it is ''economically practical'' to make them only with machine tools.

In the 1930s, the U.S. National Bureau of Economic Research (NBER) referenced the definition of a machine tool as "any machine operating by other than hand power which employs a tool to work on metal"..

The narrowest colloquial sense of the term reserves it only for machines that perform metal cutting—in other words, the many kinds of onventionalmachining and grinding. These processes are a type of deformation that produces swarf. However, economists use a slightly broader sense that also includes metal deformation of other types that squeeze the metal into shape without cutting off swarf, such as rolling, stamping with dies, shearing, swaging,