1504_lecture_5
TRANSCRIPT
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1SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
Engi 1504 – GraphicsLecture 5: Sectioning and Dimensioning Sectioning an object
– Sectioning symbols– Locating sections– conventions
Dimensioning
Class assignment 5
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2SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
Intro to Sectioning We know what the outside looks like, but
what’s going on inside? Internal details are shown by ‘removing’ a
section
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3SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
Intro to Sectioning cont. To show that the front has been removed
section lines are added Only show surface on cut line, not hole
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4SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
Intro to Sectioning cont. In orthographic view show internal details by
drawing view on cutting plane Arrows indicate direction of eye
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5SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
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
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6SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
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)
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7SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
Sectioning Symbols Symbols are
standardized (ANSI) to show different materials
Placed at 45º unless section lines appear parallel to any portion of an outline
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8SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
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
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9SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
Full Section Cutting plane cuts all the way through the
object in a straight line.
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10SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
Offset Section If internal details of a hole are required,
section should pass through centre of hole.
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11SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
Offset Section All sections shown as if the holes were in line.
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12SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
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.
Cutting plane is optional
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13SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
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.
O U TLINES CAN BE O M ITTED AT SECTIO N
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Removed Section A section located somewhere other than in a
“normal” position.A
A
B
B
SECTIO N A-A SEC TION B-B
Note: Can also be included on a separate piece of paper for large objects (like buildings).
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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.
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16SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
Conventions to make life easierBreaks 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.
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17SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
Conventions to make life easierRotations 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
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18SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
Conventions to make life easierRotations (webs) Same problem, so rotate
the webs so that they appear full size in front view.
To avoid confusion, Webs are not crosshatched!
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Conventions to make life easier
Rotations (summary) Holes, ribs, and lugs must be aligned in a
section view.
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20SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
Break Time Take 5 minute break
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21SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
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
Dimensioning
Notes on drawing
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22SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
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
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23SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
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
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24SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
Dimensioning Terms
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25SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
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
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26SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
Linear Dimensions 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)
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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!
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28SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
Tolerances Consider a shaft passing through a hole Max shaft diameter = 30.5 mm Minimum hole diameter = 29.5
Interference
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TolerancesSolution? Unilateral tolerance (can vary in only one direction).
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30SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
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
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31SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
TolerancesSolution? Use coordinate dimensioning to reduce effect of
tolerance addition
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Rules for Dimensioning1. Dimensions must be complete with no information
missing. User must not be required to make assumptions or measure anything directly on drawing.
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Rules for Dimensioning
2. Do not add extra dimensions
here
here
Not here
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Rules for Dimensioning3. Show dimensions on true profile and refer to
visible outlines, not hidden lines
Incorrect Correct
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35SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
Rules for Dimensioning
Show where shape shows best
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36SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
Rules for Dimensioning4. Dimensions should be arranged for
maximum readability
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37SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
Rules for Dimensioning
Group dimensions around features
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Rules for Dimensioning5. Should be no redundant dimensions, but
sometimes can add reference dimensions for more information (e.g. overall size).
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Place Dimensions OFF View
and don’t use visible boundary lines for extension lines*
*
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Place Dimensions BETWEEN Views
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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
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Dimensioning Features Angular Dimensions
specify angle between two points
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43SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
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
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Dimensioning Features Large diameter holes are dimensioned specifying
the diameter
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45SheetCourse: Engineering Graphics 1504 Memorial University of Newfoundland
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
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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.