1.4 Dimensioning Features: Diameters, Radii, Chamfers, Countersinks, Counterbores, Spotfaces, Keyseats & Slots
Key Takeaways
- ASME Y14.5-2009 para. 1.8.14 states that where no depth or remaining thickness is specified for a spotface, the spotface is the minimum depth necessary to clean up the surface to the specified diameter.
- Countersunk holes are dimensioned by diameter and included angle, while counterdrilled holes are dimensioned by diameter and depth, with the counterdrill angle optional (para. 1.8.12).
- The depth dimension of a blind hole or a counterdrill is the depth of the full diameter measured from the outer surface of the part, not the depth to the drill point (paras. 1.8.10 and 1.8.12).
- A radius symbol R permits any surface, including flats and reversals, inside the two-arc zone, whereas a controlled radius CR additionally requires a fair curve without flats or reversals (paras. 2.15.1 and 2.15.2).
- Keyseats are dimensioned by width, depth, location, and if required length (para. 1.8.17), and chamfers may be specified by note only when they are 45 degrees on perpendicular surfaces (para. 1.8.16.1).
1.4 Dimensioning Features: Diameters, Radii, Chamfers, Countersinks, Counterbores, Spotfaces, Keyseats & Slots
Quick Answer: Para. 1.8 of ASME Y14.5-2009, Dimensioning Features, is the paragraph the Scope category quietly mines for a third of its questions. It sets the method for each feature type: diameters take the Ⓓ diameter symbol ahead of the value; radii use
Rfor a zone between two arcs andCRfor a controlled radius whose surface must be a fair curve; counterbores dimension diameter and depth (or remaining material thickness); countersinks dimension diameter and included angle; counterdrilled holes dimension diameter and depth; a spotface with no depth stated is simply the minimum depth required to clean up the surface to the specified diameter; chamfers take a linear dimension plus an angle, or two linear dimensions; and keyseats are dimensioned by width, depth, location, and if required, length.
Why Para. 1.8 Is Worth Real Study Time
Technologist candidates routinely burn their Scope-category margin here. Datums and position feel like "real GD&T," so 1.8 gets skimmed — and then four or five items ask which value a blind counterbore depth refers to, or what a spotface callout means when no depth is given. These are pure recall questions with unambiguous answers, which makes them the cheapest points on the exam.
Diameters, Radii, and Spherical Features (Paras. 1.8.1 – 1.8.3)
- Diameters (1.8.1): The diameter symbol precedes all diametral values. A spherical diameter uses the spherical diameter symbol (
Sⵁ), and a spherical radius usesSR. Where a number of concentric cylindrical features are dimensioned, the diameters are arranged so each is clearly associated with its feature. - Radii (1.8.2): The letter
Rprecedes the value. Where the center of a radius is located by dimensions, the arc is drawn to that located center; where the center is not located, the arc is drawn with no center shown. Foreshortened radii are drawn with a jogged leader. - Rounded ends and slotted holes (1.8.4): Dimensioned either by overall length and width, or by the center-to-center distance and the end radii — but a radius on a slot end is often shown with the letter
Rand no value, because the width dimension already controls it.
The R vs. CR Distinction (Para. 2.15)
This is a frequent two-option trap and belongs in your Scope study notes even though it lives in Section 2:
| Symbol | Paragraph | Tolerance Zone | Extra Requirement |
|---|---|---|---|
R (radius) | 2.15.1 | A zone bounded by two arcs, the minimum and maximum radii; the part surface must lie within the zone | None. The surface may contain flats and reversals so long as it stays inside the crescent |
CR (controlled radius) | 2.15.2 | The same two-arc zone | The contour must be a fair curve with no flats or reversals, and radii taken at all points on the contour must fall between the limits |
Exam framing: if a question asks which callout forbids a flat spot or a reversal in a fillet, the answer is
CR, neverR.
Machined Hole Features (Paras. 1.8.10 – 1.8.15)
COUNTERBORE (1.8.11) COUNTERSINK (1.8.12) SPOTFACE (1.8.14)
┌─────┬─────┐ ╲ ╱ ┌─────┬─────┐
│ │ │ ╲ ╱ │ │ │ ← clean-up only
├──┬──┴──┬──┤ ╲─────╱ ├──┬──┴──┬──┤
│ │ │ │ │ │ │ │ │ │
│ │HOLE │ │ │ │ │ │HOLE │ │
⌀CBORE + DEPTH ⌀CSK + INCLUDED ANGLE ⌀SF; depth optional
(or remaining thickness) (counterdrill: ⌀ + depth) (default = min clean-up)
- Round holes (1.8.10): Where it is not clear that a hole goes through, the notation
THRUfollows the dimension. The depth dimension of a blind hole is the depth of the full diameter from the outer surface of the part — not to the drill point. - Counterbored holes (1.8.11): Specified by counterbore diameter and depth. Where the thickness of the remaining material has significance, that thickness rather than the depth is dimensioned. The relationship between the counterbore and the hole shall be specified — this is why counterbores frequently carry their own feature control frame.
- Countersunk and counterdrilled holes (1.8.12): For a countersink, specify the diameter and included angle. For a counterdrill, specify the diameter and depth; stating the counterdrill's included angle is optional. The counterdrill depth is the depth of the full diameter of the counterdrill from the outer surface.
- On curved surfaces (1.8.13): Where a hole is chamfered or countersunk on a curved surface, the specified diameter applies at the minor diameter of the chamfer or countersink.
- Spotfaces (1.8.14): Where the spotfaced diameter is specified, either the depth or the remaining thickness of material may be specified. If neither is specified, the spotface is the minimum depth necessary to clean up the surface to the specified diameter. A spotface may be specified by note only and need not be shown pictorially.
Chamfers, Keyseats, Knurls, and Threads (Paras. 1.8.16 – 1.8.22)
- Chamfers (1.8.16): Dimensioned by a linear dimension and an angle, or by two linear dimensions. Where an angle and a linear dimension are used, the linear dimension is the distance from the indicated surface of the part to the start of the chamfer.
- 1.8.16.1 — By note: A note may specify 45° chamfers on perpendicular surfaces only, because for 45° the linear value applies in either direction. A note such as
2 X 45°is therefore illegal on a 30° chamfer. - 1.8.16.2 — Round holes: Where the edge of a round hole is chamfered, the note practice of 1.8.16.1 is followed; the same style of control may be applied to a chamfer diameter on a shaft.
- 1.8.16.3 — Non-perpendicular intersecting surfaces: Two acceptable methods are given for surfaces meeting at other than right angles.
- 1.8.16.1 — By note: A note may specify 45° chamfers on perpendicular surfaces only, because for 45° the linear value applies in either direction. A note such as
- Keyseats (1.8.17): Dimensioned by width, depth, location, and if required, length. Depth is measured on the side of the keyseat that shows the maximum depth.
- Screw threads (1.8.20): Y14.5 does not define thread callouts; methods of specifying and dimensioning screw threads are covered in ASME Y14.6. Involute splines are covered by the ANSI B92 series (1.8.22), and surface texture by ASME Y14.36M with additional information in ASME B46.1 (1.8.21).
Trap: a Technologist item may ask which standard governs thread callouts. Y14.5 governs the geometric tolerance applied to the thread (para. 2.9), while Y14.6 governs how the thread itself is designated.
Rapid-Recall Table
| Feature | Para. | What Must Be Dimensioned | Default If Omitted |
|---|---|---|---|
| Blind hole | 1.8.10 | Diameter, depth | Depth = depth of the full diameter |
| Counterbore | 1.8.11 | Ⓓ and depth, or Ⓓ and remaining thickness | Relationship to the hole must be specified |
| Countersink | 1.8.12 | Ⓓ and included angle | — |
| Counterdrill | 1.8.12 | Ⓓ and depth; angle optional | Depth = depth of the full diameter |
| Spotface | 1.8.14 | Ⓓ; depth or remaining thickness optional | Minimum depth to clean up the surface |
| Chamfer | 1.8.16 | Linear + angle, or two linears | Note form permitted for 45° only |
| Keyseat | 1.8.17 | Width, depth, location, length if required | — |
Radius R | 2.15.1 | Value after R | Flats and reversals permitted |
Controlled radius CR | 2.15.2 | Value after CR | Fair curve required |
An engineering drawing calls out '4X ⌀10.5 THRU, ⌀20 SF' on a cast boss with no depth value and no remaining-thickness value given for the spotface. Under ASME Y14.5-2009, what depth of spotface is required?
A fillet on a highly stressed cast bracket is specified as 'CR 5 ± 0.5' rather than 'R 5 ± 0.5'. What additional requirement does the controlled radius symbol impose on the manufactured surface?
A drawing specifies a hole as '⌀8 ↧ 15' followed by a counterbore callout of '⌀14 ↧ 6'. To what geometry does the 6 mm counterbore depth apply under ASME Y14.5-2009?