Dimension symbols
Say what the blue part of the callout means. Get 10 right to finish. Pick ASME (US) or ISO (Europe) in the box below.
What does the part of the callout marked in blue mean?
The symbols
| Symbol | Meaning | Used in |
|---|---|---|
| Ø | Diameter | Both |
| R | Radius | Both |
| SØ | Spherical diameter | Both |
| SR | Spherical radius | Both |
| Square symbol | Square feature, one size for both sides | Both |
| (25) | Reference dimension, for information only | Both (ISO calls it an auxiliary dimension) |
| Boxed value | Basic dimension (ASME), theoretically exact dimension (ISO) | Both |
| 4X / 4× | Number of places | Both |
| Counterbore, countersink, depth symbols | Counterbore or spotface, countersink, depth | ASME only. ISO drawings write these out or show them in a section. |
Common mistakes
- Reading a radius as a diameter. Check for R or Ø before the number.
- Measuring to a reference dimension. It has no tolerance and is not checked.
- Missing the 4X. The callout covers every one of those features.
Questions
- What does a dimension in parentheses mean?
- It is a reference dimension. It repeats or follows from other dimensions and is there for information only. It is not used for making or inspecting the part.
- What does a dimension in a box mean?
- It is a basic dimension (ASME) or theoretically exact dimension (ISO): an exact value with no tolerance of its own. The tolerance comes from a geometric tolerance that refers to it.
- What is the difference between a counterbore and a countersink?
- A counterbore is a flat-bottomed, larger hole on top of a hole, for a socket head screw. A countersink is cone shaped, for a flat-head screw. The counterbore is given by diameter and depth, the countersink by diameter and angle.
Explained in our own words and checked against several textbooks and published tables. For the full rules, see ASME Y14.5, ASME Y14.36, ISO 286, ISO 965, ISO 1302 and ISO 21920-1.