多功能果蔬打浆机设计-带开题报告【7张CAD高清图纸 说明书】【YC系列】
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7张CAD高清图纸 说明书
YC系列
多功能果蔬打浆机设计-带开题报告【7张CAD高清图纸
说明书】【YC系列】
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装订线外文科技文献译文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 workpiece and the cutting tool and can precisely control their relative positions and the velocity of the tool with respect to the workpiece. 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 is a waste product that is considerably shorter than the workpiece from which it came but with a corresponding increase in thickness of the uncut chip. The geometrical shape of workpiece depends on the shape of the tool and its path during the machining operation. 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 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 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 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 which the tool point moves normal to 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. 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 workpiece 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 tool for holes with depths up to 5 to 10 times the drill diameter. Whether the drill turns or the workpiece rotates, relative motion between the cutting edge and the workpiece is 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 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. Basic Machine Tools 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 disposal 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 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 grinding 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 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 functions: 1. it rigidly supports 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. Speed and Feeds in Machining Speeds, feeds, and depth 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 settings 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 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 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 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 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 operation to leave a small predetermined and uniform amount of stock on the work-piece to be removed by the finishing operation. 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-usually 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 a chuck. The headstock end of the workpiece may be held in a four-jaw chuck, or in a type chuck. This method holds the workpiece firmly and transfers the power to the workpiece smoothly; the additional support to the workpiece 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 workpiece accurately in the chuck. Very precise results can be obtained by supporting the workpiece between two centers. A lathe dog is clamped to the 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 the 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 workpiece has been removed from the lathe for any reason, the center holes will accurately align the workpiece back in the lathe or in another lathe, or in a cylindrical grinding machine. The workpiece must never be held at the headstock end by both a chuck and a lathe center. While at first thought this seems like a quick method of aligning the workpiece in the chuck, this must not be done because it is not possible to press evenly with the jaws against the workpiece 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 chucks 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 workpieces 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 not 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 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-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 equipment is used with unsupported workpiece. Low setup cost for small Quantities. Machining has two applications in manufacturing. 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 equipment 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, 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 quantities 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. Internal 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 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 work 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 tool advances into the workpiece. Where the workpiece 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, 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 Temperatures 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, workpiece and chip. A typical set of isotherms is shown in figure where it can be 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 chip adjacent to the face as the chip is sheared in secondary deformation. This leads to a maximum cutting temperature a 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. 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 undeformed chip thickness tends to be a scale effect where the amounts of heat which pass to the workpiece, 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 workpiece 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 volume 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 tools. The technique is based on the metallographic examination of sectioned high-speed-steel tools which relates microstructure changes to thermal history. Trent has described measurements of cutting temperatures and temperature distributions for high-speed-steel tools 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 tempered 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 tools and twist drills. Wears of Cutting Tool 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 and 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 tool and workpiece and a condition where efficient cutting can no longer take place. On the minor cutting edge, which determines workpiece size and surface finish, flank wear can result in an oversized product which has poor surface finish. Under most practical cutting conditions, the tool will fail due to major flank wear before the minor flank wear is sufficiently large to result in the manufacture of an unacceptable component. Because of the stress distribution on the tool face, the frictional stress in the region of sliding contact between the 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 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 referred to as catering and which normally has a section in the form of a circular arc. In many respects and for practical cutting conditions, crater wear is a less severe form of wear than flank wear and consequently flank wear is a more common tool failure criterion. However, since various authors have shown that the temperature on the face increases more rapidly with increasing cutting speed than the temperature on the flank, and since the rate of wear of any type is significantly affected by changes in temperature, crater wear usually occurs at high cutting speeds. At the end of the major flank wear land 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 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 occasionally is very severe. Although the presence of the notch will not significantly affect the cutting properties of the tool, the notch is often relatively deep and if cutting were to continue there would be a good chance that the tool would fracture. 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, 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 the onset of catastrophic failure. For high-speed-steel cutting tools, however, where the wear tends to be non-uniform it has been found that the most meaningful and reproducible results can be obtained when the wear is allowed to continue to the onset of catastrophic failure even though, of course, in practice a cutting time far less than that to failure would be used. The onset of catastrophic failure is characterized by one of several phenomena, the most common being a sudden increase in cutting force, the presence of burnished rings on the workpiece, and a significant increase in the noise level. Mechanism of Surface Finish Production There are basically five mechanisms which contribute to the production of a surface which have been machined. These are:(l) The basic geometry of the cutting process. In, for example, single point turning the tool will advance 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 which replicates the shape of the tool in cut. (2) The efficiency of the cutting operation. It has already been mentioned that cutting with unstable built-up-edges will produce a surface which contains 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 cutting speeds, besides producing conditions which lead to unstable built-up-edge production, the cutting process itself can become unstable and instead of continuous shear occurring in the shear zone, tearing takes place, discontinuous chips of uneven thickness are produced, and the resultant surface is poor. This situation is particularly noticeable when machining very ductile materials such as copper and aluminum. (3) The stability of the machine tool. Under some combinations of cutting 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 vibration 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 phenomenon is known as chatter and in axial turning is characterized by long pitch helical bands on the workpiece surface and short pitch undulations on the transient machined surface. (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 under 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 it is likely that it will impinge on the cut surface and mark it. Inevitably, this marking besides looking. (5)The effective clearance angle on the cutting tool. For certain geometries of minor cutting edge relief and clearance angles 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 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 surface characteristics. Limits and Tolerances 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. To 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 because machines are not perfect, and the tools become worn. A slight variation from the exact size is always allowed. The amount of this variation depends on the kind of part being manufactured. For examples part might be made 6 in. long with a variation allowed of 0.003 (three-thousandths) in. above and below this size. Therefore, the part could be 5.997 to 6.003 in. and still be the correct size. These are known as the limits. The difference between upper and lower limits is called the tolerance. A tolerance is the total permissible variation in the size of a part. The basic size is that size from which limits of size arc derived by the application of allowances and tolerances. Sometimes the limit is allowed in only one direction. This is known as unilateral tolerance.Unilateral 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 seriously affecting the fit.When the tolerance is in both directions from the basic size it is known as a bilateral tolerance (plus and minus). Bilateral tolerancing is a system 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 shown. Thus, the tolerance is the difference between these two dimensions. Surface Finishing and Dimensional Control Products that have been completed to their proper shape and size 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 penetration 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 materials processed in different manners, may require some special surface treatment to provide uniformity of appearance. 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 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 variety of: environments. The type of protection procedure will depend largely upon the anticipated exposure, with due consideration to the material being protected and the economic factors involved. Satisfying the above objectives necessitates the use of main surface-finishing methods that involve chemical change of the surface mechanical work affecting surface properties, cleaning by a variety of methods, and the application of protective coatings, organic and metallic. In the early days of engineering, the 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, until 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 was obtained, the fitter therefore being a fitter in the literal sense. J It is obvious that the two parts would have to remain together, and m the event of one having to be replaced, the fitting would have to be done all over again. In these days, we expect to be able to purchase a replacement for a broken part, and for it to function correctly without the need for scraping and other fitting operations.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 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. Automatic Fixture Design 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 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 is fixed by a machine spindle or mould, or when tolerances a
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