Manufacturing Engineering MECH 314

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Manufacturing Engineering - MECH 314

• Lecture 12

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CSA B78.2-M91 and ASME Y14.5M
GDT Symbols, Modifiers and Control Frames

• January 31, 2007

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Basic Dimension

• Basic dimension locates a geometric tolerance
  zone or defines gage information (example 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

To complete the specification, a geometric
tolerance must be added to the feature involved
with the basic dimension.
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MODIFIERS

• Modifiers communicate additional information
  about the drawing or tolerancing of a part.

In the CSA Standard there are six common
modifiers used in geometric tolerancing The ASME
Standard uses eight modifiers
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MODIFIERS (continued)

• The MMC, LMC, RFS and E modifiers were explained
  earlier
• The next modifier, the "P" in circle stand for
  projected tolerance zone.
• The projected tolerance modifier projects the
  tolerance zone so that it exists above the part.

The ASME Y14.5M standard uses "T" in circle,
which stand for tangent plane. The tangent plane
modifier denotes that only the tangent plane of
the toleranced surface needs to be within this
tolerance zone.
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MODIFIERS in CSA and ASME Y14.5M

• A toleranced radius (symbol for a radius is "R)
  with an unlocated centre creates a tolerance zone
  defined by two arcs (the minimum and maximum
  radii) that are tangent to the adjacent surfaces
• The radius modifier is always used outside the
  feature control frame.
• The part surface may have flats or reversals
  within the tolerance zone

Use and interpretation of Radius
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MODIFIERS (contd)

• If neither flats nor reversals in the contour are
  permitted
• it is usually preferable to specify the maximum
  radius with a unilateral tolerance
• One example is when the part stresses are very
  high and reversals in the radiused surface would
  produce higher additional stresses.

Controlled Profile Example
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MODIFIERS (contd)

• The ASME uses a controlled radius
• The symbol for a controlled radius is "CR, with
  no flats or reversals allowed.
• A controlled radius is a stringent requirement
  it should only be used in special cases.

Controlled Radius Example
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Geometric Characteristic Symbols

• Set of fourteen symbols used in the language of
  geometric tolerancing.
• The symbols are divided into five categories
• form,
• profile,
• orientation,
• location,and
• runout.

Table of Geometric Characteristic Symbols
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Feature Control Frame

• A feature control frame is a rectangular box that
  is divided into compartments within which the
  geometric characteristic symbol, tolerance value,
  modifiers, and datum references are placed.
• When specifying a non-datum related control, the
  feature control frame will have two compartments.
  
• When specifying a datum related control, the
  feature control frame may have up to five
  compartments

WCB in A 12.7 WCB in B 12.72
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Feature Control Frames
Frames with two to five compartments