2. Machining

(slovní zásoba v sekci Slovíčka)

Machining is the process of removing material from a workpiece to achieve desired shape and size. The material is cut by sharp cutting tools (lathes, milling machines, drill press) into the form of chips (also called swarf) Machining is a precise procedure compared to forming, molding and casting. It is necessary where high degree tolerances on dimensions and high-quality finished products are required.

A finished product would be a workpiece that meets the specifications set out by engineering drawings. A lot of factors play a role to achieve the desired product, e.g.: tool size, tool shape, level of contact with the tool, the depth of the cuts and also relative motion. During machining process a relative motion is produced between the tool and the workpiece. The primary motion is called cutting speed (the cutting speed of the machine is the rotation of the workpiece against the tool )and the secondary motion is called feed (the distance of tool travel during one revolution of the part.)

 The type of material to be cut is a deciding factor when determining the correct speed and feed rate. Machinist also has to keep in mind that a machining allowance on specific parts of a given workpiece is needed. Unlike a tolerance, which is an unplanned deviation, machining allowance is planed deviation.

Machining has to be viewed as a system consisting of the machine – the tool – the workpiece but also as a group of manufacturing operations.

There are two categories of machining – conventional and non-conventional.

Conventional processes

– the machining operations that use single- or multi- point cutting tools (each with a clearly defined geometry) to remove material in the form of chips. They include: turning, milling, dribbling, planning, grinding, honing, loping etc.

The cutting tool must be harder than a workpiece.

Turning – the most basic machining operation used to produce cylindrical components. The main rotating cutting motion is done by a workpiece, while feed and infeed is done by a tool.  Turning is performed on a machine called a lathe (the process is controlled manually) or on a modern computer numerical controlled CNC machine (the process is controlled by computer). When turning, the workpiece is held and rotated around a horizontal axis while being formed to size and shape by a cutting tool, called a turning tool. The turning tool is stationary, single-point and parallel to the surface. The workpiece is mounted in a chuck, collet or faceplate.  typical examples of the products made by turning process include internal or external cylindrical components or threads. The lathe can be also used for other machining operation like drilling or boring.

Milling – a machining process for producing parts having flat, angular or curved shapes like grooves, helixes, geared wheels etc. In the milling process, the cutting motion is carried out by rotating tool while feed and infeed is done by a workpiece in three perpendicular directions – vertical, longitudinal or crosswise. The workpiece is commonly held in a vise or similar device clamped to a table, that can move, and fed against a rotating multiple-tooth tool known as the milling cutter.

Drilling – a machining process of making cylindrical holes and it is extensively used because most of components have one or more holes in them. The cutting motion and feed is performed by a rotating tool while the infeed is given by the radius of a drill. The drilling operation can be carried out on a drilling machine but a lathe can be used for drilling as well. The tool used for making holes is called a drill. The workpiece remains stationary and the drill is rotated as well as fed into the workpiece. The vertical motion of the drill is the feed in drilling operations.

There are various finishing operations used for enlarging an existing hole and to achieve greater accuracy of the diameter of a hole, e.g.: boring, counterboring, countersinking, reaming.

Finish machining operations are grinding, honing, lapping, superfinishing. They are used to improve surface finish (by removing scratches or machine marks), abrade hard materials and increase geometric form of a surface.

Non - conventional processes

Used for alloys that are very hard, high-strength, temperature resistant or too brittle. To evercome these problems, non-conventional operations are used. They are characterized by utilizing some form of energy for metal machining instead of a traditional tool. They can be broadly classified according to the type of energy employed: water jet, ultrasonic, electron-beam, laser-beam, plasma-beam, chemical cutting.

Cutting tools - is a device used to remove the unwanted material from a given workpiece. The materials for cutting tools must have the following properties: high hardness, heat and wear resistance, bending strength, toughness and thermal stability.

Materials used for cutting tools: high-speed steel, indexable carbide, ceramic, diamond, cubic boron nitride.