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Tolerancing

Tolerancing. Chapter 10. Objectives. Describe nominal size, tolerance, limits, and allowance of two mating parts Identify a clearance fit, interference fit, and transition fit Describe the basic hole and basic shaft systems. Objectives (cont.).

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Tolerancing

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  1. Tolerancing Chapter 10

  2. Objectives • Describe nominal size, tolerance, limits, and allowance of two mating parts • Identify a clearance fit, interference fit, and transition fit • Describe the basic hole and basic shaft systems

  3. Objectives (cont.) • Dimension mating parts using limit dimensions, unilateral tolerances, and bilateral tolerances • Describe the classes of fit and give examples of each • Draw geometric tolerancing symbols • Specify geometric tolerances

  4. Understanding Tolerance • Tolerancing is an extension of dimensioning • It allows you to specify a range of accuracy for every feature of a product so the parts will fit together and function properly when assembled

  5. Understanding Tolerance • To provide tolerances in CAD, you must: • Understand the fit required between mating parts • Have a clear picture of how inspection measurements are performed • Be able to apply tolerance symbols to a drawing

  6. Tolerance • Tolerance is the total amount a specific dimension is permitted to vary • Use generous tolerances when possible because increased precision makes parts more expensive to manufacture

  7. Quality Control • Large batches of parts may use statistical methods to control quality where a sample of parts are inspected • Specific tolerances are based on the part’s function and fit

  8. Variations in Form • Acceptable parts must not extend beyond boundary limits

  9. Implied 90 Degree Angles • When lines intersect on a drawing at angles of 90 degrees, it is customary not to dimension the angle • Implied 90 degree angles have the same general tolerances applied to them as any other angles covered by a general note

  10. Implied 90 Degree Angles

  11. Fits Between Mating Parts • Fit is the term for the range of tightness or looseness resulting from the allowances and tolerances in mating parts

  12. Clearance Fit

  13. Interference Fit

  14. Transition Fit

  15. Definitions for Size Designation • Nominal size – used for general identification and usually expressed in decimals • Basic size (basic dimension) – the theoretically exact size from which limits of size are determined

  16. Definitions for Size Designation • Actual size – the measured size of a finished part • Allowance – the minimum clearance or maximum interference specified to achieve a fit between two mating parts

  17. Basic Hole System • Toleranced dimensions are commonly determined using the basic hole system in which the minimum hole size is taken as the basic size

  18. Basic Shaft System • In this system, the maximum shaft is taken as the basic size and is used only in specific circumstances

  19. Specifying Tolerances • The primary ways to indicate tolerances in a drawing are: • A general tolerance note • A note providing a tolerance for a specific dimension • A reference on the drawing to another document that specifies the required tolerances

  20. Specifying Tolerances • (cont.) • Adding limit tolerances to dimensions • Adding direct plus/minus tolerances to dimensions • Geometric tolerances

  21. General Tolerance Notes

  22. Limit Tolerances

  23. Plus-or-Minus Tolerances

  24. Tolerance Stacking

  25. Tolerances and Machining Processes

  26. Metric System of Tolerances and Fits • The preceding material on limits and fits applies to both systems of measurement • The ISO has a system of preferred metric limits and fits • The system is specified for holes, cylinders, and shafts and has similar definitions of terms

  27. Geometric Tolerancing • Geometric tolerances state the maximum allowable variations of a form or its position from the perfect geometry implied in the drawing • The term “geometric” refers to forms such as planes, cylinders, squares, etc.

  28. Geometric Tolerancing • Tolerances of form and position (or location) control such characteristics as: • Straightness • Flatness • Parallelism • Perpendicularity (squareness) • Concentricity • Roundness • Angular displacement, etc.

  29. Geometric Characteristics and Modifying Symbols

  30. Symbols for Tolerances of Position and Form

  31. Symbols for Tolerances of Position and Form

  32. Datum Surfaces and Features • Datum surfaces and features are used as references to control other features

  33. Maximum Material Condition • Maximum material condition (MMC) means that a feature of a finished product contains the maximum amount of material permitted by the toleranced dimensions for that feature

  34. Tolerances of Angles • Bilateral tolerances have traditionally been given on angles

  35. Using Geometric Dimensioning and Tolerancing • Geometric dimensioning and tolerancing (GDT) considers an individual part’s dimensions and tolerances and that part’s relation to its related parts • GDT also simplifies the inspection process

  36. Using Geometric Dimensioning and Tolerancing • Applying GDT principles requires: • Defining the part’s functions • Listing the functions by priority • Defining the datum reference frame • Control selection • Calculating tolerances

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