重庆科创职业学院Unit 05Fundamentals of Manufacturing Accuracy.

1、Unit 5 Fundamentals of Manufacturing Accuracy1Unit 5 Fundamentals of Manufacturing AccuracyManufacturing can be defined as the transformation of raw materials into useful products through the use of the easiest and least-expensive methods. It is not enough, therefore, to process some raw materials a

2、nd obtain the desired product. It is, in fact, of major importance to achieve that goal through employing the easiest, fastest, and most efficient methods. If less efficient techniques are used, the production cost of the manufactured part will be high, and the part will not be as competitive as sim

3、ilar parts produced by other manufacturers. Also, the production time should be as short as possible to enable capturing a larger market share.Modern industries can be classified in different ways. The classification by process is exemplified by casting industries, stamping industries, and the like.

4、 When classifying by product, industries may belong to the automotive, aerospace, and electronics groups. The third method, i.e., classification based on production volume, identifies three main distinct types of production, mass, job shop, and moderate.2Unit 5 Fundamentals of Manufacturing Accuracy

5、Mass production is characterized by the high production volume of the same (or very similar) parts for a prolonged period of time. An annual production volume of less than 50 000 pieces cannot certainly be considered as mass production. As you may expect, the production volume is based upon an estab

6、lished or anticipated sales volume and is not directly affected by the daily or monthly orders. The typical example of mass-produced goods is automobiles.Job-shop production is based on sales orders for a variety of small lots. Each lot may consist of 20 up to 200 or more similar parts, depending up

7、on the customers needs. It is obvious that this type of production is most suitable for subcontractors who produce varying components to supply various industries. The machines employed must be flexible to handle variations in the configuration of the ordered components, which are usually frequent.

8、Also, the employed personnel must be highly skilled in order to handle a variety of tasks, which differ for the different parts that are manufactured.3Unit 5 Fundamentals of Manufacturing Accuracy Moderate production is an intermediate phase between the job-shop and the mass-production types. The pr

9、oduction volume ranges between 10 000 to 20 000 parts, and the machines employed are flexible and multipurpose. This type of production is gaining popularity in industry because of an increasing market demand for customized products.4Unit 5 Fundamentals of Manufacturing AccuracyA very important fact

10、 of the manufacturing science is that it is almost impossible to obtain the asked nominal dimension when processing a workpiece. This is actually caused by the inevitable, though very slight, inaccuracies inherent in the machine tool as well as by various complicated factors like the elastic deforma

11、tion and recovery of the workpiece and/or the fixture, temperature effects during processing, and sometimes the ski1l of the operator. Since it is very difficult to analyze and completely eliminate the effects of these factors, it is more feasible to establish a permissible degree of inaccuracy or a

12、 permissible deviation from the nominal dimension that would not affect the proper functioning of the manufactured part in a detrimental way. According to the ISO (International Standardization Organization) system, the nominal dimension is referred to as the basic size of the part. The deviations f

13、rom the basic size to each side (i.e.,positive or negative) determine the high and the low limits, respectively, and the difference between those two limits of size is called the tolerance. The latter is an absolute value without a sign and can also be obtained by adding the absolute values of the d

14、eviations. The magnitude of the tolerance is dependent upon the basic size and is designated by an alphanumeric symbol called the grade. There are eighteen standard grades of tolerance in the ISO system, and the tolerances can be obtained form the formulas or the tables pub1ished by the ISO. It is o

15、bvious that smaller tolerances require the use of high precision machine tools in manufacturing the parts and therefore increase production costs.5Unit 5 Fundamentals of Manufacturing AccuracyBefore two components are assembled to together, the relationship between the dimensions of the mating surfa

16、ces must be specified. In other words, the location of the zero line to which deviations are referred must be established for each of the two mating surfaces. This actually determines the degree of tightness or freedom for relative motion between the mating surfaces. There are basically three types

17、of fits, namely, clearance fit, transition fit, and interference fit. In all cases of clearance fit, the upper limit of the shaft is always smaller than the lower limit of the mating hole. This is not the case in interference fit, where the lower limit of the shaft is always larger than the upper li

18、mit of the hole. The transition fit, as the name suggests, is an intermediate fit. According to ISO, the internal enveloped part is always referred to as the shaft, whereas the surrounding surface is referred to as the hole. Accordingly, from the fits point of view, a key is referred to as the shaft

19、 and the keyway as the hole.6Unit 5 Fundamentals of Manufacturing AccuracyThere are two ways for specifying and expressing the various types of fits, the shaft-basis and the hole-basis systems. The location of the tolerance zone with respect to the zero line is indicated by a letter, which is always

20、 capital for holes and lowercase for shafts, whereas the tolerance grade is indicated by a number. Therefore, a fit designation can be H7/6, F6/g5, or any other similar form.When the service life of an electric bulb is over, all you do is buy a new one and replace the bulb. This easy operation, whic

21、h does not need a fitter or a technician, would not be possible without two main concepts, interchangeability and standardization. Interchangeability means that identical parts must be interchangeable, i.e., able to replace each other, whether during assembly or subsequent maintenance work; without

22、the need for my fitting operations. Interchangeability is achieved by establishing a permissible tolerance, beyond which any further deviation from the nominal dimension of the part is not allowed.7Words and Expressionscapture 5kAptF n.俘获，夺取俘获，夺取vt.赢得，引起赢得，引起classification 7klAsifi5keiFnn.分类，归类，类别，分

23、组分类，归类，类别，分组lot lCtn.组，块，批组，块，批subcontractor 7sQbkn5trAktn.转包人，小承包商转包人，小承包商multipurpose 5mQti5p:ps adj.通用的，多功能的，多目标的通用的，多功能的，多目标的customize5kQstmaizv.定做，按规格改制定做，按规格改制nominal 5nCminl adj.标定的，额定的，规定的，名义上的标定的，额定的，规定的，名义上的inevitable in5evitbl adj.不可避免的，必然发生的不可避免的，必然发生的 inherent in5hirnt adj.固有的，本质的，内含的，本

24、来的固有的，本质的，内含的，本来的permissible p(:)5misbl adj.容许的，许可的，准许的，安全的容许的，许可的，准许的，安全的deviation 7di:vi5eiFn n. 偏离，偏差，差异偏离，偏差，差异detrimental detri5mentl adj.有害的有害的alphanumeric Alfnju:5merik adj.字母数字的，包括文字与数字的字母数字的，包括文字与数字的tightness 5taitnisn.紧密性，松紧度紧密性，松紧度interference 7intfirnsn.干涉，干预，妨碍干涉，干预，妨碍8Words and Express

25、ionskeyway5ki:7wein.键槽，凹凸缝键槽，凹凸缝fitter 5fitn.装配工，修理工装配工，修理工fitting 5fitiNn.装配，选配，调整装配，选配，调整mass production 大批生产，大量生产大批生产，大量生产job-shop production 单件小批量生产单件小批量生产moderate production 中批生产中批生产basic size 基本尺寸，公称尺寸基本尺寸，公称尺寸mating surface 配合表面，啮合表面配合表面，啮合表面clearance fit 间隙配合间隙配合 transition fit 过渡配合过渡配合inter

26、ference fit 过盈配合过盈配合capital letter 大写字母大写字母lowercase letter 小写字母小写字母zero line 零位线，基准线零位线，基准线the shaft-basis and hole-basis systems 基轴制和基孔制基轴制和基孔制9Surface Finishing Heavy cuts in a material leave rough and torn surfaces. The lighter and milder it cuts, the better the surface and the tolerance is. Goo

27、d finishes can be obtained by operating cutting tools at light feeds, but that is slow. As has been explained, grinding is often faster for fine cutting because it removes material by the action of many grains taking small bites. Although grinding may be carried to the extreme to produce as fine fin

28、ishes and as high a degree of precision as may be desired, other abrasive operations that have slower speeds and a milder action usually prove more economical for the best finishes. Such operations are honing, lapping and superfinishing.Honing is a low-velocity abrading process. Because material rem

29、oval is accomplished at lower cutting speeds than in grinding, heat and pressure are minimized, resulting in excellent size, surface finish, and metallurgical control of the surface. The cutting action is obtained from abrasive sticks (aluminum oxide and silicon carbide) mounted on a metal mandrel.

30、Distortion is minimized because the work floats and is not clamped or chucked. The work is given a slow reciprocating motion as the mandrel rotates, thus generating a straight and round hole. Parts honed for finish remove only 0.03mm or less; however, certain inaccuracies can be corrected in amounts

31、 up to 0.51mm.Coolants are essential to flush away small chips and to keep temperatures uniform. Honing gives a smooth finish with a characteristic crosshatch appearance. The depth of hone marks can be controlled by variations in pressure, speed, and type of abrasive.10Surface Finishing Lapping prod

32、uces geometrically true surfaces, corrects minor surface imperfections, improves dimensional accuracy, or provides a very close fit between two contact surfaces. Although it is a material-removing operation, it is not economical for that purpose. The amount of material removed is usually less than 0

33、.03mm.Lapping is used on flat, cylindrical, spherical, or specially formed surfaces in contact with a lap. The two have relative motion with one another in such a way that fresh contacts between the grit and the part are constantly being made. Loose abrasive carried in some vehicle such as oil, grea

34、se, or water is used between the lap and work to do the necessary abrading. Sometimes the abrasive is in the form of a bonded wheel and the lapping operation is similar to centerless and vertical-spindle surface grinding. Metal laps must be softer than the work and for machine lapping are usually ma

35、de of gray iron. Other materials like steel, copper, lead, and wood are used where cast iron is not suitable. By having the lap softer than the work, the abrasive particles (usually boron carbide, silicon carbide, aluminum oxide in fine-screened sizes, or flour) become embedded in the lap and cause

36、the greater wear to occur on the hard surface. In lapping carbide tools and jewels, diamond particles permanently embedded in copper laps are most successful.11Surface FinishingVertical lapping machines are used for both flat and cylindrical lapping. These machines have two laps: a lower one that su

37、pports the work and rotates at relatively slow speeds, and a stationary upper lap. The upper lap floats on the work and supplies pressure for the abrading action. Either it is provided with drive pins that impart a rotary and gyratory motion, or it is given a planetary motion. In either method the w

38、ork contacts the entire surface of the lap in an ever-changing path. Commercial accuracy can be held to 0.0006mm and to even closer limits if needed. Products commonly finished by this process include gages, piston pins, valves, gears, roller bearings, thrust washers, and optical parts.All machining

39、 operations as well as the usual grinding processes leave a surface coated with fragmented, noncrystalline, or smear metal that, though easily removed by sliding contact, result in excessive wear, increased clearances, noisy operation, and lubrication difficulties. Superfinshing is a surface-improvi

40、ng process that removes this undesirable fragmentation metal, leaving a base of solid crystalline metal. It is somewhat similar to honing in that both processes use an abrasive stone, but it differs in the motions given to the stone. This process, which is a finishing process and not a dimensional o

41、ne, can be superimposed on other finishing operations.12Surface FinishingIn cylindrical superfinishing, a bonded-form abrasive stone, having a width about two-thirds of the diameter of the part to be finished and the same length, is operated at low speed and pressure. The motion given to the stone i

42、s oscillating with an amplitude of 1.66.4mm at about 450cycles per minute. The stone pressure is 21275 kPa. If the part is of greater length than the stone, an additional longitudinal movement of either stone or work is necessary. The work is rotated at a speed of about 0.25m/s and during the operation is flooded with light oil that carries away the minute particles abraded from the surface by the short, oscillating stone strokes. The stone action is similar to a scrubbing movement an