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Engi 1504 – Graphics. Lecture 5: Sectioning and Dimensioning Sectioning an object Sectioning symbols Locating sections conventions Dimensioning Class assignment 5. Intro to Sectioning. We know what the outside looks like, but what’s going on inside?

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engi 1504 graphics
Engi 1504 – Graphics

Lecture 5: Sectioning and Dimensioning

  • Sectioning an object
    • Sectioning symbols
    • Locating sections
    • conventions
  • Dimensioning
  • Class assignment 5
intro to sectioning
Intro to Sectioning
  • We know what the outside looks like, but what’s going on inside?
  • Internal details are shown by ‘removing’ a section
intro to sectioning cont
Intro to Sectioning cont.
  • To show that the front has been removed section lines are added
  • Only show surface on cut line, not hole
intro to sectioning cont4
Intro to Sectioning cont.
  • In orthographic view show internal details by drawing view on cutting plane
  • Arrows indicate direction of eye
intro to sectioning cont5
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
intro to sectioning cont6
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)
sectioning symbols
Sectioning Symbols
  • Symbols are standardized (ANSI) to show different materials
  • Placed at 45º unless section lines appear parallel to any portion of an outline
locating sections
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
full section
Full Section
  • Cutting plane cuts all the way through the object in a straight line.
offset section
Offset Section
  • If internal details of a hole are required, section should pass through centre of hole.
offset section11
Offset Section
  • All sections shown as if the holes were in line.
half section
Half Section

Cutting plane is optional

  • 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.
revolved section
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.
removed section
Removed Section
  • A section located somewhere other than in a “normal” position.

Note: Can also be included on a separate piece of paper for large objects (like buildings).

conventions to make life easier
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.
conventions to make life easier16
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.
conventions to make life easier17
Conventions to make life easier

Imagine them in rotated position!

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.

Rotate holes in section view

conventions to make life easier18
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!
conventions to make life easier19
Conventions to make life easier

Rotations (summary)

  • Holes, ribs, and lugs must be aligned in a section view.
break time
Break Time
  • Take 5 minute break
intro to dimensioning
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

intro to dimensioning cont
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
intro to dimensioning cont23
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

dimensioning terms25
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
linear dimensions
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)
tolerances
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!
tolerances28
Tolerances
  • Consider a shaft passing through a hole
  • Max shaft diameter = 30.5 mm
  • Minimum hole diameter = 29.5

Interference

tolerances29
Tolerances

Solution?

  • Unilateral tolerance (can vary in only one direction).
tolerances30
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

tolerances31
Tolerances

Solution?

  • Use coordinate dimensioning to reduce effect of tolerance addition
rules for dimensioning
Rules for Dimensioning
  • Dimensions must be complete with no information missing. User must not be required to make assumptions or measure anything directly on drawing.
rules for dimensioning33
Not here

here

here

Rules for Dimensioning
  • Do not add extra dimensions
rules for dimensioning34
Rules for Dimensioning
  • Show dimensions on true profile and refer to visible outlines, not hidden lines

Incorrect

Correct

rules for dimensioning35
Rules for Dimensioning

Show where shape shows best

rules for dimensioning36
Rules for Dimensioning
  • Dimensions should be arranged for maximum readability
rules for dimensioning37
Rules for Dimensioning

Group dimensions around features

rules for dimensioning38
Rules for Dimensioning
  • Should be no redundant dimensions, but sometimes can add reference dimensions for more information (e.g. overall size).
place dimensions off view
Place DimensionsOFFView

*

and don’t use visible boundary lines for extension lines*

other guidelines
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

dimensioning features
Dimensioning Features
  • Angular Dimensions specify angle between two points
dimensioning features43
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
dimensioning features44
Dimensioning Features
  • Large diameter holes are dimensioned specifying the diameter
dimensioning features45
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
assignment 5
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.
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