Machining (redirected from machined)

Machining

An operation that changes the shape, surface finish, or mechanical properties of a material by the application of special tools and equipment. Machining almost always is a process where a cutting tool removes material to effect the desired change in the workpiece. Typically, powered machinery is required to operate the cutting tools. See Production methods

Although various machining operations may appear to be very different, most are very similar: they make chips. These chips vary in size from the long continuous ribbons produced on a lathe to the microfine sludge produced by lapping or grinding. These chips are formed by shearing away the workpiece material by the action of a cutting tool. Cylindrical holes can be produced in a workpiece by drilling, milling, reaming, turning, and electric discharge machining. Rectangular (or nonround) holes and slots may be produced by broaching, electric discharge machining, milling, grinding, and nibbling. Cylinders may be produced on lathes and grinders. Special geometries, such as threads and gears, are produced with special tooling and equipment utilizing the turning and grinding processes mentioned above. Polishing, lapping, and buffing are variants of grinding where a very small amount of stock is removed from the workpiece to produce a high-quality surface.

In almost every case, machining accuracy, economics, and production rates are controlled by the careful evaluation and selection of tooling and equipment. Speed of cut, depth of cut, cutting-tool material selection, and machine-tool selection have a tremendous impact on machining. In general, the more rigid and vibration-free a machining tool is, the better it will perform. Jigs and fixtures are often used to support the work-piece. Since it relies on the plastic deformation and shearing of the workpiece by the cutting tool, machining generates heat that must be dissipated before it damages the workpiece or tooling. Coolants, which also acts as lubricants, are often used.

To increase the life and speed of cutting tools, they are often coated with a thin layer of extremely hard material such as titanium nitride or zirconium nitride. These materials, which are applied over the cutting edges, provide excellent wear resistance. They are also brittle, so they rely on the toughness of the underlying cutting tool to support them. Coated tools are more expensive than conventional tools, but they can often cut at much higher rates and last significantly longer. When used properly on sufficiently rigid machine tools, they are far more economical than conventional tooling. See Metal coatings