Tolerances Introduction to Engineering Design 2012 Project Lead The Way, Inc. Tolerances Variation is unavoidable No two manufactured parts are identical some degree of variation will exist
Tolerances are used in production drawings to control the manufacturing process and control the variation between copies of the same part In particular, tolerances are applied to mating parts in an assembly One advantage in using tolerances is that interchangeable parts can be used Tolerances Large tolerance may affect functionality of part
Specify tolerances to ensure proper function Small tolerance will affect the cost of the part Cost generally increases with smaller tolerances Will require precise manufacturing Will require quality control with inspection and rejection of parts Do not specify a tolerance that is smaller than necessary!
Tolerances ANSI/ASME Standard Y14.5 Each dimension shall have a tolerance, except those dimensions specifically identified as reference, maximum, minimum, or stock. The tolerance may be applied directly to the dimension or indicated by a general note located in the title block of the drawing.
Tolerances A tolerance is an acceptable amount of dimensional variation that will still allow an object to function correctly. Tolerances Three basic
tolerances that occur most often on working drawings are: limit dimensions bilateral tolerance unilateral tolerance Limit Dimensions Provide an upper
limit and lower limit for the dimension. Any size between or equal to the upper limit and/or lower limit is allowed The upper limit dimension is 0.126 The lower limit dimension is 0.125
Bilateral Tolerance Provides an equal allowable variation, larger and smaller Uses a plus/minus () symbol to specify the allowable variation Counter bore depth can be .003 larger or
smaller than .25 Hole location can be .05 larger or smaller than 1.50 Unilateral Tolerance Provides an allowable variation in only one direction (either larger or
smaller) Uses separate plus (+) and minus () signs The hole diameter may vary .004 larger but may not be smaller than .500 Tolerances
Identify the type of tolerance displayed in red Limit dimensions Bilateral Unilateral Definitions Specified Dimension is the target
dimension from which the limits are calculated Specified dimension 1.50 Definitions Limits are the maximum and minimum sizes shown by the toleranced dimension Upper limit is the maximum allowable dimension Upper Limit = Specified Dimension + positive variance
1.55 = 1.50 + 0.05 Lower limit is the minimum allowable dimension Lower Limit = Specified Dimension + negative variance 1.45 = 1.50 + ( 0.05) Definitions Tolerance is the total variance in a dimension and is equal to the difference between the upper and lower limits.
Tolerance = Upper Limit Lower Limit 0.10 = 1.55 1.45 Calculating Tolerance Tolerance .010 - .05 1.45 Lower Limit
+ .05 1.50 1.55 Upper Limit Tolerance = Upper Limit Lower Limit 0.10 = 1.55 1.45
General Tolerances General tolerances are tolerances that are assumed if no specific tolerance is given for a dimension Typically tolerances are specified based on the number of digits to the right of the decimal point in a dimension Shown on drawing Linear Dimensions Angles = .5
X.X = .020 X.XX = .010 X.XXX = .005
General Tolerances Upper Limit = 3.00 + 0.010 = 3.010 Lower Limit = 3.00 + - 0.010 = 2. 990 Tolerance = Upper Limit Lower Limit = 3.010 2.990 = 0.020 Out of Tolerance A manufactured part is said to be out of
tolerance if the part is not within specified limits Manufacturing facilities often institute quality control measures to help ensure that parts are within tolerance Types of Fit Clearance Fit limits the size of mating parts so that a clearance always results when mating parts are assembled
Interference Fit limits the size of mating parts so that an interference always results when mating parts are assembled Transition fit occurs when two mating parts can sometimes have a clearance fit and sometimes have an interference fit Types of Fit Clearance Fit Always a clearance between the
axle and the opening Types of Fit Interference Fit - Always an interference between the axle and the opening Transition Fit sometimes and interference sometimes a clearance