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Request Engineering ReviewSPI uses drawing tolerances, GD&T, datum references and critical-to-quality characteristics to define how a part should be manufactured, inspected and documented. The requirement is not complete until the feature, reference system, inspection state and acceptance evidence are clear to both manufacturing and quality teams.
General tolerances and CTQ tolerances should not be treated as two levels of the same capability claim. They serve different engineering purposes: general tolerances define routine dimensional acceptance, while CTQs identify features whose variation can directly affect fit, function, sealing, alignment, safety or customer approval.
General tolerances provide a practical default for dimensions that do not require individual functional control beyond the drawing, title-block standard or agreed manufacturing specification.
A CTQ should identify a feature that matters to product performance or acceptance and should be supported by a clear datum reference, measurement state and appropriate verification method.
Classification is not a capability promise. If the real question is whether a specific CTQ can be achieved on the selected geometry, material and manufacturing process, that decision belongs in a feature-level tolerance feasibility review.
Review Tolerance Feasibility →GD&T is most useful when it defines how a functional feature relates to the part's real assembly or mating condition. SPI reviews datum references, feature controls and measurement state so manufacturing and inspection use the same coordinate logic instead of interpreting the drawing from different reference surfaces.
Datum A, B and C should represent stable locating or mating features that reflect how the part is assembled, constrained or inspected—not simply the easiest faces to measure.
Position, profile, flatness, perpendicularity and related controls should define the feature relationship that matters to fit or function and reference a datum structure that can be reproduced during inspection.
The drawing or quality plan should clarify whether a critical requirement applies as-machined, after coating, after heat treatment, after conditioning or in an assembly-restrained state when those conditions can change the measured result.
Section boundary: this section explains how GD&T and datum requirements should be defined and reproduced for inspection. Detailed ASME Y14.5 or ISO 1101 interpretation, process capability and tolerance-achievability decisions remain outside this page.
A tolerance requirement is not fully controlled until the measurement method and evidence format are clear. SPI matches the inspection approach to the feature, datum structure, geometry, access and acceptance requirement so the reported result reflects the same condition defined on the drawing.
CMM inspection is useful for position, profile, perpendicularity and multi-datum relationships where the feature must be evaluated in a controlled coordinate system.
Plug, thread, height or functional gauges and dedicated fixtures can support efficient production checks when the gauge condition represents the drawing requirement directly.
Vision, profile, roughness or related methods may be selected when feature size, edge geometry, accessibility or surface requirements make them more suitable than contact measurement.
Section boundary: this section explains how tolerance requirements connect to inspection methods and evidence. Full PPAP, FAI workflow, control-plan design and quality-system procedures remain on the dedicated quality-document and quality-assurance pages.
CNC-machined and injection-molded parts should not use the same tolerance language by default. Machined features are typically referenced to controlled setups and defined surfaces, while molded dimensions may depend on part condition, shrinkage behavior and whether the feature is inspected free-state or under an intended assembly or fixture condition.
CNC dimensions are usually evaluated from defined machined datums or feature relationships, with the inspection state tied to the machining or finishing condition specified on the drawing.
Molded-part dimensions should identify the part state in which acceptance applies, because conditioning, free-state warpage, restraint and mating conditions can change the measured result.
Process difference does not equal an achievable-tolerance promise. If a drawing requires a specific numerical tolerance on a particular geometry, material or molded condition, review that requirement at the feature level before production commitment.
Review Tolerance Feasibility →A tolerance review is useful when the drawing requirement cannot be interpreted confidently from the nominal dimension alone. The purpose is to clarify the feature, datum, part condition and inspection basis before the requirement becomes a manufacturing or acceptance dispute.
The specified tolerance is significantly tighter than surrounding dimensions or appears critical to fit, sealing or alignment.
Position, profile or orientation is specified without a stable reference system that manufacturing and inspection can reproduce.
It is not clear whether acceptance applies as-machined, after finish, conditioned, free-state, restrained or assembled.
A critical feature is called out, but CMM, gauge, fixture or other verification method has not been agreed.
The same tolerance language is being applied across processes or part conditions that require different acceptance logic.
A tight dimension is marked critical, but the fit, interface, functional risk or customer acceptance purpose is not clear.
This page defines the standard and inspection logic. When the real question becomes whether a specific tolerance can be achieved on a particular geometry, material, process or part condition, move to a feature-level feasibility review instead of treating the requirement as a general capability statement.
Review Tolerance Feasibility →If the drawing includes CTQ dimensions, GD&T, tight fits, finish-sensitive dimensions or unclear measurement conditions, submit the drawing before production release. SPI can review the tolerance definition, datum intent, inspection state and required evidence so manufacturing and quality teams work from the same acceptance basis.
