Form & position
Geometric tolerances (ISO 1101) tell on the drawing how much a surface may deviate in form and position from the ideal. Below are the key symbols, what they mean and how they appear on the drawing.
How to read the feature control frame
Read left→right: tolerance type symbol, tolerance value (Ø prefix if the zone is cylindrical), and one or more datums.
- Symbol: which type of geometric tolerance (form, location, runout …).
- Value: width of the tolerance zone in mm. Ø = cylindrical/spherical zon.
- Datum: reference surfaces/axes (A, B, C). Required for orientation, location and runout — not for pure form tolerances.
What is a datum (reference)?
A datum is a surface, edge or axis on the part that we measure from. It is the 'zero point' that all orientation, location and runout tolerances are referenced to.
The letter (A, B, C …) is placed on the surface used as the reference. The same letter then appears inside the feature control frame.
- Sit the part on datum A (bottom) — fixes the height.
- Push against datum B (left side) — fixes the X position.
- Push against datum C (top edge) — fixes the Y position.
- Measure 80 mm and 50 mm to the actual hole axis. It must lie within the Ø0.2 circle.
Straightness
A line on the surface (or an axis) must not deviate more than the tolerance value from a perfect straight line. No datum needed.
E.g. ⏤ 0.05 — every line on the surface must lie between two straight lines 0.05 mm apart.
Flatness
The entire surface must lie between two parallel planes separated by the tolerance value. No datum.
E.g. ⏥ 0.1 — the surface fits between two planes 0.1 mm apart.
Roundness / Circularity
Each cross-section of a cylindrical or conical feature must lie between two concentric circles whose radial difference = the tolerance value.
E.g. ○ 0.02 — every cross-section must lie between two circles 0.02 mm apart.
Cylindricity
The whole cylindrical surface must lie between two coaxial cylinders whose radial difference equals the tolerance. Combines roundness, straightness and parallelism.
E.g. ⌭ 0.03
Profile of a line
Each line along a curved contour must lie within a band of the tolerance width, centred on the nominal profile.
Used on curved edges and form features.
Profile of a surface
The entire 3D surface must lie within a tolerance volume of the given width, centred on the nominal surface.
Common on formed sheet parts and castings.
Parallelism
A surface or axis must be parallel to a datum. The tolerance zone is two planes parallel to the datum.
E.g. ∥ 0.1 A — the surface lies between two planes 0.1 mm apart, parallel to datum A.
Perpendicularity
Feature must be 90° to the datum. Tolerance zone is two parallel planes perpendicular to the datum.
E.g. ⟂ 0.05 A
Angularity
Surface must sit at a given angle to the datum (other than 0° or 90°). Tolerance zone is two parallel planes at the specified angle.
E.g. ∠ 0.2 A 30°
Position
A feature (typically a hole or axis) must lie within a circle/cylinder of diameter = the tolerance, centred on the theoretically exact location from the datums.
E.g. ⌖ Ø0.2 A B C — the hole axis within a Ø0.2 mm cylinder.
Concentricity
The centre/axis of a feature must coincide with the datum axis, within a circle/cylinder equal to the tolerance.
E.g. ⊚ Ø0.05 A
Symmetry
The median plane of a feature must coincide with a datum, within a band of the tolerance value.
E.g. ⌯ 0.1 A
Circular runout
When the part is rotated about the datum axis, the dial indicator reading in each cross-section must not exceed the tolerance. Combines roundness and coaxiality in one check.
E.g. ↗ 0.05 A — per revolution, max 0.05 mm reading.
Total runout
Like runout, but the dial also traverses the full surface during rotation. The whole surface's deviation from the datum axis must stay within the tolerance.
E.g. ⌰ 0.1 A B