Manufacturing Engineering MECH 314

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Manufacturing Engineering - MECH 314

• Lecture 11

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CSA B78.2-M91
Feature and Feature of Size

• January 30, 2007

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FEATURE

• Feature means a specific characteristic portion
  of a part, such as a surface, hole, slot, screw
  thread, radius, chamfer, or profile.
• Note While a feature may include one or more
  surfaces, the term is generally used in a more
  restrictive sense, to indicate a specific point,
  line, or surface to which reference is being made
  or which forms the basis for a datum.

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FEATURES OF SIZE

• A feature of size (FOS) is one cylindrical or
  spherical surface, or a set of two opposed
  elements or opposed parallel surfaces, associated
  with a size dimension
• An axis, median plane or center point can be
  derived from a feature of size.

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FEATURES OF SIZE

• Applies to
• diameter
• thickness

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Rule 1

• Rule 1 is a dimensioning rule used to ensure
  that features of size will assemble with one
  another.
• When Rule 1 applies, the maximum boundary (or
  envelope) for an external FOS is its MMC. The
  minimum envelope for an internal FOS is its MMC.
• To determine if two features of size will
  assemble, the designer can then compare the MMCs
  of the features of size.

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Rule 1 (contd)

• There are two components to Rule 1
• the envelope principle and,
• the effects on the form of a FOS as it departs
  from MMC.
• In industry, Rule 1 is often paraphrased as
  "perfect form at MMC" or the "envelope rule."

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Controlling the form of a FOS

• The form of a FOS is controlled by its limits of
  size
• The surfaces of a feature of size shall not
  extend beyond a boundary (envelope) of perfect
  form at MMC. (Rule 1)
• When the actual local size of a FOS has departed
  from MMC toward LMC, the form is allowed to vary
  by the same amount.
• The actual local size of an individual feature of
  size must be within the specified tolerance of
  size.
• There is no requirement for a boundary of perfect
  form at LMC. If a feature of size is produced at
  LMC, it can vary from true form by the amount
  allowed by the MMC boundary. (If MMC is specified)

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Inspecting a Feature of Size

• When inspecting a FOS that is controlled by Rule
  1, both its size and form need to be verified.
• The MMC size and the Rule 1 envelope can be
  verified with a Go gage.
• A Go gage is a gage that is intended to fit into
  (for an internal FOS) or fit over (for an
  external FOS) the FOS.
• A Go gage is made to the MMC limit of the FOS and
  has perfect form.
• A Go gage can verify the MMC limit and Rule 1
  form envelope of a FOS.
• To fully verify the Rule 1 effects, a Go gage
  must be at least as long as the FOS it is
  verifying.

The figure above shows examples of a Go gages for
a pin and a hole.
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Inspecting a Feature of Size (contd)

• The minimum size (LMC) of a FOS can be measured
  with a No-Go gage.
• A No-Go gage is a gage that is not intended to
  fit into or over a FOS.
• A No-Go gage is made to the LMC limit of the FOS.
  
• A No-Go gage makes a two-point check a caliper
  or snap gage could be used as a No-Go gage.
• The two-point check is made at various points
  along the cross section to insure that the FOS
  does not violate the LMC limit.

The figure above shows examples of a No-Go gages
for a pin and a hole.
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General rules in CSA B78.2-M91(continued)
Rule 2 (paragraph 11.3.2)

• If no relationship is specified between the
  feature size and the geometrical tolerance, the
  tolerance applies regardless of feature size
  (RFS) and the two characteristics are treated as
  unrelated requirements.
• MMC or LMC must be specified on the drawing where
  required.

The figure above shows examples of Rule 2 and 2a
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Basic Dimension

• A basic dimension is a numerical value used to
  describe the theoretically exact size, true
  profile, orientation, or location of a feature or
  gage information (i.e., datum targets).
• On engineering drawings there are two uses for
  basic dimensions.
• One is to define the theoretically exact
  location, size, orientation, or true profile of a
  part feature
• the other use is to define gage information
  (example datum targets).
• In simple terms, a basic dimension locates a
  geometric tolerance zone or defines gage
  information (example datum targets).
• When basic dimensions are used to describe part
  features, they must be accompanied by geometric
  tolerances to specify how much tolerance the part
  feature may have.

A good way to look at the basic dimension is that
it only specifies half the requirement. To
complete the specification, a geometric tolerance
must be added to the feature involved with the
basic dimension.
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Basic Dimension (contd)

• When basic dimensions are used to specify datum
  targets, they are considered gage dimensions.
• Gage-makers' tolerances (a very small tolerance
  compared to product tolerances) apply to gage
  dimensions.
• Titleblock tolerances do not apply to basic
  dimensions.
• Basic dimensions must get their tolerances from a
  geometric tolerance or from a special note.

A.
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Example