11.4 Equal and Unequal Disposition

11.4 Equal and Unequal Disposition

Quick Answer: The profile tolerance zone is a uniform boundary disposed about the true profile. ASME Y14.5-2009 defaults to equal bilateral — half the tolerance on each side, no symbol needed. Unilateral puts the whole zone on one side. Unequal bilateral splits the zone asymmetrically and uses the circled-U (unequally disposed) modifier with a number giving the additional material-side allowance. Reading the U-modifier correctly — total zone width first, side allowance second — is a common Senior trap.

Equal bilateral disposition (default)

When a profile callout gives only a tolerance value — e.g., profile of a surface 0.4 — the standard interprets the zone as equally disposed about the true profile: 0.2 on each side. No additional symbol is required, and any note saying "0.2 each side" is informational, not a different specification. This is the most common disposition and the one assumed when no other indication is present.

Unilateral disposition

A unilateral profile zone places the entire tolerance on one side of the true profile: either entirely outside (adding material) or entirely inside (removing material). Under Y14.5-2009 the unequally disposed modifier is used to express this: a 0.4 zone with the entire allowance on the outside is written with the U-modifier value equal to the total — for example, profile 0.4 with the U modifier indicating 0.4 on the added-material side and 0 on the other. Unilateral disposition is used when one side of the contour must be protected absolutely:

• A casting cleanup allowance: the as-cast contour must remain outside the true profile so that machining can clean up to the true profile; the inside is not allowed to encroach.
• A mating surface that must not interfere: the produced surface must remain on the non-interfering side of the true profile.

Unequal bilateral disposition

Unequal bilateral splits the zone unequally: more allowance on one side than the other, but some on both. The unequally disposed modifier (circled U) is required. The value following the U modifier specifies the additional material-side allowance (the long side), and the total zone width is the profile tolerance value.

The unequally disposed modifier in Y14.5-2009

Y14.5-2009 introduced the circled-U symbol as the unequally disposed profile modifier. In a feature control frame it appears after the tolerance value, with a number indicating the additional allowance on the side of the arrow (the "plus" side, by convention the outside/material-addition side). Reading the callout:

• Profile 0.4 U 0.3 (with the arrow indicating the material-addition side) means a total zone 0.4 wide, with 0.3 on the material-addition side and 0.1 on the other side. The 0.4 is the total; the 0.3 is the long side.
• The short side is computed: total minus long side = 0.4 − 0.3 = 0.1.
• A U value equal to the total (e.g., 0.4 U 0.4) means the entire zone is on one side — a unilateral zone.
• A U value equal to half the total (e.g., 0.4 U 0.2) reproduces the default equal bilateral and is redundant.

The Senior trap is reading the U value as an addition to the total rather than as the long side within the total. The total zone width is always the tolerance value stated first. The U value never makes the zone wider than the stated tolerance.

Disposition table

Worked example: casting cleanup allowance

A cast pump housing has a contoured flange face that will be finish-machined to the true profile. The casting must carry enough stock on the outside so the machinist can clean up to the true profile; the casting must not encroach inside the true profile or the cleanup will not be complete.

• True profile: the machined flange face contour.
• Callout: profile of a surface 0.8 U 0.8 | A | B | C, with the U arrow pointing outward (material-addition side).
• Interpretation: total zone 0.8 wide; 0.8 on the outside (added-material side), 0 on the inside.
• Produced cast surface must lie entirely in the 0.8-wide zone outside the true profile — every point on the as-cast contour must be between the true profile and 0.8 mm beyond it. Any encroachment inside the true profile rejects the part.
• After machining, the surface is brought to the true profile, and a separate tighter profile callout governs the finished face.

Worked example: unequal bilateral with bias

A contoured airfoil is controlled by profile of a surface 0.5 U 0.35, with the U arrow pointing toward the convex (outside) side. Interpretation: total zone 0.5 wide; 0.35 on the convex (outside) side, 0.15 on the concave (inside) side. The designer wants to allow more stock on the convex side (where the manufacturing process tends to leave material) while still permitting a small concave deviation. Reading the callout as "0.5 plus 0.35 = 0.85" would be wrong — the total zone is 0.5, and 0.35 is the long side within it.

Worked example: equal bilateral (default)

A molded housing face is controlled by profile of a surface 0.4 | A | B | C with no U modifier. The zone is 0.2 on each side of the true profile. Any point on the produced face within 0.2 of the true profile (in either direction) is acceptable. The default requires no symbol and no note.

Senior-level traps

Trap 1: misreading the U value

Profile 0.4 U 0.3 is a 0.4-wide zone with 0.3 on the long side and 0.1 on the short side — NOT a 0.4 zone plus an additional 0.3 (which would be 0.7 total). The first number is the total; the second is the long side.

Trap 2: assuming a U value is required for equal bilateral

Equal bilateral is the default and needs no symbol. Adding U 0.2 to a 0.4 profile is redundant and confusing — not wrong, but unnecessary.

Trap 3: confusing the U-modifier with the Y14.5-2018 "U" conventions

This guide follows Y14.5-2009 Senior, where the circled U is the unequally disposed profile modifier and the value is the long-side (material-addition side) allowance. (The 2018 revision changed some disposition conventions; do not mix them on the 2009 exam.)

Trap 4: forgetting which side the U value applies to

The U value applies to the side indicated by the arrow in the feature control frame. A U value of 0.3 on a 0.4 zone where the arrow points outward means 0.3 outward, 0.1 inward. If the arrow direction is reversed, the 0.3 applies inward instead. Always read the arrow before assigning the long side.

Summary

Disposition controls where the profile zone sits relative to the true profile. Equal bilateral is the no-symbol default. Unilateral protects one side entirely. Unequal bilateral biases the zone asymmetrically and requires the circled-U modifier with the long-side allowance. The U value is always within the total zone width, never in addition to it — and the arrow direction fixes which side is the long side.