Parallelism
Datum-based orientation tolerances for surfaces, axes, and median features, with practical interpretation and inspection limits.
Tags: parts-and-design, specifications, gd-and-t, metrology
On this page
Parallelism constrains orientation to a datum with a linear zone. Datum separation is a separate requirement. GD&T Basics, parallelism
In the ideal face example, slope 0.0004 over a 100 mm horizontal span gives 0.040 mm parallelism and zero flatness. Translating the entire plane 1 mm upward leaves its 0.040 mm datum-normal range unchanged. This example uses a fixed ideal datum.
Read the complete requirement
Start with the drawing or model's governing standard, edition, units, feature-control-frame attachment, datum references, zone-shape symbol, and modifiers. ASME Y14.5-2018 (R2024) is the publication identified by ASME; ISO 1101:2017, edition 4, is identified by ISO as confirmed in 2022. These public catalog records establish editions and scope; they do not make the systems' defaults interchangeable. Reaffirmation and confirmation dates are distinct from technical publication dates. ASME Y14.5, ISO 1101:2017
Identify both the controlled feature and the datum feature. A datum is an ideal reference, such as a plane or line; the actual datum feature is the imperfect physical geometry used to establish it. A manufacturing setup's machine coordinates are not automatically the drawing's datum reference. KEYENCE's datum introduction makes this ideal-reference distinction explicit. KEYENCE, understanding datums
Surface, axis, and median-feature zones
| Application | Zone or evaluation | Interpretation check |
|---|---|---|
| Ordinary surface parallelism | Two planes separated by the tolerance and oriented to the datum | Evaluate the surface, not only its fitted plane's angle. |
| Axis with a diameter zone symbol | A cylindrical zone with its axis oriented to the datum reference | The numerical value is the zone diameter, not a radial allowance. |
| Axis with a planar zone | Parallel planes controlling deviation in the indicated direction | Establish the zone orientation; this is not interchangeable with a cylindrical zone. |
| Width or midplane application | A planar zone applied to the specified derived feature | Measure enough of both opposing surfaces to construct the required feature. |
GD&T Basics illustrates the usual surface, cylindrical-axis, and midplane zones. Hexagon's PC-DMIS 2023.2 documentation additionally supports diametric or planar zones for axial features and requires the planar zone's orientation to be specified. These software capabilities explain why a shorthand instruction such as “check the hole parallel” is incomplete. GD&T Basics, Hexagon, parallelism
What parallelism does and does not constrain
An ordinary surface zone limits both form and orientation. Flatness permits independent zone orientation: a flat inclined face can fail parallelism. A tighter flatness callout can refine form separately. Do not extend this relationship uncritically to derived-axis or tangent-plane requirements. GD&T Basics, surface parallelism and flatness
Size and location remain separate requirements. GD&T Basics' February 14, 2021 datum-reference-frame example explains that orientation zones retain translational freedom and require additional dimensions to locate the holes. Its example uses two mutually perpendicular datum planes to constrain the direction of a hole axis: parallelism to only one plane leaves an in-plane direction free. Datum references must be interpreted in their stated order. Position may be appropriate when both location and orientation need control, but replacing a callout requires design authority. GD&T Basics, feature-of-size parallelism
Material-condition indications also require feature-specific interpretation. Do not apply a hole's size-dependent allowance to an ordinary surface requirement or infer extra tolerance from a caliper diameter without checking the applicable size definition and governing standard. The same teaching source distinguishes material modifiers on features of size from surface applications. GD&T Basics, orientation rules
Inspection and association
For a suitable rigid part with a planar datum feature, a surface plate and indicator can provide a practical surface check. With the datum properly established, scan the controlled region and take the highest minus lowest reading in the datum-normal direction. Cover the area rather than assuming one traverse represents the complete face. KEYENCE notes that stylus force can deform non-rigid parts and that a reference face which cannot be seated properly makes this setup difficult. Its page is undated; its illustrative point counts are not universal sampling requirements. KEYENCE, measuring parallelism
For coordinate measurement, distinguish datum construction from controlled-feature construction. PC-DMIS 2023.2's parallelism help, dated August 30, 2023, requires correct nominal geometry and optimizes the zone subject to datum constraints. Its sparse-sampling illustration produces a measured value below the actual value. Accordingly, retain acquisition coverage on both the controlled and datum features; a dense target scan cannot compensate for a poorly established reference. Hexagon, parallelism evaluation
Hexagon's companion datum documentation, also dated August 30, 2023, distinguishes DEFAULT and LSQ datum math. For its ASME implementation, DEFAULT applies void filtration to planar surfaces and constrained least-squares fitting; LSQ uses an ordinary unfiltered least-squares fit. The ISO implementation offers different DEFAULT constructions. These are edition-specific software descriptions, not permission to choose the most favorable result. Record the selected standard and datum math, and justify the measurement strategy against the required datum simulation and uncertainty. Hexagon, how PC-DMIS solves datums
An axis is likewise not simply whatever cylinder fit was used for a size report. Hexagon's feature-derivation documentation distinguishes ASME fitted/extrapolated axes from its ISO treatment, which can construct an imperfect axis from sectional centers under stated conditions. Acquisition layout, feature math, and evaluated length therefore belong in an axis-parallelism report. Avoid assuming that the straightness of one fitted axis proves the form of the actual bore. Hexagon, deriving the toleranced feature
Original calculation: form, orientation, and location
Assume an ideal datum plane z = 0 and a perfectly planar controlled face:
z = 20.000 + 0.0004 x, over 0 ≤ x ≤ 100 mm.
Its datum-normal height range is 0.0004 × 100 = 0.040 mm. Thus its surface parallelism value to this ideal datum is 0.040 mm, while its mathematical flatness is zero. A hypothetical 0.030 mm parallelism requirement is exceeded by 0.010 mm. The corresponding inclination is arctan(0.0004) ≈ 0.0229°; that angle explains this ideal ramp but does not replace the distance-based tolerance.
Raise the entire face by 1 mm: z = 21.000 + 0.0004 x. The parallelism value remains 0.040 mm because both extremes increased equally. Whether the new separation is acceptable depends on a separate size or location requirement. This reconciled example assumes perfect surfaces and datum, complete coverage, and no measurement error; real acceptance also requires the applicable uncertainty and decision rule.
Information to retain
Retain the controlled feature and extent, datum sequence, standard edition, zone shape and direction, modifiers, size results where relevant, acquisition coverage, feature/datum association, software version, support condition, and decision rule. Document restraint for flexible parts rather than silently clamping them into a different shape. Diagnose differing results by comparing these inputs before changing the tolerance or manufacturing process.