ISO 1101 parallelism tolerance and how to measure it
The drawing of a machine tool guide slide specifies a geometric tolerance of 0.015 A on the top face. The machine setter has to make sure the top face stays perfectly parallel to the base datum plane A. A parallelism error makes the carriage jam under load.
Inspecting an ISO 1101 parallelism tolerance on a surface plate
| Feature inspected | Datum surface A | Tolerance zone | Recommended instrument |
|---|---|---|---|
| Flat top face | Base plane A | Two parallel planes 0.015 mm apart | Dial indicator on a stand + surface plate |
| Hole axis | Base plane A | Two planes 0.020 mm apart | CMM |
| Cylinder generating line | Datum axis A-B | Cylinder 0.010 mm in diameter | Bench between centres + dial indicator |
All values are given for the reference environmental conditions of 20 °C.
How the parallelism sweep is done on a surface plate
To measure the parallelism of a face relative to a datum plane, face A is placed directly on the surface plate. The probe of a digital indicator is zeroed on the first corner of the top surface. The operator slides the indicator stand to sweep the whole face. The largest deviation found gives the parallelism value. To look at the related symbols, see our guide on ISO 1101 geometric symbols.
Reading the zone between two parallel planes
The specification 0.015 A means that the actual surface must lie between two planes parallel to datum plane A and 0.015 mm apart. All orientation deviations and local flatness errors are contained in this 15 micron thickness. The absolute height of the part is governed separately by the linear dimensional tolerance.
Worked example: inspecting a guide block
An inspector measures a 150 mm long guide block, toleranced 0.015 A relative to face A, at 20 °C. 1. Clean the surface plate. 2. Place face A on the plate. 3. Sweep the indicator lengthways over 140 mm. 4. Low point read: 0.000 mm, high point read: +0.019 mm. Verdict: the value found (0.019 mm) exceeds the 0.015 mm maximum by 4 microns. The block is declared non-conforming.
Classic pitfalls when inspecting parallelism
The first pitfall comes from burrs or chips under datum face A, which tilt the part artificially. The second is confusing parallelism with average thickness. The third comes from the part bending under clamping force during machining. To record your geometric readings, CoteCote makes entries quick on your dimensional inspection sheet.