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Introduction to Symbols, Terms,
and CADD
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Objective
To recognize the various types of symbols by
their:
Name
Shape
Size
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Five Basic Types of Symbols
1. Dimensioning Symbol
2. Datum Feature and Datum Target Symbols
3. Geometric Characteristic Symbols
4. Material Condition Symbols
5. Feature Control Frame.
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Dimensioning Symbols
SymbolsRepresent specific information that would
otherwise be difficult and time consuming to duplicate in
note form.
Recommended because they are an international
language and are read the same way in any country.
Makes the communication process uniform.
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Dimensioning Symbols
Symbols aid in clarity, ease of drawing, and time
saving, especially when used with a CADD
system.
Example 2-1 shows the recommended
dimensioning symbols.
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Datum Feature Symbol
Considered theoretically perfect surfaces,
planes, points, or axes.
Identifies which edge is considered to be the 0,0
location, just like on a CNC machine.
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Datum Feature Symbol
Any letter of the alphabet can be used to identify
a datum except for I,O,or Q because the could
be confused with numbers.
Each datum must have its own identification
letter.
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Datum Feature Symbol
Letters A, B, and C are commonly used to
identify datums.
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Datum Target Symbols
Used to specify points, lines, or areas of contact
on a part that establish datums when it is not
possible to use a surface.
The symbol is drawn as a circle with a horizontal
line through the center.
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Datum Target Symbols Top half is left blank unless a
target area is specified.
Lower half identifies the datum
reference letter and the targetnumber.
B 2
Datum Target Area
Datum ReferenceDatum Target Number
.005
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Datum Target SymbolsExample of datum target symbols:
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Geometric Characteristic
Symbols
Geometric Symbols are separated into 5 basic
types:
1. Form
2. Profile
3. Location
4. Orientation
5. Runout
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Material Condition Symbols
Often referred to as modifying symbolsbecause they modify the geometric tolerance in
relationship to the produced size.
Only used in geometric dimensioning.
The different material conditions are MMC, LMC,and RFS
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Maximum Material Condition
Referred to as MMC
Indicates the largest amount of material that
may be left on a feature.
M
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Least Material Condition
Referred to as LMC
Indicates the least amount of material that may
be left on a feature.
L
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Regardless of Feature Size
Referred to as RFS
Indicates that the geometric tolerance is going to
stay the same no matter what the size of the
feature is.
No symbol is used.
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Feature Control Frame
Order of Elements:
.005 M A B C
Geometric Symbol Primary DatumTertiary Datum
Geometric Tolerance Material Condition
Secondary Datum
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Basic Dimensions
Considered a theoretically perfect dimension.
Used to describe the exact size, profile,
orientation, or location of a feature or datumtarget.
Basically tells you where the tolerance zone or
datum target is located.
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Basic Dimensions
Shown by placing a rectangle around the
dimension.
Sometimes a note is used that says:Untoleranced Dimensions Locating True
Position Are Basic.
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Additional Symbols
Free State: Describes distortion of a part after
the removal of forces applied during
manufacture.
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Additional Symbols
Tangent Plane: Placed after geometric
tolerance when it is necessary to control a
feature by contacting points of tangency.
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Additional Symbols
Projected Tolerance Zone: Used to inform the
reader that the geometric tolerance zone is
projected away from the primary datum.
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Additional Symbols
Between: Used with profile tolerances to
identify where the profile tolerance has been
applied.