Quantcast Chapter 4-Machine Drawing

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CHAPTER 4 MACHINE  DRAWING When you have read and understood this chapter, you should be able to answer the following learning objectives: 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  machine drawings. COMMON TERMS AND SYMBOLS In learning to read machine drawings, you must first become familiar with the common terms, symbols, and conventions  defined  and  discussed  in  the  following paragraphs. GENERAL  TERMS The  following  paragraphs  cover  the  common  terms most used in all aspects of machine drawings. Tolerances Engineers   realize   that   absolute   accuracy   is impossible, so they figure how much variation is permissible. This allowance is known as tolerance. It is stated on a drawing as (plus or minus) a certain amount, either by a fraction or decimal. Limits are the maximum and/or minimum values prescribed for a specific dimension, while tolerance represents the total amount by which a specific dimension may vary. Tolerances may be shown on drawings by several different methods; figure 4-1 shows three examples. The unilateral method (view A) is used when variation from the design size is permissible in one direction only. In the bilateral method (view B), the dimension figure shows the plus or minus variation that is acceptable. In the limit dimensioning method (view C), the maximum and minimum measurements are both stated The  surfaces  being  toleranced  have  geometrical characteristics  such  as  roundness,  or  perpendicularity  to another surface. Figure 4-2 shows typical geometrical characteristic symbols. A datum is a surface, line, or Figure 4-1.—Methods of indicating tolerance. Figure 4-2.—Geometric characteristic symbols. 4-1

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