Major Rules of GD&T 311

The class “Major Rules of GD&T” provides an overview of the rules and concepts of geometric dimensioning and tolerancing, or GD&T, including Rule #1, Rule #2, bonus tolerance, the 3-2-1 Rule, and virtual and resultant conditions, as well as the datum reference frame (DRF). The DRF limits all six degrees of freedom by mapping three perpendicular planes onto the part using datum feature simulators. GD&T standards offer specific guidelines on a wide range of part features, including projected tolerance zones, radii, controlled radii, tapers, threads, and gears. Special rules also apply for composite or single segment tolerancing and statistical tolerancing. GD&T functions as a complex language used in blueprints to convey all necessary information about a part. To accurately read a GD&T print, the student must understand its many guidelines. After taking this class, users should be able to explain key GD&T concepts and various approaches for situating a part within the DRF.

Class Details

Class Name:
Major Rules of GD&T 311
Version:
2.0
Difficulty:
Advanced
Number of Lessons:
24
Related 1.0 Class:
Interpreting GD&T 315 (2009)

Class Outline

  • Key Terminology of GD&T
  • Rule #1
  • Rule #1: An Example
  • Gaging for Rule #1
  • Rule #2
  • The Rules of GD&T
  • Bonus Tolerance
  • Virtual and Resultant Conditions
  • Virtual and Resultant Conditions: Examples
  • Projected Tolerance Zones
  • The Fit of Mating Parts
  • The 3-2-1 Rule
  • Enacting the 3-2-1 Rule
  • Datum Feature Simulators
  • Datum Selection Criteria
  • Datum Order
  • Datum Targets
  • Datum Target Symbols
  • Composite and Single-Segment Tolerancing
  • Statistical Tolerancing
  • Terms
  • Dimensioning Radii
  • Dimensioning Conical and Flat Tapers
  • Dimensioning Threads and Gears

Objectives

  • Distinguish among 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 how bonus tolerance is applied to a hole.
  • Define virtual condition and resultant condition.
  • Identify the correct virtual condition formula for mating parts.
  • Describe the characteristics of a projected tolerance zone.
  • Explain how the 3-2-1 rule 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 how datum targets affect the gaging of a part.
  • Describe the characteristics of composite and single-segment tolerancing.
  • Describe how statistical tolerancing affects the gaging of a part.
  • Distinguish between a radius and a controlled radius.
  • Distinguish between dimensioning rules for conical and flat tapers.
  • Distinguish between dimensioning rules for threads and gears.

Job Roles

Certifications

NIMS
  • CNC Lathe Operations
  • CNC Milling Operations
  • CNC Milling Programming, Setup, & Operation
  • CNC Turning Programming, Setup, & Operation
  • Drill Press I
  • Grinding I
  • Job Planning, Benchwork, & Layout I
  • Measurement, Materials, & Safety I
  • Milling I
  • Turning Operations: Turning Chucking Skills

Glossary

Vocabulary Term Definition
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 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.
axes An imaginary straight line or circle used to describe the location or movement of an object in three-dimensional space. Three axes make up the Cartesian coordinate system: the X axis, Y axis, and Z axis.
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 multi-segment 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 with a flat polished table and a suspended probe that measures parts in three-dimensional space. CMMs can measure using either contact or noncontact methods.
Cpk A measure of process capability, Cpk measures the amount of variation 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 symbols 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.
DRF Datum reference frame. 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.
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 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. The CMM, which uses software to construct the datum planes during inspection, acts as a flexible datum reference frame simulator.
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 feature of size establishes an axis, median plane, or center point.
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.
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.
geometric dimensioning and tolerancing GD&T. 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 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.
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.
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.
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.
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.
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 lack of material condition modifier that indicates 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 tolerance for a datum applies regardless of its actual size within an acceptable size limit.
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, and 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 lines One of a series of diagonal lines drawn close together. Section lines can identify a datum target area in a GD&T print.
single-segment tolerancing A combination of more than one geometric tolerance applied to the same feature using a multi-segment feature control frame. Single-segment 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.
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, and for an internal feature, the VC is the inner boundary.