321 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

AME. 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.

Axis

An imaginary straight line that lies in the center of an object. When a cylindrical surface forms a feature of size in GD&T, it establishes the axis of that cylinder.

Basic Dimension

A dimension that is theoretically perfect. A basic dimension has no direct tolerance and is denoted on a GD&T blueprint as a number enclosed in a rectangular box.

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.

Center Point

A single imaginary point located in the center and equally distant from the exterior of a circular feature. When a spherical surface forms a feature of size, it establishes the center point of that sphere.

Composite Tolerancing

A combination of more than one geometric tolerance applied to the same feature using a multisegment feature control frame. Composite tolerancing is used with position or profile tolerances, and the tolerance symbol is entered into the feature control frame once and applies to both segments.

Conical Taper

A cylindrical feature that gradually changes from a larger diameter to a smaller diameter at a constant ratio. Conical tapers include the standard machine tapers used throughout the machine tool industries.

Controlled Radius

A radius that yields a circle, arc, or sphere with no flat sections or reversals. In GD&T, controlled radius occurs on a blueprint with the symbol "CR."

Cpk

A measure of process capability, Cpk measures the amount of variatioin in a process relative to the limits supplied by its specifications. Cpk only applies to processes that use SPC.

Datum Feature 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 Feature Symbol

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

Datum Reference Frame

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

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.

Envelope Principle

Also called the Taylor Principle or Rule #1, the Envelope Principle is 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. As the material of the feature of size decreases, variation is allowed.

Feature of Size

FOS. A cylindrical surface, spherical surface, or two opposed parallel elements or surfaces that can be associated with a size dimension. A feature of size establishes an axis, median plane, or center point.

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 changes from a larger height to a smaller height at a constant slope or incline. A GD&T print may specify a flat taper with either a tolerance slope, with a tolerance height at one end, or an incline ratio of height of incline and distance between start and stop of the incline.

Flexible Datum Feature Simulator

A datum reference frame simulator that can reposition the datum reference frame without losing any datum relationships. The CMM, which uses software to construct the datum planes during inspection, acts as a flexible datum reference frame simulator.

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 Tolerance

A tight or strict tolerance that ensures 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.

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 Feature 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

An angle found in both thread and gear anatomy. For a thread, it is the angle formed by the helix or spiral of the thread at its pitch point. For gears, it is the angle between the axis of a helical gear and an imaginary line that is tangent to the gear tooth.

Least Material Condition

LMC. 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

The largest diameter feature of a workpiece. On a thread, it is the diameter from crest to crest of an external thread or from root to root of an internal thread. On a gear, it is the widest diameter formed by the gear.

Maximum Material Condition

MMC. 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. These surfaces form a feature of size in GD&T, establishing the median plane between them.

Minor Diameter

The smallest diameter feature of a workpiece. On a thread, the minor diameter is the diameter from root to root of an external thread or from crest to crest of an internal thread. On a gear, it is the base diameter matching the root of the teeth.

No Go Gage

A gage on or in which a good part should not fit. A nogo gage quickly checks a part's features without providing a measurement value.

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

A diameter found on both threads and gears that is measured using pitch points. On a thread, it is the diameter between the pitch points taken in the groove between the threads. On a gear, it is the diameter of the circle passing through the pitch points on the gear teeth.

Projected Tolerance Zone

A tolerance zone that transfers beyond a feature by a specified distance. Symbolized by a "P" enclosed in a circle, projected tolerance zones help ensure that mating parts fit during assembly.

Radius

The distance from a center point to a point on a circle or arc. In GD&T, radius creates a curved feature formed by identifying a uniform distance from a center point to the edge of a circle or arc.

Regardless of Feature Size

RFS. 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.

Regardless of Material Boundary

RMB. A lack of material condition modifier that indicates the stated datum reference applies regardless of its actual size within an acceptable size limit. Rule #2 of GD&T states that all tolerances are RFS and all datum references are RMB, unless a material condition modifier is specified.

Resultant Condition

RC. A constant "worst case" imaginary boundary used for wall thickness concerns that is generated by the collective effects of a feature's size and geometric tolerance, including bonus tolerance. For an external feature, the RC is an inner boundary. For an internal feature, the RC is an outer boundary.

Rule #1

Also called the Taylor Principle or the Envelope Principle, 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. As the material of the feature of size decreases, variation is allowed.

Rule #2

The GD&T rule stating that, for all applicable geometric tolerances, the regardless of feature size modifier applies to the individual tolerance, datum reference, or both where no modifier symbol is specified. Rule #2 addresses how features and datums relate to one another.

Section Line

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

SingleSegment Tolerancing

A combination of more than one geometric tolerance applied to the same feature using a multisegment feature control frame. Composite tolerancing is used with position or profile tolerances, and each segment receives its own tolerance symbol.

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.

Standard Deviation

A number representing the degree of variation within a numerical set. The lower the standard deviation, the more reliable the numerical data.

Statistical Process Control

SPC. The use of statistics and control charts to measure key quality characteristics and control how the related process behaves. In SPC, sample data allows predictions of the overall process.

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.

Taylor Principle

Also called the Envelope Principle or 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. As the material of the feature of size decreases, variation is allowed.

Virtual Condition

VC. A constant "worst case" imaginary boundary used when parts are toleranced for assembly that is defined by the collective effects of a feature's size and geometric tolerance. For an external feature, the VC is the outer boundary. For an internal feature, the VC is the inner boundary.
