Tolerances

EML 2023 Computer Aided Design Introduction tolerancing –technique of dimensioning parts within a required range of variation to ensure interchangeability A tolerance should be as large as possible without interfering with the function of the part to minimize production costs. 2

EML 2023 Computer Aided Design Tolerance Dimensions unilateral tolerance bilateral tolerance limit tolerance ±

EML 2023 Computer Aided Design Mating Parts 4

EML 2023 Computer Aided Design Cylindrical Fits 5

EML 2023 Computer Aided Design Cylindrical Fits – English Units 6

EML 2023 Computer Aided Design Cylindrical Fits – English Units 7

EML 2023 Computer Aided Design Cylindrical Fits – English Units 8

EML 2023 Computer Aided Design Cylindrical Fits – English Units 9

EML 2023 Computer Aided Design Cylindrical Fits – English Units tolerance - difference between the limits of size for a single feature ; shaft.0025 ; hole.0040 limits of tolerance – the max and min sizes of a single feature allowance – the tightest fit between two mating parts ;.0050 nominal size – the general size of a shaft or hole ; 1.5 basic size – the size to which the plus-and minus is applied actual size – the measured size of the finished part fit - the degree of tightness or looseness between two assembled parts 10

EML 2023 Computer Aided Design Cylindrical Fits – English Units clearance fit – gives a clearance between two assembled mated parts ; min clearance.005 ; max clearance.0115 transition fit interference fit line fit 11

EML 2023 Computer Aided Design Cylindrical Fits – English Units clearance fit interference fit – a binding fit that requires the parts to be forced together, much as if they were welded transition fit – may range from a interference fit to a clearance fit between the mated parts line fit – results in surface contact or clearance when limits are reached 12

EML 2023 Computer Aided Design Cylindrical Fits – English Units The ANSI B4.1 standard defines a series of fits between cylindrical features in inches for the basic hole system. The types of fits covered in the standard are: –RC: running or sliding clearance fits –LC: clearance locational fits –LT: transition locational fits –LN: interference locational fits –FN: force and shrink fits 13

EML 2023 Computer Aided Design Cylindrical Fits – English Units RC: running or sliding clearance fits – RC1 to RC9 LC: clearance locational fits LC1 to LC11 LT: transition locational fits LT1 to LT6 LN: interference locational fits LN1 to LN3 FN: force and shrink fits FN1 to FN5 14

EML 2023 Computer Aided Design Example – RC9 Fit hole and shaft have a basic diameter of 2.5” From Tables in Appendix 29 thousands of an inch 15

EML 2023 Computer Aided Design Example – LT3 Fit hole and shaft have a basic diameter of 2.5” From Tables in Appendix 31 thousands of an inch

EML 2023 Computer Aided Design Example – LN2 Fit hole and shaft have a basic diameter of 2.5” From Tables in Appendix 32 thousands of an inch

Example Problem 18

EML 2023 Computer Aided Design Cylindrical Fits – Metric Units ANSI B4.2 standard basic size – the diameter from which limits are calculated upper and lower deviation – the difference between the hole or shaft size and the basic size tolerance - the difference between the maximum and minimum sizes 19

EML 2023 Computer Aided Design Cylindrical Fits – Metric Units 20

EML 2023 Computer Aided Design Cylindrical Fits – Metric Units fundamental deviation – a letter grade that describes the deviation closest to the basic size International Tolerance (IT) grade – a series of tolerances that vary with the basic size to provide a uniform level of accuracy within a given grade there are 18 IT grades: IT01, IT0, IT1, …, IT16 21

EML 2023 Computer Aided Design Cylindrical Fits – Metric Units Hole basis –a system of fits based on the minimum hole size as the basic diameter – the fundamental deviation for a hole-basis system is “H” – Appendices 35 and 36 give hole-basis data for tolerances Shaft basis –a system of fits based on the maximum shaft size as the basic diameter – the fundamental deviation for a hole-basis system is “h” – Appendices 37 and 38 give shaft-basis data for tolerances 22

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EML 2023 Computer Aided Design Cylindrical Fits – Metric Units 24

EML 2023 Computer Aided Design Cylindrical Fits – Metric Units 25

EML 2023 Computer Aided Design Example 1 determine the shaft and hole limits for: –hole-basis system –a close running fit –a basic diameter of 49 mm 26

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EML 2023 Computer Aided Design Example 1 use a preferred basic diameter of 50 mm use a fit of H8/f7 28

EML 2023 Computer Aided Design Example 2 determine the shaft and hole limits for: –hole-basis system –a location transition fit –a basic diameter of 57 mm 29

EML 2023 Computer Aided Design 30

EML 2023 Computer Aided Design Example 2 use a preferred basic diameter of 60 mm use a fit of H7/k6 31

EML 2023 Computer Aided Design Example 3 determine the shaft and hole limits for: –hole-basis system –a medium drive fit –a basic diameter of 96 mm 32

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EML 2023 Computer Aided Design Example 3 use a preferred basic diameter of 100 mm use a fit of H7/s6 34

EML 2023 Computer Aided Design Nonstandard Fits, Nonpreferred Sizes determine the shaft and hole limits for: –hole-basis system –a close running fit –a basic diameter of 45 mm (do not change to a preferred size) 35

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EML 2023 Computer Aided Design Nonstandard Fits, Nonpreferred Sizes use a fit of H8/f7 37