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Tolerancing

Tolerancing. Definition: Allowance for specific variation in the size and geometry of a part Need for Tolerancing It is IMPOSSIBLE to manufacture a part to an EXACT size or geometry Since variation from the drawing is inevitable we must specify the acceptable degree of variation

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Tolerancing

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  1. Tolerancing • Definition: Allowance for specific variation in the size and geometry of a part • Need for Tolerancing • It is IMPOSSIBLE to manufacture a part to an EXACT size or geometry • Since variation from the drawing is inevitable we must specify the acceptable degree of variation • Large variation may affect part functionality • Small allowed variation affects the part cost • requires precise manufacturing • may result in incorrect rejection of otherwise functional parts

  2. Tolerance Follows Function • Assemblies: • Parts will not fit together if their dimensions do not fall with in a certain range of values • Interchangeable Parts: • If a replacement part is used it must duplicate the original part within certain limits of deviation • The relationship between functionality and size/shape of an object varies with the part • Automobile transmission is very sensitive to the size & shape of the gears • Bicycle is not as sensitive to the size & shape of the gears

  3. Two Forms of Physical Tolerance • Size • Limits specifying the allowed variation in each dimension (length, width, height, diameter, etc.) are given on the drawing • Geometry • Geometric Dimensioning & Tolerancing (GD&T) • Allows for specification of tolerance for the geometry of a part separate from its size • GD&T uses special symbols to control different geometric features of a part

  4. Tolerance Spec Hierarchy • Generally Three Levels of Tolerances • DEFAULT: Placed in the Drawing Title-Block by The Engineering Firm • Typically Conforms to Routine Tolerance Levels • GENERAL: Placed on the Drawing By the Design-Engineer as a NOTE • Applies to the Entire Drawing • Supercedes the DEFAULT Tolerance • SPECIFIC: Associated with a SINGLE Dimension or Geometric Feature

  5. Range Limit Dimensions • Range Limit dimensions specify the upper and lower value. • An acceptable part may be at the upper limit, lower limit, or any value in between • Advantage: Fabricators measure total distance, and limit dimensions show these

  6. Deviation Limit Dimensions • Basic size of Dimension is Given With Tolerances Noted As A Plus/Minus (±) Range.

  7. Symmetrical (Equal) Bi-lateral Tolerance Forms • Most Easily Fabricated • Most Fabrication Processes Vary Randomly • “Target” Value Given by BiLat Dims • Advantage: Target Value stated – Fabricator can set Machine to Target

  8. Geometric Dimensioning & Tolerancing • Uses Standard Symbols To Indicate Tolerances That Are Based On The Feature’s Geometry. • Sometimes Called Feature-Based Dimensioning & Tolerancing, Or True Position Dimensioning & Tolerancing • Uses “Feature Control Frames” To Indicate Tolerance(s) • “State of the Art” for Tolerances

  9. History of Tolerancing • In the 1800’s, manufacturing used the “cut & try, file & fit” approach. • The plus-minus (or coordinate) system of tolerancing was next developed. • In the 1900’s, the first GD&T standards came out to improve the quality & utility of engineering drawings. • In 1966, the united GD&T standard was published

  10. Envelope Principle • Proper Tolerancing establishes the ENVELOPE of an acceptable part that satisfies functionality • Any deviation in form is acceptable, as long as it remains within the limits of size

  11. Limits of Size • A variation in form is allowed between the least material condition (LMC) and the maximum material condition (MMC). EnvelopePrinciple defines the size and form relationships between mating parts.

  12. GD&T Feature Control Frame

  13. TYPE OF TYPE OF FEATURE TOLERANCE Form & Orientation Controls GEOMETRIC CHARACTERISTIC CONTROLS 14 characteristics that may be controlled CHARACTERISTIC SYMBOL FLATNESS INDIVIDUAL (No Datum Reference) STRAIGHTNESS FORM CIRCULARITY CYLINDRICITY INDIVIDUAL or RELATED FEATURES LINE PROFILE ORIENTATION SURFACE PROFILE

  14. Orientation and Location Controls GEOMETRIC CHARACTERISTIC CONTROLS 14 characteristics that may be controlled TYPE OF TYPE OF CHARACTERISTIC SYMBOL FEATURE TOLERANCE PERPENDICULARITY ORIENTATION ANGULARITY PARALLELISM RELATED FEATURES (Datum Reference Required) CIRCULAR RUNOUT TOTAL RUNOUT LOCATION CONCENTRICITY POSITION SYMMETRY

  15. Basic Dimension • A theoretically exact dimension used as a starting point for the application of tolerances. • No tolerance specified • Identified by Enclosure in a FRAME

  16. Cylindrical Tolerance Zone • Line Connecting the Centers of the Circles at the Top & Bottom Surfaces Must Fall Completely Within The Tolerance Cylinder

  17. Material Conditions • Maximum Material Condition (MMC) • largest acceptable size for external feature • smallest acceptable size for internal feature • object weighs the most • Least Material Condition (LMC) • smallest acceptable size for external feature • largest acceptable size for internal feature • object weighs the least • Regardless of Feature Size (RFS) • indicates the material condition is not to be considered.

  18. A A ANSI1982 ASME1994 A ISO Datum • A Datum is a starting point for a dimension. Datums are theoretically ideal locations in space such as a plane, centerline, or point. • Not every GD&T feature requires a datum. • Datum Reference Symbols

  19. ANSI Datum Frame • Used By ACAD

  20. Flatness

  21. Straightness

  22. Circularity (Roundness)

  23. Cylindricity

  24. Parallelism

  25. Concentricity

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