Parallelism and perpendicularity

Parallel, square, and tied to a datum

Parallelism and perpendicularity control direction relative to a datum. Parallelism asks a surface, centre plane or axis to follow the datum direction; perpendicularity asks it to stand at 90° to that direction. NIST classifies both as orientation tolerances, distinct from form and location tolerances.

Flatness asks only whether one surface fits between two parallel planes. It has no datum. Parallelism and perpendicularity add a second object to the question: the datum established from another feature. A face may be flat and still lean by 0.01 mm across its width, so it can pass flatness and fail parallelism.

Read the zone from the datum

For the surface-to-plane case used in this lesson, a parallelism tolerance t creates two parallel planes t apart and parallel to the datum plane. A perpendicularity tolerance creates the same two-plane shape, but the planes stand at 90° to the datum. The controlled surface must fit inside the zone.

Read the feature control frame in order: identify the controlled feature, read the tolerance value, then follow the datum letter to the feature that establishes the direction. If the drawing names datum A, seating some more convenient face does not create a substitute datum A.

Axes, centre planes and a tolerance preceded by the diameter symbol can use different zone shapes. This lesson does not expand those cases because the full GB/T 1182-2018 and GB/T 1958-2017 texts were not available for clause-level checking.

Parallelism on a surface plate

A basic plate check begins by seating the datum face on the clean plate. An indicator on a rigid stand contacts the controlled face. The stylus then traverses the face while the datum contact remains unchanged; one point cannot supply a range.

KELIBRON’s planned teaching model samples a 5 × 3 grid and reports the spread, maximum reading minus minimum reading. This is a product teaching model awaiting the GB/T 1958 text check, not a claim that the standard prescribes a 5 × 3 grid.

The planned pass part has a 0.01 mm spread against a 0.02 mm teaching tolerance: 0.01 ≤ 0.02, leaving 0.01 mm. The planned reject part spreads 0.03 mm, which is 0.01 mm above the same limit. These are draft lesson values, not general shop limits.

Perpendicularity needs a square direction

A perpendicularity check still starts from the datum. The datum feature must be seated first; the controlled surface is then assessed in a direction 90° to the established datum. Changing which face sits on the plate changes the reference and answers a different question.

A square or angle plate can help establish and observe the right-angle relationship, but a visible light gap is not automatically a perpendicularity value in millimetres. A numerical result needs a defined setup, traverse, sampling rule and evaluation method.

KELIBRON has no released perpendicularity exercise. G4b still needs its method and evaluation rule to pass metrology review, so this article supplies no invented command, point grid, tolerance or pass/fail example for it.

Four mistakes that spoil the result

Most bad checks lose the datum or lose the scan. The consequence is not a slightly worse number; it is a number that answers the wrong question.

In the planned domain model, seating without scanning gives incomplete_scan and no verdict. If any scan point loses contact or exceeds the indicator travel, the measurement is invalid; an invalid pose never passes.

What the standards support here

GB/T 1182-2018 is the current Chinese standard for indicating form, orientation, location and run-out tolerances; it is a modified adoption of ISO 1101:2017. ISO confirmed its 2017 edition in 2022. GB/T 1958-2017 is the current Chinese standard for detection and verification of geometrical tolerances.

Those records establish the standards and their scope. The unavailable full texts are why this page stops at the basic surface zones and labels the 5 × 3 sweep as KELIBRON’s draft teaching model rather than quoting a standard inspection rule.

In KELIBRON

The released lab does not yet offer the G4 orientation lesson. The DI-10 indicator exists, but the parallel blocks, top-face scan and orientation lesson scene remain development-gated; perpendicularity is earlier still, awaiting G4b method approval. No orientation reenactment is attached to this draft.

Until G4 is released, the indicator comparison scene can rehearse contact, zero and reading a difference. It cannot practise or certify parallelism or perpendicularity. The planned parallelism exercise will add the 0.01 mm pass part, the 0.03 mm reject part and the incomplete-scan refusal after the release gates are met.