ISO 1101 straightness tolerance and how to inspect it
The drawing of a 500 mm hydraulic cylinder rod specifies a straightness tolerance of 0.03 mm on its generating line. The inspector has to check that the actual line doesn’t stray beyond two parallel straight lines 0.03 mm apart. A straightness error makes the rod bend when it pushes under pressure.
Definition and measuring methods for a straightness tolerance
| Feature specified | Tolerance zone | Measuring instrument | Application |
|---|---|---|---|
| Surface generating line | Two parallel lines 0.03 mm apart | Knife-edge straightedge + feeler gauges / dial indicator | Rods, slides |
| Median axis of the cylinder | Cylinder 0.03 mm in diameter (if ⌀ in front) | CMM / V-block bench | Drive shafts |
All values are given for the reference environmental conditions of 20 °C.
Measuring the straightness of a generating line on a surface plate
The rod sits on two adjustable V-blocks. The probe of a digital indicator travels along the top generating line over its full length. The difference between the highest and lowest points gives the straightness. See our piece on ISO 1101 geometric symbols to go further into the framing.
Straightness of a generating line versus straightness of the axis
If the ⌀ symbol comes before the tolerance value (⌀0.03), the tolerance zone becomes a cylinder around the median axis. Without the ⌀ symbol, the specification only applies to the inspected surface line.
Example: inspecting a guide rod
A 400 mm rod with a 0.03 mm straightness tolerance is measured. 1. Sweep over 380 mm of usable length. 2. Largest variation recorded: 0.038 mm. Verdict: the value (0.038 mm) exceeds the tolerance by 0.008 mm. The rod is declared non-conforming and must be straightened in a press.
Common pitfalls when inspecting straightness
The first pitfall is the part sagging under its own weight during measurement. The second is confusing straightness with cylindricity. The third comes from setting the V-blocks at the wrong spacing. To record your inspection data, discover our dimensional inspection tools.