The document outlines the essential principles of Geometric Dimensioning and Tolerancing (GD&T) in accordance with ASME Y14.5 standards, focusing on metric and inch dimensioning rules. It discusses the significance of datum features, symbols, and the placement of feature control frames, particularly in relation to irregular surfaces and complex geometries. The emphasis is on maintaining clarity and precision in engineering drawings through appropriate notation and measurement conventions.

1. IMPORTANT POINTS IN GD&T
Sreelakshmy V.U

2. Unit for measure in ASME GD&T Y14.5
SI Linear unit used in engineering drawing is MILLI METER (mm).
{However, the drawing shall contain a note stating UNLESS OTHERWISE SPECIFIED,
ALL DIMENSIONS ARE IN MILLIMETERS (or in INCHES, as applicable)}

3. MILLIMETER DIMENSIONING
• When the value is LESS THAN
one, a zero is used before the
decimal point
• When the dimension is a whole
number, NEITHER zero NOR
decimal is used
• When the dimension exceeds
with a fraction, the last digit is
not followed with zero

4. INCH DIMENSIONING
• When the value is LESS THAN
one, a zero is NOT used before
the decimal point
• When the dimension exceeds
with a fraction, the last digit is
followed with zero

5. Reading (Counter bore, Countersunk, depth, Spot face) symbols in a drawing

6. VIRTUAL CONDITION & RESULTANT CONDITION
(CALCULATION)

8. ORDER OF PRECEDENCE OF DATUM

9. Datum target symbols
Datum target symbols
are mostly used in
irregular surfaces OR
complex surfaces
where we cannot give
datum feature symbols

10. Determining a datum feature
DATUM FEATURE - SURFACE
Datum placed on the outline of a feature
surface or on an extension line of the feature
outline, clearly separated from the
dimension line
DATUM FEATURE – AXIS
Datum placed on an extension of the
dimension line of a feature of size

11. Determining a datum feature - Continued
DATUM - AXIS
Datum feature symbol placed on the
outline of a cylindrical feature surface or
extension line of the feature outline
separated from the size dimension
DATUM – AXIS or CENTER PLANE
DATUM can be established if the datum
feature symbol is placed above or below
the feature control frame.

12. PLACING FEATURE CONTROL FRAME
1) Frame attached below the dimension (with a
leader line) pertaining to the feature
2) Frame directly attached to the feature
3) End (or) One side of the frame is attached to the
extension line from the feature, provided it is a
plane surface
4) End (or) One side of the frame is attached to the
extension of a dimension line pertaining to a
feature of size.

13. DATUM REFERENCE FRAME - DERIVATION

14. Feature Control Frame

15. COMPOSITE TOLERANCE FRAME
First frame controls the
location and the second
frame controls the
orientation.
The second frame
refines the control of
first frame

17. Two Single feature control frame
First frame controls the location
and the second frame controls the
location and orientation.
The second frame refines the
control of first frame

19. Additional drawings:

20. Thank you 
(Referred from concerned websites)