GEOMETRIC DIMENSIONING AND TOLERANCING (GD&T)

MAIN FEATURES OF GD & T

DEFINITION: -

                     GD & T is a systematic way of representing the dimensions and providing additional tolerances, geometric characteristics to the dimensions accordingly.

WHY IT IS USED:-

                  To avoid wastage of parts while assembling, due to minor error in dimensions.

BASIC RULES OF GD & T:-       

1. Limits of size of a feature control the amount of variation in size and geometric form.

2.  RFS (Regardless of Feature of Size) applies with respect to the individual tolerance, datum             reference, or both, where no modifying symbol is required. MMC or LMC must be specified on the drawing wherever required.

FEATURE CONTROL FRAME:-

                                   Feature Control Frame is used for representing the tolerances, bonus tolerances or modifiers, geometric characteristic and datum. This frame may or may not be attached with dimensions. In the frame, the arrangement of datum follows in descending order. Datum is used for giving constraints to the part, so as to restrict the motion of body. The primary, secondary and tertiary datum is applied to the body by following the 3-2-1 principle.

GEOMETRIC CHARACTERISTIC SYMBOLS:-

                                 The symbols in the given table are used as geometric characteristic symbols in GD&T. Altogether; there are 14 types of geometric characteristics as per ASME standards. The classification of tolerances is based on the use of primary, secondary or tertiary datums for the specific geometric characteristics. Form tolerances are specified in feature control frame without datums, whereas profile tolerances may be specified with or without datums. Orientation, location and runout tolerances are always specified in relation with datums.

MODIFIERS USED WITH GEOMETRIC CHARACTERISTIC SYMBOLS:-

                    Apart from the geometric characteristics, there are modifiers which are used along with dimensions or feature control frame. Datum modifiers are the ones which are shown along with datum, so as to mention that modifiers like MMC (Maximum Material Condition) and LMC (Least Material Condition) are used in specified datum. MMC or LMC are also used in tolerance zone of feature control frame to provide bonus tolerances to the dimensions.          

                                          Projected tolerance zone modifier is used when non-perpendicular threaded or press-fit holes could cause interference with mating parts. Free State modifier is used to describe distortion of part after removal of force applied during manufacture. Tangent plane is a modifier for a theoretical plane derived from geometric counterpart of the specified feature surface. All other modifiers are additional symbols used to describe nature of dimensions. 
                  Diameter and radius are used for specifying parameters of hole or shaft or similar bodies. Spherical diameter and spherical radius are used for specifying diameter and radius of sphere. Controlled radius creates a tolerance zone defined by two arcs that are tangent to adjacent surface. Statistical tolerance is a tolerance assigned to related components of an assembly on the basis of statistics.     

APPLICATION OF GD&T FOR A SAMPLE DRAWING:-