GD&T position tolerance is one of the most useful controls for mechanical drawings because it defines how accurately a feature such as a hole must be located relative to a datum reference frame.
What Is Position Tolerance?
Position controls the location of a feature from its theoretically exact position. Instead of relying only on plus/minus coordinate dimensions, GD&T creates a controlled tolerance zone around the true position.
Understanding Datums
Datums establish the reference system used to inspect a component. A typical machined part may use a primary surface as datum A, a secondary feature as datum B and a third reference as datum C. Together they orient and locate the part for inspection.
Feature Control Frame
A position callout commonly contains the position symbol, tolerance value and datum references. For example, a drawing may specify a position tolerance of 0.20 relative to A, B and C. The exact requirement depends on the drawing standard and feature condition.
What Does MMC Mean?
Maximum Material Condition (MMC) describes the condition containing the most material. For an internal hole, MMC is the smallest permitted hole size. When a position tolerance is applied at MMC, bonus tolerance can become available as the actual hole departs from MMC.
Why Position Is Better Than Simple Coordinates
Coordinate dimensions can control location but may not communicate functional relationships as clearly. Position tolerance creates a cylindrical tolerance zone and can support functional inspection and assembly requirements.
Practical Example
Imagine a mounting plate with four bolt holes. The hole pattern must align with a mating component. Position tolerance can control the location of each hole relative to the mounting datums, helping ensure that the complete pattern works during assembly.
Common Mistakes
- Ignoring the datum reference frame.
- Confusing size tolerance with location tolerance.
- Applying MMC without understanding its functional purpose.
- Inspecting position without correctly establishing the datum system.
Design Engineer Takeaway
Use GD&T position tolerance when feature location matters functionally. Always connect the tolerance to the way the part is assembled, manufactured and inspected.