Major Rules of GD&T 311
Major Rules of GD&T provides an overview of the rules and concepts of geometric dimensioning and tolerancing, or GD&T. The major rules include Rule #1, or the Envelope Principle, which specifies how size controls form, Rule #2, or RFS and RMB defaults, and the 321 Rule, which defines the minimum number of points of contact in the datum reference frame. In addition to these major rules, other concepts described are bonus tolerances, virtual and resultant conditions, and the components of the datum reference frame. GD&T functions as a complex language used in blueprints to convey necessary information about a part. GD&T standards offer specific guidelines for part features, including projected tolerance zones, radii, controlled radii, tapers, threads, and gears. To accurately read a GD&T print, users must understand its many rules and principles. After taking this class, users should be able to explain key GD&T concepts and approaches for part inspection.

Difficulty Advanced

Format Online

Number of Lessons 24

Language English
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 Geometric Dimensioning and Tolerancing
 Key Terminology of GD&T
 Rule #1: Size Controls Form
 Rule #1: An Example
 Gaging for Rule #1
 Rule #2
 Review: Rules of GD&T
 Bonus Tolerance
 Virtual and Resultant Conditions
 Projected Tolerance Zones
 Review: Fit of Mating Parts
 The 321 Rule
 Demonstrating the 321 Rule
 Datum Feature Simulators
 Datum Selection Criteria
 Datum Review
 Datum Targets
 Datum Target Symbols
 Composite and SingleSegment Tolerancing
 Statistical Tolerancing
 Review: GD&T Terminology
 Dimensioning Radii
 Dimensioning Conical and Flat Tapers
 Dimensioning Threads and Gears
 Describe geometric dimensioning and tolerancing.
 Explain key terms used to describe GD&T relationships.
 Define Rule #1.
 Describe how Rule #1 affects the tolerancing of a feature.
 Describe how Rule #1 affects the gaging of a feature.
 Define Rule #2.
 Describe a bonus tolerance.
 Describe virtual condition and resultant condition.
 Describe the characteristics of a projected tolerance zone.
 Describe the 321 rule and how it restricts the six degrees of freedom for a part.
 Describe the 321 rule and how it restricts the six degrees of freedom for a part.
 Define datum feature simulator.
 Describe how the feature control frame locates a part in the datum reference frame.
 Describe datum targets and their effect on gaging.
 Describe datum targets and their effect on gaging.
 Describe composite and singlesegment tolerancing.
 Describe statistical tolerancing and its role in GD&T.
 Describe a radius and a controlled radius.
 Describe dimensioning rules for conical and flat tapers.
 Describe dimensioning rules for threads and gears.
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 actual mating envelope 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 14.5
The standard for geometric dimensioning and tolerancing published by the American Society of Mechanical Engineers (ASME). The ASME 14.5 2018 revision is the most recent version of the standard.
attribute data
Data that represents a characteristic or individual count. Attribute data can be gathered from gaging.
axis
A theoretical 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 applied to a feature as its size shifts from a stated material condition. Bonus tolerance is permissible as a hole deviates from MMC to LMC.
caliper
A measuring instrument with a pair of jaws on one end and a long beam containing a marked scale of unit divisions. The jaws can measure both internal and external features.
center point
A single theoretical 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 tapers
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."
coordinate measuring machine
CMM. A sophisticated measuring instrument that uses a suspended probe to measure parts in threedimensional space. Coordinate measuring machines operate using either contact or noncontact methods.
datum feature simulator
A gaging surface, machine component, or other device that establishes a datum plane in the datum reference frame. Datum feature simulators adhere to gaging tolerances and act as a reasonable substitute for the datum reference frame.
datum feature symbols
A GD&T symbol in a print indicating a part feature that acts as a datum feature. Datum feature symbols represent features that contact a datum reference frame simulator.
datum features
A physical feature that acts as an acceptable substitute for a datum. Datum features relate the various features of the part to each other.
datum reference frame
Three theoretical planes perpendicular to one another that are mapped onto the part. The datum reference frame 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.
datums
A point of reference for machine tools, programs, and fixtures from which measurements are taken. A datum can be a hole, line, or any threedimensional shape.
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
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 feature without size can be a single flat surface.
features
A defining physical characteristic. Features include shapes, lines, and other elements machined into a workpiece.
fixed CMMs
A stationary coordinate measuring machine. Fixed CMMs have a large base with a suspended probe and are operated in a controlled environment.
flat tapers
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. A flexible datum feature simulator is the coordinate measuring machine.
form tolerance
A geometric tolerance that limits the amount of error in the shape of a feature. The form tolerances include straightness, flatness, circularity, and cylindricity.
FOS
Feature of size. A cylindrical surface, spherical surface, or two opposed parallel elements or surfaces that can be associated with a size dimension. A FOS establishes an axis, median plane, or center point.
functional gage
A gage representing a worstcase 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.
GD&T
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.
gear
A round or cylindrical mechanical component with teeth that is used to transmit power. Gears are designed to mesh with one another in order to alter the speed, torque, or direction of mechanical energy.
geometric dimensioning and tolerancing
GD&T. An international standard for communicating instructions about the design and manufacturing of parts. Geometric dimensioning and tolerancing 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 feature simulator
A datum reference frame simulator that is rigidly fixed in place in one or more planes. Hard datum feature simulators can be the combination of a surface plate and angle plate.
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, and for gears, it is the angle between the axis of a helical gear and an imaginary line that is tangent to the gear tooth.
independency symbol
An indication that a regular feature of size does not require perfect form at maximum material condition or least material condition. The independency symbol overrides Rule #1 of GD&T.
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.
LMC
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.
location tolerance
A geometric tolerance that limits the location or placement of features. Location tolerances are related tolerances.
major diameter
The largest diameter feature of a workpiece. Major diameter is measured from crest to crest on an external thread, from root to root on an internal thread a gear, and is the widest diameter formed by the gear.
material condition modifiers
Defines the tolerance of a feature in relation to its acceptable size limits. There are three material condition modifiers in GD&T: maximum material condition, least material condition, and regardless of feature size.
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 maximum material condition.
median plane
A theoretical, 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.
micrometer
A Ushaped measuring instrument with a threaded spindle that slowly advances toward a small anvil. Micrometers are available in numerous types for measuring assorted dimensions and features.
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, from crest to crest of an internal thread, and the base diameter matching the root of the teeth on a gear
MMC
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.
modifiers
An element that communicates information about the tolerance of a feature. In GD&T, each modifier has a symbol that distinguishes it from other modfiers on a blueprint.
multiple singlesegment tolerancing
A combination of more than one geometric tolerance applied to the same feature using a multisegment feature control frame. Singlesegment tolerancing is used with position or profile tolerances, and each segment receives its own tolerance symbol.
no go gage
A gage on or in which a good part should not fit. A no go gage quickly checks a part 's features without providing a measurement value.
orientation tolerance
A geometric tolerance that limits the direction, or orientation, of a feature in relation to other features. Orientation tolerances are related tolerances.
phantom line
A line drawn by alternating long dashes followed by two short dashes. A phantom line can indicate a datum target line in a GD&T print.
pitch diameter
The diameter of an imaginary cylinder that passes through the thread form in such a way that it intersects the thread groove and the thread ridge equally. A gear's pitch diameter can be determined by measuring from the top of one gear tooth to the bottom of the opposite gear tooth.
planar
Having the character of a flat surface. Planar surfaces are often used as datum features.
plug gage
A hardened, cylindrical gage used to inspect the size of a hole. Plug gages are available in standardized diameters.
portable CMMs
A coordinate measuring machine that can be easily carried or moved. Portable CMMs can be used on the factory floor.
process capability index
Cpk. The ability of a process to produce output within tolerances. Process capability index is used in statistical process control to measure variation in processes.
profile of a surface
A threedimensional profile tolerance that describes the allowable variability in the contour of a surface. Profile of a surface can be either an individual or a related tolerance.
profile tolerance
A geometric tolerance that controls the size, location, orientation, and form of a feature. Profile tolerances can be either individual or related.
projected tolerance zone
A tolerance zone that extends 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 forms a curved feature formed by identifying a uniform distance from a center point to the edge of a circle or arc.
RC
Resultant condition. A constant worst case theoretical 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 resultant condition is an inner boundary, and for an internal feature, the resultant condition is an outer boundary.
reference dimension
A dimension that is provided for informational purposes only. Reference dimensions are enclosed in parentheses.
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. Regardless of feature size does not permit bonus tolerance.
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. Regardless of feature size is no longer listed on drawings since it is the assumed default material condition modifier.
regardless of material boundary
RMB. A lack of material condition modifier that indicates the stated tolerance for a datum 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 worstcase theoretical 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 resultant condition is an inner boundary, and for an internal feature, the resultant condition is an outer boundary.
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, and as the material of the feature of size decreases, variation is allowed. Rule #1 is also called the Taylor Principle or the Envelope Principle.
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.
runout tolerance
A geometric tolerance that simultaneously limits the form, location, and orientation of cylindrical parts. Runout tolerances are related tolerances requiring a datum axis.
section lines
One of 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.
SPC
Statistical process control. A method of measuring and controlling the processes that yield a product. In SPC, statistics are used to collect sample data and predict outcomes.
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. A method of measuring and controlling the processes that yield a product. In statistical process control, statistics are used to collect sample data and predict outcomes.
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.
taper
The uniform increase or decrease in the width or diameter of a part or a part feature. Tapers are often manufactured based on industry standards.
thread
A long, spiral ridge around the exterior or interior of a cylindrically shaped object. Threads are used to fasten or provide motion.
tolerance zones
A theoretical zone in which a part feature must be completely contained for the part to pass inspection. This zone contains the dimensions between the maximum and minimum limits of a feature's location.
tolerances
An unwanted but acceptable variation or deviation from a desired dimension. A part within tolerances will meet specifications and pass inspection.
twopoint inspection device
A tool used to measure the distance between two opposite sides of an object. Examples of twopoint inspection devices include calipers and micrometers.
variable data
Data that can be measured on a scale and compared with other data. Variable data, also known as continuous data, can be added to or subtracted from other variable data sets.
VC
Virtual condition. A constant worst case theoretical 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, and for an internal feature, the virtual condition is the inner boundary.
virtual condition
VC. A constant worstcase theoretical 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 virtual condition is the outer boundary, and for an internal feature, the virtual condition is the inner boundary.
X axis
The linear axis representing sidetoside movement or coordinate position in a feature, relative to the origin. Xaxis movement is restricted by the primary and tertiary datums.
Y axis
The linear axis representing backandforth movement or coordinate position in a feature, relative to the origin. Coordinate tolerancing uses the x and y axes to describe location. Yaxis movement is restricted by the primary and secondary datums.
Z axis
The linear axis representing upanddown movement or coordinate position in a feature, relative to the origin. Zaxis movement is restricted by the primary and secondary datums.