Boundy8e ppt ch02

Copyright 2012 McGraw-Hill Australia Pty Ltd PPTs t/a Engineering Drawing 8e by Boundy 2-1

Chapter Two

Dimensioning Drawings:Symbols, Methods, CommonFeatures and Screw Threads


Purpose

This chapter provides an overview of how to:apply linear and angular dimensions to engineering drawingsuse a range of symbols representing common featuresrepresent screw threads according to standard practiceindicate standard procedures when applying dimensions.


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.


Table 2.1


Table 2.1 (cont)


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).



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).



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.



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.



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.



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).


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).



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.



Figure 2.12


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).



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.



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.


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).



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)



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).



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).



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.

Boundy8e ppt ch02

Education

engineering drawing

drawings dimensions

angular dimensions

mcgrawhill australia

drawings linear dimensions

overall dimensions

functional dimensions

auxiliary dimensions