3-2-1 rule

A rule that defines the minimum number of contact points necessary to properly locate a part within the datum reference frame. The primary datum requires three points, the secondary datum two points, and the tertiary datum one point.

actual mating envelope

A geometrically perfect shape that is a "best fit" around a feature. The AME is the smallest possible cylinder contacting a shaft at its highest points or the largest possible cylinder contacting a hole at its highest points.

ASME Y14.5M

The standard for geometric dimensioning and tolerancing published by the American Society of Mechanical Engineers (ASME). The 1994 revision is the most recent version of the standard.

axis

An imaginary, perfectly straight line used to indicate measurements or positions. An axis runs through the center of a cylindrical object.

basic dimension

A measurement that describes the theoretical and perfect size, shape, or location of a feature.

bolt-hole pattern

A series of holes with centers equally positioned around the circumference of a larger imaginary circle. Bolt-hole patterns can be controlled with composite tolerancing.

bonus tolerance

Additional tolerance that applies to a feature as its size shifts from a stated material condition. Both MMC and LMC allow bonus tolerance.

Cartesian coordinate system

A positioning system that uses three linear axes perpendicular to each other to locate positions in three-dimensional space. The three axes in the Cartesian coordinate system are the X-axis, Y-axis, and Z-axis.

casting

A part that has been made by melting the material, pouring it into a mold or cavity, and removing the part after it has cooled and solidified. Metal castings tend to have rough surfaces.

center point

A single imaginary point located in the center and equally distant from the exterior of a circular feature.

circularity

A two-dimensional geometric tolerance that controls how much a feature can deviate from a perfect circle.

composite tolerancing

A combination of more than one geometric tolerance applied to the same feature. Composite tolerancing is used with position or profile tolerances and is applied with a multi-segment feature control frame.

conical taper

A cylindrical feature that gradually changes from a larger diameter to a smaller diameter at a constant ratio.

conicity

A three-dimensional geometric tolerance that controls how much a feature can deviate from a perfect tapered cone.

controlled radius

A radius that yields a circle, arc, or sphere with no flat sections or reversals.

cylindricity

A three-dimensional geometric tolerance that controls how much a feature can deviate from a perfect cylinder.

datum feature symbol

A GD&T symbol in a print indicating a part feature that acts as a datum feature and that contacts a datum reference frame simulator.

datum reference frame

Three imaginary planes perpendicular to one another that are mapped onto the part and provide an anchor for relating part features to each other.

datum reference frame simulator

A gaging surface, machine component, or other device that establishes a datum plane in the datum reference frame. These devices adhere to gaging tolerances and act as a reasonable substitute for the datum reference frame.

datum target

A GD&T symbol in a print indicating the size, shape, and location for a matching gaging point, line, or surface that is used to position the part in the datum reference frame. Datum targets are most often used with rough or irregular parts.

feature control frame

A series of compartments containing symbols and values that describe the geometric tolerance of a feature. The order and purpose of these compartments follow a consistent standard.

feature of size

A cylindrical surface, spherical surface, or two opposed parallel elements or surfaces that can be associated with a size dimension.

feature without size

A feature that cannot be associated with a size dimension. A single flat surface is a feature without size.

flat taper

A flat surface that gradually change from a larger height to a smaller height at a constant slope or incline.

flatness

A three-dimensional geometric tolerance that controls how much a feature can deviate from a flat plane.

flexible datum reference frame simulator

A datum reference frame simulator that can reposition the datum reference frame without losing any datum relationships.

functional gage

A gage representing a "worst case" mating part that provides a simple pass/fail assessment of the inspected part. Functional gages often can quickly inspect several features at once.

gaging tolerances

Tight or strict tolerances that ensure an inspection gage provides a reasonable amount of measurement certainty. Gaging tolerances are typically ten times more accurate than the part that the gage inspects.

geometric dimensioning and tolerancing

An international standard for communicating instructions about the design and manufacturing of parts. GD&T uses universal symbols and emphasizes the function of the part.

go gage

A gage on or in which a good part should fit easily. A go gage quickly checks a part's features without providing a measurement value.

hard datum reference frame simulator

A datum reference frame simulator that is rigidly fixed in place in one or more planes. The combination of a surface plate and angle plate is a hard DRF simulator.

helix angle

The angle formed by the helix or spiral of the thread at its pitch point.

least material condition

The point at which a feature contains the least amount of material within its acceptable size limit. The largest acceptable hole and the smallest acceptable shaft are examples of LMC.

major diameter

On a thread, the diameter from crest to crest of an external thread or the diameter from root to root of an internal thread. On a gear, the widest diameter formed by the gear.

maximum material condition

The point at which a feature contains the greatest amount of material within its acceptable size limit. The smallest acceptable hole and the largest acceptable shaft are examples of MMC.

median plane

An imaginary, perfectly flat plane positioned in the middle between two opposing flat surfaces.

minor diameter

On a thread, the diameter from root to root of an external thread or the diameter from crest to crest of an internal thread. On a gear, the base diameter matching the root of the teeth.

perpendicular tolerance

A three-dimensional geometric tolerance that controls how much a surface, axis, or plane can deviate from a 90° angle.

phantom line

A line drawn by alternating a long dash, followed by two short dashes. A phantom line can indicate a datum target line in a GD&T print.

pitch diameter

On a thread, the measured diameter between the pitch points taken in the groove between the threads. On a gear, the diameter of the circle passing through the pitch points on the gear teeth.

position tolerance

A three-dimensional geometric tolerance that controls how much the location of a feature can deviate from its true position.

primary datum

The datum feature that first situates the part within the datum reference frame. The primary datum is the first feature to contact a fixture or surface during assembly.

profile tolerance

A geometric tolerance that controls either how much the two-dimensional outline or three-dimensional surface of a feature can deviate from its true profile.

projected tolerance zone

A tolerance zone that extends beyond a feature by a specified distance. Projected tolerance zones help ensure that mating parts fit during assembly.

radius

A straight line extending from the center to the periphery of a circle, arc, or sphere.

regardless of feature size

A modifier indicating that the stated tolerance for a feature applies regardless of its actual size within an acceptable size limit. RFS does not permit bonus tolerance.

resultant condition

A variable "worst case" boundary opposite the virtual condition that equals the feature's actual mating envelope and its location tolerance.

Rule #1

The GD&T rule stating that, when a tolerance for a feature of size is specified, the surfaces of that feature cannot extend beyond its boundary of perfect form at its maximum material condition (MMC).

Rule #2

The GD&T rule stating that, for all applicable geometric tolerances, the regardless of feature size (RFS) modifier applies to the individual tolerance, datum reference, or both where no modifier symbol is specified.

secondary datum

The datum feature that situates the part within the datum reference frame after the primary datum. The secondary datum is the second feature to contact a fixture or surface during assembly.

section lines

A series of diagonal lines drawn close together. Section lines can identify a datum target area in a GD&T print.

six degrees of freedom

The six basic possible linear and rotational movements that a part can have if left unrestricted. Positioning a part in the datum reference frame restricts all six degrees of freedom.

sphericity

A three-dimensional geometric tolerance that controls how much a feature can deviate from a perfectly round sphere.

statistical process control

The use of statistics and control charts to measure key quality characteristics and control how the related process behaves. With SPC, operators typically track the variation of part measurements over time.

statistical tolerancing

The assigning of tolerances for mating parts of an assembly that applies the principles of statistics. Statistical tolerancing is used in conjunction with SPC.

straightness

A two-dimensional geometric tolerance that controls how much a feature can deviate from a straight line.

tertiary datum

The datum feature that situates the part within the datum reference frame after the secondary datum.

virtual condition

A constant "worst case" imaginary boundary defined by the collective effects of a feature's size, geometric tolerance, and material condition.