ASME GDTP Technologist cheat sheet

ASME Y14.5-2018 is the current GD&T standard; Y14.5-2009 and Y14.5M-1994 are superseded editions.

Rule #1 (envelope principle): unless otherwise specified, the limits of size of a feature of size define a boundary of perfect form at MMC that no surface element may violate.

Rule #2: RFS is the default condition; MMC or LMC applies only when the circled M or circled L modifier is placed after the tolerance value.

Fourteen geometric characteristic symbols are grouped into five categories: form, orientation, location, profile, and runout.

Form tolerances (flatness, straightness, circularity, cylindricity) never carry datum references; orientation, location, profile-of-surface with datums, and runout do.

Basic dimensions are boxed, untoleranced, theoretically exact values that define true profile and true position.

Feature control frame order: characteristic symbol | tolerance zone value (with diameter symbol if the zone is cylindrical) | modifiers | datum references in order of precedence.

A feature of size is a cylindrical surface or a set of two opposed parallel planes that can be associated with an actual mating envelope.

A tolerance value without a diameter symbol defines a zone of two parallel planes; a diameter symbol means a cylindrical zone.

Precedence: the primary datum constrains three degrees of freedom, the secondary two, and the tertiary one, totaling six in the datum reference frame.

The primary datum plane contacts its simulator at a minimum of three points, the secondary at two points, and the tertiary at one point.

A datum is theoretical (plane, axis, point); the datum feature is the actual part surface; the simulator is the inspection equipment surface (plate, mandrel, chuck, collet).

A datum feature of size referenced RMB allows no shift; referenced at MMB it allows shift equal to its departure from MMB.

The datum translation modifier (T) permits the simulator to translate away from its basic location and is used with an MMB datum reference.

A common datum (A-B) establishes a single axis or plane from two coaxial or coplanar datum features, and both must be referenced together.

Datum targets (point, line, area) define a stable datum from irregular or non-planar surfaces where full surface contact is not repeatable.

All geometric tolerances that cite the same datum reference frame are evaluated in that single frame, fixed to the part by simulator contact.

Selecting a datum feature: choose a functional, sufficiently large, stable surface that contacts the mating part, and avoid long unsupported spans.

Flatness: zone is two parallel planes; applied to a surface only; no datum is used.

Straightness of a surface: zone is two parallel lines in the direction of the leader; may apply to each element separately or to the whole surface.

Straightness of a feature of size: zone is a cylinder (diameter symbol) around the derived median line; it is the only form control that may carry an MMC modifier.

Circularity (roundness): zone is two concentric circles with radial separation equal to the tolerance; each cross-section is evaluated independently; always RFS.

Cylindricity: zone is two coaxial cylinders with radial separation equal to the tolerance; it controls roundness, straightness, and taper simultaneously; always RFS.

Rule #1 caps form error at the size tolerance; a form tolerance equal to or larger than the size tolerance adds no restriction.

Circularity does not control taper or straightness; cylindricity does.

Form tolerances are verified without datum alignment; the feature is evaluated in its own best-fit frame.

Parallelism, perpendicularity, and angularity all require at least one datum, and the zone is oriented to that datum at a basic angle.

Perpendicularity is 90 degrees; parallelism orients the zone parallel to the datum (0/180 degrees); angularity covers any basic angle other than 90 degrees.

Zone shape: two parallel planes for a surface, a cylinder for an axis when the diameter symbol precedes the tolerance value.

Orientation controls constrain tilt only; the zone is free to translate, so location is not controlled.

Applied to a feature of size at MMC, an orientation tolerance gains bonus tolerance equal to the feature's departure from MMC.

Angularity requires a basic angle dimension; the angle itself is not toleranced, the FCF controls the deviation from it.

An orientation tolerance equal to or larger than the size tolerance of a feature of size is absorbed by Rule #1 and adds nothing.

Perpendicularity applied to a hole axis controls tilt only; to control both tilt and location, use position with the same datums.

Position: zone is centered at true position derived from basic dimensions; a diameter symbol gives a cylindrical zone; datum references orient and locate the pattern.

Bonus tolerance = absolute value of (actual mating size minus MMC size); it exists only when the MMC modifier follows the tolerance value.

Total permissible variation = stated geometric tolerance + bonus tolerance + datum shift (when the datum feature of size is at MMB).

Zero position tolerance at MMC produces a virtual condition boundary equal to MMC size and converts the entire size tolerance into bonus.

Concentricity: zone is two coaxial cylinders centered on the datum axis, controlling median points of diametrically opposed elements; always RFS.

Symmetry: zone is two parallel planes centered on the datum center plane, controlling median points of opposed elements; always RFS.

The LMC modifier grants bonus as the feature departs from LMC, protecting minimum wall thickness or clearance.

Position is not applied to a single surface; surface location is controlled with profile of a surface.

Profile tolerances use basic geometry as the true profile; the profile dimensions themselves carry no plus/minus tolerance.

The default profile zone is bilateral and equally disposed about the true profile; the tolerance value is the total zone width.

Unilateral profile: zone entirely on one side of the true profile, shown by a phantom line at the true profile plus the U modifier and offset value in the FCF.

Profile of a surface controls form, orientation, and location, and controls size when applied to a feature of size.

Without datum references, a profile tolerance controls form only; with datums it also orients and locates the feature.

Profile of a line applies to each line element in a cross-section; profile of a surface applies to the entire surface at once.

Profile tolerances are RFS; the MMC and LMC modifiers do not apply to them.

The all-around symbol (small circle on the leader) extends the profile zone around all surfaces in the view; the all-over symbol applies it to the entire part surface.

Circular runout: zone is two concentric circles at each cross-section; each circular element is evaluated independently during one full rotation.

Total runout: zone is two coaxial cylinders (or two parallel planes for a face); the entire surface is evaluated while the part rotates and the indicator traverses.

Runout always requires a datum axis and is always RFS; MMC and LMC modifiers are not permitted.

Runout error is a composite of form, orientation, and location error of the surface relative to the datum axis.

Total runout is always greater than or equal to circular runout on the same surface because it also controls taper, straightness, and cumulative deviation.

Face runout applied to a flat shoulder controls perpendicularity of that face to the datum axis and is read with an axial indicator.

Circular runout on a cylinder controls roundness and coaxiality of each element but does not control taper or straightness of the whole surface.

Runout is verified as a single indicator reading in one direction (radial or axial), unlike the symmetric zone of a position tolerance.

Virtual condition, external feature: VC = MMC size + the geometric tolerance stated at MMC; internal feature: VC = MMC size - that tolerance.

Resultant condition, external feature: RC = LMC size + the geometric tolerance; internal feature: RC = LMC size - the tolerance (opposite worst-case boundary).

Virtual condition defines the envelope the mating part must clear; a zero geometric tolerance at MMC makes virtual condition equal to MMC size.

Composite position FCF: the upper segment locates the pattern to datums; lower segments control feature-to-feature spacing and orientation with tighter values.

In a composite FCF the lower-segment tolerance must be less than or equal to the upper, and its datum references must be a subset of the upper datums in the same order.

Two single-segment FCFs are independent: each segment has its own tolerance value and datum references and is verified separately.

Combined controls apply two characteristics (for example position and profile of a surface) to the same feature when their zones coincide, shown in one FCF.

At MMB, datum shift = absolute value of (datum feature actual mating size minus its MMB size); shift adds translation only, never rotation.

Bonus tolerance belongs to the toleranced feature and datum shift belongs to the datum feature of size; the two allowances are computed and added separately.