Geometric Dimensioning Geometric Dimensioning and Tolerancingand Tolerancing
GD&TGD&T
What is GD & T?What is GD & T? Geometric dimensioning and tolerancing is an international Geometric dimensioning and tolerancing is an international
language used on drawings to accurately describe a part. language used on drawings to accurately describe a part. The language consists of a well-defined set of symbols, The language consists of a well-defined set of symbols, rules, definitions, and conventions that can be used to rules, definitions, and conventions that can be used to describe the size, form, orientation, and location tolerances describe the size, form, orientation, and location tolerances of part features. of part features.
Geometric tolerancing is an exact language that enables Geometric tolerancing is an exact language that enables
designers to “say what they mean” on a drawing, thus designers to “say what they mean” on a drawing, thus improving product designs. Production uses the language to improving product designs. Production uses the language to interpret the design intent, and inspection looks to the interpret the design intent, and inspection looks to the language to determine set up. By providing uniformity in language to determine set up. By providing uniformity in drawing specifications and interpretation, GD&T reduces drawing specifications and interpretation, GD&T reduces controversy, guesswork, and assumptions throughout the controversy, guesswork, and assumptions throughout the manufacturing and inspection process. manufacturing and inspection process.
Why do I need GD & T?Why do I need GD & T?
Whenever two parts are expected to Whenever two parts are expected to fit together and function without re-fit together and function without re-work or adjustment, the parts must work or adjustment, the parts must be clearly defined. be clearly defined.
You can’t build what you can’t You can’t build what you can’t measure because you don’t know measure because you don’t know when you’re finished.when you’re finished.
Learning ObjectivesLearning Objectives Label datum features on a drawingLabel datum features on a drawing Place proper feature control frames on Place proper feature control frames on
drawings, establishing geometric drawings, establishing geometric tolerances tolerances
Establish basic dimensions where Establish basic dimensions where appropriate appropriate
Use and interpret material condition Use and interpret material condition symbols symbols
Use and interpret datum targets and Use and interpret datum targets and areasareas
DatumDatumDatums are considered to be theoretically perfect Datums are considered to be theoretically perfect
surfaces, planes, points or axes to which surfaces, planes, points or axes to which additional material conditions are referenced.additional material conditions are referenced.
Feature Control FrameFeature Control Frame
Material ConditionsMaterial Conditions
ParallelismParallelism - is the - is the condition of a condition of a surface, line, or axis, surface, line, or axis, which is equidistant which is equidistant at all points from a at all points from a datum plane or axis.datum plane or axis.
PerpendicularityPerpendicularity - is the - is the condition of a surface, condition of a surface, axis, or line, which is 90 axis, or line, which is 90 deg. From a datum plane deg. From a datum plane or a datum axis. or a datum axis.
Material Conditions (cont’d.)Material Conditions (cont’d.)
Material Conditions (cont’d.)Material Conditions (cont’d.)
AngularityAngularity - is the - is the condition of a surface, condition of a surface, axis, or center plane, axis, or center plane, which is at a specified which is at a specified angle from a datum angle from a datum plane or axis. plane or axis.
ConcentricityConcentricity - - describes a describes a condition in condition in which two or which two or more features, more features, in any in any combination, combination, have a common have a common axisaxis
Material Conditions (cont’d.)Material Conditions (cont’d.)
StraightnessStraightness - a condition where an element of a surface or an - a condition where an element of a surface or an
axis is a straight line. 2D axis is a straight line. 2D FlatnessFlatness - is the condition of a surface having all elements in - is the condition of a surface having all elements in
one plane. 3Done plane. 3D
RoundnessRoundness - describes the condition on a surface of revolution - describes the condition on a surface of revolution (cylinder, cone, sphere) where all points of the surface (cylinder, cone, sphere) where all points of the surface intersected by any plane. 2Dintersected by any plane. 2D
CylindricityCylindricity - describes a condition of a surface of revolution in - describes a condition of a surface of revolution in which all points of a surface are equidistant from a common which all points of a surface are equidistant from a common axis. 3Daxis. 3D
Profile of a LineProfile of a Line - is the condition permitting a uniform amount - is the condition permitting a uniform amount of profile variation, ether unilaterally or bilaterally, along a of profile variation, ether unilaterally or bilaterally, along a line element of a feature. 2Dline element of a feature. 2D
Profile of a SurfaceProfile of a Surface - is the condition permitting a uniform - is the condition permitting a uniform
amount of profile variation, ether unilaterally or bilaterally, amount of profile variation, ether unilaterally or bilaterally, on a surface. 3Don a surface. 3D
Material Conditions (cont’d.)Material Conditions (cont’d.)
All Around SymbolAll Around Symbol - indicating that a tolerance applies to surfaces all around the part. - indicating that a tolerance applies to surfaces all around the part. Position TolerancePosition Tolerance (True Position)- defines a zone within which the axis or center plane (True Position)- defines a zone within which the axis or center plane
of a feature is permitted to vary from true (theoretically exact) position. of a feature is permitted to vary from true (theoretically exact) position. SymmetrySymmetry - is a condition in which a feature (or features) is symmetrically disposed - is a condition in which a feature (or features) is symmetrically disposed
about the center plane of a datum feature.about the center plane of a datum feature. RunoutRunout - is the composite deviation from the desired form of a part surface of revolution - is the composite deviation from the desired form of a part surface of revolution
through on full rotation (360 deg) of the part on a datum axis. through on full rotation (360 deg) of the part on a datum axis.
Total Runout Total Runout - is the simultaneous composite control of all elements of a surface at all - is the simultaneous composite control of all elements of a surface at all circular and profile measuring positions as the part is rotated through 360.circular and profile measuring positions as the part is rotated through 360.
Datum TargetDatum Target - is a specified point, line, or area on a part that is used to establish the - is a specified point, line, or area on a part that is used to establish the Datum Reference Plane for manufacturing and inspection operations. Datum Reference Plane for manufacturing and inspection operations.
Target Point - Target Point - indicates where the datum target point is dimensionally located on the indicates where the datum target point is dimensionally located on the direct view of the surface.direct view of the surface.
Target Area - Target Area - indicates where the datum target area is dimensionally located on the indicates where the datum target area is dimensionally located on the direct view of the surface.direct view of the surface.
Material Conditions (cont’d.)Material Conditions (cont’d.)
Maximum Material Condition (MMC) Maximum Material Condition (MMC) - is that condition of a part feature - is that condition of a part feature wherein it contains the maximum amount of material within the stated limits wherein it contains the maximum amount of material within the stated limits of size. That is: minimum hole size and maximum shaft size. of size. That is: minimum hole size and maximum shaft size. (Condition (Condition where part weighs the most)where part weighs the most)
Least Material Condition (LMC)Least Material Condition (LMC) - implies that condition of a part feature of - implies that condition of a part feature of size wherein it contains the least (minimum) amount of material, examples, size wherein it contains the least (minimum) amount of material, examples, largest hole size and smallest shaft size. It is opposite to maximum material largest hole size and smallest shaft size. It is opposite to maximum material condition.condition.
Basic DimensionBasic Dimension - used to describe the exact size, profile, orientation or - used to describe the exact size, profile, orientation or location of a feature. A basic dimension is always associated with a feature location of a feature. A basic dimension is always associated with a feature control frame or datum target. control frame or datum target.
Reference DimensionReference Dimension - a dimension usually without tolerance, used for - a dimension usually without tolerance, used for information purposes only. It does not govern production or inspection information purposes only. It does not govern production or inspection operations. operations.
Feature Control FrameFeature Control Frame - is a rectangular box containing the geometric - is a rectangular box containing the geometric characteristics symbol, andcharacteristics symbol, andthe form, run out or location tolerance. If necessary, datum references and the form, run out or location tolerance. If necessary, datum references and modifiers applicable to the feature or the datums are also contained in the modifiers applicable to the feature or the datums are also contained in the box. box.
Material Conditions (cont’d.)Material Conditions (cont’d.)
Datum targets are typically used in Datum targets are typically used in situations where it is inappropriate to situations where it is inappropriate to specify an entire surface as a datum specify an entire surface as a datum feature. feature.
There are six datum targets shown in There are six datum targets shown in this diagram. Four of these datum this diagram. Four of these datum targets are datum target points, targets are datum target points, each of which is represented by an each of which is represented by an ×. The other two datum targets are ×. The other two datum targets are datum target areas, each of which is datum target areas, each of which is represented by a cross-hatched represented by a cross-hatched circular area. circular area.
Datum Targets, Target Areas and Datum Targets, Target Areas and Target PointsTarget Points
Material Conditions and PositionMaterial Conditions and Position
When the MMC symbol appears after a geometric tolerance number, it means that the given tolerance only applies when the feature is made at its MMC. So if a hole is given a size of 12.0 – 12.1, and also a position tolerance of Ø.02, it means that the position of .02 is to be held if the hole is made to a size of 12.0 (its MMC)! But suppose that we make a hole of 12.05. This is not the MMC size, but it is still within legal range. So here's where it gets interesting -- a hole of 12.05 has deviated from MMC by .05 inch. So we can adjust the position tolerance by .05 also! The print said position of Ø.02, but our part really gets a position tolerance of Ø.07! This trend continues until the hole reaches its LMC (12.1); at that size the position tolerance would be Ø.12. This is the original Ø.02 plus a "bonus" of .10, which comes from the deviation in hole size. Essentially, it boils down to this: a smaller hole has to be positioned pretty accurately, but as the hole gets larger, its center may deviate more from the true position.
Revising a Drawing with GD&TRevising a Drawing with GD&T
Revising a Drawing with GD&T Revising a Drawing with GD&T (cont’d.)(cont’d.)
