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? • Internal details are shown by ‘removing’ a section
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 cont. • In orthographic view show internal details by drawing view on cutting plane • Arrows indicate direction of eye
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 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 • Symbols are standardized (ANSI) to show different materials • Placed at 45º unless section lines appear parallel to any portion of an outline
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 • Cutting plane cuts all the way through the object in a straight line.
Offset Section • If internal details of a hole are required, section should pass through centre of hole.
Offset Section • All sections shown as if the holes were in line.
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 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 • 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 • 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 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 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 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 easier Rotations (summary) • Holes, ribs, and lugs must be aligned in a section view.
Break Time • Take 5 minute break
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. • 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 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 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 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 • 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!
Tolerances • Consider a shaft passing through a hole • Max shaft diameter = 30.5 mm • Minimum hole diameter = 29.5 Interference
Tolerances Solution? • Unilateral tolerance (can vary in only one direction).
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
Tolerances Solution? • Use coordinate dimensioning to reduce effect of tolerance addition
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.
Not here here here Rules for Dimensioning • Do not add extra dimensions
Rules for Dimensioning • Show dimensions on true profile and refer to visible outlines, not hidden lines Incorrect Correct
Rules for Dimensioning Show where shape shows best
Rules for Dimensioning • Dimensions should be arranged for maximum readability
Rules for Dimensioning Group dimensions around features
Rules for Dimensioning • Should be no redundant dimensions, but sometimes can add reference dimensions for more information (e.g. overall size).
Place DimensionsOFFView * and don’t use visible boundary lines for extension lines*
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 • Angular Dimensions specify angle between two points
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 Features • Large diameter holes are dimensioned specifying the diameter
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 • In your workbook, complete question 32 in Chapter 3. • Note: the question is fairly simple, so make sure it is neat and complete.