外文翻譯--基本加工工序和切削技術(shù)

1、<p>　　Basic Machining Operations and Cutting Technology</p><p>　　Basic Machining Operations </p><p>　　Machine tools have evolved from the early foot-powered lathes of the Egyptians and John

2、 Wilkinson's boring mill. They are designed to provide rigid support for both the workpiece and the cutting tool and can precisely control their relative positions and the velocity of the tool with respect to the wor

3、kpiece. Basically, in metal cutting, a sharpened wedge-shaped tool removes a rather narrow strip of metal from the surface of a ductile workpiece in the form of a severely deformed chip. The chip i</p><p>　　

4、Most machining operations produce parts of differing geometry. If a rough cylindrical workpiece revolves about a central axis and the tool penetrates beneath its surface and travels parallel to the center of rotation, a

5、surface of revolution is produced, and the operation is called turning. If a hollow tube is machined on the inside in a similar manner, the operation is called boring. Producing an external conical surface uniformly vary

6、ing diameter is called taper turning, if the tool point trav</p><p>　　Flat or plane surfaces are frequently required. They can be generated by radial turning or facing, in which the tool point moves normal t

7、o the axis of rotation. In other cases, it is more convenient to hold the workpiece steady and reciprocate the tool across it in a series of straight-line cuts with a crosswise feed increment before each cutting stroke.

8、This operation is called planning and is carried out on a shaper. For larger pieces it is easier to keep the tool stationary and draw the workp</p><p>　　Multiple-edged tools can also be used. Drilling uses a

9、 twin-edged fluted tool for holes with depths up to 5 to 10 times the drill diameter. Whether the </p><p>　　drill turns or the workpiece rotates, relative motion between the cutting edge and the workpiece is

10、 the important factor. In milling operations a rotary cutter with a number of cutting edges engages the workpiece. Which moves slowly with respect to the cutter. Plane or contoured surfaces may be produced, depending on

11、the geometry of the cutter and the type of feed. Horizontal or vertical axes of rotation may be used, and the feed of the workpiece may be in any of the three coordinate directions.</p><p>　　Basic Machine To

12、ols </p><p>　　Machine tools are used to produce a part of a specified geometrical shape and precise I size by removing metal from a ductile material in the form of chips. The latter are a waste product and v

13、ary from long continuous ribbons of a ductile material such as steel, which are undesirable from a disposal point of view, to easily handled well-broken chips resulting from cast iron. Machine tools perform five basic me

14、tal-removal processes: I turning, planning, drilling, milling, and grinding. All other </p><p>　　The amount and rate of material removed by the various machining processes may be I large, as in heavy turning

15、 operations, or extremely small, as in lapping or super finishing operations where only the high spots of a surface are removed. </p><p>　　A machine tool performs three major functions: 1. it rigidly support

16、s the workpiece or its holder and the cutting tool; 2. it provides relative motion between the workpiece and the cutting tool; 3. it provides a range of feeds and speeds usually ranging from 4 to 32 choices in each case.

17、 </p><p>　　Speed and Feeds in Machining </p><p>　　Speeds, feeds, and depth of cut are the three major variables for economical machining. Other variables are the work and tool materials, coolant

18、 and geometry of the cutting tool. The rate of metal removal and power required for machining depend upon these variables. </p><p>　　The depth of cut, feed, and cutting speed are machine settings that must b

19、e established in any metal-cutting operation. They all affect the forces, the power, and the rate of metal removal. They can be defined by comparing them to the needle and record of a phonograph. The cutting speed (V) is

20、 represented by the velocity of- the record surface relative to the needle in the tone arm at any instant. Feed is represented by the advance of the needle radially inward per revolution, or is the differen</p>&l

21、t;p>　　Turning on Lathe Centers </p><p>　　The basic operations performed on an engine lathe are illustrated. Those operations performed on external surfaces with a single point cutting tool are called turn

22、ing. Except for drilling, reaming, and lapping, the operations on internal surfaces are also performed by a single point cutting tool. </p><p>　　All machining operations, including turning and boring, can be

23、 classified as roughing, finishing, or semi-finishing. The objective of a roughing operation is to remove the bulk of the material as rapidly and as efficiently as possible, while leaving a small amount of material on th

24、e work-piece for the finishing operation. Finishing operations are performed to obtain the final size, shape, and surface finish on the workpiece. Sometimes a semi-finishing operation will precede the finishing operati&l

25、t;/p><p>　　Generally, longer workpieces are turned while supported on one or two lathe centers. Cone shaped holes, called center holes, which fit the lathe centers are drilled in the ends of the workpiece-usual

26、ly along the axis of the cylindrical part. The end of the workpiece adjacent to the tailstock is always supported by a tailstock center, while the end near the headstock may be supported by a headstock center or held in

27、a chuck. The headstock end of the workpiece may be held in a four-jaw chuck, or i</p><p>　　Very precise results can be obtained by supporting the workpiece between two centers. A lathe dog is clamped to the

28、workpiece; together they are driven by a driver plate mounted on the spindle nose. One end of the Workpiece is mecained;then the workpiece can be turned around in the lathe to machine the other end. The center holes in t

29、he workpiece serve as precise locating surfaces as well as bearing surfaces to carry the weight of the workpiece  and to resist the cutting forces. After the workpi</p><p>　　While very large diameter wo

30、rkpieces are sometimes mounted on two centers, they are preferably held at the headstock end by faceplate jaws to obtain the smooth power transmission; moreover, large lathe dogs that are adequate to transmit the power n

31、ot generally available, although they can be made as a special. Faceplate jaws are like chuck jaws except that they are mounted on a faceplate, which has less overhang from the spindle bearings than a large chuck would h

32、ave. </p><p>　　Introduction of Machining </p><p>　　Machining as a shape-producing method is the most universally used and the most important of all manufacturing processes. Machining is a shape-

33、producing process in which a power-driven device causes material to be removed in chip form. Most machining is done with equipment that supports both the work piece and cutting tool although in some cases portable equipm

34、ent is used with unsupported workpiece. </p><p>　　Low setup cost for small Quantities. Machining has two applications in manufacturing. For casting, forging, and press working, each specific shape to be prod

35、uced, even one part, nearly always has a high tooling cost. The shapes that may he produced by welding depend to a large degree on the shapes of raw material that are available. By making use of generally high cost equip

36、ment but without special tooling, it is possible, by machining; to start with nearly any form of raw material, so tong as t</p><p>　　Close accuracies, good finishes. The second application for machining is b

37、ased on the high accuracies and surface finishes possible. Many of the parts machined in low quantities would be produced with lower but acceptable tolerances if produced in high quantities by some other process. On the

38、other hand, many parts are given their general shapes by some high quantity deformation process and machined only on selected surfaces where high accuracies are needed. Internal threads, for example, are se</p>&l

39、t;p>　　Primary Cutting Parameters </p><p>　　The basic tool-work relationship in cutting is adequately described by means of four factors: tool geometry, cutting speed, feed, and depth of cut. </p>&

40、lt;p>　　The cutting tool must be made of an appropriate material; it must be strong, tough, hard, and wear resistant. The tool s geometry characterized by planes and angles, must be correct for each cutting operation.

41、Cutting speed is the rate at which the work surface passes by the cutting edge. It may be expressed in feet per minute. </p><p>　　For efficient machining the cutting speed must be of a magnitude appropriate

42、to the particular work-tool combination. In general, the harder the work material, the slower the speed. </p><p>　　Feed is the rate at which the cutting tool advances into the workpiece. "Where the work

43、piece or the tool rotates, feed is measured in inches per revolution. When the tool or the work reciprocates, feed is measured in inches per stroke, Generally, feed varies inversely with cutting speed for otherwise simil

44、ar conditions. </p><p>　　The depth of cut, measured inches is the distance the tool is set into the work. It is the width of the chip in turning or the thickness of the chip in a rectilinear cut. In roughing

45、 operations, the depth of cut can be larger than for finishing operations. </p><p>　　The Effect of Changes in Cutting Parameters on Cutting Temperatures </p><p>　　In metal cutting operations hea

46、t is generated in the primary and secondary deformation zones and these results in a complex temperature distribution throughout the tool, workpiece and chip. A typical set of isotherms is shown in figure where it can be

47、 seen that, as could be expected, there is a very large temperature gradient throughout the width of the chip as the workpiece material is sheared in primary deformation and there is a further large temperature in the ch

48、ip adjacent to the face as th</p><p>　　Since virtually all the work done in metal cutting is converted into heat, it could be expected that factors which increase the power consumed per unit volume of metal

49、removed will increase the cutting temperature. Thus an increase in the rake angle, all other parameters remaining constant, will reduce the power per unit volume of metal removed and the cutting temperatures will reduce.

50、 When considering increase in unreformed chip thickness and cutting speed the situation is more complex. An incr</p><p>　　The most direct and accurate method for measuring temperatures in high -speed-steel c

51、utting tools is that of Wright &. Trent which also yields detailed information on temperature distributions in high-speed-steel cutting tools. The technique is based on the metallographic examination of sectioned hig

52、h-speed-steel tools which relates microstructure changes to thermal history. </p><p>　　Trent has described measurements of cutting temperatures and temperature  distributions for high-speed-steel tools

53、when machining a wide range of workpiece materials. This technique has been further developed by using scanning electron  microscopy to study fine-scale microstructure changes arising from over tempering of the temp

54、ered martens tic matrix of various high-speed-steels. This technique has also been used to study temperature distributions in both high-speed -steel single point turning </p><p>　　Wears of Cutting Tool </

55、p><p>　　Discounting brittle fracture and edge chipping, which have already been dealt with, tool wear is basically of three types. Flank wear, crater wear, and notch wear. Flank wear occurs on both the major an

56、d the minor cutting edges. On the major cutting edge, which is responsible for bulk metal removal, these results in increased cutting forces and higher temperatures which if left unchecked can lead to vibration of the to

57、ol and workpiece and a condition where efficient cutting can no longer take pl</p><p>　　Because of the stress distribution on the tool face, the frictional stress in the region of sliding contact between the

58、 chip and the face is at a maximum at the start of the sliding contact region and is zero at the end. Thus abrasive wear takes place in this region with more wear taking place adjacent to the seizure region than adjacent

59、 to the point at which the chip loses contact with the face. This result in localized pitting of the tool face some distance up the face which is usually referre</p><p>　　At the end of the major flank wear l

60、and where the tool is in contact with the uncut workpiece surface it is common for the flank wear to be more pronounced than along the rest of the wear land. This is because of localised effects such as a hardened layer

61、on the uncut surface caused by work hardening introduced by a previous cut, an oxide scale, and localised high temperatures resulting from the edge effect. This localised wear is usually referred to as notch wear and occ

62、asionally is very severe</p><p>　　If any form of progressive wear allowed to continue, dramatically and the tool would fail catastrophically, i. e. the tool would be no longer capable of cutting and, at best

63、, the workpiece would be scrapped whilst, at worst, damage could be caused to the machine tool. For carbide cutting tools and for all types of wear, the tool is said to have reached the end of its useful life long before

64、 the onset of catastrophic failure. For high-speed-steel cutting tools, however, where the wear tends to be</p><p>　　Mechanism of Surface Finish Production </p><p>　　There are basically five mec

65、hanisms which contribute to the production of a surface which have been machined. These are:</p><p>　　(l) The basic geometry of the cutting process. In, for example, single point turning the tool will advanc

66、e a constant distance axially per revolution of the workpiecc and the resultant surface will have on it, when viewed perpendicularly to the direction of tool feed motion, a series of cusps which will have a basic form wh

67、ich replicates the shape of the tool in cut. </p><p>　　(2) The efficiency of the cutting operation. It has already been mentioned that cutting with unstable built-up-edges will produce a surface which contai

68、ns hard built-up-edge fragments which will result in a degradation of the surface finish. It can also be demonstrated that cutting under adverse conditions such as apply when using large feeds small rake angles and low c

69、utting speeds, besides producing conditions which lead to unstable built-up-edge production, the cutting process itself can beco</p><p>　　(3) The stability of the machine tool. Under some combinations of cut

70、ting conditions; workpiece size, method of clamping ,and cutting tool rigidity relative to the machine tool structure, instability can be set up in the tool which causes it to vibrate. Under some conditions this vibratio

71、n will reach and maintain steady amplitude whilst under other conditions the vibration will built up and unless cutting is stopped considerable damage to both the cutting tool and workpiece may occur. This pheno</p>

72、;<p>　　(4)The effectiveness of removing swarf. In discontinuous chip production machining, such as milling or turning of brittle materials, it is expected that the chip (swarf) will leave the cutting zone either u

73、nder gravity or with the assistance of a jet of cutting fluid and that they will not influence the cut surface in any way. However, when continuous chip production is evident, unless steps are taken to control the swarf

74、it is likely that it will impinge on the cut surface and mark it. Inevitab</p><p>　　(5)The effective clearance angle on the cutting tool. For certain geometries of minor cutting edge relief and clearance ang

75、les it is possible to cut on the major cutting edge and burnish on the minor cutting edge. This can produce a good surface finish but, of course, it is strictly a combination of metal cutting and metal forming and is not

76、 to be recommended as a practical cutting method. However, due to cutting tool wear, these conditions occasionally arise and lead to a marked change in the s</p><p>　　Limits and Tolerances </p><p&

77、gt;　　Machine parts are manufactured so they are interchangeable. In other words, each part of a machine or mechanism is made to a certain size and shape so will fit into any other machine or mechanism of the same type. T

78、o make the part interchangeable, each individual part must be made to a size that will fit the mating part in the correct way. It is not only impossible, but also impractical to make many parts to an exact size. This is

79、because machines are not perfect, and the tools become worn. A sl</p><p>　　A tolerance is the total permissible variation in the size of a part. </p><p>　　The basic size is that size from which

80、limits of size arc derived by the application of allowances and tolerances. </p><p>　　Sometimes the limit is allowed in only one direction. This is known as unilateral tolerance.</p><p>　　Unilat

81、eral tolerancing is a system of dimensioning where the tolerance (that is variation) is shown in only one direction from the nominal size. Unilateral tolerancing allow the changing of tolerance on a hole or shaft without

82、 seriously affecting the fit.</p><p>　　When the tolerance is in both directions from the basic size it is known as a bilateral tolerance (plus and minus). </p><p>　　Bilateral tolerancing is a sy

83、stem of dimensioning where the tolerance (that is variation) is split and is shown on either side of the nominal size. Limit dimensioning is a system of dimensioning where only the maximum and minimum dimensions arc show

84、n. Thus, the tolerance is the difference between these two dimensions. </p><p>　　Surface Finishing and Dimensional Control </p><p>　　Products that have been completed to their proper shape and s

85、ize frequently require some type of surface finishing to enable them to satisfactorily fulfill their function. In some cases, it is necessary to improve the physical properties of the surface material for resistance to p

86、enetration or abrasion. In many manufacturing processes, the product surface is left with dirt .chips, grease, or other harmful material upon it. Assemblies that are made of different materials, or from the same materia&

87、lt;/p><p>　　Surface finishing may sometimes become an intermediate step processing. For instance, cleaning and polishing are usually essential before any kind of plating process. Some of the cleaning procedures

88、 are also used for improving surface smoothness on mating parts and for removing burrs and sharp corners, which might be harmful in later use. Another important need for surface finishing is for corrosion protection in a

89、 variety of: environments. The type of protection procedure will depend largely up</p><p>　　Satisfying the above objectives necessitates the use of main surface-finishing methods that involve chemical change

90、 of the surface mechanical work affecting surface properties, cleaning by a variety of methods, and the application of protective coatings, organic and metallic. </p><p>　　In the early days of engineering, t

91、he mating of parts was achieved by machining one part as nearly as possible to the required size, machining the mating part nearly to size, and then completing its machining, continually offering the other part to it, un

92、til the desired relationship was obtained. If it was inconvenient to offer one part to the other part during machining, the final work was done at the bench by a fitter, who scraped the mating parts until the desired fit

93、 was obtained, the fitter</p><p>　　When one part can be used 'off the shelf' to replace another of the same dimension and material specification, the parts are said to be interchangeable. A system of

94、 interchangeability usually lowers the production costs as there is no need for an expensive, 'fiddling' operation, and it benefits the customer in the event of the need to replace worn parts. </p><p&g

95、t;　　Automatic Fixture Design </p><p>　　Traditional synchronous grippers for assembly equipment move parts to the gripper centre-line, assuring that the parts will be in a known position after they arc picked

96、 from a conveyor or nest. However, in some applications, forcing the part to the centre-line may damage cither the part or equipment. When the part is delicate and a small collision can result in scrap, when its location

97、 is fixed by a machine spindle or mould, or when tolerances are tight, it is preferable to make a gripper comply</p><p>　　Production is characterized by batch-size becoming smaller and smaller and greater va

98、riety of products. Assembly, being the last production step, is particularly vulnerable to changes in schedules, batch-sizes, and product design. This situation is forcing many companies to put more effort into extensive