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Geometric Dimensioning and Tolerancing GD&T. What is GD & T?.
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What is GD & T? • Geometric dimensioning and tolerancing is an international language used on drawings to accurately describe a part. The language consists of a well-defined set of symbols, rules, definitions, and conventions that can be used to describe the size, form, orientation, and location tolerances of part features. • Geometric tolerancing is an exact language that enables designers to “say what they mean” on a drawing, thus improving product designs. Production uses the language to interpret the design intent, and inspection looks to the language to determine set up. By providing uniformity in drawing specifications and interpretation, GD&T reduces controversy, guesswork, and assumptions throughout the manufacturing and inspection process.
Why do I need GD & T? • Whenever two parts are expected to fit together and function without re-work or adjustment, the parts must be clearly defined. • You can’t build what you can’t measure because you don’t know when you’re finished.
Learning Objectives • Label datum features on a drawing • Place proper feature control frames on drawings, establishing geometric tolerances • Establish basic dimensions where appropriate • Use and interpret material condition symbols • Use and interpret datum targets and areas
Datum Datums are considered to be theoretically perfect surfaces, planes, points or axes to which additional material conditions are referenced.
Material Conditions Parallelism - is the condition of a surface, line, or axis, which is equidistant at all points from a datum plane or axis.
Material Conditions (cont’d.) Perpendicularity - is the condition of a surface, axis, or line, which is 90 deg. From a datum plane or a datum axis.
Material Conditions (cont’d.) Angularity - is the condition of a surface, axis, or center plane, which is at a specified angle from a datum plane or axis.
Material Conditions (cont’d.) Concentricity - describes a condition in which two or more features, in any combination, have a common axis
Material Conditions (cont’d.) Straightness - a condition where an element of a surface or an axis is a straight line. 2D Flatness - is the condition of a surface having all elements in one plane. 3D Roundness - describes the condition on a surface of revolution (cylinder, cone, sphere) where all points of the surface intersected by any plane. 2D Cylindricity - describes a condition of a surface of revolution in which all points of a surface are equidistant from a common axis. 3D Profile of a Line - is the condition permitting a uniform amount of profile variation, ether unilaterally or bilaterally, along a line element of a feature. 2D Profile of a Surface - is the condition permitting a uniform amount of profile variation, ether unilaterally or bilaterally, on a surface. 3D
Material Conditions (cont’d.) All Around Symbol - indicating that a tolerance applies to surfaces all around the part. Position Tolerance (True Position)- defines a zone within which the axis or center plane of a feature is permitted to vary from true (theoretically exact) position. Symmetry - is a condition in which a feature (or features) is symmetrically disposed about the center plane of a datum feature. Runout - 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. Total Runout - 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. Datum Target - is a specified point, line, or area on a part that is used to establish the Datum Reference Plane for manufacturing and inspection operations. Target Point - indicates where the datum target point is dimensionally located on the direct view of the surface. Target Area - indicates where the datum target area is dimensionally located on the direct view of the surface.
Material Conditions (cont’d.) Maximum Material Condition (MMC) - is that condition of a part feature wherein it contains the maximum amount of material within the stated limits of size. That is: minimum hole size and maximum shaft size. (Condition where part weighs the most) Least Material Condition (LMC) - implies that condition of a part feature of size wherein it contains the least (minimum) amount of material, examples, largest hole size and smallest shaft size. It is opposite to maximum material condition. Basic Dimension - used to describe the exact size, profile, orientation or location of a feature. A basic dimension is always associated with a feature control frame or datum target. Reference Dimension - a dimension usually without tolerance, used for information purposes only. It does not govern production or inspection operations. Feature Control Frame - is a rectangular box containing the geometric characteristics symbol, andthe form, run out or location tolerance. If necessary, datum references and modifiers applicable to the feature or the datums are also contained in the box.
Datum Targets, Target Areas and Target Points Datum targets are typically used in situations where it is inappropriate to specify an entire surface as a datum feature. There are six datum targets shown in this diagram. Four of these datum targets are datum target points, each of which is represented by an ×. The other two datum targets are datum target areas, each of which is represented by a cross-hatched circular area.
Material 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.
