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1. MECHANICAL DRAUGHTING

2. Lecture 1
SUBJECT AIMS
The student must be able to:
 Identify the function of Mechanical
Draughting in the engineering industry
 Interpret Mechanical Draughting as an
accurate and meaningful manner of
communication, since drawing is the
language used in the engineering industry

3. SPECIFIC SUBJECT AIMS
The student must be equipped with the knowledge and skills in
the following content:
Conventional representation of spur
gears, square threads and helical springs
Cam profiles
Sectional drawing
Detail drawing
Assembly drawing

4. SPECIFIC SUBJECT AIMS
Emphasis is placed on the following elements:
Correctness
Linework
Speed
Accuracy
Balanced layout and neatness

5. LEARNING CONTENT
MODULE DESCRIPTION
1 Spur gears, square threads, helical springs
2 CAM profiles
3 Sectional drawing
4 Detail drawing
5 Assembly drawing

6. MODULE 1: CONVENTIONAL REPRESENTATION OF A SINGLE SPUR
GEAR, SPUR GEARS IN MESH, SQUARE THREADS AND HELICAL
SPRINGS
LEARNING OUTCOMES:
1. Draw according to first or third angle orthographic projection, the conventional views of
single spur gears as well as spur gears in mesh
 outside and sectional views
 dedendum circle must be drawn
 gears based on metric standard, clearance =0,25xmodule, dedendum =1,25x module
2. Draw according to first or third angle the primary views of internal and external left-hand
and right-hand square threads and helical springs bearing in mind:
 outside views and sectional views
 helical springs made from square or round material

7. MODULE 2: CAM PROFILES
CONSTANT VELOCITY CAMS, UNIFORM ACCELERATION AND
RETARDATION, SIMPLE HARMONIC MOTION
 Displacement diagram must be drawn
 Centreline of the follower must be on the cam
 Only cams with knife edge or roller followers
 Follower need not be drawn
 Construction lines drawn and marked with compass
 Lift must occur within 180 degrees of cam rotation
 Fall (return) must occur in last 180 degrees
 Dwell may be included
 Clockwise and anticlockwise rotation is asked
 Angle of lift and fall must and type of motion must be indicated CV,UAR,SHM.
 Displacement diagram must be drawn
 Shaft is shown as an outline (line type A) and is hatched

8. MODULE 3: SECTIONAL DRAWING
DRAW PRIMARY VIEWS OF MACHINE COMPONENTS ACCORDING TO SCALE IN
FIRST OR THIRD ANGLE ORTHOGRAPHIC PROJECTION
 Full section, half section, cutting planes that change direction, partial section, revolved sections, removed
sections.
 Cutting plane lines
 Outside views
 Machine parts with interpretation radii
 Join interpenetration views with flex curve
 Convention representation of drilled and tapped holes, bolts, nuts and studs
 Internal and external chamfering, spotfacing, couterboring, countersinking, drilling and tapping specification
 Machining and surface finish texture symbols
 Limits and fits
 Hidden detail

9. MODULE 4: DETAIL DRAWING
DISMANTLE ASSEMBLY DRAWING AND DRAW PRIMARY VIEWS
ACCORDING TO SCALE IN FIRST OR THIRD ANGLE OTHOGRAPHIC
PROJECTION
 Dimensioning
 Sectional and outside views
 Machining and surface finish texture symbols and ISO limits and fits
 Conventional representation of drilled and tapped holes as well as manufacturers
bolts and nuts

10. MODULE 5: ASSEMBLY DRAWING
CORRECTLY ASSEMBLE COMPONENTS IN DETAIL DRAWINGS AND
DRAW THE PRIMARY VIEWS TO SCALE IN FIRST OR THIRD ANGLE
ORTHOGRAPHIC PROJECTION
 Include item numbers and a parts list
 Sectional or outside views
 A maximum of ten items may be given
 Conventional representation of bolts, nuts, studs, drilled and treaded holes.
 Nuts referred to in item list
 Complete assembly drawing will not necessarily be asked

11. 1. INTRODUCTION
WHAT IS ENGINEERING DRAWING?
Engineering drawing is the “language” engineers
and draughtsmen use to communicate their designs
Think of having to explain to someone how to
assemble a motor car – a drawing is a simpler way
of communicating to the factory floor what is to be
done in the process

12. 1. INTRODUCTION
Another example is building a house, you need a drawing to
communicate to the builder where the rooms must be
THE PURPOSE OF ENGINEERING DRAWING
The purpose is to convey the details of the draughtsman's
object or design to the reader of the drawing. The drawing
must represent the object so that it is understood.
It needs to communicate all the necessary information for
manufacturing or assembly and must be free of errors

13. 1. INTRODUCTION
DRAWING STANDARD
The line of the drawing need to be according to a standard.
The standard used in South Africa is the
Code of Practice for Engineering Drawing, SABS 0111
Lines need to be the correct thickness, density and colour
Writing also needs to keep to the standard

14. 1. INTRODUCTION
METHODS OF REPRESENTATION
Drawings can be done using instruments or
freehand sketches
REQUIRMENTS FOR DRAWING
A good quality drawing set is required

15. 1. INTRODUCTION
Components of a drawing set:
1. Pencil compass with 120mm legs
2. Dividers with approximately 100-120mm legs
3. A small springbow compass
4. A scale rule (scales 1:1; 1:2; 1:5)
5. A T-square (plastic approx. 650mm long)
6. Set squares 200mm sides (one 30-60 degree and one 45 degree)
7. A protractor (radius of approx. 75mm)

16. 1. INTRODUCTION
Components of a drawing set:
8. Pencils
9. Lead for a compass
10. Lead for compass
11. Eraser
12. Drawing clips
13. Eraser shield
14. Stencils – for lettering, figures, nuts etc.

17. 1. INTRODUCTION
The use of instruments
The T- Square
 The T-square is used to draw horizontal lines, not for
vertical lines

18. 1. INTRODUCTION
The set square
 The set square is used in combination with the T-
square to draw vertical lines upwards from the T-square

19. 1. INTRODUCTION
The pencil compass
 Pencil compass is used to draw circles or mark off arcs.
 The lead is adjusted till the turning knob of the compass is vertical.
 The sharp point of the compass is used to mark the centre of the circle
while you use your hands to turn the compass in an arc to complete the
circle.

20. 1. INTRODUCTION
Dividers
Dividers are used to mark the same dimension off at
more than one place.

21. 1. INTRODUCTION
The scale rule
 Mark off dimensions directly from the scale rule onto
the drawing paper
 Be careful to check the scale – is it 1:1(same size), or
1:2..(half the size)

22. 1. INTRODUCTION
USEFUL ADVICE WHEN DRAWING:
 Keep your instruments in good condition-clean, use them for the correct purpose.
 Wash set squares regularly.
 Do not erase too much as it damages the paper.
 Try to stand when drawing.
 Use a smaller compass for smaller circles.
 Neatness is mot important – was your hands before drawing, keep the paper clean.
 Draw according to the standard.
 Be proud of your work.

23. 2. LETTERS AND FIGURES
 Write letters and figures so they are clear and uniform
HEIGHT OF LETTERS AND FIGURES
For drawing on an A2 sheet (420x594mm):
1. 7mm – for the name and title of the object/drawing, min. height of 7mm, max. line
thickness of 0.7mm
2. 3,5mm - for all other printing such as holes, dimensions and instruction notes, min.
height is 2,5mm, with max. line thickness of 0,18mm. Line thickness of 0,3-0,5mm is
suitable for a print height of 3,5mm.

24. 2. LETTERS AND FIGURES
 Letters and figures need to be drawing according to the Code of practice
for Engineering Drawing (SABS 0111)
 Letters may be inclined up to 15 degrees
 Captial letters are normally only used on drawings

25. 2. LETTERS AND FIGURES
Guidelines:
 Printing must be done between two faint guidelines which
ensure that the size of the lettering is the same
 To setup the guidelines do the following
1. Set the divers according to the writing height – 3.5mm or 7mm
2. With the dividers mark 2 small holes at the required position
3. Draw a faint thin line with a pencil through each point
4. Write the letters between the guidelines

26. 3. TYPES OF LINES
GENERAL:
 Lines according to SABS 0111
 Line thickness cannot be changed at will, needs to be
consistent
 Quality and standard of lines is very important

27. 3. TYPES OF LINES
THICKNESSES OF LINES
 Use lines of two thicknesses for an entire drawing. Pencil - use
0.3; 0.5; 0.7 and 0.9mm.
Recommendation:
1. 0.5mm for full lines (Continuous thick)
2. 0.3mm for other types of line
Whn drawing first draw thin lines that can be erased, final lines
can be drawn later.

28. 3. TYPES OF LINES
Types of lines (table)
Dashed lines are appro.3mm long with a space of 1-2mm in between

29. End of lecture 1
3. TYPES OF LINES
Order of precedence of types of lines if lines overlap:
1. Full lines
2. Dashed lines
3. Centre lines or cutting plane lines
4. Break lines
5. Dimension and projection lines
6. Hatching