8.1 Position Tolerancing and True Position

8.1 Position Tolerancing and True Position

Position tolerancing (ASME Y14.5-2009 Section 7) is the geometric control that locates a feature of size relative to a datum reference frame (DRF). It is the workhorse of location control and the most heavily tested topic on the Senior exam because it ties together datums, material condition modifiers, virtual condition, and bonus tolerance.

True Position: The Theoretically Exact Location

True position is the theoretically exact location of a feature, defined by basic dimensions (boxed dimensions shown on the drawing). Basic dimensions carry no plus/minus tolerance of their own; they only define where the ideal feature sits. All permissible variation is delivered through the position tolerance zone called out in the feature control frame (FCF).

A feature control frame for position reads, left to right:

This FCF means: "The axis of the feature shall be contained within a cylindrical zone of diameter 0.2 at MMC, located by basic dimensions relative to datum A (primary), B (secondary), and C (tertiary)."

The Position Tolerance Zone

The shape of the zone depends on the feature:

• Cylindrical zone (with the diameter symbol ⌀ in the FCF) — applied to the axis of a hole, pin, or cylinder. The produced axis must lie inside a cylinder of the tolerance diameter, centered on true position.
• Two parallel planes (no diameter symbol) — applied to the center plane of a slot, tab, keyseat, or wide feature of size. The produced center plane must lie between two parallel planes separated by the tolerance value, symmetric about true position.

For a hole at MMC with a cylindrical zone, the zone diameter is the stated tolerance. As the hole departs from MMC toward LMC, bonus tolerance (Section 8.3) expands the zone diameter.

Implied 90-Degree Basic Relationships

When a drawing shows a center line, axis, or surface of a feature at 90 degrees to a datum axis or plane and no basic angle is dimensioned, a 90-degree basic angle is implied. This implied basic angle still has to be respected — position tolerance zones are centered on true position, which depends on those implied basics. Senior exam items often hide an implied 90-degree relationship inside a pattern location problem and expect you to know it is basic, not free.

RFS vs. MMC vs. LMC on the Position Control

The material condition modifier in the position FCF (after the tolerance value) determines how size variation interacts with position:

In ASME Y14.5-2009, RFS is the default when no modifier is shown (the 2018 standard flips this to RMB/MMB/LMB language, but this guide follows 2009). On the Senior exam, the modifier is almost always given explicitly because the virtual-condition and bonus calculations depend on it.

Position vs. Coordinate (Plus/Minus) Tolerancing

Consider locating a hole with coordinate tolerancing of ±0.2 in X and ±0.2 in Y. The produced center can land anywhere in a 0.4 × 0.4 square — a diagonal displacement of up to √(0.4² + 0.4²) ≈ 0.566 from true position, yet a hole exactly on the X or Y boundary (0.2 off) is only 0.2 off. Coordinate tolerancing gives a non-uniform zone: more tolerance along the diagonal than along the axes.

Position tolerancing with ⌀0.4 gives a circular zone of radius 0.2 — uniform in every direction — and the worst-case displacement is exactly 0.2 in every direction. For a mating pin/hole this uniform zone is what the mating part actually cares about (radial clearance). Position also decouples location (in the FCF) from size (in the size tolerance) instead of double-counting the size tolerance inside a ± location stack.

Worked Example: Four-Hole Pattern

A plate has a 4-hole pattern on a 50 × 50 basic bolt circle, each hole ⌀10.0 ± 0.2, with position ⌀0.3 at MMC to datums A | B | C.

• The basic 50 × 50 dimensions define true position of each hole center — they have no tolerance.
• Each hole's axis must lie inside a cylinder of ⌀0.3 centered on its true position, at MMC (hole at 9.8).
• At MMC (9.8), bonus = 0, so the zone is ⌀0.3.
• If a hole is produced at 10.0 (0.2 away from MMC), bonus = 0.2, so the effective zone = ⌀0.3 + 0.2 = ⌀0.5.
• If a hole is produced at LMC (10.2), bonus = 0.4, so the effective zone = ⌀0.3 + 0.4 = ⌀0.7.

The datum reference frame for the pattern is A (the primary planar datum — the plate's bottom face, constraining 2 rotational DOF), B and C (the two edges, constraining the remaining DOF). Each hole is located identically relative to the DRF; the pattern is simultaneously located to the same DRF, so a pattern-location error (the whole pattern shifted) is judged once against the DRF, not hole-to-hole.