 The choice of the most suitable dimensioning method depends on how the part will be produced  Unit production: each part is made separately  Mass production: parts are produced in quantity

 Linear dimensions are measured paralle or perpendicular to reference axes or datum planes that are perpendicular to one another (fig.8-36)  Arbitrary points (fig.8-37,8-38)  Without dimension lines (fig. 8-39)  Tabular dimensioning (fig.8-40)

 Circular planes or circular configurations of features (fig.8-42a)  Position is determined by a linear dimension and angle  Chordal dimensioning (fig.8.42b)

 A series of dimensions is applied on a point-to- point basis (fig. 8-44)  Undesirable accumulation of tolerances between individual features can result

 Several dimensions emanate from a common reference point or line (fig.8-45)  Parallel method  Superimposed method-dimensions should be placed near the arrowhead in line with the corresponding extension line and the origin is indicated by a circle (fig.8-46)

 Why?  Exact dimensions and shapes can not be attained in the manufacture of materials and products  Slight variations in size can be tolerated without impairing its function  Interchangeable parts need not be identical  Restrict the variations with limits and tolerances

 Definition: Tolerances are the permissible variations in the specified form, size, or location of individual features of a part from that shown in the drawing.  Definition: Limits are the largest and smallest permissible sizes

 Actual size-the measured size  Basic size-the theoretical size from which the limits are derived  Nominal size-the designation used for the purpose of general identification  Tolerance-tolerance of a dimension is the total permissible variation in size of a dimension…the mathematical difference between the limits of size

 Bilateral tolerance-variation is permitted in both directions from the specified dimension [1.5±.004]  Unilateral tolerance-variation is permitted in only one direction from the specified dimension [ ]  Maximum material size-the limit of size of a feature that results in the part containing the maximum amount of material (GDT)

 Limits of tolerance directly on the drawing (fig.8-48)  +/- tolerancing (equal and unequal)  General tolerance note referring to all dimensions or specific dimensions

 The high limit (maximum value) is placed above the low limit (minimum value)  When expressed in a single line, the low limit precedes the high limit and they are seperated by a dash  The digits to the right of the decimal place should match for both limits (English & SI)

 Specified size given first followed by a +/- expression of tolerancing.  Plus value placed above minus value

 The dimension need not be shown to the same number of decimal places as its tolerance  1.5±0.04 NOT 1.50 ±0.04  10 ±0.1 NOT 10.0 ±0.1  Metric bilateral tolerances-both +/-must have the same number of decimal places using zeros if necessary  NOT  When a unilateral tolerance is used and either value is nil, a single zero is used 

 The dimension is given to the same number of decimals as its tolerance .500 ±.004NOT.50 ±.004  NOT

 A chain of tolerances can build up a cumulative tolerance between surfaces that have an important relation to one another (fig.8-52)  Chain (greatest tolerance accumulation)  Datum (lesser tolerance accumulation)  Direct (least tolerance accumulation)

 Design intent  Completely describe part geometry  Avoid unnecessary accumulation of tolerances  Manufacturing methods are not specified  Dimension to visible lines (NOT hidden lines)  In general, place dimensions outside the outline of the part and between views  Dimensions should be aligned, if practicable, and should be grouped for uniform appearance