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Date: 2015-10-07; view: 459.

Material removal processes, which include machining, cutting, grind­ing, and various nonmechanical chipless processes, are desirable or even necessary for the following basic reasons: closer dimensional tolerances, surface roughness, or surface-finish characteristics may be required than are available by casting, forming, powder metallurgy, and other shaping processes; and part geometries may be too complex or too expensive to be manufactured by other processes. However, mate­rial removal processes inevitably waste material in the form of chips, production rates may be low, and unless carried out properly, the pro­cesses can have detrimental effects on the surface properties and per­formance of parts.

Traditional material removal processes consist of turning, boring, drilling, reaming, threading, milling, shaping, planing, and broaching, as well as abrasive processes such as grinding, ultrasonic machining, lapping, and honing.

A factor of great significance in metal cutting is tool wear. Many factors determine the type and rate at which wear occurs on the tool. The major critical variables that affect wear are tool temperature, type and hardness of tool material, grade and condition of workpiece, abrasiveness of the micro-constituents in the workpiece material, tool geometry, feed speed, and cutting fluid. The type of wear pattern that develops depends on the relative role of these variables.

Tool wear can be classified as (1) flank wear; (2) craterwear on the tool face; (3) localized wear, such as the rounding of the cutting edge; (4) chipping or thermal softening and plastic flow of the cutting edge; (5) concentrated wear resulting in a deep groove at the edge of a turning tool, known as wear notch.

The general types of machine tools are lathes; turret lathes; screw, bor­ing, drilling, reaming, threading, milling, and gear-cutting machines; planers and shapers; broaching, cutting-off, grinding, and polishing ma­chines. Each of these is subdivided into many types and sizes.

Turning is a machining operation for all types of metallic and nonmetallic materials and is capable of producing circular parts with straight or various profiles. The cutting tools may be single-point or form tools. The most common machine tool used is a lathe; modern lathes are computer-controlled and can achieve high production rates with little labor.

Boring is a machining process for producing internal straight cylindrical surfaces or profiles, with process characteristics and tooling similar to those for turning operations. Boring machines are of two general types, horizontal and vertical, and are frequently referred to as horizontal boring machines and vertical boring and turning mills. A classification of boring machines comprises horizontal boring, drilling, and milling machines; vertical boring and turning mills; vertical multi-spindle cylinder boring mills; vertical cylin­der boring mills; vertical turret boring mills; car- wheel boring mills; diamond or precision boring machines; and jig borers.

Drilling is a commonly employed hole-making process that uses a drill as a cutting tool for producing round holes of various sizes and depths. Drilled holes may be subjected to additional operations for better sur­face finish and dimensional accuracy, such as reaming and honing. Drilling machines are intended for drilling holes, tapping, counter boring, reaming, and general boring operations. They may be classified into a large variety of types: twist drills, straight-shank twist drills, taper-shank drills, heavy-duty drill, and radial drill.

A reaming is a multiple-cutting edge tool used to enlarge or finish holes, and to provide accurate dimensions as well as good finish. Reamers are of two types: 1)rose and 2) fluted. The rose reamer is a heavy-bodied tool with end cutting edges. It is used to remove considerable metal and to true up a hole preparatory to flute reaming. It is similar to the three- and four-fluted drills. Wide cylindrical lands are provided back of the flute edges.

Threads may be formed on the outside or inside of a cylinder or cone (1) with single-point threading tools, (2) with threading chasers, (3) with taps, (4) with dies, (5) by thread milling, (6) by thread rolling, and (7) by grinding. There are numerous types of taps, such as hand, machine screw, pipe, and combined pipe tap and drill. Small taps usually have no radial relief. Large taps may have still more flutes.

Milling is one of the most versatile machining processes and is capable of producing a variety of shapes involving flat surfaces, slots, and con­tours (Fig. 1)

Fig. 1 Basic types of milling cutters and operations. (a) Slab (peripheral) milling; (b) face milling; (c) end milling.

Milling machines use cutters with multiple teeth in contrast with the single-point tools of the lathe and planer. The workpiece is generally fed past the cutter perpendicular to the cutter axis. Milling usually is face or peripheral cutting. Standard spindle noses and arbors for milling machines provide inter- changeability of arbors and face-milling cutters, regardless of make or size of machine. Milling-machine classification is based on design, operation, or pur­pose. Planers are used to rough and finish large flat surfaces, although arcs and special forms can be made with proper tools and attachments. Sur­faces to be finished by scraping, such as ways and long dovetails and, particularly, parts of machine tools, are, with few exceptions, planed.

Shapers are used for miscellaneous planing, surfacing, notching, key seating, and production of flat surfaces on flat parts. They are virtually obsolete, but some are still in use. The tool is held in a holder supported on a clapper on the end of a ram which is reciprocated hydraulically or by crank and rocker arm, in a straight line.

Broaching is a production process whereby a cutter, called a broach, is used to finish internal or external surfaces such as holes of circular, square, or irregular section, keyways, the teeth of internal gears, multi­ple spline holes, and flat surfaces. In broaching, the action of the broach itself serves as a clamping medium so that in many cases the operation may be completed in the time ordinarily taken to chuck the piece The broach is usually long and is provided with many teeth so graded in size that each takes a small chip when the tool is pulled or pushed through the previously prepared leader hole or past the surface.