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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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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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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.

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