Profile of a line
Interpreting sectional profile zones, datum constraints, tolerance distribution, and measurement coverage without substituting whole-surface or best-fit results.
Tags: parts-and-design, specifications, gd-and-t, metrology
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Profile of a line controls a specified section relative to its theoretically exact contour. The section can contain straight or curved features. KEYENCE, profile tolerances
In the fixed-alignment example, an equal bilateral 0.20 mm zone runs from −0.100 to +0.100 mm. A section with deviations +0.030 to +0.120 mm has only 0.090 mm peak-to-valley range but exceeds the positive boundary by 0.020 mm. This interpretation assumes signed normal deviations and ignores uncertainty.
Establish the governing definition
Read the drawing or model's governing standard and edition, units, exact nominal geometry, datum references, tolerance distribution, section direction, and controlled extent. ASME currently identifies Y14.5-2018 (R2024); ISO identifies ISO 1101:2017, edition 4, confirmed in 2022. Reaffirmation or confirmation does not create a rewritten technical edition. These public records identify the standards; detailed rules and defaults still need to come from the specified publication and contractual drawing requirements. ASME Y14.5, ISO 1101:2017
A nominal radius alone is not a complete inspection definition for an arbitrary profile. Establish the contour's position, orientation, transition geometry, and applicable datum constraints from the definition. For a CAD-defined part, retain the model revision and units used for comparison. Do not replace a specified contour with a convenient fitted circle or spline and then report the residuals as compliance to the original contour.
Section direction and extent
The direction of sectioning changes the curve being evaluated. KEYENCE's illustrated requirement concerns cross-sections parallel to the drawing's projection plane. It is not permission to select any visually convenient slice. A longitudinal section, transverse section, and silhouette can describe different geometry on the same part. Identify the drawing's section direction and applicable region before selecting a scan path. KEYENCE, line-profile illustration
Record the start and end of the controlled contour, any restricted region, and indications extending the requirement around a perimeter. Resolve ambiguous coverage with the design authority. An “all around” indication does not turn a line-profile symbol into a surface-profile symbol; sectioning and the distinction between sectional and whole-surface evaluation still matter.
GD&T Basics' April 6, 2021 teaching article describes independent cross-section evaluation and explains that line profile does not, by itself, relate one section to another as surface profile does. Its sampling discussion does not prescribe a universal section count. A passed inspection at two ends should therefore be reported as two evaluated sections, not proof that every intervening contour conforms. GD&T Basics, line-profile tolerance zone
Datum constraints and zone distribution
Datums affect the freedom available when comparing measured and nominal contours. A datum-free profile can act as a form control; referenced datums can constrain orientation and location. Do not infer a fully locked alignment merely by counting datum letters: the actual features and constraint directions matter. GD&T Basics distinguishes datum-free form control from datum-related location and orientation in its profile discussion. GD&T Basics, profile of a line
The tolerance is the total zone width. For an equally disposed zone of 0.20 mm, the boundaries are 0.10 mm to either side of the nominal profile, measured normal to it. This is not ±0.20 mm. GD&T Basics explains the normal-direction interpretation and equal bilateral distribution in its 2021 article. A fixed Cartesian offset is generally not equivalent to an offset normal to a curved contour. GD&T Basics, zone construction
Do not assume equal distribution when the drawing specifies an offset or another modifier. PC-DMIS 2023.2, help dated August 30, 2023, describes equal bilateral zones by default and distinguishes ASME unequally disposed/dynamic profile indications from ISO UZ/OZ indications. It explicitly says the analogous modifiers are not equivalent. Decode the drawing under its own standard before entering software offsets. Hexagon, profile of a line
Measurement and software implementation
An optical comparator can compare a visible contour with boundary lines; KEYENCE notes that focus and lighting direction/intensity affect apparent edge position. A projected outline must represent the required section rather than an unrelated silhouette. CMM or scanner data likewise require the correct section extraction, datum alignment, and nominal correspondence. KEYENCE, measuring line profile
PC-DMIS 2023.2 recommends separate geometric-tolerance commands for separate sections. Combining them in one command makes the fit simultaneous; its simultaneous-tolerance option warns that behavior becomes equivalent to surface profile. This is an implementation-specific reason to review the command structure, not merely the report's label. The same help cautions that insufficient point density can understate the result and that piecewise-planar nominal approximations can be poor near sharp corners or radii. Hexagon, recommended usage and exposed options
Association of the datum is another input. Hexagon's companion documentation distinguishes constrained datum-simulator fitting from plain least-squares fitting, with differences between its ASME and ISO implementations. Record datum math and permitted alignment freedoms. A free best fit must not remove location or orientation error that the datum-based requirement retains. Hexagon, PC-DMIS 2023.2 datum construction
Use coverage suited to the contour and manufacturing risks: include transitions, local damage, likely waviness, and sections where the process changes. Retain section stations and point/scan spacing. Filtering, smoothing, point rejection, and interpolation should be explicit rather than silently eliminating an adverse peak. These are practical inspection-planning controls, not universal numerical sampling requirements. Support and restraint must also be defined for flexible parts so a fixture does not change the contour without that change being reported.
Original calculation: range is not a centered profile result
Assume a smooth, datum-located contour with an equally disposed 0.20 mm zone, and signed normal deviations from the fixed nominal profile. Ignore uncertainty and assume the required alignment is fully established.
| Evaluated section | Minimum deviation | Maximum deviation | Peak-to-valley range | Twice largest absolute deviation |
|---|---|---|---|---|
| A | −0.040 mm | +0.080 mm | 0.120 mm | 0.160 mm |
| B | +0.030 mm | +0.120 mm | 0.090 mm | 0.240 mm |
Section A lies within the boundaries −0.100 and +0.100 mm. Section B exceeds the positive boundary by 0.020 mm, despite having the smaller peak-to-valley range. Re-centering B to make its range appear acceptable would alter the stipulated fixed comparison. The centered enclosing-zone calculation is 2 × max(abs(minimum), abs(maximum)), yielding 0.160 and 0.240 mm respectively.
This original example illustrates reporting arithmetic, not a universal result formula for every profile modifier. Hexagon's PC-DMIS Blade 6.0 help, generated March 26, 2021, distinguishes paired signed min/max results from single profile values and requires their acceptance limits to match the actual zone distribution. Offset or dynamic zones require their own interpretation. Neither row establishes the condition of unsampled sections or points. Hexagon, Blade 6.0 profile reporting
Information to retain
A useful result identifies the nominal definition/revision, standard edition, section direction/stations, contour extent, datum sequence, permitted fit freedoms, distribution and sign convention, acquisition/filtering settings, software version, signed extremes, and applicable decision rule. Preserve individual section results when the requirement is sectional. Do not average a failing section into a passing overall number.