7.3a Tolerances
Tolerance
A tolerance is an acceptable amount of dimensional variation that will still allow an object to function correctly. •limit dimensions •bilateral tolerance •unilateral tolerance
Transition Fit
occurs when two mating parts can sometimes have a clearance fit and sometimes have an interference fit
Maximum material condition (MMC)
the condition of a part when it contains the largest amount of material. -The MMC of an external feature, e.g., the length of a plate, is the upper limit of the dimension -The MMC of an internal feature, e.g., the diameter of a hole, is the lower limit of the dimension
Least material condition (LMC)
the condition of a part when it contains the smallest amount of material. -The LMC of an external feature, e.g., the length of a plate, is the lower limit of the dimension -The LMC of an internal feature, e.g., the diameter of a hole, is the upper limit of the diameter dimension
Allowance
the minimum clearance or maximum interference between parts Allowance = MMC internal feature -MMC external feature
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
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
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
Lower Limit
Specified Dimension + negative variance 1.45 = 1.50 + (-0.05)
Upper Limit
Specified Dimension + positive variance 1.55 = 1.50 + 0.05
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
Large tolerance
may affect functionality of part -Specify tolerances to ensure proper function
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.
Limits
The maximum and minimum sizes shown by the toleranced dimension
Specified Dimension
The target dimension from which the limits are calculated
General tolerances
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
Tolerance
Upper Limit -Lower Limit 0.10 = 1.55 -1.45
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
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
Note About Dimension Tolerance
•In general, the more significant figures in the dimension, the tighter the tolerance •Overly precise dimensions and overly tight tolerances increase manufacturing costs •Specify dimensions only to the precision and tolerance necessary for the part to function properly
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