Dimensioning and tolerancing
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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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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

  • Shaft in hole fits are very common

  • Both shaft and hole diameter vary

  • Dimensions and tolerances must be specified to guarantee the desired fit


Tolerance

Minimum clearance

Shaft in Hole


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


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


Transition Fits

  • Used to accurately locate parts during assembly

  • Tradeoff between ease of assembly/disassembly and accuracy of location

  • Example: locating dowels or pins


Interference Fits

  • Used for force or press fits

  • Results in permanent assembly without need for fasteners or other joining operations

  • High locational accuracy


Multiple holes and shafts

  • Often parts are assembled with multiple shafts mating with multiple holes

  • Examples?


Objectives

  • Close fit with no play and good locational accuracy

  • High probability of parts fitting despite variations in dimensions


Types of Variations

  • Size tolerances on hole diameters

  • Size tolerances on shaft diameters

  • Positional tolerances on hole locations


Example


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


Maximum

Smallest hole

Minimum

Largest shaft

Worst Case


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


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.


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


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


Geometric Controls

  • Form controls

    • compare feature to ideal geometry

  • Orientation controls

    • compare orientation of features to datums

  • Location controls

    • compare location to datums


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


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


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


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


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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