Why is Straightness used?
Straightness is used where bow or waviness along a surface line could disturb sliding contact, sealing or assembly. It states a form limit for the controlled line rather than relying on size alone.
FORM CONTROL / GD&T SYMBOL GUIDE
STRAIGHTNESS
Straightness limits deviation of individual surface line elements or, with a different drawing attachment, the derived median line of a feature of size. The two examples here show surface straightness on flat and cylindrical parts; neither requires a datum.
INTERACTIVE APPLICATION
Surface straightness: flat and cylindrical examples
Dashed blue: zone boundariesOrange: controlled surface line element
FLAT SURFACE STRAIGHTNESS
A line on the flat face is controlled
- The leader attaches the ⏤ | 0.05 frame to the top surface.
- Each specified line element lies between two parallel straight lines 0.05 apart.
Interpretation
- The orange line illustrates one surface element within the blue boundaries.
- The zone can follow the element's direction; no datum is referenced.
CYLINDRICAL SURFACE STRAIGHTNESS
A line along the cylinder is controlled
- The leader attaches the ⏤ | 0.05 frame to the cylindrical surface.
- Longitudinal surface line elements must fit between parallel lines 0.05 apart.
Interpretation
- The end view identifies the cylindrical feature and its centre lines.
- This is a surface-line control, different from straightness of the derived axis.
Where is Straightness used?
Apply it to flat guides and machined faces or to longitudinal line elements on shafts and pins. If the design needs the derived median line controlled, attach the frame to the feature-of-size dimension instead.
How is Straightness used?
Attach the feature control frame to the surface for a line-element control, or to the size dimension for a derived median-line control. State the value in the drawing’s units and define the controlled extent. Neither straightness frame uses a datum.
⏤ | 0.05 mm surface line
The specified surface line must fit between two parallel lines separated by the tolerance value. This is a form control and needs no datum.
⏤ | ⌀0.03 mm derived median line
The derived median line must fit within a cylindrical zone when ⌀ precedes the value. The resulting form limit can affect the functional envelope, depending on the drawing standard and any material modifier; it differs from a surface-line control.
How to read a straightness tolerance
Read the straightness symbol, then the zone value. A plain value controls a surface line element when the frame attaches to that surface. A ⌀ prefix on a size-dimension attachment controls the derived median line inside a cylindrical zone. Neither example references a datum.
Straightness vs Flatness: Which one to use?
Straightness controls individual surface line elements, or a derived median line when the frame is applied to a size dimension. Surface flatness controls an entire face; feature-of-size flatness controls its derived median plane. Use straightness for line or axis control and flatness for the complete face or median plane. Compare the flatness example →
Commonly defined variable values
When specifying straightness, define the tolerance t, the controlled feature and extent, and whether the zone applies to a surface line or derived median line. The examples below are illustrative starting ranges in millimetres, not values required by ASME Y14.5-2018 (R2024).
Scroll the table sideways to see every column.
| Application | Typical straightness tolerance | Example feature | Notes / caution |
|---|---|---|---|
| General machined surface | 0.05–0.20 mm over 100 mm | Milled mounting face | Specify measurement direction and controlled line length. |
| Precision ground surface | 0.005–0.03 mm over 100 mm | Ground guide face | Check achievable process and inspection uncertainty. |
| Short locating pin | ⌀0.01–0.05 mm over 50 mm | Cylindrical dowel | Median-line example; include size and mating-hole limits. |
| Long shaft | ⌀0.05–0.20 mm over 500 mm | Transmission shaft | Check deflection, support during inspection and operating load. |
| Sliding guide rail | 0.02–0.10 mm over 300 mm | Sliding track edge | Consider mating rail, alignment and travel length. |
| Bearing / bush fit shaft | ⌀0.01–0.05 mm over 100 mm | Bearing journal | Fit and alignment may demand tighter or additional controls. |
| Welded or fabricated member | 0.5–2.0 mm over 1 m | Fabricated beam edge | Account for distortion, restraint and post-weld processing. |
| Extruded profile | 0.2–1.0 mm over 1 m | Extruded rail edge | Agree free-state or restrained inspection with supplier. |
These are planning examples, not universal design rules or guaranteed process capabilities. Set the actual tolerance from function, process capability, inspection method, material, feature length and the drawing standard. Values marked ⌀ illustrate derived median-line controls; unmarked values illustrate surface-line controls.
How is straightness measured in real life?
Choose the setup from the callout: a surface-line requirement is checked along the indicated surface element, while feature-of-size straightness needs the derived median line or axis evaluated from opposing surface measurements.
- Surface plate + dial indicator: Support and level the part, then sweep the indicator along the specified surface line. Record the full variation for that line; repeat at relevant positions.
- V-blocks + dial indicator: Support a shaft in V-blocks and scan along its length for a quick bow check. Rotation and roundness affect the reading, so this alone is not a direct median-line result.
- CMM inspection: Sample surface lines or opposite points along the feature. Software can evaluate the extracted line or construct the derived median line against its stated zone.
- Straightedge or optical methods: Use a calibrated straightedge, light gap or optical scan for suitable long features. Confirm resolution and setup before judging a tight tolerance.
ASME Y14.5-2018 (R2024)
ASME’s Rule #1 provides a perfect-form boundary at maximum material size for a regular feature of size unless an exception applies. A separate straightness control can further define line-element or derived-median-line form. Its placement, any material modifier and the drawing’s Y14.5 edition affect interpretation.
ASME Y14.5 edition reference. Examples are learning schematics; use the complete requirements on the drawing.
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