ASME GDTP Senior Level cheat sheet

ASME Y14.5 establishes uniform practices for stating and interpreting dimensioning, tolerancing, and related requirements on engineering drawings; it does not cover welding symbols, surface texture, or screw threads in detail.

GD&T controls apply to the as-drawn ideal geometry; the actual produced part must satisfy every specified control independently unless a stated relationship exists.

Rule #1 (Envelope Principle): where only a size dimension is given for a regular feature of size, the limits of size define a perfect-form envelope at MMC that the feature may not violate.

Rule #2: where a geometric tolerance is applied to a feature of size, perfect form is required at MMC only when the tolerance is an orientation, location, or form control that invokes the boundary; the size limits still apply independently.

The drawing is a legal contract: dimensions and tolerances define the acceptable limits; implied 90-degree angles apply to surfaces shown perpendicular unless otherwise toleranced.

Basic dimensions are theoretically exact values shown in a rectangular frame, used to establish the true profile or true position of features; they carry no tolerance themselves.

GD&T is a design specification language; inspection methods (functional gaging, CMM) are chosen to verify conformance, not to redefine the tolerance.

Units and decimal-place conventions follow the drawing's general tolerance block; a dimension without a tolerance is governed by that block, not by GD&T controls.

Feature control frame reads left to right: geometric characteristic symbol, then tolerance zone value, then material condition modifier (if any), then datum references in order of precedence.

Common characteristic symbols: straightness, flatness, circularity, cylindricity (form); perpendicularity, angularity, parallelism (orientation); position, concentricity, symmetry (location); profile of a line, profile of a surface; circular runout, total runout.

Material condition modifiers: MMC (M in a circle), LMC (L in a circle), RFS (no modifier, default).

Feature of size: a cylindrical or spherical surface, or a set of two opposed parallel surfaces, each associated with a size dimension.

Regular feature of size: a feature of size with a single size dimension and no datums; irregular feature of size has a non-uniform cross-section or is not a single size.

Datum feature symbol attaches to a surface or feature of size; the datum feature is the actual part feature, the datum is the theoretically exact plane/axis/point derived from it.

Basic dimension is enclosed in a rectangle; reference dimension is enclosed in parentheses and is for information only.

Free state, restrained condition, and statistical tolerancing (ST) symbols modify interpretation; ST indicates a statistical tolerance allocation, not a geometric control.

Datum reference frame is established by three mutually perpendicular planes (or axes/points) derived from the primary, secondary, and tertiary datum features in the order listed in the feature control frame.

Primary datum feature contacts the datum simulator at a minimum of three points (a plane); secondary at two points; tertiary at one point, establishing six degrees of freedom constrained.

Datum feature precedence in the frame determines which degrees of freedom are constrained first; the order is not interchangeable.

A datum axis is derived from a cylindrical datum feature of size; a datum plane from a planar surface; a datum point from a spherical or conical feature.

Datum targets (point, line, area) are specified when the datum feature is irregular or when a specific contact area is required; they are shown with basic dimensions locating the target.

A compound datum (e.g., A-B) creates a single datum axis or plane from two features acting together; the datum is centered between them.

Datum feature simulator is the inspection equipment (surface plate, mandrel, chuck) that contacts the datum feature to establish the datum; it must be of sufficient accuracy.

When a datum feature is referenced at MMB (M), the datum simulator is a fixed-size boundary at the virtual condition of the datum feature, allowing a shift tolerance.

Form controls (straightness, flatness, circularity, cylindricity) are independent of datums; no datum references are permitted in the feature control frame.

Straightness of a surface: the surface must lie between two parallel lines (in the view) or within a cylindrical zone if applied to an axis (straightness of a feature of size).

Flatness: the surface must lie between two parallel planes separated by the tolerance value; the tolerance zone is not related to any datum.

Circularity (roundness): each cross-section perpendicular to the axis must lie between two concentric circles whose radial separation equals the tolerance; evaluated per cross-section.

Cylindricity: the entire surface must lie between two coaxial cylinders whose radial separation equals the tolerance; controls roundness, straightness, and taper simultaneously.

Rule #1 imposes an implicit form control at MMC for features of size; an explicit form tolerance tighter than the Rule #1 envelope must be specified to tighten form.

Form tolerances are always RFS; material condition modifiers do not apply to form characteristics.

A straightness tolerance applied to a feature of size with an M modifier creates a virtual condition boundary; the tolerance may be exceeded as the feature departs from MMC.

Orientation controls (perpendicularity, angularity, parallelism) relate a feature to a datum and control the angle; they do not locate the feature.

Perpendicularity: the toleranced surface or axis must lie within a zone perpendicular to the datum plane (or axis) within the stated tolerance.

Angularity: the toleranced feature must lie within a zone oriented at the basic angle relative to the datum; the basic angle is shown without tolerance.

Parallelism: the toleranced feature must lie within a zone parallel to the datum; the tolerance zone is oriented, not located.

Orientation tolerances require at least one datum; the datum reference frame constrains the orientation of the tolerance zone.

When applied to a feature of size at MMC, orientation tolerance creates a virtual condition boundary and permits bonus tolerance as the feature departs from MMC.

Orientation tolerance must be less than or equal to the size tolerance when applied to a feature of size; otherwise the size limits govern.

A perpendicularity control on a feature of size also controls the orientation of the axis; it does not control location relative to other features unless combined with position.

Location controls (position, concentricity, symmetry) locate features relative to datums; position is the most common and is typically applied to features of size.

Position tolerance defines a zone (cylindrical for holes/shafts, two parallel planes for slots) centered on the true position established by basic dimensions.

Concentricity controls the median points of opposed elements of a feature of size relative to a datum axis; it is difficult to verify and rarely used.

Symmetry controls the median points of opposed elements relative to a datum plane; also rarely used and difficult to inspect.

Location tolerances require basic dimensions to locate the true position; the tolerance zone is centered on the true position.

When position is applied at MMC, bonus tolerance equals the amount the actual size departs from MMC (for internal features, actual size minus MMC size).

A composite position tolerance (two single-segment frames) controls the pattern location (PLTZF) and feature-to-feature location (FRTZF) separately.

Location tolerances do not control form; form is controlled by Rule #1 or by separate form controls.

Profile of a surface controls the location, orientation, and form of a surface relative to a true profile defined by basic dimensions; the tolerance zone is bilateral by default (equal on both sides).

Profile of a line controls the profile of a cross-section (line element) relative to the true profile; it is a 2D control and does not control the surface between cross-sections.

A unilateral profile tolerance is shown with a phantom line and a tolerance value on one side of the true profile; the zone lies entirely on that side.

Profile tolerances may reference datums to locate and orient the true profile; without datums, profile controls only form and orientation (not location).

Profile can be applied to a feature of size; when applied at MMC or LMC, bonus tolerance applies and the profile tolerance creates a boundary.

Profile of a surface tolerance zone is bounded by two surfaces offset from the true profile by the tolerance value (bilateral) or by the stated amount (unilateral).

Composite profile (two single-segment frames) controls the profile relative to datums (PLTZF) and the profile between features (FRTZF) separately.

Profile tolerances are often used for irregular or non-analytic surfaces where position or orientation cannot be applied.

Circular runout controls the variation of a surface element (circular element) during one revolution about a datum axis; the indicator reading (FIM) must not exceed the tolerance.

Total runout controls the entire surface (all elements) during rotation about a datum axis; the indicator reading must not exceed the tolerance over the full surface.

Runout controls both form and location (coaxiality) of a surface relative to a datum axis; it is a composite control.

Runout requires a datum axis; the datum feature is typically a cylindrical feature of size that establishes the axis of rotation.

Runout tolerances are always RFS; material condition modifiers do not apply.

Circular runout does not control taper or axial location; total runout controls the entire surface including taper and axial variation.

Runout tolerance zone for a cylindrical surface is two coaxial cylinders separated by the tolerance value, centered on the datum axis.

Runout is typically verified with a dial indicator and a precision spindle or centers; the part is rotated and the total indicator movement is recorded.

Virtual condition (VC) = MMC size minus (for internal features) or plus (for external features) the geometric tolerance; it is the boundary the feature may not violate.

Bonus tolerance = actual mating size minus MMC size (internal feature) or MMC size minus actual mating size (external feature); it is added to the stated geometric tolerance.

At MMC, the geometric tolerance is at its minimum; as the feature departs from MMC toward LMC, bonus tolerance increases the allowable geometric tolerance.

LMC modifier (L) creates a boundary at LMC; bonus tolerance is the departure from LMC toward MMC; used when minimum wall thickness or clearance is critical.

RFS (default) means the geometric tolerance is fixed regardless of feature size; no bonus tolerance applies.

Zero geometric tolerance at MMC is permitted; it requires perfect form at MMC and provides bonus tolerance as the feature departs from MMC.

Position tolerance zone for a hole is a cylinder of diameter equal to the stated tolerance (plus bonus) centered on the true position.

The MMC modifier in the feature control frame applies to the toleranced feature; the MMB modifier in the datum reference applies to the datum feature and allows datum shift.

Functional gaging verifies MMC requirements by simulating the virtual condition boundary; a functional gage checks conformance to the worst-case boundary, not the actual size.

CMM inspection measures actual feature geometry and computes the deviation from the true position/profile; it can verify RFS and MMC requirements with software.

When a tolerance is specified at MMC, the inspector may use either a functional gage or a variable measurement with bonus tolerance calculation.

Inspection must use the datum reference frame specified; the datum feature simulator must contact the datum feature in the order of precedence.

A feature that violates the virtual condition boundary is nonconforming even if its actual size is within limits, because the combined effect exceeds the allowable envelope.

Interpretation: all dimensions and tolerances apply to the finished part in its free state unless a restrained condition is specified.

Statistical tolerancing (ST) modifies the tolerance allocation but does not change the inspection method; it indicates a statistical process control requirement.

When a drawing is ambiguous, the order of precedence is: the feature control frame, the datum reference frame, basic dimensions, and general notes/tolerance block.