It is not possible to build a part that exactly planed but
its fitment can match it. The engineering drawing is the controlling document
that ensures the manufacturability of part. It creates with GD&T and
controlled precisely so that machinists and quality engineers will use, print
dimensions, and drawing notes to develop a manufacturing process and inspection
methodology. It will construct high-precision components and matching the
designer’s original vision.
Drawing is a
graphical and GD&T is a symbolic language that
communicates ideas and information from one engineer to another.
Levels of Design
Three levels of design are considered in engineering design,
which are as follows:
System Design: Design of a
system which fulfill the specific function and purpose.
Parameters Design: Mechanical
parameters, electrical parameters, thermal parameters, quantity parameters
designing... of a system.
Tolerance Design: Design
for tolerances for fitment of assembly.
Specification and Tolerance
- 10 ± 0.5:
Specification is 10 and
tolerance is 1.
- Part
to Part variation is control by Size tolerance
- Within
Part variation is control by Geometric Tolerance (Shape)
- Size Tolerance > Geometric (Shape) Tolerance e.g. ±1 > ± 1/32 (0.03)
Tolerance: Allowance for specific
variation
Size tolerance is independent tolerance while Geometric tolerance
controlled by its Feature Control Frame (FCF).
14 GD&T characteristics in 5 categories = 14.5
1.
FORM = (4)
1.
Flatness,
2. Straightness
3. Circularity
4. Cylindricity
2. ORIENTATION =
(3)
1. Perpendicularity
2. Parallelism
3. Angularity
3. LOCATION = (3)
1. Symmetry
2. Position
3. Concentricity
4. RUNOUT = (2)
1. Circular run-out
2. Total run-out
5. PROFILE = (2)
1. Profile of a line
2. Profile of a surface
8/4/2 Rule for Datums : 8Yes / 4No / 2Yes or No
(Orientation+Location +Runout) / Form / Profile
Symbols except for the Form tolerances (straightness,
flatness, circularity and cylindricity) can use datums.
Basic Rules of Drawing
- Dimensions
are measured at 68°F (degree fahrenheit) or 20°C in mechanical engineering system design.
(68°F − 32) × 5/9 = 20°C
- Minimum, Maximum, Basic, Stroke and Reference dimensions
never have any tolerances limit. These dimensions are free from
tolerances.
- Dimensions
shall have only one interpretation in engineering drawing. It never gives
you conflict in between the interpretation and understanding of drawing.
- Reference dimensions
should be kept as minimum value.
- Centerlines and featurelines are at right angle and angle is
not mentioned in drawing.
- No
zero allowed before decimal and digits must be equal after decimal
in Inch unit system. For example; .12, .25 and .50 . It should not be 0.12, 0.25 and .5 in
this case.
- Zero
is must before decimal and no extra zero allowed after decimal in MM unit system. For example; 0.12, 0.25 and 0.5 It should not be .12, .25 and .50
- Primary
datum control the Orientation of
the feature in the drawing.
- All
associates dimensions are basic dimensions
(tolerance free) in profile tolerance.
