GD N T DETAILS FOR STUDY OF EVERY ONE ACEDEMIC

Gd & t

1. 1
THE INSTITUTE FOR ENHANCEMENT OF TECHNOLOGY
DATUMS AND THE
THREE PLANE CONCEPTS

2. 2
THE INSTITUTE FOR ENHANCEMENT OF TECHNOLOGY
Datum
“ A datum is a point, line, plane, or other
geometrical surface, from which
dimensions are measured when so
specified, or to which geometrical
tolerances are referenced. A datum has
an exact form, and represents an exact
or fixed location, for purposes of
manufacture or measurement”

3. 3
THE INSTITUTE FOR ENHANCEMENT OF TECHNOLOGY
Datum Feature
“A datum feature is a feature of a part,
such as an edge, a surface, or a hole,
which forms the basis for a datum, or is
used to establish the location of a
datum”

4. 4
THE INSTITUTE FOR ENHANCEMENT OF TECHNOLOGY
Theoretical Datums
True datums are theoretical as datum features
are created by manufacturing.

Even these surfaces are not perfect
geometrical surfaces, but they are intended to
be of sufficiently high quality that any errors
that may introduce are insignificant in
comparison with part tolerances.

5. 5
THE INSTITUTE FOR ENHANCEMENT OF TECHNOLOGY
DATUMS AND THE THREE PLANE CONCEPT
Geometrical form tolerances such as
straightness & flatness refer to unrelated
lines and surfaces, do not require use of
datums.
Orientation and locational tolerances
refer to related features.
Usually only one datum is required for
orientation purposes.

6. 6
THE INSTITUTE FOR ENHANCEMENT OF TECHNOLOGY
DATUMS AND THE THREE PLANE CONCEPT
Positional relationships may require a
datum system consisting of two or three
datums.
These are designated as primary,
secondary & tertiary datums.
These are refer to as three plane datum
system/datum reference system when
datums are mutually perpendicular
surfaces.

7. 7
THE INSTITUTE FOR ENHANCEMENT OF TECHNOLOGY
Primary Datum
•If the primary datum is a flat surface,
theoretically minimum three high spots on the
flat surface will come in contact with the
surface of gauge.

8. 8
THE INSTITUTE FOR ENHANCEMENT OF TECHNOLOGY
Secondary datum
•If the part, while lying on the primary plane, is brought
into contact with the secondary plane, it will
theoretically touch at minimum of two points.

9. 9
THE INSTITUTE FOR ENHANCEMENT OF TECHNOLOGY
Tertiary Datum
•The part can slide along while maintaining contact with
primary & secondary planes till it contacts third
plane.
•The third plane becomes the tertiary datum and
theoretically touch at one point.

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THE INSTITUTE FOR ENHANCEMENT OF TECHNOLOGY
•Three planes constitute a datum system from which
measurements can be taken.
DATUMS AND THE THREE PLANE CONCEPT

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THE INSTITUTE FOR ENHANCEMENT OF TECHNOLOGY
•Datum symbols are required to serve two
purposes:
•To Indicate the datum surface or feature on
the drawing. 
•To identify the datum features so that it can
easily be referred to in other requirements.
DATUMS AND THE THREE PLANE CONCEPT
IDENTIFICATION OF DATUMS

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THE INSTITUTE FOR ENHANCEMENT OF TECHNOLOGY
•It is a solid right-angled triangle, with a leader
projecting from the 90º apex.

•The base of the triangle should be slightly greater
than the height of the lettering used on the
drawing.
DATUM INDICATOR SYMBOL

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THE INSTITUTE FOR ENHANCEMENT OF TECHNOLOGY
•Datum feature is identified by a capital letter
placed in a square frame, and connected to the
datum indicator symbol.
DATUM IDENTIFICATION SYMBOL

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THE INSTITUTE FOR ENHANCEMENT OF TECHNOLOGY
POSITION OF IDENTIFIED DATUMS
If two or more datum references are involved they must be shown in
correct order, i.e. in primary, secondary, and tertiary datums.
Each datum feature must be identified, using a capital
letter under the following circumstances;
When two or more datum features are associated with the control of a
single feature, the datum letter or letters are then placed in the feature
control symbol frame by adding an extra compartment for each datum
reference.

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THE INSTITUTE FOR ENHANCEMENT OF TECHNOLOGY
MULTIPLE DATUM FEATURES
If a single datum is established by two datum
features, such as two ends of a shaft, the features
are each identified by separate letters. Both letters
are then placed in the same compartment of the
feature control symbol, with a dash between them.