影响加工精度的基本因素、油压泵简介.docx

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1、Fundamentals of Manufacturing AccuracyManufacturing can be defined as the transformation of raw materialsinto useful products through the use of the easiest and least-expensivemethods. It is not enough, therefore, to process some raw materials andobtain the desired product.It is, in fact, of major i

2、mportance to achieve that goal throughemploying the easiest, fastest, and most efficient methods. If less efficienttechniques are used, the production cost of the manufactured part will behigh, and the part will not be as competitive as similar parts produced byother manufacturers. Also, the product

3、ion time should be as short aspossible to enable capturing a larger market share.Modem industries can be classified in different ways. There includeclassification by process, classification by product, and classificationbased on the production volume and the diversity of products. Theclassification

4、by process is exemplified by casting industries, stampingindustries, and the like. When classifying by product, industries maybelong to the automotive, aerospace, and electronics groups. The thirdmethod, i.e., classification based on production volume, identifies threemain distinct types of producti

5、on, mass, job shop, and moderate. Let usbriefly discuss the features and characteristics of each type.Mass production is characterized by the high production volume ofthe same (or very similar) parts for a prolonged period of time. An annualproduction volume of less than 50 000 pieces cannot certain

6、ly beconsidered as mass production. As you may expect, the productionvolume is based upon an established or anticipated sales volume and isnot directly affected by the daily or monthly orders. The typical exampleof mass-produced goods is automobiles.Job-shop production is based on sales orders for a

7、 variety of small lots.Each lot may consist of 20 up to 200 or more similar parts, dependingupon the customers needs. It is obvious that this type of production ismost suitable for subcontractors who produce varying components tosupply various industries. The machines employed must be flexible tohan

8、dle variations in the configuration of the ordered components ,whichare usually frequent. Also, the employed personnel must be highly skilledin order to handle a variety of tasks, which differ for the different partsthat are manufactured.Moderate production is an intermediate phase between the job-s

9、hopand the mass-production types. The production volume ranges between10 000 to 20 000 parts, and the machines employed are flexible andmultipurpose. This type of production is gaining popularity in industrybecause of an increasing market demand for customized products.A very important fact of the m

10、anufacturing science is that it is almostimpossible to obtain the desired nominal dimension when processing aworkpiece. This is actually caused by the inevitable, though very slight,inaccuracies inherent in the machine tool as well as by variouscomplicated factors like the elastic deformation and re

11、covery of theworkpiece and/or the fixture, temperature effects during processing, andsometimes the skill of the operator. Since it is very difficult to analyzeand completely eliminate the effects of these factors, it is more feasible toestablish a permissible degree of inaccuracy or a permissible de

12、viationfrom the nominal dimension that would not affect the proper functioningof the manufactured part in a detrimental way. According to the ISO(International Standardization Organization) system, the nominaldimension is referred to as the basic size of the part. The deviations fromthe basic size t

13、o each side (i.e. , positive or negative ) determine the highand the low limits, respectively, and the difference between those twolimits of size is called the tolerance. The tolerance is an absolute valuewithout a sign. As you may expect, the magnitude of the tolerance isdependent upon the basic si

14、ze and is designated by an alphanumericsymbol called the grade. There are eighteen standard grades of tolerancein the ISO system, and the tolerances can be obtained from the formulasor the tables published by the ISO. It is obvious that smaller tolerancesrequire the use of high-precision machine too

15、ls in manufacturing the partsand therefore increase production costs.Before two components are assembled together, the relationshipbetween the dimensions of the mating surfaces must be specified. In otherwords, the location of the zero line to which deviations are referred mustbe established for eac

16、h of the two mating surfaces. This actuallydetermines the degree of tightness or freedom for relative motion betweenthe mating surfaces. There are basically three types 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 themating hole.