Table 3.1 The standard linetypes and their applications'

1 2 3 Desig- Examples of nating , Description of linetype Example of line Typical application application letter (Figure number's

AContinuous — thick Visible outlines 3.1 Busbars and transmission paths 3.19

BContinuous — thin Fictitious outlines 3,1 • Dimension lines, projection

lines or extension lines 3.1, 3.13 Hatching 3 1, 3.9 Leaders 3.1, 113 Outlines of revolved sections 3.1 Imaginary intersection of - surfaces 3.8 Fold or tangent bend lines 3.7 Short centrelines 3.10 General purpose electrical conductors and symbols 3.19

CContinuous — thin, freehand c------------------------------ Break lines (other than on an axis) 3.1, 3.9

DContinuous — thin, ruled, zig-zag

Break lines (other than on an axis) 3.1, 3.8

E Dashed—thin lmm MINIMUM Hidden outlines 3.10 S—II I

2s I --I 4s q

F Dashed — medium 1mm MINIMUM Assemblies, subaSsemblies, boxes and containers for

S 11

- 2s - 1 1 electrotechnology only 4s '

Chain — thin Centrelines, pitch lines 3.10 Developed views Repeated detail

3.7 3.11

q I I

3 q Material to be removed =•.)

3.12

10q

H Chain — thick at ends and change of direction,

Cutting planes 3.6 ,

thin elsewhere (Proportions as for linetype G)

J Chain — thick Indication of surfaces requiring 3.1. 3.13 — —

(Proportions as for special treatment

linetype G)

Outlines of adjacent parts 3.1, 3.14 K Chain — thin, double - Alternative and extreme

dashed positions of movable parts 3.1 (Proportions as for Centroidal lines 3.6 linetype G) Tooling outlines 3.15

1.50 45 . + 2 . ® 1.48 -

HEAT CODE MUST APPEAR HERE

15.01 15.00

0° ®

0 40.01 0 39.99

1 07.51

- 07.49 0

LOCATE

r j A es I

Figure 11.1 Process drawing

Chapter 11

DEFINITION OF SIZE - Dimensioning and Tolerancing

11.1 Introduction Engineering drawings provide a total definition of a component or an assembly of several components. Previous chapters have shown how a Visual image can be created using shape elements. However, 'while a drawing containing shape elements alone is a useful indication of shape and form, it is incomplete without precise information on size. If the scale of the drawing is known it is possible to measure sizes from the drawing using a scale rule. This practice, referred to as "scaling" dimensions, would be a very tedious process to use in all but the most simple drawings. Inaccuracies in drawing and measurement combined with the dimensional instability of most drawing sheets render the process very unreliable. A general note which states:-

DIMENSIONS SHOULD NOT BE SCALED FROM THE DRAWING

should be placed on all engineering drawings to ensure that sizes are never determined in this manner.

Having outlawed the practice of scaling, the responsibility falls on the designer to place sufficient information on the drawing to clearly and unambiguously indicate all necessary sizes. Dimensioning should be consistent with the purpose of the drawing. For process drawings this may mean limiting dimensions to those indicating particular processing details (See Figure 11.1), while for assembly or layout drawings, usually only overall dimensions and some critical assembly dimensions are given (see Figure 11.2). However, the majority of drawings are detail drawings and require complete specification of the size, form and location of every feature of the item. This chapter describes the variety of ways, by which this task can be accomplished. Figure 11.3 shows a typical detail drawing of a simple item giving complete definition of that item.

114 DEFINITION OF SIZE

FOR KEY SEAT USE NO. 7 WOODRUFF KEY CUTTER

1.5 x 45°

35

h- A 1.5 x 45°

05 TYP

014.275 014.260

A

34.80 17.5 SECTION A—A 34.55

104.5 103.5

NOTE: UNLESS OTHERWISE STATED ALL DIMENSIONS IN MILLIMETRES

Figure 11.3 Detail drawing

015.140 015.125 0.1255"

0.1240"

F. 0.2505" 0.2455"

Figure 11.2 Assembly drawing pump and motor

The drawings shown in Figures 11.1, 11.2 and 11.3 indicate the size of all features requiring definition. In most cases these sizes also include some variation, indicating the acceptable spread in the actual component dimension. These variations are referred to as tolerances. Tolerances

are necessary because there are no production processes which can produce features on components with size, form and position exactly matching the design requirements. Without tolerances, modern industry could never produce acceptable products.

DEFINITION OF SIZE 115

Figure 11.4 Application of dimensions to suit design requirements

11.2 Functional dimensioning and tolerances

Dimensions are necessary to establish sizes, while tolerances account for the inaccuracies of the production processes employed to produce those sizes. The choice of which particular dimensions should be used, and the magnitude of the tolerances ascribed to them, must never be dictated by the production processes. Dimensions and tolerances should always be chosen to ensure the proper function of the item in question. It is critical at this point that the concept of "functional dimensioning" be fully understood. Most items do not "function" by themselves but do so as part of an assembly of items. Figure 11.4(a) shows a sectional arrangement of a rotating lever, item 3, which is fixed to a machine, item 1, by a stepped screw, item 2. This arrangement is only a small part of an overall assembly of many items but illustrates well the principles of functionality. The major functional requirement of this assembly is that the lever be able to rotate freely, but without excessive movement either radially, or along the screw axis. The designer must determine how much clearance there should be between the bore of the lever and the screw body to ensure free movement and decide what constitutes excessive movement. The level of axial play allowable for proper function should also be assessed. These decisions, although made on the most rational basis possible, will often involve a large degree of subjective assessment based on previous design experience.

The requirement stated above for axial play might be summarised by the dimensions shown in Figure 11.4(b). On any actual assembly of components the sum of dimensions A and B must not violate the axial play limits. Dimension C must be nori-zero. These dimensions are, however, assembly dimensions and are of no use for controlling the manufacture of the individual items. Figure 11.4 (c) shows each item with the dimensions that control the axial play requirements labelled F (functional). The dimensions given to complete the definition of the items should be chosen to meet other subordinate functional requirements, or to give a convenient basis for manufacture and measurement if they are non-functional. An example of a subordinate functional requirement might be in dimensioning the hexagon head of the screw to ensure a snug fit to a standard spanner size. The example quoted here is ob,viously extremely simple. For assemblies of reasonable complexity, this form of functional analysis is more difficult and requires specialised training.

11.3 Methods of indicating size form and position

Having established that each feature must be specified for size, form and position, it would be a very cluttered drawing if each of these was specified directly for every feature. There are many ways of specifying size form and position and each drawing invariably contains a mixture of the types discussed below. Figure 11.5 indicates the various types on a simple drawing.

116 DEFINITION OF SIZE

60 ± 0.05 +0.3

4 HOLES 015 0 PCD 80 ± as 40 ± 0.05 -

15 ± 0.1

0 0120 —0.1

0 0.05 — 029.9 —

0 049.6 —0.07—

11.3.1 Direct dimensioning Dimensions are given directly for the feature concerned. This may be by way of a length, diameter or angle. All functional dimensions should be shown using this method. In Figure 11.5 there are three shaft diameters and three lengths shown with direct dimensions. No direct dimensions should be capable of being deduced from other direct dimensions. If such an error were committed, it would lead to the possibility of confusing the tolerances which are applicable and the.paitis unnecessarily overconstrained. This overspecification is called redundancy and one of the dimensions is referred to as being "redundant" and should be omitted from the drawing. In every case the dimension chosen for omission should be the one which is least critical to function.

11.3.2 Indirect dimensioning Where a length, diameter or angle is not specified directly, it should always be possible to deduce it indirectly from the dimensions given for other features of the • item. Only dimensions which are not functionally critical should be dimensioned in this way. The overall length of the shaft in Figure 11.5 is an example of an indirect dimension. Obviously, such dimensions do not carry, a tolerance. The maximum possible error in manufacture is determined by the extremes of the dimensions whiCh define it indirectly.

11.3.3 Dimensions determined by convention This method of dimensioning is basically for form and position. If features are obviously drawn with a particular geometric relationship, that geometric relationship is assumed. For example, where faces are shown at right angles, squareness is assumed. Where no information

exists to the contrary, components are assumed to be symmetrical if they appear to have been drawn symmetrical. This convention dramatically reduces the amount of explicit dimensioning on a drawing. Imagine what a drawing would look like if such assumptions could not be made. The tolerances for such dimensions are those specified in the general tolerance note or, if none exists, they are determined by the "normal workshop practice" convention (Section 11.4.3). - In Figure 11.5 the faces of the flange are thus assumed to be square to the axis of the shaft and the various segments of the shaft are assumed to be concentric.

11.3.4 Dimensioning by notes Notes are often used to dimension features where it would be inconvenient or unnecessarily repetitive to dimension them directly. In Figure 11.6 the notes applied to the hole diameters are unambiguous and drastically reduce the number of dimensions shown on the drawing.

11.4 Methods of indicating tolerances The size form and position of every feature on a drawing carries a tolerance whether given explicitly or implied. Tolerances are expressed or implied by one of the following methods.

11.4.1 Direct tolerancing Where it is important to control the size, form or position of a feature to within a tolerance zone to ensure function, that tolerance should be stated directly on the drawing. This can be done by appending the tolerance to the dimension, or by adding it to a table with direct reference to the dimension concerned. As a general rule, all functional dimensions should be toleranced directly. The tolerances given on the diameter and length dimensions of . the flanged shaft in Figure 11.5 are examples of direct tolerances

ALL CHAMFERS 1.5 X 45° Figure 11.5 Methods of indicating dimensions -

DEFINITION OF SIZE 117

30 10

10

20

15 • • 0 •

4 HOLES 010

2 HOLES 05

Figure 11.6 Notes for holes

ALL SCREW THREADS TO AS 1275

Figure 11.7 Tolerancing by reference to a standard

TOLERANCES ON DIMENSIONS (EXCEPT WHERE OTHERWISE STATED):

UP TO 10 + 0.1 OVER 10 UP TO 100 + 0.2 OVER 100 + 0.5 ALL ANGLES + 1°

11.4.2 Reference to standard - In some cases a tolerance is , applied simply by referencing an international, national or local standard or specification. An example of a note referencing such a standard is shown in Figure 11.7.

11.4.3 General tolerance notes - Where a size is non-functional it may be covered by a general note placed on the drawing to cover dimensions without specific tolerances. Examples of such general notes are given in Figure 11.8. The use of general tolerances which relate the tolerance to the magnitude of the dimensions is preferred and the tolerances should, where possible, be chosen as a standard tolarance grade from AS1654 "Limits and Fits". Where there are untoleranced dimensions on a drawing, a general tolerance note should always be included. Without such a note it is implied that the specification allows as acceptable, any component dimensions manufactured to normal workshop practice. This is of course difficult to define and can lead to arguments over acceptability of product. The use of general tolerance notes for geometry tolerances implies that any two features on the item with the relationship in question should fulfill the stated tolerance. There is no implication that either be the datum for the other. For these reasons a general note for geometry tolerances should be used carefully and should give very generous tolerances.

11.5 Dimensioning and tolerancing entities

After deciding which dimensions and tolerances are required to define an item, and having determined their magnitude, it only remains to insertthem on the engineering drawing. To enable this to be achieved clearly and unambiguously a number of dimensioning entities are defined within AS 1100,201, and certain conventions have been laid down for their use. Figure 11.9 shows a partly dimensioned drawing containing each of the dimensioning entities. Their use is discussed in some detail below.

11.5.1 Projection lines Projection lines are the link between the item and the dimension line. They indicate the precise locations between which the dimension applies and allow the dimension to be conveniently arranged outside the item outline where possible. It is important that projection lines are not confused with the component outline and conventions have evolved to ensure this:- (i) Projection lines are drawn as thin full lines (Type B) and

in every case extend approximately 2 mm beyond the dimension line.

TOLERANCE ON CASTING THICKNESSES ± 1%

TOLERANCE EXCEPT WHERE OTHERWISE STATED: LINEAR + 0.2 ANGULAR +

Figure 11.8 Examples of general tolerance notes

118 DEFINITION OF SIZE

Min 15°

Min 15°

Trojection line passes through or terminates at point

(a)

Projection line posses through or terminates at point

(b)

Figure 11.10 Projection lines from points on surfaces

75 50

Projection line extends /Dimension and tolerance

beyond dimension line 30 ± 0.1 Projection line

Minimum lmm gap between-----e projection line and outline Dimension line

Arrowhead

Figure 11.9 The dimensioning entities

Character height = h Minimum gap between first dimension line and second

dimension line to be 21-1 Minimum gap between outline

and first dimension line to be 3h

Figure 11.12 Minimum spacing of dimension lines

DEFINITION OF SIZE 119

Zia Figure 11.11 Imaginary points of intersection

220 170

120 90

40

0-0 r

Figure 11.13 Dimensions from a common datum

(ii) When projection lines are extensions of outlines, a gap of approximately 1mm is left between the outline and the projection line as shown in Figure 11.9.

Where projection lines refer to points on surfaces or lines i.e. to a general point on a surface or line, rather than an edge, they may pass through, or terminate on, the point as shown in Figure 11.10. For clarity, oblique projection lines are used. In such cases a projection line should never be at an angle of less than 15 degrees to the part outline, or its tangent, at the point in question. Two, examples of oblique projection lines satisfying this criterion are shown in Figure 11.10. If projection lines terminate at non-existent points such as imaginary points of intersection they should be shown as in Figure 11.11. The outlines are extended using Type B lines with a small gap between the extension and the outline. The projection line should pass through or terminate at a dot drawn at the intersection point with a diameter approximately equal to the thickness of the outline, or 1 mm, whichever is greater.

11.5.2 Dimension lines Dimension lines are thin full lines (Type B), usually terminated by arrowheads, or in some special cases, by prominent dots. They indicate the extent of the dimension and are oriented to be parallel to the direction of measurement of the dimension. For clarity, dimension lines should always be placed outside the item outline where possible, and spaced to allow insertion of dimensions and tolerances, without crowding. AS1100,201 recommends the spacings shown in Figure 11.12 as minimum values. Arrowheads should be formed as detailed in Section 4.7.1. and should touch the projection lines.

Where there are several dimensions from a common datum, dimension lines may be arranged according to any of the four methods shown in Figure 11.13 In cases (b), (c) and (d) a prominent dot (of a diameter approximately three times the dimension line thickness, or 1.5mm, whichever is greater) is used to terminate the dimension string at the datum end. To avoid misinterpretation on a drawing, centre lines, extensions of centrelines and extensions of outlines should never be used as dimension lines. This common mistake is illustrated clearly in Figure 11.14(a). Note how much drawing clarity is improved when the correct dimensioning techniques are applied (Figure 11.14(b)). Dimension lines may be broken in some instances to allow dimensions to be inserted. For guidance on breaking dimension lines see Section 11.10.1. Angular dimension lines are similar to those for linear dimensions but formed as an arc centred on the vertex of the angle being dimensioned. The rules for their use are the same as for linear dimensions. Examples of angular dimension lines are shoWn in Figure 11.23:

120 DEFINITION OF SIZE

(b) Acceptable

(a) Not acceptable

Figure 11.14

Centerlines and extension lines not to be used as dimension lines

0 20 ± 0.02

THIS FACE TO BE POLISHED.

ENGRAVE PART NO

15 ± 0.02

(a) At Outlines

(b) On Surfaces

(c) At Dimension Lines

Figure 11.15 Termination of leaderlines

11.5.3 Leaders Leaders are the lines used to indicate where dimensions or notes apply. They are a thin, full line (Type B) and terminate at the note on one end and the point of application at the other. The termination of leaders at the point of application depends on circumstances but the general rules are: (a) At outlines, leaders should terminate with an arrowhead,

as shown in Figure 11.15(a).. (b) Within outlines, such as on surfaces, leaders should

terminate with a prominent dot as shown in Figure 11.15(b). The diameter of the dot sh6uld be equal to the thickness of the component `outline but not less than 1 mm.

(c) At dimension lines where direct placement of a dimension is precluded by overcrowding, the leader from the dimension to the dimension line is terminated without arrowhead or dot, as shown in Figure 11.15(c).

To avoid confusion between leaders and outlines a leader should never terminate at an outline such that the included angle between the two is less that 45°. The acceptable region is shown diagrammatically in Figure 11.16. Considerable drafting time can be saved by connecting one note or dimension to a number of points on the drawing where it applies. However, this should never be done where it requires long leaders Figure 11.17 and Figure 11.18 show two examples of long leaders which are not

DEFINITION OF SIZE 121

An acceptable leader)

45°

Leaders should not fall within shaded regions.

Figure 11.16 Leaders touching lines

3 HOLES X 05

3 HOLES Y 010

(a) Acceptable

\ LES3 HO

444tio

\ 3 HOLES 010

(b) Unacceptable

Figure 11.18

Use of letter symbols to avoid long leaders

R 2.5 (a) Not acceptable

(b) Acceptable Figure 11.17

Use of repeated dimensions to avoid long leaders

CASTING RADII R 10 TYP

Figure 11.19 Use of notes to avoid repeated dimensions

acceptable. One alternative is to repeat the dimension or note each time it is required as in Figure 11.17. The problem can also be overcome bylabelling the multiple features with distinguishing letters as in Figure 11.18, or by showing one dimension and indicating that this is typical of other similar points on the drawing. The letters TYP are used to indicate this condition as shown in Figure 11.19. In other simple cases where the intention is absolutely clear, a single feature of a group may be referenced with an indication of the number of features in the group as in Figure 11.6. These last two practices are not specifically recommended in AS 1100 but are used in practice.

122 DEFINITION OF SIZE

Figure 11.20 Dimensions placed within a view

Figure 11.21 Partially dimensioned object showing

dimensions placed in relation to views and features

11.6 Placement of dimensions and notes

The placement of dimensions and notes to convey information clearly and unambiguously is an art. However the following guidelines should assist the beginner.:

(a) Dimensions and notes should be placed where the reader of the drawing will naturally look for them and where they are easily found.

(b) As far as possible, dimensions should be placed outside views. Dimensions may be placed within views if long projection lines crossing many other lines would be otherwise needed (Figure 11.20). When inscribed within a view, dimensions must be well clear of other lines. If necessary, centre lines may be interrupted to allow insertion of dimensions.

(c) Dimensions should be placed between views where the dimension projection lines tend to direct the eye to corresponding features in adjacent views and thus help interpretation.

(d) Related dimensions should, where possible, be placed on the same view.

(e) Dimensions relating to shape, contour, etc. should be placed on the view which shows the shape or contour concerned.

An example of the application of these recommendations is shown in the partly dimensioned item of Figure 11.21. The dimensions defining the inclined surface are related and are all given on the view which shows the surface as inclined. In addition, the dimensions are placed between that view and the adjacent views, thus helping to identify on those views the outlines which represent the intersection of the inclined surface with the horizontal and vertical surfaces of the object

The rectangular recess is dimensioned and located on the top view in which the shape of the recess is shown.

The two holes are related functionally and all dimensions relating to size and location are conveniently placed on the right-hand view where the holes are immediately identified by their shape.

11.7 Dimensions Dimensions are generally made up- of two elements, a numerical value and a unit. They should be clearly indicated, in an unambiguous fashion, adjacent to the dimension line or leader, or placed within a note. For details of the arrangement of dimensions on drawings consult Section 11.10.

11.7.1 Numerical values Numerical values of dimension should always be expressed to the number of decimal places necessary for complete definition of the design requirements. The decimal sign, where applicable, should be a dot placed level with the base of the character. Where a quantity is less than unity, the decimal sign should be preceded by the zero numeral

e.g. 0.25 not .25

To allow easier reading of long numbers there is a convention that a full space should be used to divide each group of three numerals to the right or left of the decimal sign

e.g. 25 000 2 500 2.498 5 427.624 85

If there are only four numerals to the left or right of the decimal sign the space is optional. For all numbers having trailing zeros to the right of the decimal sign these redundant zeros must be omitted. Where a dimension is an integral number of units, the decimal sign (as in the second example above) must also be omitted.

e.g. 50.2 not 50.20 50 not 50.0 or 50.

These rules for trailing zeros do not apply to numerical values expressing limits of size as described in Section 11.8.1.

DEFINITION OF SIZE- 123

0 39.90 039.78

040.22 0 40.00

(a) Lim its of 1

Size

0 039.90 —0.12

+0.22 0 40.0 0

(b) Size and Unilateral Tolerance

(c) Size and Bilateral Tolerance

(d) Single Lim it of Size

Figure 11.22 Linear tolerance presentation

11.7.2 Units 11.7.2.1 Linear dimensions • . The units for all linear dimensions should be from the SI system (millimetres preferred) unless there are very good reasons for using imperial, i.e. inch, units. SI units are listed in Appendix D.

It is definitely not good practice to mix linear units on the one drawing (particularly mixing inches and millimetres). However, in very exceptional circumstances, this may be necessary. An example of when it might be necessary was given in Figure 11.3. The specification of dimensions and tolerances for the Woodruff keyway were given in imperial units on . a metric gaming as Woodruff keys are not commonly available in metric sizes. If all, or most, units are of the one type, it creates unnecessary crowding on a drawing to include a unit with every numerical value. In practice, one of the following systems may be adopted in order of preference. (a) Where only one unit is used, the drawing should

include a prominent note, preferably in or near the title block, indicating which units are applicable eg.

DIMENSIONS IN MILLIMETRES In this case no linear units should be included in the body of the drawing..

(b) Where the units are mostly of one type, with occasional use of another, a prominent note should be, included to indicate the more common units e.g.

UNLESS OTHERWISE STATED DIMENSIONS IN MILLIMETRES. •

In this case the units which are in the minority should be expressly stated by placing the appropriate unit abbreviation after the numerical value, with a half space separation.

e.g. - 14m 265m • (c) In the situation where neither (a) nor (b) is applicable

i.e. where there is a general mixture of units, all units should be included following numerical values. As in (b) above, the appropriate unit abbreviation should follow the numerical value separated by a half space.

11.7.2.2 Angular dimensions Angular dimensions should always be expressed in one of two ways

(a) Degrees and decimal parts. The numerical value expressed as a decimal number is followed by a degree symbol. e.g. 22.5° 120.47°

(b) Degrees, minutes, seconds. The angle is expressed as a combination of degrees and minutes didegrees minutes and seconds with the numerical values for each separated by the appropriate symbol. e.g. 22° 30' 2° 4' 5" Leading zeros may be used before minutes and seconds when the numerical value is less than 10. e.g. 2° 04' 05"

In either system when an angle is less than one degree it should be preceded by 0°.

e.g. 0.5° 0° 30' 0° 21' 12"

Unless it is absolutely necessary, as in the case of a toleranced functional angle, an angle of 90 degrees need not be dimensioned. Remember that if it looks to be drawn as 90 degrees, it is assumed to be 90 degrees. (See Section 11.3.3).

124 DEFINITION OF SIZE