Engi 1504 – Graphics
Lecture 5: Sectioning and Dimensioning
 Sectioning an object
– Sectioning symbols
– Locating sections
– conventions

Dimensioning

Class assignment 5
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet
1
Intro to Sectioning


We know what the outside looks like, but
what’s going on inside?
Internal details are shown by ‘removing’ a
section
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet
2
Intro to Sectioning cont.


To show that the front has been removed
section lines are added
Only show surface on cut line, not hole
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet
3
Intro to Sectioning cont.

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In orthographic view show internal details by
drawing view on cutting plane
Arrows indicate direction of eye
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet
4
Intro to Sectioning cont.


Draw view on section A-A
Section lines show cut surface and only show
surface formed by cutting plane, not hole
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet
5
Intro to Sectioning cont.

Note: still have to show all visible lines. Hidden
lines are omitted, but must show all visible
lines (i.e. back of hole)
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet
6
Sectioning Symbols


Symbols are
standardized (ANSI) to
show different materials
Placed at 45º unless
section lines appear
parallel to any portion of
an outline
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet
7
Locating Sections



Locate section to show the required internal details
Sections can be taken anywhere and need not be
taken through middle of object.
Examples include:
–
–
–
–
–
Full section
Half section
Offset section
Revolved section
Removed section
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet
8
Full Section

Cutting plane cuts all the way through the
object in a straight line.
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet
9
Offset Section

If internal details of a hole are required,
section should pass through centre of hole.
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 10
Offset Section

All sections shown as if the holes were in line.
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 11
Half Section


So far both full and
offset sections have cut
all the way through the
object.
If there is an axis of
symmetry only one
side needs to be
drawn.
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Cutting plane is
optional
Sheet 12
Revolved Section


Revolved sections are the same as full sections, but
drawn at a different location.
A revolved section is drawn directly on the view,
rather than in a different view.
OUTLINES CAN BE OMITTED AT SECTION
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 13
Removed Section

A section located somewhere other than in a
“normal” position.
A
B
A
B
SECTION A-A
SECTION B-B
Note: Can also be included on a separate piece of
paper for large objects (like buildings).
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 14
Conventions to make life easier


Some features are simplified to make them
easier to draw and not shown as they would
actually appear.
Important to know these conventions in order
to understand a drawing.
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 15
Conventions to make life easier
Breaks
 If a part is long (say a
shaft), only need to show
the ends and a part of
the centre with a
conventional break
 Length is specified, but
full length is not drawn.
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 16
Conventions to make life easier
Rotations

If side view were drawn using
principles of projection it
would be confusing, and time
consuming.

Section is drawn as if the
holes were rotated to where
they would show a true cross
section and diameter can be
seen.
Imagine them in
rotated position!
Rotate holes
in section
view
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 17
Conventions to make life easier
Rotations (webs)
 Same problem, so rotate
the webs so that they
appear full size in front
view.
 To avoid confusion,
Webs are not
crosshatched!
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 18
Conventions to make life easier
Rotations (summary)
 Holes, ribs, and lugs must be aligned in a
section view.
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 19
Break Time

Take 5 minute break
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 20
Intro to Dimensioning

Before you can build
something need to
know:
– How big it will be
– Size and location of any
features
– The material it is to be
made of
– How many to make
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Dimensioning
Notes on
drawing
Sheet 21
Intro to Dimensioning cont.



Various organizations publish standard
methods for dimensioning and tolerancing
engineering documents
Canadian Standards Association (CSA) B78.2
American Society of Mechanical Engineers
Standard Dimensioning and Tolerancing
(ASME) Y14.5M
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 22
Intro to Dimensioning cont.
Units
 SI units. Common linear unit is mm (e.g. 5 mm)
 Imperial units. Customary linear unit is the decimal
inch (0.25 in)
 If all dimensions are in either millimetres or inches, the
symbol after each dimension can be omitted. Put a
note on the drawing:
ALL DIMENSIONS IN MILLIMETRES
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 23
Dimensioning Terms
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 24
Dimensioning terms

Extension lines
– Indicate length to which dimension applies
– Do not touch the object (gap)
– Should not cross other lines

Dimension lines
– Show extent of the dimension
– Should not cross other lines

Notes
– Give information about object
– Always in uppercase letters

Leaders
– Point to a feature, terminate with arrowhead
– Point to a surface, terminate with dot
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 25
Linear Dimensions
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Linear dimensions apply to straight lines or distances.
Chain (starting point for one dimension is the end of previous
dimension)
Coordinate dimensions (referenced from one point)
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 26
Tolerances



Tolerance is the maximum amount by which a
length can vary and still be acceptable.
In general, the smaller the tolerance, the more
it will cost to manufacture
But parts still must fit together!
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 27
Tolerances
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Consider a shaft passing through a hole
Max shaft diameter = 30.5 mm
Minimum hole diameter = 29.5
Interference
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 28
Tolerances
Solution?
 Unilateral tolerance (can vary in only one direction).
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 29
Tolerances


Also tolerances on dimensions
Tolerances can add up, and parts may be too tight (or
loose).
Edge A could be
1.5mm too big
Edge B could be
2mm too short
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 30
Tolerances
Solution?
 Use coordinate dimensioning to reduce effect of
tolerance addition
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 31
Rules for Dimensioning
1.
Dimensions must be complete with no information
missing. User must not be required to make
assumptions or measure anything directly on
drawing.
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 32
Rules for Dimensioning
2.
Do not add extra dimensions
Not here
here
here
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 33
Rules for Dimensioning
3.
Show dimensions on true profile and refer to
visible outlines, not hidden lines
Incorrect
Course: Engineering Graphics 1504
Correct
Memorial University of Newfoundland
Sheet 34
Rules for Dimensioning
Show where shape shows best
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 35
Rules for Dimensioning
4.
Dimensions should be arranged for
maximum readability
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 36
Rules for Dimensioning
Group dimensions around features
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 37
Rules for Dimensioning
5.
Should be no redundant dimensions, but
sometimes can add reference dimensions for more
information (e.g. overall size).
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 38
Place Dimensions OFF View
*
Course: Engineering Graphics 1504
and don’t use visible
boundary lines for
extension lines*
Memorial University of Newfoundland
Sheet 39
Place Dimensions BETWEEN Views
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 40
Other Guidelines
…
Place the largest dimension farthest from the part
boundary
Avoid:
long extension lines;
dimensioning to hidden lines;
crossing dimension lines with extension lines
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 41
Dimensioning Features

Angular Dimensions
specify angle between
two points
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 42
Dimensioning Features


Circular Dimensions are defined by specifying the location of the
centre and either the radius or diameter
Diameter
– a solid cylinder is dimensioned where both length and diameter are in
same view with visible outlines
– A hole (a negative cylinder) is dimensioned where the circular shape is
seen
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 43
Dimensioning Features

Large diameter holes are dimensioned specifying
the diameter
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 44
Dimensioning Features

Radius
– Incomplete circular features are specified by the location of the centre,
the starting point, the end point, and the radius
– The location may not be specified by the drawing, other information
such as tangent points must be given to locate its centre
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 45
Assignment #5


In your workbook, complete question 32 in
Chapter 3.
Note: the question is fairly simple, so make
sure it is neat and complete.
Course: Engineering Graphics 1504
Memorial University of Newfoundland
Sheet 46