Location Controls

What are Location Controls?

The primary location control is:

Note: Concentricity and Symmetry were deprecated in ASME Y14.5-2018 in favor of Position with appropriate modifiers.

True Position

Basic dimensions are shown in boxes and have no tolerance:

         ┌────┐    ┌────┐
         │ 50 │    │ 30 │
         └────┘    └────┘
            │         │
            ▼         ▼
    ════════╬════════════════ Datum B
            ║
            ║    ⊙ ← Hole's true position
            ║
    ════════╬════════════════ Datum A

Position Tolerance

The position tolerance defines a cylindrical (or planar) zone within which the feature's axis must lie:

FCF: ┌───┬────────────┬────────┬────────┬────────┐
     │ ⊕ │ ⌀ 0.25 (M) │ A      │ B      │ C      │
     └───┴────────────┴────────┴────────┴────────┘

The Cylindrical Tolerance Zone

For holes and pins, position creates a cylindrical zone:

                  ╭─────╮
    True         │  ⌀   │  ⌀ 0.25 tolerance zone
    Position ──► │  ○   │
                  ╰─────╯
                     │
    Actual axis must lie within this cylinder

Calculating Position Deviation

The position deviation formula:

Position Deviation = 2 × √(ΔX² + ΔY²)

Where:

• ΔX = deviation from true position in X
• ΔY = deviation from true position in Y

• Hole center is 0.08mm off in X
• Hole center is 0.06mm off in Y
• Position deviation = 2 × √(0.08² + 0.06²) = 2 × 0.10 = 0.20mm

If tolerance is ⌀ 0.25, the part passes (0.20 < 0.25).

Bonus Tolerance (MMC/LMC)

When (M) is specified, you get bonus tolerance as the feature departs from MMC:

For a Hole (Internal Feature)

• MMC = Smallest hole (most material)
• LMC = Largest hole (least material)

Example:
Hole size: ⌀ 10.0 ± 0.2
MMC = 9.8mm (smallest)
LMC = 10.2mm (largest)

Position: ⌀ 0.25 (M)

If actual hole = 10.0mm:
Bonus = 10.0 - 9.8 = 0.2mm
Total position tolerance = 0.25 + 0.2 = 0.45mm

Bonus Tolerance Table

Position for Hole Patterns

Position is commonly used for bolt hole patterns:

    ○───────○
    │       │
    │   ⊕   │  Basic dimensions locate pattern
    │       │
    ○───────○

FCF: ┌───┬────────────┬────────┬────────┐
     │ ⊕ │ ⌀ 0.50 (M) │ A      │ B      │
     └───┴────────────┴────────┴────────┘

All holes in the pattern must meet the position tolerance.

Composite Position Tolerance

For patterns requiring both pattern location AND hole-to-hole relationship:

┌───┬────────────┬────────┬────────┬────────┐
│ ⊕ │ ⌀ 0.50 (M) │ A      │ B      │ C      │  ← Pattern to datums
├───┼────────────┼────────┴────────┴────────┤
│ ⊕ │ ⌀ 0.15 (M) │                          │  ← Hole-to-hole
└───┴────────────┴──────────────────────────┘

• Upper segment: Pattern location relative to datums (0.50)
• Lower segment: Feature-to-feature relationship (0.15)

Position vs. Concentricity

Key Takeaways

• Position controls feature location relative to datum reference frame
• Basic dimensions define true position (shown in boxes)
• Position tolerance creates a cylindrical zone for axes
• MMC modifier provides bonus tolerance as feature departs from MMC
• Composite position controls both pattern location and feature-to-feature
• Position replaces the deprecated Concentricity and Symmetry

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