CHAPTER 4MACHINE DRAWINGWhen you have read and understood this chapter,you should be able to answer the following learningobjectives:Describe basic machine drawings.Describe the types of machine threads.Describe gear and helical spring nomenclature.Explain the use of finish marks on drawings.This chapter discusses the common terms, tools,and conventions used in the production of machinedrawings.COMMON TERMS AND SYMBOLSIn learning to read machine drawings, you must firstbecome familiar with the common terms, symbols, andconventions defined and discussed in the followingparagraphs.GENERAL TERMSThe following paragraphs cover the common termsmost used in all aspects of machine drawings.TolerancesEngineers realize that absolute accuracy isimpossible, so they figure how much variation ispermissible. This allowance is known as tolerance. Itis stated on a drawing as (plus or minus) a certainamount, either by a fraction or decimal. Limits are themaximum and/or minimum values prescribed for aspecific dimension, while tolerance represents the totalamount by which a specific dimension may vary.Tolerances may be shown on drawings by severaldifferent methods; figure 4-1 shows three examples.The unilateral method (view A) is used when variationfrom the design size is permissible in one direction only.In the bilateral method (view B), the dimension figureshows the plus or minus variation that is acceptable. Inthe limit dimensioning method (view C), the maximumand minimum measurements are both statedThe surfaces being toleranced have geometricalcharacteristics such as roundness, or perpendicularity toanother surface. Figure 4-2 shows typical geometricalcharacteristic symbols. A datum is a surface, line, orFigure 4-1.—Methods of indicating tolerance.Figure 4-2.—Geometric characteristic symbols.4-1
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