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Cutting and Machining of Metals

Some metal shaping processes are forging, rolling, extrusion, etc., that do not involve the removal of metal by means of cutting tools. Many important shaping processes are based on cutting and similar operations. The tools used are made of special steels (tool steels), hard metals (Cemented carbide alloys), oxide ceramic materials, and diamonds.

The principles of the various methods are briefly outlined, without detailed descriptions of the machines used for performing the shaping operations. For each of these methods a whole range of tools has been developed, each type of tool being employed for a particular purpose. In chiseling (Fig.1), the cutting edge of the tool is driven into the surface of the workpiece by the action of blows. To ensure even and regular removal of the chips it is essential to hold the chisel correctly and take care that it does not slip on the metal surface or dig too deeply into it. Chiseling is used chiefly for cutting off and for the removal of edges, burrs, fins, etc.

Machining operations like planing, shaping and slotting (Fig.2) are comparable to chiseling, characterized by the removal of the chips in one direction, the tool being moved to and fro or up and down in relation to the workpiece. In sawing (Fig.3) the removal of metal is effected by a series of saw teeth. With power-driven band saws and circular saws, cutting can be performed in continuous operation.

The shape, spacing and number of teeth vary greatly for different saws. Large diameter circular saws may have interchangeable teeth or interchangeable segments comprising a number of teeth. Sawing is used mainly for cutting off and for cutting plate material of not too great a thickness. Thick pieces of metal can, while still hot from the furnace, be cut with hot sawing machines or with cutting discs. The latter achieve very high cutting rates, cutting being effected by melting of the metal due to frictional heating. Band-type cutting devices are based on the same principle of heat generated at the cutting surface.

Filing is another important basic process (Fig.4). By using suitably shaped files it is possible to cut metal to any desired shape. The actual cutting is performed by the teeth of the file. Roughing out the shape is first done by means of coarse files, followed by finishing with finer files. Files are available in great variety of shapes, sizes and grades. They are classified and named according to sectional shape (e.g., half-round, square, triangular, round), length, and the relative fineness of cut of the teeth. With regard to fineness, the following classes of file are distinguished: bastard, second-cut, smooth, dead smooth. If there is only one series of parallel teeth, the file is known as a single-cut file. If the first series is intersected by a second and finer series, so as to form diamond shaped teeth, the file can also be called as double-cut.

A broach (Fig.5) is a tapered tool provided with a series of cutting teeth which are lower at one end of the tool than they are at the other. Broaching is mainly employed for machining out holes or other internal surfaces, but can also be used for external surfaces and for burnishing already-formed holes. The cut starts with the smaller teeth, which enter the hole, and finishes with the larger teeth, which bring the hole to the finished size. Fig.5 shows an internal broach. Its cross-sectional shape may be round, rectangular, etc., depending on the desired shape of the hole. The broaching operation is performed by a machine that pulls or pushes the broach through the workpiece.

Turning (Fig.6) is one of the most important machining processes. It is the process of reducing the diameter of material held in a lathe. The workpiece is attached to a driven spindle and, while rotating, is brought into contact with a cutting tool. The position of the tool in relation to the axis of rotation can be varied so as to cut the workpiece to the desired shape. In longitudinal turning, the tool is moved parallel to the axis of the rotation, so that cylindrical shapes are obtained.

A screw thread can be cut by a tool forming a spiral groove. In transverse turning, also known as facing, and in forming, the tool is moved at right angles to the axis. Workpieces of any desired tapered or other axially symmetrical shape can be produced by suitable combinations of longitudinal and transverse tool movements.

Turning tools are available in a wide range of shapes and types. The cheapest are made of high carbon steel, hardened and tempered. Alloys known as high speed steels are used for tools that can be operated at much higher cutting speeds. In tipped tools the cutting tip is made of a special hard material. E.g., a cemented carbide, particularly tungsten carbide.

Drilling (Fig.7) is a rotary cutting operation for producing holes. The tool most widely used for the purpose is the twist drill, provided with helical cutting edges, which rotates and is fed forward into the material under pressure. The combination of rotary and feed motion cuts away chips of the material, which are removed from the hole. For drilling a hole in a solid workpiece, it is necessary first to make an indentation for the center of the drill to revolve in. A tool called center punch is used for the purpose. It is advisable first to use a smaller drill and then follow up with a drill of larger diameter.

Counterboring (Fig.8) is a process related to drilling and is employed to form a cylindrical hole of large diameter at the end of an existing hole. E.g., to receive the head of a screw or bolt. If the enlarged hole is formed with tapered sides, the process is called countersinking.

Milling (Fig.9) is another important machining process, in which the workpiece is shaped by means of a rotating cutter provided with a number of teeth. Usually the work is fed against the teeth, the work-feed direction (in relation to the cutter) being longitudinal, transverse or vertical. Milling machines are very versatile and can be used for a great variety of work, including screw-thread cutting. In circular milling the cutter and the workpiece are both rotated; in straight milling the cutter rotates while the workpiece performs a straight feed motion

Grinding (Fig.10) is the operation in which an abrasive wheel or disc is used to remove metal. It is employed as a finishing treatment to give parts already machined the necessary precision by the removal of excess material. It is also employed as a machining process in its own right – e.g., for roughly forged or cast parts or for the shaping of hard materials. Centerless grinding is used for small cylindrical parts and is performed between two grinding wheels. Grinding wheels are made from artificial abrasives, usually of the aluminum oxide or the silicon carbide type, embedded in suitable bonding agents. Wheels are available in a vast number of different combinations of abrasive, grain size, type of bond, hardness of bond, and structure.