Boundy8e ppt ch02

Chapter Two
Dimensioning Drawings:
Symbols, Methods, Common
Features and Screw Threads

Copyright © 2012 McGraw-Hill Australia Pty Ltd
PPTs t/a Engineering Drawing 8e by Boundy

2-1

Purpose
This chapter provides an overview of how to:
apply linear and angular dimensions to engineering
drawings
use a range of symbols representing common
features
represent screw threads according to standard
practice
indicate standard procedures when applying
dimensions.

2-2

Apply linear and angular dimensions
to engineering drawings
 Dimensions are characteristics such as length or
angle who’s magnitude is identified using an
appropriate unit of measurement.
 Standard dimension symbols are utilised to
represent geometrical features and these are
proportional to the height of characters (text) used
on a particular drawing.


2-3

Table 2.1

2-4

Table 2.1 (cont)


2-5

 Dimension lines are thin, continuous lines that
indicate the extent of a measurement.
 Projection lines are thin continuous lines that
transfer detail from one view to another or allow
dimensions to be inserted (indicate the limit of
measurement).


2-6

 Linear dimensions normally expressed in
millimetres without the ‘mm’ symbol.
 Angular dimensions can be expressed either as
degrees, minutes and seconds or decimal degrees.
 Dimensions can be ‘unidirectional’ (drawn parallel
to bottom of drawing) or ‘aligned’ (drawn parallel to
dimension line) as shown in Figure 2.3, p.23
(Boundy, 2012).


2-7

 If a number of parallel dimensions are grouped
together they should be ‘staggered’ to enable ease
of reading.
 ‘Functional’ dimensions are inserted on detail
drawings to show the proper working relationship
of mating parts and are necessary for the operation
of the product.


2-8

 For ease of reading, ‘overall’ dimensions are
provided on the outside of a group of linear
measurement; however, one or more of the
dimensions that make up the overall length is
omitted to allow variations of size (see Figure 2.5,
p.24).
 ‘Auxiliary’ dimensions (indicated by enclosing the
dimension in brackets) are overall dimensions
which are added while still including all dimensions
that add up to the overall value.

2-9

 A dimension underlined with a thick continuous line
is not drawn to scale.
 When a dimension is too large to fit on a drawing
the free end is terminated in a double arrow head.
 No more dimensions than necessary are included
on a drawing.
 Dimensioning should lead readers to a clear
understanding of the relationship of parts and their
real magnitude.


2-10

Use a range of symbols representing
common features
 This symbol Ø represents diameter and is placed
preceding the dimension indicating a hole or
cylinder.
 A radius dimension is preceded by the letter R.
 Methods of dimensioning diameters and radii are
illustrated in Figures 2.7 and 2.8, p.25 (Boundy,
2012).


2-11

common features
 Spherical dimensions are preceded by the letter S
and either Ø or R depending on the dimension.
 The □ symbol indicates the feature is a square and
is followed by its ‘across the flats’ dimension;
however, if the  symbol is included in a hole
dimension then it indicates the Envelope Principle
(described on pages 96 and 98) has been applied.
 Examples of both these are shown in Figures 2.10
and 2.11, p.26 (Boundy, 2012).


2-12

common features
 Holes, form or shape should be indicated by an
appropriate symbol, e.g. □ or
 The depth of the hole (indicated by the symbol )
relates to the full form depth, if the depth is
unspecified they are considered through holes.
 Figure 2.12 on the next slide indicates methods of
dimensioning holes using both end and side views.


2-13

Figure 2.12

2-14

common features
 Hole position may be indicated by specifying pitch
diameter or rectangular coordinates (e.g. Figures
2.13 and 2.14, p.27).
 The methods for indicating countersinks ( ),
counterbores ( ) and chamfers is illustrated in
figures 2.16, 2.17 and 2.18, p.28 (Boundy, 2012).


2-15

common features
 Dimensioning rectangular and square keyways in
shafts and hubs is illustrated in Figure 2.19, p.29,
and tolerance dimensions for keyways (not
considered at this stage) are provided in Tables 2.2
and 2.3, pp.30–31 (Boundy, 2012).
 Woodruff keys require an overall linear dimension
and the diameter of the cut (as shown Figure 2.22,
p.32)
 Taper ( ) dimensioning is illustrated in Figure
2.23 p.32.

2-16

Represent screw threads according
to standard practice
Screw threads may be represented by:
 end view
 side view – external threads and sectional internal
threads
 side view – internal threads
 limit of useful length of threads
 the diameter of a metric thread is the nominal size
of the thread; for example, an M12 thread has a
nominal diameter of 12mm.

2-17

 When showing a thread in section, the hatching is
continued to the minor diameter of an internal
thread and the outer diameter of an external thread
(Figure 2.24, p.33 Boundy 2012).
 When threads are assembled and sectioned
hatching is omitted over the length of common
contact (Figure 2.25 (a) and (b), p.33).
 Special threads are often shown as a partial
section illustrating the form of the thread (Figure
2.25 (c), p.33).

2-18

 Full threads are dimensioned to the end of true
shape of the thread.
 Runout of the thread (where thread gradually
reduces shape) can be measured if required (Figure
2.26, p.34).
 The diameter of metric threads is always preceded
by the capital letter M which indicates metric thread.
 If the metric thread is not a coarse series thread the
pitch is added to the dimension (fig 2.27b, p.34)


2-19

 For through holes, thread length is not required
unless the design requires a thread length to be
added (i.e. thread does not go all the way through).
 In a blind hole it is important to nominate full thread
depth and an allowance for thread/ production
runout (Figure 2.27 (c) and (d), p.34).


2-20

 The minor diameter of a thread is effectively its
tapping size, which is calculated by ‘outside
diameter – pitch’; the pitch is obtained from charts
(e.g. Table 2.4, p.37).
 The depth of thread (the distance between the two
lines representing the thread in a drawing) can be
calculated by:
depth = 0.577 x pitch (internal thread)
depth = 0.604 x pitch (external thread).


2-21

Summary
 To facilitate drawing interpretation a standard
approach to dimensioning is required. AS1100.101
provides a structured methodology for indicating
linear and angular dimensions; in addition, to
simplify identification of common features, symbols
may be used.
 Furthermore, the common thread form in Australia
is metric and care must be taken to identify its pitch
and thread length to enable accurate interpretation
of manufacturing requirements.

2-22

Boundy8e ppt ch02

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