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Dimensioning and Tolerancing

Dimensioning and Tolerancing. Tolerancing. No manufacturing process can produce parts with exact dimensions Allowable variations or tolerances must be specified by the designer, with two objectives: ensure fit and function minimize manufacturing cost. Example - shaft in hole fit.

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Dimensioning and Tolerancing

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  1. Dimensioning and Tolerancing

  2. Tolerancing • No manufacturing process can produce parts with exact dimensions • Allowable variations or tolerances must be specified by the designer, with two objectives: • ensure fit and function • minimize manufacturing cost

  3. Example - shaft in hole fit • Shaft in hole fits are very common • Both shaft and hole diameter vary • Dimensions and tolerances must be specified to guarantee the desired fit

  4. Tolerance Minimum clearance Shaft in Hole

  5. Types of Fit • Clearance fit • largest shaft diameter is smaller than smallest hole diameter • there is always clearance • Interference • smallest shaft diameter is larger than largest hole diameter • there is always interference • Transition • there could be either interference or clearance

  6. Clearance Fits • Loose running • lots of play, where accuracy is not important • Free running • less play, good for moving parts • Close running • close fit for moving parts, high accuracy required

  7. Transition Fits • Used to accurately locate parts during assembly • Tradeoff between ease of assembly/disassembly and accuracy of location • Example: locating dowels or pins

  8. Interference Fits • Used for force or press fits • Results in permanent assembly without need for fasteners or other joining operations • High locational accuracy

  9. Multiple holes and shafts • Often parts are assembled with multiple shafts mating with multiple holes • Examples?

  10. Objectives • Close fit with no play and good locational accuracy • High probability of parts fitting despite variations in dimensions

  11. Types of Variations • Size tolerances on hole diameters • Size tolerances on shaft diameters • Positional tolerances on hole locations

  12. Example

  13. Must Fit in Worst Case • Holes at minimum size • Shafts (e.g. bolts) at maximum size • Holes at minimum distance on one part • Holes at maximum distance on the other part

  14. Maximum Smallest hole Minimum Largest shaft Worst Case

  15. Geometric Dimensioning and Tolerancing (GD&T) • GD&T symbols specify additional tolerancing information for 3D geometry • Used in addition to standard +/- dimensioning • Used properly, GD&T can allow looser tolerances to minimize manufacturing cost

  16. Datums • A datum is a plane, centerline or point used as a reference starting point for dimensions • Often flat faces of a part or centerlines of holes are used as datums • There can be several datums, labeled A, B, C, etc.

  17. Maximum Material Condition (MMC) • MMC is the condition where a feature has the maximum volume or material • For a hole, it is the smallest size • For a shaft, it is the largest size

  18. Departure from MMC • As a feature departs from MMC, it moves away from the worst case • This may permit tolerances on other dimensions to be increased while still achieving fit • For example, if the holes in the previous example are larger than the minimum, the tolerance on the spacing can be increased and the parts will still fit

  19. Geometric Controls • Form controls • compare feature to ideal geometry • Orientation controls • compare orientation of features to datums • Location controls • compare location to datums

  20. Form Controls • Straightness • compares a line or axis to a perfectly straight line • Circularity • compares a circular cross section to a perfect circle • Flatness • compares a flat surface to a perfect plane • Cylindricity • compares a cylindrical feature to a perfect cylinder • includes axis straightness, circularity and taper

  21. Orientation Controls • Parallelism • a line or surface must be parallel to a datum • Perpendicularity • a line or surface must be perpendicular to a datum • Angularity • a line or surface must be at an angle to a datum • Line profile and Surface profile • line and surface profiles compare features to ideal profiles

  22. Location Controls • Concentricity • controls deviation of concentric axes of cylindrical elements • Runout • measures “wobble” in surface of cylindrical feature as it is rotated about an axis • Position • Locates features relative to datums • allows larger “bonus” tolerances as features depart from MMC

  23. Feature Control Frames • GD&T controls are added to drawings as feature control frames, using standard symbols • In I-DEAS, they are defined by completing a form

  24. Tolerancing in I-DEAS • Tolerances can be applied to constraints • I-DEAS tolerance analysis uses variational geometry to analyze the effect of tolerance stack-up • GD&T symbols can be applied as annotations in modelling mode • Annotations are automatically included in drawings

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