Dimensions and Tolerances mine

Document Sample
Dimensions and Tolerances mine Powered By Docstoc
					Dimensions and Tolerances
Objective

   To learn how to effectively tolerance
    parts such that parts function correctly
    and cost is kept to a minimum
Tolerancing
   Definition: Allowance for specific variation in the
    size and geometry of a part
   Why is tolerancing necessary?
    – It is impossible to manufacture a part to an exact size or
      geometry
    – Since variation from the drawing is inevitable the
      acceptable degree of variation must be specified
    – Large variation may affect the functionality of the part
    – Small variation will effect the cost of the part
         requires precise manufacturing
         requires inspection and the rejection of parts
Functionality
   Assemblies: Parts will often not fit together if their dimensions
    do not fall within a certain range of values
   Interchangeability: If a replacement part is used it must be a
    duplicate of the original part within certain limits of deviation
   The relationship between functionality and size or shape of an
    object varies from part to part
     – the usefulness of eyeglasses is extremely sensitive to size
        and shape
     – the usefulness of glass marbles are not very sensitive to size
        and shape
   By specifying tolerances in
    manufacturing, you can control the
    degree of accuracy needed for an
    object.
   You can apply tolerances directly to a
    dimension by attaching the tolerances
    to the dimension text. These
    dimension tolerances indicate the
    largest and the smallest permissible
    size of the dimension.
How Is Tolerance
Specified?
   Size
     – Limits specifying the allowed variation in each
       dimension (length, width, height, diameter, etc.)
       are given on the drawing
   Geometry
     – Geometric Tolerancing
          Allows for specification of tolerance for the
           geometry of a part separate from its size
          GDT (Geometric Dimensioning and Tolerancing)
           uses special symbols to control different
           geometric features of a part
   Tolerances can be specified from
    theoretically exact measurements.
    These are called basic dimensions and
    have a box drawn around them.
   If the dimension value can vary in both
    directions, the plus and minus values you
    supply are appended to the dimension value
    as deviation tolerances.
    If the deviation tolerance values are equal,
    AutoCAD displays them with a ± sign and
    they are known as symmetrical. Otherwise,
    the plus value goes above the minus value.
   If the tolerances are applied as limits,
    AutoCAD uses the plus and minus
    values you supply to calculate a
    maximum and minimum value.
    These values replace the dimension
    value. If you specify limits, the upper
    limit goes above the lower.
   Tolerances are used to control the
    variation that exists on all
    manufactured parts.
   Tolerances dimensions control the
    amount of variation on each part of an
    assembly.
General Tolerances
   A note may be placed on the drawing which
    specifies the tolerance for all dimensions
    except where individually specified

   Specific tolerances given to a dimension on a
    drawing always supersede general tolerances
The more accuracy needed for the parts
the higher is the cost.

   The tolerance allowed on each part
    depends on the function of the part
    and of the assembly.
    For example the tolerances placed on
    an electrical hand drill parts are not as
    important as those place on a jet
    engine parts.
Example:
   A tolerance of 5.650  0.003 means that the final
    measurement of the part can be from 5.653 to
    5.647 and the part can be acceptable.
   The upper and lower sizes are referred to as the
    limit dimensions, and the tolerance is the difference
    between the limits.
   Upper limit (largest value) = 5.653
   Lower limit (smallest value) = 5.647
   Tolerance = 5.653 – 5.647
   Tolerance = 0.006
Definitions of terms
   Nominal size – a dimension used to describe the
    general size.
   Basic size - the theoretical size used as a starting
    point for the application of tolerances.
   Actual size – the measured size of the finished
    part after machining.
   Limits – the maximum and minimum sizes shown
    by the toleranced dimension.
   Allowance – the minimum clearance or maximum
    interference between parts, or the tightest fit
    between two mating parts.
   Tolerance – the total amount allowable variance in a
    dimension; the difference between the upper and lower limits.

   Maximum material condition (MMC) – the condition of a
    part when it contains the greatest amount of material. The
    MMC of an external feature, such as a shaft, is the upper
    limit. The MMC of an internal feature, such as a hole, is the
    lower limit.

   Least material condition (LMC) – the condition of a part
    when it contains the least amount of material possible. The
    LMC of an external feature is the lower limit.The LMC of an
    internal feature is the upper limit.
Holes and Tolerancing Shafts
   Types of Fit
    – Clearance fit
         The parts are toleranced such that the largest shaft is
          smaller than the smallest hole
         The allowance is positive and greater than zero

    – Transition fit
         The parts are toleranced such that the allowance is
          negative and the max. clearance is positive
         The parts may be loose or forced together

    – Interference fit
         The max. clearance is always negative

         The parts must always be forced together

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:7
posted:4/27/2010
language:English
pages:19