12.4 Combined Controls and Profile as a Refinement

Combined Controls and Profile as a Refinement

Quick Answer: A combined control stacks frames on one feature: a broad control for location, a tighter one refining orientation or form. Two rules make a stack valid — the refinement's tolerance must be smaller than its parent's, and its datum references must be the same as, or a subset of, its parent's. Profile of a line legitimately refines profile of a surface.

Combined controls appear by name on the Y14.5-2009 Senior body of knowledge under Tolerances of Profile, and Y14.5-2018 adds profile of a line as a refinement as its own item. The underlying idea runs through the whole standard: a feature usually has a loose functional requirement in one respect and a tight one in another, and forcing both into a single tolerance value wastes money.

The refinement hierarchy

Geometric controls form a natural hierarchy from broadest to tightest:

Location ⊃ Orientation ⊃ Form

A position tolerance already limits orientation and form, because a derived median line confined to a cylindrical zone cannot be arbitrarily tilted or bent. A profile tolerance to a full datum reference frame already limits form, orientation, and location of the surface. So a second, tighter control is not adding a new kind of restriction — it is subdividing the budget the first control granted.

A typical stack on a machined face:

| profile of a surface | 0.5 | A | B | C |     <- locates the surface
| flatness             | 0.1 |               <- refines its form

The surface must lie within the 0.5-wide profile zone located by the datums, and additionally be flat within 0.1 anywhere inside that zone. The face may sit anywhere in the 0.5 band; it just may not be wavy while it does.

The two validity rules

Rule 1 — the refinement must be tighter. A flatness of 0.5 beneath a profile of 0.5 controls nothing, because the profile zone already limits flatness to 0.5. A flatness of 0.8 beneath a profile of 0.5 is worse than meaningless: it appears to add a requirement while actually being unreachable as a constraint. Both are drawing errors, and exam questions present them as "what is wrong with this drawing?"

Rule 2 — the refinement's datums must be the same or a subset. A refining control may reference:

• No datums — refining form only (flatness, straightness, circularity, cylindricity).
• A subset of the parent's datums — refining orientation to fewer datums than the parent used for location.
• The same datums — refining the same relationship more tightly.

It may never reference a datum feature the parent does not reference. Doing so introduces a new relationship rather than refining an existing one, and the two frames then say independent things that may conflict.

Profile of a line refining profile of a surface

Profile of a surface controls the entire surface within a three-dimensional zone. Profile of a line controls individual cross-sections within a two-dimensional zone at each section.

Stacking them expresses a real and common design intent: the overall surface may wander within a broad band, but each individual cross-section must be a clean shape. An extruded aerofoil, a sheet-metal channel, and a moulded seal groove all behave this way — a slow bow along the length is harmless, while a ripple within any one cross-section is not.

| profile of a surface | 1.0 | A | B | C |
| profile of a line    | 0.2 |             <- each cross-section

Y14.5-2018 lists profile of a line as a refinement explicitly on the Senior body of knowledge; the practice is equally valid under 2009.

Composite profile is a refinement, not a combined control

Distinguish two things that look similar on the page:

• A composite profile frame is one frame with one profile symbol and two stacked tolerance segments. The upper segment locates the feature to the full DRF; the lower segment refines orientation and form within that located zone and never relocates the feature. Datums repeated in the lower segment control orientation only.
• A combined control is two separate frames, each with its own symbol, each a complete requirement in its own right.

The practical difference: in a composite frame the lower segment's zone is free to translate within the upper zone but is locked in orientation to the repeated datums. In a combined control the second frame is an independent requirement that happens to be tighter.

Combining profile with position

A slot that must both locate and present a controlled contour is often given position on the slot as a feature of size plus profile on the slot's surfaces:

| position           | Ø0.3 (M) | A | B | C |    <- locates the median plane, MMC bonus available
| profile of a surface | 0.15    | A | B | C |    <- controls the contour of the walls

These are not parent and refinement in the strict sense; they control different geometry — one the derived median plane, the other the surfaces. Both apply, and the part must satisfy both. This is legitimate and common, and a Senior question may ask you to identify what each frame governs.

Traps

1. A refinement equal to its parent. Adds nothing; a drawing error.
2. A refinement looser than its parent. Worse than nothing; a drawing error.
3. Introducing a new datum in the refinement. Not a refinement at all.
4. Reading a composite lower segment as relocating the feature. It refines orientation and form only.
5. Assuming two frames on one feature must be parent and refinement. Position on a feature of size and profile on its surfaces control different geometry and both apply independently.