Tolerancing Engr 135

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TolerancingEngr 135
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Tolerance Theory

• Tolerances are used to control the variation that
  exists on all manufactured parts.
• The amount that each part is allowed to vary in
  size and shape depends on the end use of the
  part.
• How would the tolerances of parts for a
  childrens bicycle compare to the tolerances of
  parts for a supersonic jet aircraft?

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Tolerance Values

• The dimension for a part is given as
• 4.250 0.002
• What is the upper limit (largest value) allowable
  for the part?
• What is the lower limit (smallest value)
  allowable for the part?
• The upper and lower allowable sizes are called
  limit dimensions.

4.252
4.248
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Tolerancing and Design

• Tolerancing enables an engineer to design parts
  that are interchangeable, or parts that may be
  replaced.
• For example, you expect to be able to purchase
  new brushed aluminum wheels for your car and have
  them be an identical replacement of your factory
  rims.

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Interchangeability

• Tolerances are assigned to the dimensions of
  parts that are mated together. This allows for
  the interchangeability of parts with like parts.
• We are not only concerned with the size of a part
  but also the variation in the size of a part.

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Manufacturing Parts

• Tolerances are necessary because it is impossible
  to manufacture parts with some degree of
  variation.
• We want to be able to manufacture parts in
  different locations and different time and still
  be able to put them together in a functional
  assembly. (For example, Boeing 787)

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Tolerancing Standards

• ANSI/AMSE
• American National Standards Institute
• American Society of Mechanical Engineers
• ISO
• International Standards Organization

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Manufacturing Tolerances

• Why are manufacturing tolerances import for
  LEGOs?
• LEGO reports their tolerance in some cases to be
  0.002 mm

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Wikipedia on LEGOs

• Bricks, beams, axles, mini figures, and all other
  elements in the Lego system are manufactured to
  an exacting degree of tolerance. When snapped
  together, pieces must have just the right amount
  of "clutch power" they must stay together until
  pulled apart. They cannot be too easy to pull
  apart, or the resulting constructions would be
  unstable they also cannot be too difficult to
  pull apart, since the disassembly of one creation
  in order to build another is part of the Lego
  appeal. In order for pieces to have just the
  right "clutch power", Lego elements are
  manufactured within a tolerance

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Tolerance Types

• Limit Dimensions
• Plus-Minus Dimensions
• Page or Block Tolerances

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Limit Dimensions

• Limit Dimensions are the maximum and minimum size
  a part can be and still be a functional part.
• The largest number is always place first or on
  top.

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Plus or Minus Dimensions

• Plus or minus dimensions give a basic size and
  the variation that can occur around that basic
  size.

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Page or Block Dimensions

• A page tolerance is actually a general note that
  applies to all dimensions not covered by some
  other tolerancing type.
• These are usually found in the general notes or
  in the title block of a drawing.