Hole and Feature Callouts
Read radii, diameters, slots, repeated features and hole notes, including the units and decimal places on an inch drawing.
Use a diameter for a full circle and a radius for an arc
Use a diameter for a full circle and a radius for an arc
⌀ before a value means diameter. R before a value means radius. Thus ⌀.60 describes a full circular diameter of .60, while R.30 describes an arc radius of .30. The values are related, but the callouts describe different kinds of feature.
4X ⌀.20 THRU callout applies the same size and end condition to four identified holes. It does not locate them.n X means the requirement applies in n places. A repeated size still needs an unambiguous pattern: center coordinates, spacing, or a fully defined bolt circle. A sketch that merely looks evenly spaced is not enough.
A round-ended slot needs size, location and direction
A round-ended slot needs size, location and direction
For a straight slot with semicircular ends, give its width and either its overall length or the distance between the end centers. Make the chosen meaning clear. Then locate the slot and establish its direction relative to the part.
Do not control the width, end radius, center spacing and overall length all at once. That repeats the relationships already built into the semicircular-ended shape. If a slot is not semicircular-ended, this shortcut does not apply.
Read a hole callout from the main hole to the added recess
Read a hole callout from the main hole to the added recess
The first size usually describes the main hole. Read the remaining symbols with the values immediately following them. All examples below are invented decimal-inch teaching values.
| Callout | How it reads |
|---|---|
⌀.30 THRU | A .30 diameter opening goes through the part. THRU is an end condition, not a numeric depth. |
⌀.30 ↧ .55 | A .30 diameter blind hole has .55 depth to the full-diameter portion. A drill-point cone, if present, extends farther unless otherwise specified. |
⌀.30 THRU | The .30 through hole has a cylindrical, flat-bottomed counterbore: .60 diameter and .15 deep from the entry surface. |
⌀.30 THRU | The .30 through hole has a conical countersink: .60 major diameter at the entry and a 90° included angle, measured across both sides of the cone. |
Read the leader and section view to identify the entry side. A hole callout supplies size and end condition; it still needs location dimensions. A countersink angle is not automatically 90°.
A spotface provides a local flat seat
A spotface provides a local flat seat
A spotface provides a flat circular seating surface around a hole. Its modern symbol puts SF inside the counterbore-shaped mark. Older drawings may use the counterbore mark with a note or context identifying the spotface.
A spotface may have a stated depth or remaining material thickness. If neither is specified, the ASME convention is the minimum depth needed to clean up the specified diameter. Do not invent a depth from the sketch.
Reading old and new symbol styles
Legacy notes may say .30 DIA, .55 DP, C'BORE, C'SINK or SP FACE. Modern symbol notation moves that meaning into the symbols shown above. A through hole may be clear from the drawing without THRU, but an isolated diameter value does not prove a through condition. Read the views and notes together.
Use the standard and edition named on the drawing. The current ASME edition governs new ASME work unless another edition is specified; the ASME catalog lists Y14.5-2018 (R2024). A reference sheet headed “current 2009” is historical, not evidence of today’s edition. Surface finish requirements are a separate language: see Surface Texture and Finish.