# Dimensions and Tolerances mine

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```					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

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 views: 7 posted: 4/27/2010 language: English pages: 19