Technical Drawing Program

General dimensioning rules and techniques are necessary for uniformity and clarity on Detail Drawings. Products or parts of products that must fit together or move within each other must be dimensioned more precisely using tolerances. The following terms and conditions are based on the ANSI B4.1 - 1967, R1987 Standard Fit Tables in Appendix B of your textbook and should serve as guides to good precision dimensioning techniques:

• The size of a part can be expressed in three ways: nominal size, design size or actual size.
  
• Nominal size is used to refer to the descriptive size of the part.
  Example: A 1"Ø steel rod that is actually .995"Ø or a 2" x 4" wooden stud that is actually 1.5" x 3.5".
  
• Design size is used to describe the optimum or ideal size of the part or parts before tolerances are applied.
  Example: A 2"Ø hole is to be drilled in a wheel for a 2"Ø axle with a loose fit. The hole would actually be dimensioned as 2.000" +/-.001" and the axle would be dimensioned as 1.995" +/-.001" for a clearance of .003" and an allowance of .007".
  
• Actual size is used to describe the measured size of the part or parts after they are produced. The hole in the wheel described above could be anywhere between 1.999" to 2.001" (maybe 1.9995") and the axle could be anywhere between 1.994" to 1.996" (maybe 1.9955").
  
• A tolerance is the total amount of variation in the size of a part or feature on a part. To find the tolerance of a part or feature substract the smallest size from the largest size.
  Example: Given a dimension of 2.50 +/- .05 - the largest size would be 2.55, the smallest size would be 2.45, and the tolerance would be .10.
  
• A bilateral tolerance is the amount of variation above or below the design size.
  Example: 3.75 +/-.05 which means the feature or parts can be as small as 3.70 or as large as 3.80.
  
• A unilateral tolerance is the amount of variation in one direction either above or below the design size.
  Example: 3.75 +.05/-.00 which means the feature or parts can be as small as 3.75 or as large as 3.80.
  
• Tolerance dimensions can be express as plus or minus (+/-) values or as limits (largest size over smallest size) values.
  Example: 10.50 +/- .05 or 10.55 / 10.45.
  
• Different tolerances or limits can be specified for each of the three types of dimensions:
  overall length, width and height limit dimensions might have a tolerance of +/- .05
  specific size limit dimensions of features might have a tolerance of +/- .025
  locational limit dimensions might have a tolerance of +/- .0125
  
• Generall tolerances can be specified on a drawing as a note using the number of decimal places to indicate the degree of accuracy.
  Example:
  .xx = +/- .05
  .xxx = +/- .025
  .xxxx = +/- .0125
  
• Tolerances are determined by industry standards and are influenced by the use of the part, the type of material the part is constructed from and how the part fits together with other parts.
  
• Two parts such as a wheel and an axle that must roll freely about each other will have an allowance (tightest fit) and a clearance (loosest fit). Example: If the axle size is .76 / .74 and the hole size is .80 / .78, the clearance would be .80 minus .74 or .06 and the allowance would be .78 minus .76 of .02.
  
• Datums are edges, surfaces or features on an object that are considered accurate starting points for referencing the location or size of other edges, surfaces or features. Datums are represented by a rectangle (.375" h. x .75" w.) with dashes and a letter -A- inside the rectangle. The datum symbol can be attached to an edge or surface by using an extension line or to feature by using a leader.
  
• The fit of two or more parts can be classified as:
  Clearance Fit - when the largest shaft is always smaller than the smallest hole
  Interferance Fit - when the sizes of the shaft is always larger than the sizes of the hole
  Transition Fit - when largest shaft is larger than the largest hole but the lsmallest shaft may fit in the largest hole
  Running or Sliding Fit - there are nine classes of running and sliding (RC) fits from RC1 (fit together, no play, slow speeds) to RC9 (fit loosely, higher operating temperatures, higher speeds)
  Force and Shrink Fits - there are five classes of force and shrink (FN) fits from FN1 (light drive fit and pressures, thin sections or long engagement) to FN5 (high stresses and pressures)
  Transition Locational Fit - there are six classes of transition locational (LT) fits from LT1 (small amount of interference and large amount of clearance) to LT6 (small amount of clearance and large amount of interference)
  Clearance Locational Fit - there are eleven classes of clearance locational (LC) fits from LC1 (small amounts of clearance) to LC11 (large amounts of clearance)
  Interferance Locational Fit - there are three classes of interference locational (LN) fits from LN1 (small amounts of intereference) to LN3 (large amounts of intereference)
  
• Least Material Condition - when the least material is available on the part (lower limit for a shaft or higher limit for a hole)
  
• Maximum Material Condition - when the most material is available on the part (higher limit for a shaft or lower limit for a hole)
  
• Basic Hole System - Tolerances are applied to a hole and a shaft using the nominal size of the hole as a starting point
  
• Basic Shaft Systems - Tolerances are applied to the shaft and hole using the nominal size of the shaft as a starting point
  

NOTE: The American National Standard, ANSI Y14.5M, for the "Dimensioning and Tolerancing of Engineering Drawings and Related Documentation Practices" should be adhered to for uniformity and acceptance by other concerns. It must be remembered that there are no absolutely hard and fast rules, nor any practice, not subject to change or modification under special conditions or requirements of a particular industry. When there is a variation of any rule, there must always be a reason which can be completely justified.