Rules of Dimensioning and Tolerancing

Flashcards for key vocabulary related to dimensioning standards, rules, and geometric dimensioning and tolerancing (GD&T).

ANSI Y14.5

The accepted national standard in the United States for Dimensioning and Tolerancing, published by the American Society of Mechanical Engineers (ASME).

Dimension

A numerical value expressed in appropriate units of measure used to define the size, location, geometric characteristic, or surface texture of a part or part feature.

Arrowhead

A small triangle at the end of dimension lines and leaders to indicate the direction and extent of a dimension.

Dimension Line

A thin, dark, solid line that terminates at each end with arrowheads.

Extension Line

A thin, dark, solid line extending from a point on an object, perpendicular to a dimension line.

Leader

A thin, dark, solid line terminating with an arrowhead at one end and a dimension, note, or symbol at the other end.

Contour Rule

A drawing practice where each dimension should be placed in the view where the contour shape is best shown.

Contour Dimensioning

Placing each dimension in the view where the contour or shape of the feature shows up best.

Baseline Dimensioning

A system of dimensioning where each feature is dimensioned from the same origin (used on CNC machines).

Chain Dimensioning

A system of dimensioning where features are dimensioned from one another instead of from an origin.

Chamfers

Beveled or angled edges that typically appear on the ends of shafts or fasteners.

Counterbore

The space where the head of a screw is set (drilled straight).

Countersink

The space where the head of a screw is set (drilled at an angle).

Spotface

A flat surface drilled around a hole for fasteners to set properly

General notes

Appear to the lower right hand corner of a drawing.

Local notes

Appear on the drawing views and are usually specified with a leader line.

Limit Tolerance

Displayed with the high limit above the low limit; if displayed on one line, the low limit appears before the high limit.

Statistical Tolerancing

A way to assign tolerances based on sound statistical practices rather than conventional tolerancing practices. It can be applied only when appropriate statistical process control methods are used for manufacturing.

Clearance Fit

Specifying the limits of size in such a way that a clearance or space always exists between mating parts.

Interference Fit

Specifying the limits of size in such a way that an interference of material always exists between mating parts.

Transition Fit

Specifying the limits of size in such a way that either a clearance or interference fit will exist when mating parts are assembled.

Allowance

The difference between the maximum material limits of mating parts (Subtract the upper limit of the shaft dimension from the lower limit of the hole dimension).

Tolerance Dimension

The total permissible variation of a size, which is the difference between the upper limit and lower limit.

Basic Size

The size from which the limit dimensions were derived.

Clearance

A fit where there is space between the two mating parts.

Interference

A fit where the two mating parts have intersecting nominal volumes, requiring the deformation of the parts.

Hole Basis/Basic Hole System

The basic size applied to the lower limit of the hole.

Shaft Basis/Basic Shaft System

The basic size applied to the upper limit of the shaft. Used when several parts with different fits are required to fit on a particular shaft.

RC (Running or Sliding Clearance Fit)

A type of fit between two parts where they have a slight clearance, allowing for free or easy movement and rotation

LC (Locational Clearance Fit)

A type of fit that is intended for parts that are normally stationary but can be freely assembled or disassembled.

LT (Locational Transition Fit)

A type of fit where accuracy of location is important but a small amount of clearance or interference is permissible.

LN (Locational Interference Fit)

A type of fit where accuracy of location is of prime importance and parts require rigidity and alignment.

FN (Force or Shrink Fit)

A type of interference fit that is characterized by maintenance of constant bore pressures throughout the range of sizes. Interference varies with diameter and difference between its minimum and maximum value is small to maintain the resulting pressures

Geometric Dimensioning and Tolerancing (GD&T)

A 3-D mathematical system that allows a designer to describe the form, orientation, and location of features on a part within precise tolerance zones (uses symbols instead of words).

Datum Reference Frame

A theoretical system made up of three mutually perpendicular planes, or datums, established by real features on the object.

Origin or datum point

The intersection of the three planes

Datum axis

The intersection of two of the planes

Feature Control Frame

The geometric characteristic symbol, the geometric tolerance, and the relative datums.

Maximum Material Condition (MMC)

The condition in which a feature of size contains the maximum amount of material within the stated limits of size

Flatness

Specifies a 3D tolerance zone defined by two parallel planes; the tolerance specified in the feature control frame must be less than the tolerance on the size dimension for the part.

Circularity (Roundness)

Specifies a tolerance zone bounded by two concentric circles; tolerance must be less than the size tolerance

Cylindricity

Controls a cylindrical surface so that all points are equidistant from a common axis; the tolerance zone is a 3D zone defined by two concentric cylinders

Profile of a Line

Specifies a 2D tolerance zone defined by two contours

Profile of a Surface

Specifies a 3D tolerance zone defined by two contoured surfaces.

Parallelism

Specifies a 3D tolerance zone that can control the orientation of a surface or axis of a hole or cylinder relative to a datum; when applied to the surface, the 3D zone is defined by the area between two parallel planes; when applied to the axis, the 3D zone is defined by the area within a cylinder

Perpendicularity

Specifies a 3D tolerance zone that can control the orientation of a surface or axis of a hole or cylinder relative to a datum; when applied to a surface, the 3D zone is defined by the area within a cylinder; when applied to an axis, the tolerance zone is a cylinder.

Angularity

Specifies a 3D tolerance zone that can control the orientation of a surface or axis of a hole or cylinder relative to a datum; when applied to a surface, the 3D zone is defined by the area between two parallel planes; when applied to an axis, the 3D zone is defined by the area within a cylinder; the tolerance must include a datum reference and a basic dimension specifying the angle from one or more datums

Position

Used to control the orientation and location of a center, an axis, or center plane of a feature of size.

Concentricity

Geometric tolerance used to control the axis-to-axis relationship between two features

Symmetry

The position of a center plane relative to an axis or a center plane of a datum feature

Circular Runout

Controls a surface relative to a datum axis.

Total Runout

A 3D control for rotating parts relative to a datum axis; when applied to a cylindrical surface, it controls circulartity, concentricity, straightness, taper, and surface profile; when applied to a flat surface it controls wobble, perpendicularity, and flatness.

Model Based Definition

A practice of embedding Product Manufacturing Information (PMI) data at the 3D model level (dimensions, tolerances, general notes, parts lists, information related to manufacturing, inspection, test, etc).