1504_lecture_5

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


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 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


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


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).


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 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.


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


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!


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


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


TolerancesSolution? 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


TolerancesSolution? Use coordinate dimensioning to reduce effect of

tolerance addition


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.


Rules for Dimensioning

2. Do not add extra dimensions

here

here

Not here


Rules for Dimensioning3. Show dimensions on true profile and refer to

visible outlines, not hidden lines

Incorrect Correct



Show where shape shows best


Rules for Dimensioning4. Dimensions should be arranged for

maximum readability



Group dimensions around features


Rules for Dimensioning5. Should be no redundant dimensions, but

sometimes can add reference dimensions for more information (e.g. overall size).


Place Dimensions OFF View

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

*


Place Dimensions BETWEEN Views


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.

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