Geometric tolerancing

  • Helps ensure interchangeability of parts (repeatability in the manufacturing process)
  • Specifies requirements more precisely than conventional tolerancing
    • Allows designer to control the finished part

 GEOMETRIC TOLERANCING SYMBOLS AND TERMS

  • Feature- general term applied to a physical portion of a part, such as a surface or slot-- think of a feature as a part surface
  • Feature of size (FOS)- one cylindrical or spherical surface, or a set of two opposed elements or opposed parallel surfaces, associated with a size dimension; an axis, median plane, or centerpoint can be derived from a feature of size.
    • Every feature of size contains one or more features
    • FOS can be internal or external
  • Feature of size dimension- a dimension that is associated with a feature of size
  • Non-feature of size dimension- a dimension that is not associated with a feature of size
  • Actual local size- the value of any individual distance at any cross section of a FOS
  • Actual mating envelope (AME)- perfect geometric counterpart of a FOS

Feature Control Frame

  • Geometric tolerances are specified on a drawing through the use of a feature control frame
    • Contains geometric characteristic symbol, tolerance value, modifiers, and datum references
    • See Figure 2-12 in Krulikowski
  • When a feature control frame is associate with a surface, it applies to the feature
  • When a feature control frame is placed beneath or behind a FOS dimension, it applies to the FOS

 

MATERIAL CONDITION SYMBOLS

  • Establishes the relationship between the SIZE of the feature within its given dimensional tolerance and the geometric tolerance
  • Referred to as modifiers because they modify the geometric tolerance in relationship to the actual produced size (measured size after production)
  • Material condition modifying elements include Maximum Material Condition (MMC), Regardless of Feature Size (RFS), Least Material Condition (LMC), Projected Tolerance Zone, Tangent Plane, Diameter, Radius, Controlled Radius, and Reference
  • Example 4-2, Madsen

 

REGARLESS OF FEATURE SIZE (RFS)

  • "Rule 2" (in ASME Y14.5M): RFS applies, with respect to the individual tolerance, datum reference, or both, where no modifying symbol is specified.   MMC or LMC must be specified on the drawing where required
  • RFS means that the geometric tolerance applies regardless of the actual produced size of the feature

 

MAXIMUM MATERIAL CONDITION (MMC)

  • Condition where a feature contains the maximum amount of material within the stated limits of size (Ex. Max. shaft diameter, min hole diameter)
  • MMC symbol must be placed in the feature control frame after the geometric tolerance or datum reference
  • When MMC is specified (in the feature control frame), the given geometric tolerance is maintained when the feature is produced at MMC. As the actual produced size departs from MMC, the geometric tolerance is allowed to get larger up to the amount of departure from MMC (creating a bonus tolerance)
    • Bonus Tolerance: additional tolerance for geometric control created when a geometric tolerance is applied to a FOS and it contains an MMC (or LMC) modifier.

LEAST MATERIAL CONDITION (LMC)

  • Condition where the feature of size contains the least amount of material (Ex.: Min shaft diameter, Max hole diameter)
  • LMC symbol must be placed in the feature control frame after the geometric tolerance or datum reference
  • When LMC is specified (in the feature control frame), the given geometric tolerance is maintained when the feature is produced at LMC. As the actual produced size departs from LMC, the geometric tolerance is allowed to get larger up to the amount of departure from LMC (creating a bonus tolerance)


CONVENTIONAL TOLERANCING, LIMITS OF SIZE, and RULES OF GD&T

  • Conventional toleraning: tolerances related to dimensioning practices without regard to geometric tolerancing
  • Limits of a size of a dimension determine the given variation allowed in the size of the feature
  • Example 4-3, Madsen
  • Permits a degree of variation in form, profile, or location because of the tolerance
  • Degree of form or location control can be increased or decreased by altering the tolerance
  • Limits of size: boundary between MMC and LMC
  • Extreme form variation: Variation of the form of a feature between the upper limit and the lower limit of a size dimension
  • Example 4-4, Madsen, Figure 3-1 in Krulikowski
  • "Rule #1" (in ASME Y14.5M): Limits of size of a feature control the amount of variation in size and geometric form
    • Rule 1 can be paraphrased as "perfect from at MMC"
    • The form of a FOS is controlled by its limits of size in the following ways:
      • The surfaces of a FOS shall not extend beyond a boundary (envelop) of perfect form (perfect flatness, straightness, circularity, and cylindricity) at MMC
      • When the actual size of a FOS has departed from MMC toward LMC, the form is allowed to vary by the same amount
      • The actual local size of an individual feature of size must be within  the specified tolerance of size
      • There is no requirement for a boundary of perfect form at LMC.   If a feature is produced at LMC, the geometric form may vary between the LMC and MMC boundaries
  • How to Override Rule # 1: 1) Applying a straightness control to a FOS, 2) If a note such as "PERFECT FORM AT MMC NOT REQUIRED" is specified next to a FOS dimension.
  • Rule # 1 Limitation: It does not control the location, orientation, or relationship between features of size.
  • Rule #1 Exceptions: 1) A FOS on a non-rigid part, 2) Stock sizes
  • Inspecting a Feature of Size (that is controlled by Rule #1)
    • Both size and form need to be verified
    • MMC size and Rule #1 envelope can be verified with a Go gage that is at least as long as the FOS
    • Minimum size (LMC) can be verified with a No-Go gage
    • Figure 3-4 in Krulikowski
       
  • Basic dimension: theoretically perfect dimension- specified with a rectangle around the dimension
 

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