Parallelism — Orientation Tolerance

Controls how parallel a surface or axis is to a datum.

Definition

Parallelism controls how parallel a surface or an axis is to a datum. It is an Orientation control, and orientation controls always require a datum reference frame — there is no self-referencing parallelism. The angle is exactly 0°, implied by the symbol, so no basic angle dimension is needed. Parallelism is the 0° member of the orientation family — perpendicularity at 90°, angularity at any other angle — and it is the member most often confused with flatness. The distinction is the datum: flatness is self-referencing and says nothing about the surface's direction, while parallelism is measured from the datum and would fail a surface that is perfectly flat but tilted. The same surface can pass flatness and fail parallelism, or pass both — they answer different questions.

The zone: two parallel planes parallel to the datum

The parallelism tolerance zone is the space between two parallel planes separated by the tolerance t, held parallel to the datum surface. The entire actual surface must lie between the planes. A surface that is perfectly flat but leans relative to the datum fails — parallelism is an orientation control, and the datum is what it is measured from. For an axis — a shaft, a bore, a slide rod — the zone is a cylinder of diameter Øt around the true axis, and an axis control may carry a material-condition modifier, because a feature of size has a size to depart from. The zone itself is the same shape as flatness's; the difference is entirely in what anchors it.

Reading a feature control frame

∥ 0.1 A reads as: a parallelism tolerance of 0.1 zone width, referenced to primary datum A. No angle dimension appears — 0° is implied by the symbol. Compare the flatness frame ⏥ 0.2: flatness references only the surface itself and never carries a datum, while parallelism always carries one. The two frames look similar, and the presence or absence of the datum reference is exactly what separates an orientation control from a Form control.

Measurement

Surface plate + height gauge — rest the part on the datum surface (or on a fixture that reproduces it), sweep an indicator over the controlled face, and read the total range; it must stay within t. The classic method for faces that must ride parallel to a reference.

CMM point sampling — probe the surface and fit the tightest pair of planes parallel to the datum that bounds the points. Used for axes and bores, and whenever several parallel faces are checked against the same datum in one setup.

When to use it

• Sliding and riding surfaces — slide ways, guide rails, carriage faces — where the moving part must stay parallel to its reference over the full travel
• Mating faces that must hold a consistent gap: a cover plate parallel to its housing face, where a wedge gap would leak or rattle
• Datum-to-datum parallelism for stack-up control, where each part's lean compounds into an unacceptable total
• Two faces that must be parallel to each other for uniform wall thickness or uniform bearing contact, referenced to a common datum