3.4 Feature Control Frame Placement, Combined Frames, Tolerance Zone Definition & Tabulated Tolerances
Key Takeaways
- ASME Y14.5-2009 para. 3.6 requires the diameter or spherical diameter symbol to precede the tolerance value only where the zone is a cylinder or sphere; without it the value is the distance between two parallel straight lines or planes or two uniform boundaries.
- A composite feature control frame (para. 3.4.4) uses a single entry of the geometric characteristic symbol, restricted to position or profile, while two single-segment frames (para. 3.4.5) repeat the symbol in each row and create independent requirements.
- Where a single datum is established by two or more datum features, all datum feature reference letters separated by a dash go in one compartment, such as A-B (para. 3.4.2).
- Para. 3.5 lists five permitted ways to attach a feature control frame, and attaching it to an extension line of a plane surface controls the surface while attaching it to an extension of a size dimension line controls the feature of size.
- Tabulated tolerances are entered as a letter preceded by the abbreviation TOL (para. 3.7), and para. 3.2 permits a supplementary note wherever symbology alone cannot convey the requirement.
3.4 Feature Control Frame Placement, Combined Frames, Tolerance Zone Definition & Tabulated Tolerances
Quick Answer: Section 3 of ASME Y14.5-2009 does more than list symbols — paras. 3.4 through 3.7 define how a feature control frame is built, attached, read, and abbreviated. The five permitted placement methods are enumerated in para. 3.5 and Fig. 3-29. Para. 3.6 supplies the single most consequential reading rule on the exam: where the tolerance value represents the diameter of a cylindrical or spherical zone, the diameter or spherical diameter symbol shall precede the tolerance value; where the zone is other than a diameter, the diameter symbol is omitted and the value represents the distance between two parallel straight lines or planes, or between two uniform boundaries. Para. 3.7 allows a tabulated tolerance letter preceded by the abbreviation
TOL, and para. 3.2 permits a note where symbology alone cannot convey the requirement.
Building the Frame (Paras. 3.4.1 – 3.4.3)
A feature control frame is a rectangle divided into compartments containing the geometric characteristic symbol, followed by the tolerance value or description, modifiers, and any applicable datum feature references.
┌─────┬──────────────────┬────────┬────────┬────────┐
│ ⌖ │ ⌀ 0.25 Ⓜ Ⓟ 18 │ A │ B Ⓜ │ C │
└─────┴──────────────────┴────────┴────────┴────────┘
1 2 3 4 5
1 Geometric characteristic symbol
2 Diameter/spherical-diameter symbol (if the zone is diametral),
tolerance value, material condition modifier, projected zone symbol + height
3-5 Datum feature references in ORDER OF PRECEDENCE, each optionally
followed by a MATERIAL BOUNDARY modifier (Ⓜ = MMB, Ⓛ = LMB)
- 3.4.2 — one datum feature reference: the datum feature reference letter goes in a compartment following the tolerance, optionally followed by a material boundary modifier. Where a single datum is established by two or more datum features — for example, an axis established from two coaxial diameters — all of the datum feature reference letters, separated by a dash, are entered in a single compartment (
A-B). - 3.4.3 — two or three datum feature references: each letter, followed where applicable by its material boundary modifier, is entered in a separate compartment in order of precedence, not alphabetical order.
| C | A | B |is a perfectly legal frame in which C is primary.
Multi-Segment Frames: Composite vs. Two Single-Segment (Paras. 3.4.4 – 3.4.5)
| Construction | Para. | Symbol Layout | Meaning |
|---|---|---|---|
| Composite feature control frame | 3.4.4 | One entry of the geometric characteristic symbol (position or profile only) spanning both rows | Lower segment refines per paras. 7.5.1 / 8.6 — datums constrain orientation only |
| Two single-segment feature control frames | 3.4.5 | The symbol is repeated in each row | Each row is an independent requirement per para. 7.5.2 — lower-segment datums constrain orientation and location |
The one-glance test: count the geometric characteristic symbols. One symbol spanning both rows = composite. Two symbols, one per row = two single-segment frames. Everything else about the interpretation follows from that single visual difference, and the standard restricts composite construction to position and profile.
Combined Frame and Datum Feature Symbol (Para. 3.4.6)
Where a feature or pattern of features that is controlled by a geometric tolerance also serves as a datum feature, the feature control frame and the datum feature symbol may be combined — the datum feature symbol is attached to the frame. This is how a drawing says "these four holes are located to A and B, and the resulting pattern is datum feature C."
Projected Tolerance Zone in the Frame (Para. 3.4.7)
Where a positional or orientation tolerance is specified as a projected tolerance zone, the projected tolerance zone symbol shall be placed in the feature control frame along with the dimension indicating the minimum height of the zone, following the stated tolerance and any modifier. Where clarification is needed, the projected zone is shown with a chain line and the minimum height dimensioned in a drawing view — in which case the height may be omitted from the frame.
Placement of the Frame (Para. 3.5)
The frame is related to the considered feature by one of the following methods, all shown in Fig. 3-29:
- Locating the frame below or attached to a leader-directed note or dimension pertaining to the feature.
- Attaching a leader from the frame pointing to the feature.
- Attaching a side, corner, or end of the frame to an extension line from the feature, provided it is a plane surface.
- Attaching a side, corner, or end of the frame to an extension of the dimension line pertaining to a feature of size.
- Placing the frame in a note, chart, or the general tolerance block.
The distinction that matters for interpretation is method 3 versus method 4: an extension line from a plane surface means the control applies to the surface; an extension of the dimension line of a feature of size means the control applies to the feature of size and therefore to its derived axis or center plane.
Defining the Tolerance Zone (Para. 3.6) — The Diameter Symbol Rule
Where the specified tolerance value represents the diameter of a cylindrical or spherical zone, the diameter or spherical diameter symbol shall precede the tolerance value. Where the tolerance zone is other than a diameter, the diameter symbol shall be omitted, and the specified value represents the distance between two parallel straight lines or planes, or the distance between two uniform boundaries, as the case may be. Some zones are nonuniform and are specified by other means (see para. 8.3.2).
| Callout | Zone Geometry | Typical Application |
|---|---|---|
⌖ ⌀0.25 Ⓜ A B C | Cylinder, ⌀0.25 | Round hole located in two directions |
⌖ 0.25 A B C | Two parallel planes, 0.25 apart | Slot or tab center plane; bidirectional position (para. 7.4.4) |
⌖ S⌀0.25 A B C | Sphere, S⌀0.25 | Spherical feature (para. 7.4.6) |
⏥ 0.05 | Two parallel planes, 0.05 apart | Surface flatness |
⌭ 0.1 | Two coaxial cylinders, 0.1 radial separation | Cylindricity |
Why examiners love this rule: a stem shows
⌖ 0.4 | A | B | Con a round hole and asks for the zone. The absent diameter symbol means the zone is two parallel planes 0.4 apart, not a ⌀0.4 cylinder — a bidirectional control. Nothing about the feature being round overrides the frame.
Tabulated Tolerances and Supplementary Notes (Paras. 3.7 and 3.2)
- 3.7 Tabulated tolerances: where the tolerance in a feature control frame is tabulated, a letter representing the tolerance, preceded by the abbreviation
TOL, shall be entered in the frame — for example⌖ | ⌀ TOL A | A | B | C, with the value ofTOL Asupplied in a table. This is standard practice on tabulated part families where one drawing covers several dash numbers. - 3.2 Use of notes to supplement symbols: situations arise where the desired geometric requirements cannot be completely conveyed by symbology. In such cases a note may be used to describe the requirement, either separately or to supplement a geometric symbol.
SEP REQT,EACH ELEMENT,TWO SURFACES, and thread-feature designations such asMAJOR DIAall rely on this permission.
A drawing controls a single ⌀12.0 plus or minus 0.1 round through-hole with the feature control frame '⌖ | 0.4 | A | B | C', with no diameter symbol shown before the 0.4. Under ASME Y14.5-2009 para. 3.6, what is the shape of the tolerance zone?
Two stacked feature control frames appear on a drawing. In the first, one position symbol spans both rows; in the second, a position symbol appears separately in each row. What is the significance of this difference under ASME Y14.5-2009?
A family drawing covers five dash numbers whose position tolerance values differ. Under ASME Y14.5-2009 para. 3.7, how should the feature control frame express the tolerance, and what does para. 3.2 permit where symbology alone is insufficient?