Clearance Holes

See this  conversion chart  for a comparison of a positional (cylindrical) tolerance zone to a coordinate tolerance zone. This is an excerpt from Lowell Foster's, Geo-Metrics II, 1986 edition.

 


	
			Hole size calculations

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   The formula for calculating hole sizes is the same, whether
          using true position or coordinate dimensioning.
   ______________________________________________________________________
         FLOATING CONDITION                     FIXED CONDITION                       
 
						 
	     H = T + S                           H = T1 + T2 + S            
						 
       H = min clearance hole               S = max fastener size
      T1 = tolerance on first part          T = tolerance on each
      T2 = tolerance on second part             mating part
    
   ----------------------------------------------------------------------    
 EXAMPLE:Coordinate tolerancing          True position tolerancing
    (relys on probability)          (takes all tolerances into account)
	 H = T + S                           H = T + S
	 H = .005 + .138                     H = .014 + .138
	 H = .142                            H = .152 
       HOLE SIZE = .142 MIN               HOLE SIZE = .152 MIN
	     (Select drill 1 to 2 sizes over min hole)
 
To convert true position to coordinate: Multiply .70711 times 1/2 the total
true position tolerance zone[.e.g. .70711x.5(.014)=.0049 OR (+/- .005)]

To convert coordinate to true position: Multiply 1.4142 times twice the 
coordinate tolerance. e.g. 1.4142 x 2(.005) = .0141 OR (.014)

It should be noted that the industry is leaning towards the latest
revision of ANSI-Y14.5, therefore; the true position tolerancing
method is preferred.
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Conversion Chart

 

 

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