ISO 1101 perpendicularity tolerance on a surface plate
On the manufacturing drawing of a turbine bearing bracket, the machined side face carries a feature control frame specifying 0.02 A. The setter has to make sure the vertical face stays strictly perpendicular to the datum seating face A. Without a rigid set-up on a calibrated granite surface plate, the geometry can’t be approved before the part goes for grinding.
Inspecting an ISO 1101 perpendicularity tolerance on a surface plate
| Feature inspected | Datum surface A | Allowed tolerance zone | Recommended measuring means |
|---|---|---|---|
| Flat side face | Ground base plane | Two parallel planes 0.02 mm apart | Grade 0 precision square + dial indicator |
| Cylinder generating line | Locating hole axis | Cylinder 0.02 mm in diameter, perpendicular to A | Height gauge or V-block set-up |
| Shaft shoulder | Bearing seat | Two planes 0.015 mm apart | Bench between centres + axial indicator |
All values are given for the reference environmental conditions of 20 °C.
How the perpendicularity set-up works on a surface plate
To measure the perpendicularity of a face relative to a datum surface, datum surface A is placed directly on the granite surface plate.
A grade 0 steel square or a cylindrical square is held against the seating face.
The dial indicator or LVDT probe is mounted on a rigid magnetic stand that runs along the reference square.
The probe sweeps the inspected surface from bottom to top over its full usable height. The largest variation on the digital indicator gives the perpendicularity error directly.
To go over the rules for framing the symbols, see the guide on ISO 1101 geometric symbols.
Calculating and reading the tolerance zone
The specification 0.02 A defines a tolerance zone bounded by two parallel planes 0.02 mm apart and perpendicular to datum plane A.
Every waviness and tilt of the actual face must fit inside this 20 micron thick space.
If the face is 100 mm high and the indicator shows a 0.018 mm offset between bottom and top, the actual tilt still conforms.
This measurement must not be confused with the flatness of the face itself, which has to be approved separately under ISO 1101.
Worked example: inspecting a support flange
An inspector measures the perpendicularity of a 120 mm high aerospace support flange, toleranced 0.02 A relative to plane A.
The check is done on a granite surface plate in a room held at 20 °C.
- Clean the surface plate and the seating faces with isopropyl alcohol.
- Place face A on the plate with magnetic clamping.
- Probe the lowest point of the vertical face: zero the display.
- Move the height gauge vertically over 100 mm of travel.
- Read the highest point recorded: +0.024 mm.
Verdict: the reading (0.024 mm) exceeds the maximum tolerance (0.020 mm) by 4 microns. The flange is declared non-conforming and needs resurfacing on a surface grinder.
Shop-floor pitfalls when measuring perpendicularity
The first pitfall is machining burrs on datum face A. A single 0.01 mm burr under the base tilts the whole part and doubles the error in the vertical reading.
The second comes from using an inspection square that is distorted or has been dropped. Periodic calibration of the shop square against a master is mandatory.
The third is confusing a perpendicularity deviation with a form error. A face bulging in the middle looks like a perpendicularity deflection when it is really a local flatness error.
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