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1、精选优质文档-倾情为你奉上附录附录1英文原文Basic Machining Operations and Cutting TechnologyBasic Machining Operations Machine tools have evolved from the early foot-powered lathes of the Egyptians and John Wilkinsons boring mill. They are designed to provide rigid support for both the work piece and the cutting tool an
2、d can precisely control their relative positions and the velocity of the tool with respect to the work piece. Basically, in metal cutting, a sharpened wedge-shaped tool removes a rather narrow strip of metal from the surface of a ductile work piece in the form of a severely deformed chip. The chip i
3、s a waste product that is considerably shorter than the work piece from which it came but with a corresponding increase in thickness of the uncut chip. The geometrical shape of work piece depends on the shape of the tool and its path during the machining operation. Most machining operations produce
4、parts of differing geometry. If a rough cylindrical work piece revolves about a central axis and the tool penetrates beneath its surface and travels parallel to the center of rotation, a surface of revolution is produced, and the operation is called turning. If a hollow tube is machined on the insid
5、e in a similar manner, the operation is called boring. Producing an external conical surface uniformly varying diameter is called taper turning, if the tool point travels in a path of varying radius, a contoured surface like that of a bowling pin can be produced; or, if the piece is short enough and
6、 the support is sufficiently rigid, a contoured surface could be produced by feeding a shaped tool normal to the axis of rotation. Short tapered or cylindrical surfaces could also be contour formed. Flat or plane surfaces are frequently required. They can be generated by radial turning or facing, in
7、 which the tool point moves normal to the axis of rotation. In other cases, it is more convenient to hold the work piece steady and reciprocate the tool across it in a series of straight-line cuts with a crosswise feed increment before each cutting stroke. This operation is called planning and is ca
8、rried out on a shaper. For larger pieces it is easier to keep the tool stationary and draw the work piece under it as in planning. The tool is fed at each reciprocation. Contoured surfaces can be produced by using shaped tools. Multiple-edged tools can also be used. Drilling uses a twin-edged fluted
9、 tool for holes with depths up to 5 to 10 times the drill diameter. Whether the drill turns or the work piece rotates, relative motion between the cutting edge and the work piece is the important factor. In milling operations a rotary cutter with a number of cutting edges engages the work piece. Whi
10、ch moves slowly with respect to the cutter. Plane or contoured surfaces may be produced, depending on the geometry of the cutter and the type of feed. Horizontal or vertical axes of rotation may be used, and the feed of the work piece may be in any of the three coordinate directions. Basic Machine T
11、ools 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 vary from long continuous ribbons of a ductile material such as steel, which are undesirable from a dis
12、posal point of view, to easily handled well-broken chips resulting from cast iron. Machine tools perform five basic metal-removal processes: I turning, planning, drilling, milling, and grinding. All other metal-removal processes are modifications of these five basic processes. For example, boring is
13、 internal turning; reaming, tapping, and counter boring modify drilled holes and are related to drilling; bobbing and gear cutting are fundamentally milling operations; hack sawing and broaching are a form of planning and honing; lapping, super finishing. Polishing and buffing are variants of grindi
14、ng or abrasive removal operations. Therefore, there are only four types of basic machine tools, which use cutting tools of specific controllable geometry: 1. lathes, 2. planers, 3. drilling machines, and 4. milling machines. The grinding process forms chips, but the geometry of the abrasive grain is
15、 uncontrollable. The amount and rate of material removed by the various machining processes may be I large, as in heavy turning operations, or extremely small, as in lapping or super finishing operations where only the high spots of a surface are removed. A machine tool performs three major function
16、s: 1. it rigidly supports the work piece or its holder and the cutting tool; 2. it provides relative motion between the work piece and the cutting tool; 3. it provides a range of feeds and speeds usually ranging from 4 to 32 choices in each case. Speed and Feeds in Machining Speeds, feeds, and depth
17、 of cut are the three major variables for economical machining. Other variables are the work and tool materials, coolant and geometry of the cutting tool. The rate of metal removal and power required for machining depend upon these variables. The depth of cut, feed, and cutting speed are machine set
18、tings that must be 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 represented by the velocity of- the record surface relative to
19、 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 difference in position between two adjacent grooves. The depth of cut is the penetration of the needle into the record or the depth of the grooves. Turning on Lathe
20、Centers The basic operations performed on an engine lathe are illustrated. Those operations performed on external surfaces with a single point cutting tool are called turning. Except for drilling, reaming, and lapping, the operations on internal surfaces are also performed by a single point cutting
21、tool. All machining operations, including turning and boring, can be 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 the work-piece
22、 for the finishing operation. Finishing operations are performed to obtain the final size, shape, and surface finish on the work piece. Sometimes a semi-finishing operation will precede the finishing operation to leave a small predetermined and uniform amount of stock on the work-piece to be removed
23、 by the finishing operation. Generally, longer work pieces 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 work piece-usually along the axis of the cylindrical part. The end of the work piece a
24、djacent to the tail stock is always supported by a tail stock center, while the end near the head stock may be supported by a head stock center or held in a chuck. The head stock end of the work piece may be held in a four-jaw chuck, or in a type chuck. This method holds the work piece firmly and tr
25、ansfers the power to the work piece smoothly; the additional support to the work piece provided by the chuck lessens the tendency for chatter to occur when cutting. Precise results can be obtained with this method if care is taken to hold the work piece accurately in the chuck. Very precise results
26、can be obtained by supporting the work piece between two centers. A lathe dog is clamped to the work piece; together they are driven by a driver plate mounted on the spindle nose. One end of the Work piece is mecained;then the work piece can be turned around in the lathe to machine the other end. Th
27、e center holes in the work piece serve as precise locating surfaces as well as bearing surfaces to carry the weight of the work piece and to resist the cutting forces. After the work piece has been removed from the lathe for any reason, the center holes will accurately align the work piece back in t
28、he lathe or in another lathe, or in a cylindrical grinding machine. The work piece must never be held at the head stock end by both a chuck and a lathe center. While at first thought this seems like a quick method of aligning the work piece in the chuck, this must not be done because it is not possi
29、ble to press evenly with the jaws against the work piece while it is also supported by the center. The alignment provided by the center will not be maintained and the pressure of the jaws may damage the center hole, the lathe center, and perhaps even the lathe spindle. Compensating or floating jaw c
30、hucks used almost exclusively on high production work provide an exception to the statements made above. These chucks are really work drivers and cannot be used for the same purpose as ordinary three or four-jaw chucks. While very large diameter work pieces are sometimes mounted on two centers, they
31、 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 not generally available, although they can be made as a special. Faceplate jaws are like chuck jaws except that they are mounted on
32、a faceplate, which has less overhang from the spindle bearings than a large chuck would have. Introduction of Machining Machining as a shape-producing method is the most universally used and the most important of all manufacturing processes. Machining is a shape-producing process in which a power-dr
33、iven 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 equipment is used with unsupported work piece. Low setup cost for small Quantities. Machining has two applications in manuf
34、acturing. For casting, forging, and press working, each specific shape to be produced, 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 equi
35、pment but without special tooling, it is possible, by machining; to start with nearly any form of raw material, so tong as the exterior dimensions are great enough, and produce any desired shape from any material. Therefore .machining is usually the preferred method for producing one or a few parts,
36、 even when the design of the part would logically lead to casting, forging or press working if a high quantity were to be produced. Close accuracies, good finishes. The second application for machining is based on the high accuracies and surface finishes possible. Many of the parts machined in low q
37、uantities would be produced with lower but acceptable tolerances if produced in high quantities by some other process. On the 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. Inter
38、nal threads, for example, are seldom produced by any means other than machining and small holes in press worked parts may be machined following the press working operations. Primary Cutting Parameters The basic tool-work relationship in cutting is adequately described by means of four factors: tool
39、geometry, cutting speed, feed, and depth of cut. 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. Cutting speed is the rate at which the wo
40、rk surface passes by the cutting edge. It may be expressed in feet per minute. For efficient machining the cutting speed must be of a magnitude appropriate to the particular work-tool combination. In general, the harder the work material, the slower the speed. Feed is the rate at which the cutting t
41、ool advances into the work piece. Where the work 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 similar conditions. The depth of cut
42、, 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 operations, the depth of cut can be larger than for finishing operations. The Effect of Changes in Cutting Parameters on Cutting Temp
43、eratures In metal cutting operations heat is generated in the primary and secondary deformation zones and these results in a complex temperature distribution throughout the tool, work piece and chip. A typical set of isotherms is shown in figure where it can be seen that, as could be expected, there
44、 is a very large temperature gradient throughout the width of the chip as the work piece material is sheared in primary deformation and there is a further large temperature in the chip adjacent to the face as the chip is sheared in secondary deformation. This leads to a maximum cutting temperature a
45、 short distance up the face from the cutting edge and a small distance into the chip. 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 removed will increase the cutting temperature.
46、 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. When considering increase in unreformed chip thickness and cutting speed the situation is more complex. An increase in undefo
47、rmed chip thickness tends to be a scale effect where the amounts of heat which pass to the work piece, the tool and chip remain in fixed proportions and the changes in cutting temperature tend to be small. Increase in cutting speed; however, reduce the amount of heat which passes into the work piece
48、 and this increase the temperature rise of the chip m primary deformation. Further, the secondary deformation zone tends to be smaller and this has the effect of increasing the temperatures in this zone. Other changes in cutting parameters have virtually no effect on the power consumed per unit volu
49、me of metal removed and consequently have virtually no effect on the cutting temperatures. Since it has been shown that even small changes in cutting temperature have a significant effect on tool wear rate it is appropriate to indicate how cutting temperatures can be assessed from cutting data. The most direct and accurate method for measuring temperatures in high -speed-steel cutting tools is that of Wright &. Trent which also yields detailed information on temperature distributions in high-speed-steel cutting to