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Geometric Dimension & Tolerance

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Limits and Fits

Limits terminology

  1. Basic size = Theoretical size — e.g. 48.00
    1. MMC for shaft/hole — Nicest value / least d.p
  2. Tolerance notation — 48.00 ± 0.02
  3. Tolerance = difference between limits of size — e.g. 0.04
  4. Limits of size = Largest and Smallest size permitted — 48.02 (largest) 47.98 (smallest)
  5. Actual size = measured size
  6. Maximum material condition (MMC) — limit of size resulting in maximum material involved
  7. Minimum material consition (LMC) — limit of size resulting in minimum material involved

Fits terminology

  1. Clearance fit — worst case scenario will still have clearance — size range do not overlap
  2. Interference fit — all cases have interference — size range always overlap
  3. Transition fit — cases can either have clearance or interference — partial overlap of size range
  4. Upper deviation =
  5. Lower deviation =
  6. Fundamental deviation — deviation closest to basic size
  7. Allowance =

GD&T Fundamental

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Surface texture

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Notations

  • Specifying max roughness

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  • Removal of material instructions

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  • Lay symbol =

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  • Lay symbol X (honing)

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  • Lay symbol C (turning)

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  • Specifying max & min roughness

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  • Machining allowance

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  • Lay symbol (grinding / shaping)

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  • Lay symbol M (lapping)

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  • Lay symbol R

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Tolerances

Form tolerances

Straightness

For parallel line element on surface, its allowed to vary in depth within straightness tolerance

  • Flatness: Same as straightness but for surface Virtual condition (VC) VC = MMC (Shaft) + Geometric tolerance
  • The modifier allows the feature surface(s) to exceed the maximum material boundary by the amount of form tolerance, meaning that VC = MMC + tolerance. Angularity

Deviation of the angle of a feature — e.g. center line of hole

  • Parallelism and Perpendicularity is similar

Location tolerance

Position

Graph View

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