UNIT 1 1.1 Introduction The term draughting is used to ...

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UNIT 1

GUIDELINES FOR DRAFTING

1.1 Introduction The term draughting is used to describe the language of drafting in this book. It defines the terminology, symbology,

conventions, and standards used in drafting. Drafting is the process of creating technical drawings consisting of 2D

images and annotations. Draughting is the universal technical language that is used for clearly and accurately

describing the form, size, finish, and color of a graphic design model for construction or recording. Draughting

guidelines deal with standards and conventions in drawing media, lettering, linestyes, projection standards, plot

scales, dimensioning rules, sectioning rules, etc. In this chapter, we will concentrate mainly on drawing media,

lettering, and linestyles while others will be discussed in the appropriate chapters.

Drafting skills involves learning to correctly apply the rules of draughting in creating acceptable or industry standard

technical drawings. Proficiency in drafting involves being able to create high quality technical drawings. Becoming

proficient in drafting must be a commitment executed with determined effort. The images in drafting are constructed

from lines and curves while annotations are composed from characters. Drafting uses different linestyles and symbols

to describe object models. Some standards and convention apply to both lines and characters in drafting. 2D technical

drawings may be created using axonometric and perspective principles. Axonometric drawings are 2D drawings

obtained by orthogonal projections of 3D objects and include orthographic, isometric, dimetric, and trimetric

drawings. Pictorial drawings such as isometric and perspective drawings mimic 3D objects in appearance but are

made of 2D entities by composition.

1.2 Conventions and Standards Draughting principles, conventions, rules, and standards help to minimize misinterpretations of drawing contents and

eliminate errors in the communication of technical ideas. Conventions are commonly accepted practices, methods or

rules used in technical drawings. Standards are sets of rules that govern the representation of technical drawings.

They are established through voluntary agreements. Standards ensure clear communication of technical ideas. The

design drafter must study and understand these conventions and standards and learn to apply them correctly in

practice. For example, good technical drawings are achieved by following some principles such as:

1. Keeping all lines black, crisp and consistent.

2. Using different linestyles.

3. Ensuring clarity in linestyle differences such as in thickness or line weight.

4. Ensuring dashes have consistent spacing with definite endpoints.

5. Keeping guide or construction lines very thin.

6. Ensuring that corners are sharp and without overlap in drawing views.

7. Placing dimension with thoughtfulness and adequate spacing.

8. Making notes simple and concise.

9. Making drawing readability a high priority.

10. Ensuring a pleasing drawing layout.

Principles 1 to 6 are largely built into Computer-Aided Drafting (CAD) software. This means the CAD operator need

not worry about them, except know what linestyle to use for different features of objects and assign appropriate line

weight or thickness. However, principles 7 to 10 must be mastered and consistently applied. These have bearings on

accuracy, legibility, neatness, and visual pleasantness of drawings.

There are national and international organizations that develop or manage the development of standards. Examples

are the American National Standards Institute (ANSI) and the International Standardization Organization (ISO).

ANSI is a federation of government, many private companies, professional, technical, trade, labor and consumer

organizations that serves as a clearinghouse for nationally coordinated voluntary standards. The standards may deal

with dimensions, rating, test methods, safety and performance specifications for equipment, products and

components, symbols and terminology, etc. Major contributors to ANSI standards include American Society of

Mechanical Engineers (ASME), Institute of Electrical and Electronic Engineers (IEEE), American Society for

Testing Metals (ASTM), etc. Drafting standards are specified in ANSI-Y14 documents which give only the character

of the graphic language. It is to contain twenty-seven or more separate sections when completed. ANSI/ASME

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Y14.2, Y14.3, and Y14.5M are popular draughting standards in the U.S.A. Sample sections of the standard are given

Table 1.1.

Table 1.1: Some ANSI-Y14 standards

Item Section

Size and format Y14.1

Lettering and linestyles Y14.2

Projections Y14.3

Pictorial Drawings Y14.4

Dimensioning and Tolerancing Y14.5M

Screw Threads Y14.6

Gears, Splines and Serrations Y14.7

Mechanical Assemblies Y14.14

ISO is a non-governmental worldwide body that coordinates standards development process in virtually every area of

human activities. It is located in Switzerland and was founded in 1947. Membership includes over 150 countries with

each country represented by one national standards institution. ANSI is the U.S. representative to ISO. ANSI

standards are usually similar but not identical to ISO standards. The design drafter must be diligent in adhering to the

standards that are relevant to a particular work. Table 1.2 gives some ISO drawing standards documents.

Table 1.2: Some ISO drawing standards

Item Section

Technical drawings: sizes and layout of drawing sheets ISO 5457

Technical drawings: general principles of presentation ISO 128

Technical drawings: methods of indicating surface texture ISO 1302

General tolerances ISO 2768

1.3 Drawing Units All engineering drawings must carry a unit of measure. This is required so that the drawing size can be correctly

interpreted. Since graphics have linear and angular attributes, the units of length and angles are indispensable in

drafting and design.

Units of Length The SI unit of length is the meter. The English or U.S customary (I-P) unit of length is the foot (ft). Table 1.3 shows

the length denominations for SI and I-P units.

Table 1.3: Drawing Units SI: meter (m) Customary: Inch (in)-foot (ft)

1 m = 1000 mm = 103 mm

1 m = 100 cm = 102 cm

1 km = 1000 m = 103 m

1 in = 16 lines

1 ft = 12 inches

1 in = 25.4 mm

The SI linear unit for drafting is the millimeter. Mechanical drawings are dimensioned in millimeter (mm).

Architectural drawings may be dimensioned in millimeter (mm) and meters (m). Meters and kilometers (km) are used

for civil dimensioning. Only decimals are used in metric dimensioning, fractions are not allowed. For numbers less

than 1.0 which must be expressed as decimals, a zero before the decimal marker is preferred. For example 0.234 is

preferred to .234. The period symbol is the decimal marker in this example.

In I-P units, mechanical drawings are dimensioned in decimal inches, architectural drawings are commonly

dimensioned in feet (‘) and fractional inches and civil drawings are dimensioned in decimal feet and inches.

Drawings in metric units carry a general note such as “all dimensions are in millimeter, unless otherwise stated” or

the label “METRIC”.

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Units of Angle Angle refers to the relative orientation of lines on a plane or the relative orientation of planes in space and is

measured in degrees (o) or radians. There are 360 degrees in a circle; 60 minutes in a degree; and 60 seconds in a

minute. The radian is the SI unit of angular measure. One radian is approximately 57.3o. However, the degree is the

unit of angular measure in technical drawings.

1.4 Drawing Media

Drawing media are physical materials that can retain graphic and textual information for a reasonable time period.

The information is put on their surfaces. Physical drawing media are used to produce hard or paper copies of models

and drawings. Certain characteristics make these media suitable for drawings and include smoothness, eraseability,

dimensional stability, transparency, durability, and cost. Smoothness describes the ease of the media to accept lines

and letters without excessive effort. Eraseability describes the ease of the media to allow lines and letters to be erased

and cleaned up. Ghosting is a term used to describe the mark left after lines are erased. The more visible they are, the

poorer the eraseability. Dimensional stability refers to the ability of the media to retain size in varying weather

conditions. Transparency allows drawings on one side of the media to be visible on the other side. This used to be an

important characteristic in traditional drafting but photocopying technology and plotter capabilities today, make this

requirement a non-critical factor. Durability refers to the ability of the media to resist normal usage wear and tear.

Wear and tear is ever present because wrinkles develop with usage that renders drawings difficult to read or

reproduce. Drawing media include bond stationary, vellum, mylar, grid papers, and tracing papers.

Bond stationary or plain paper is good for all types of technical drawing. They are made from wood pulp of higher

quality than newsprint. However, they have low durability. There are different grades of plain paper in the market.

The better ones are whiter and smoother. Plain papers should be preferably used for sketches, exploratory design

drawings, and check prints.

Vellum is the most popular drafting paper. It is specially designed to accept pencil marks and ink. It has good

smoothness and transparency but susceptible to humidity and other weather conditions. This makes it not to be very

stable dimensionally. Some brands have better eraseability.

Mylar is a plastic type (polyester) drafting material that has excellent dimensional stability, eraseability, durability

and transparency. It takes ink easily but it is expensive and requires special polyester lead for drawing on it. It is thus

used for very high quality jobs or when cost is not a factor. Mylar may have single or double working (mat) surfaces.

The single mat surface is more common.

Tracing paper is a translucent medium that is good when the need to reduce manual repetitive work is considerable.

It can also be used to obtain a final sketch if the original sketch was drawn on a grid paper. The grid background is

not traced in this case. Tracing is a fast and accurate method of reproducing existing drawing manually.

Grid papers are especially helpful for good alignment and proportioning of features on drawings when sketching.

Advantage should be taken of them whenever available. The square grid is used for sketching orthographic views and

isometric grid is used for sketching isometric views. These grid papers are very common.

Drawing Sheet or Paper Sizes

Paper or sheet sizes have been standardized by ANSI and ISO. Standard drafting papers are available in sheet or roll

form. Table 1.4 summarizes the standard paper or sheet sizes for English (ANSI) and Metric (ISO) applications. The

sizes are the overall dimensions of the sheets without allowance for margins. Note that Metric is the preferred units.

Roll sheets come in different widths and lengths with the width usually corresponds to one of the standard sheet

dimensions as shown in Table 1.4. Metric roll sizes vary from 297 mm to 420 mm in width. Large Metric sheet sizes

are cut from Metric rolls. Roll sizes in English unit vary in width from 18” to 48” and the usual length of a roll is

100’ length. In English unit, large sheet sizes F, G, H, J, and K are cut from rolls. In most situations, the paper size is

specified by the company or stated in a given problem.

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Table 1.4 Standard Paper Sizes

Metric Sizes (mm) English Sizes (Inches)

A4 210 x 297 A 8.5 x 11

A3 297 x 420 B 11 x 17

A2 420 x 594 C 17 x 22

A1 594 x 841 D 22 x 34

A0 841 x 1189 E 34 x 44

Sheet Orientation

Standard drawing sheet may be oriented with the long side horizontal and the short side vertical as shown in Fig.

1.1a. This type of orientation is known as landscape and is generally preferred for sheet sizes B, C, D, and E in

English unit or sheet sizes A3, A2, A1, and A0 in Metric unit. Occasionally, portrait orientation as shown in Fig. 1b,

is used, but is largely limited to A-size sheet in English unit and A4-size sheet in Metric unit. In this layout, the short

length of the sheet is horizontal and the long side is vertical.

a) Landscape b) Portrait

Fig. 1.1: Drawing sheet orientations

1.5 Sheet Layout Drafting paper layout refers to the arrangement of information on the paper. Fig. 11-1 shows the general layout of a

template drawing sheet. Broadly, the information in a drawing sheet may be classified into two groups of technical

and administrative. The technical information consists of drawing views and annotations. The annotation depends on

the amount of details desired in a drawing and may include dimensions and tolerances, notes, and bill of materials in

assembly drawings. The technical information usually takes the greater portion of the drawing sheet. Administrative

information on a standard drawing sheet includes title block and revision block information. A margin is provided at

each edge of the sheet and is defined by the border line (not shown in Fig. 11- 1) that is drawn at some

distance from the edge. Top, bottom, left and right-side margins are provided. They provide spaces for filing and

handling the sheet. A more detail discussion of the content of a drawing sheet follows. Specifics will vary from

company to company and from one country to another.

Sections on a Sheet Drafting paper layout refers to the arrangement of information on the paper. Fig. 1.2 shows a general sheet layout.

The content of a drawing paper includes margins, drawing views and annotations. The margins are defined by border

lines drawn at some distance from the edges of the paper. Top, bottom, and right-side margins are in the range of

12.5 mm (1/2”) to 25 mm (1”), depending on the paper size. The left-side margin is between 12.5 mm (1/2”) to

40mm (1-1/2”) wide to allow for binding of sheets. The right margin is about 25% to 50% smaller than the left

margin. Drawing views depend on the type of documentation required and annotation content will vary accordingly.

Zoning

Zoning is a technique used in large paper sizes to aid in quickly locating information on a drawing. It involves

assigning spaced numbers on the top and bottom margins of a sheet and spaced letters on the left and right margins as

shown in Fig. 1.2. This creates a grid system on the drafting paper that is similar to that used for reading information

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on maps. A zone is defined by the intersection of a letter segment and a number segment. Since a zone is a very small

section of the drawing paper, locating a piece of information in it is fast. The hatched block in Fig. 1.2 above is for

zone “B3”.

Fig. 1.2 Sheet layout elements

Title Block

By ANSI standard, title block should be located on the lower right corner of the drawing sheet. Though different title

block designs are used by companies, the information contained in it is fairly general. Most information in a title

block includes:

1. Company: Name, address, phone number.

2. Project/Client: Project number and title/ Client’s name and address

3. Drawing: Name or title/number.

4. Personnel: Designer, Drafter, Checker, Approver.

5. Scale: Ratio of design and drawing sizes.

6. Date: Completion date of drawing or project.

7. Sheet: Size and number (page) of sheet in drawing set.

8. Revisions block: A block for revision notes.

9. General tolerance: Tolerance applied to a size when unspecified.

10. Projection type symbol: First or Third angle.

Bill of Materials (BOM)

An assembly drawing should have a bill of materials or parts list. It is usually a table list

of the parts or components in an assembly. Table 1.4 shows a sample of a simple bill of

materials. By ANSI standard, it should be located on the lower right corner of the

drawing sheet. Important information in BOM is part name, item number, part material,

and quantity, part number or catalog number for standard parts. The item number is the

number assigned to a component in a particular assembly drawing, a form of local

identification and can change with different assembly drawings. The part number is a

fixed number assigned to that specific component, a form of company/global

identification and should not change for different drawings. Other information like

weight, stock size, etc, may also be included in the parts list. Fig 1.3: A simple bill of materials

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1.6 Lettering and Annotations Annotation is the addition of textual information to a drawing view. When annotation is done manually, it is called

lettering used to be a tedious and time consuming task, but things are quite different now with computers. Computers

have greatly increased the speed and quality of lettering. Text information consists of groups of characters that

express meaning which could be words, phrases, and or sentences. In technical graphics, the aim is to communicate

clearly and legibly so as to avoid misinterpretation of intent and purpose. The factors that can greatly affect legibility

are:

1. Font 2. Character size (text height) 3. Character spacing

4. Word spacing 5. Line spacing (leading)

Lettering Conventions Characters have different model designs known as fonts. A font is a set or family of character design with specific

attributes which determine the print appearance of the characters. The attributes hold the information about the

character set. Simpler font styles are easier to read; therefore open clean-cut characters are the best for drafting.

ANSI standard font for lettering in technical graphics is single-stroke Gothic font. Each character in this font is made

up of a single straight or curved line element. This makes it easy to draw the characters and make them clear to read.

There are upper-case, lower-case, and inclined Gothic letters. However, the vertical Gothic letters have become

industry standard. Fig. 1.5a shows vertical uppercase letters, Fig. 1.4b shows numbers, and Fig. 1.4c shows

lowercase letters and proportion and H is the symbol for text or character height in the figure. Characters in

annotations may be inclined from the horizontal at an angle defined by 5/2 (rise over run), approximately 68% per

ANSI as shown in Fig.1.5.

Fig. 1.4: Vertical characters Fig. 1.5: Inclined characters

An important attribute of a font is the text height or font size. Text height is measured in linear unit of mm (inch).

The ANSI recommended text height is 3 mm (1/8”). The width of characters varies depending on the specific font.

Some are narrow like “I” and others wide like “W”. The ratio of a character height to the width is described as width

factor or aspect ratio. Common aspect ratios for characters are 5/6, 1, and 4/3. The spacing between words should be

approximately equal and a minimum of 1/16” (1.5 mm) is recommended. A full character height for word spacing is

preferred. The spacing between lines should be at least half the text height but preferably a full text height. Sentences

should be separated by at least one text height; however, if space allows, two text heights should be used.

CAD Annotations The textual information and symbols added to models and drawing views for complete documentation of design are

commonly called annotations. Annotation information may be divided into two categories of technical and

administrative information. Administrative information includes revision notes and title block. Revision notes are

used for document control and record keeping of changes in design. The title block contains vital information about

the company and the drawing. Technical information includes bill of materials, dimensions, notes, and specifications.

Dimensions are the size values of objects and tolerances are permissible variations on object sizes. In Fig. 1.6, the

diameter size of 20 mm has a tolerance of 0.05 mm. Annotation symbols are commonly used for geometric

tolerancing and dimensioning. Notes are explanatory or required information needed on models and drawings for

proper interpretation. There are two types of notes found in drawings: general and local notes. General notes apply to

the whole drawing and may be placed in the title block or at the bottom of a drawing view area. Local notes apply

only to a portion or specific features in a drawing and are placed close to the feature referenced. A leader line can

link a local note to a feature or portion of a drawing; callouts and balloons are special formats of placing local notes.

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Fig. 1.7 shows examples of a leader, balloon and callout. Balloons are local notes placed inside a shape (circle,

diamond, etc). Callouts are local notes placed without a shape. Notes should be made simple and concise.

Specifications are technical requirements and are usually about material type, processing, and finishing. They often

appear as general notes or are put together as separate documents. Leader lines are thin continuous lines used to

direct information to specific features in a drawing. A leader line has an arrow head, an inclined segment and a

horizontal segment as a tail. The inclined segment connects the arrow head with the horizontal segment.

Fig. 1.6: Drawing with tolerances Fig. 1.7 Leader, balloon, and callout

Annotation in CAD is much easier than lettering. CAD letters are neat, consistent, stylish, and can be created with

speed and accuracy. Many fonts are available in the CAD software so there is a tendency to use several fonts in CAD

lettering. However, this should be limited, perhaps to two or three. Fig. 1.8 shows a sample of fonts. In architectural

drawings, Country blueprint and City Blueprint are popular fonts while simplex font is popular in mechanical

drafting. Placing text in CAD drawings requires decisions on text height and inclination angle at the least. The

inclination angle of text is 90o by default, but this could be changed. The recommended inclination angle is about 68

o.

The position of the text is often selected by clicking with a mouse. Text alignment or justification is important in

CAD lettering because it affects document appearance and readability. Text can be aligned to the left (left justified),

aligned to the center (center justified), or aligned to the right (right justified). Texts that are aligned on both left and

right edges are referred to as fully justified. In technical notes, text should be left justified. Character, word, and line

spacing have been discussed above. Fonts can be formatted by applying different treatments like bold, italic,

underline, etc. These are called special effects. They add aesthetics and emphasis to annotations.

Font Name Lower Case Upper Case

Arial Lettering LETTERING

Century Gothic Lettering LETTERING Helvetica Lettering LETTERING Impact Lettering LETTERING

Simplex Lettering LETTERING

City blue Lettering LETTERING Country blue Lettering LETTERING

Fig. 1.8: Samples of fonts

The plot height of a character is the actual size on a printed sheet and may be small print, normal print, or large print.

The normal print is the recommended ANSI text height of 3 mm (0.125”). Normal print is used within the drawing

views area and works fine for average-sized sheets such as A4 (A-size) and A3 (B-size). Dimensions, notes and

specifications should be printed in normal print or standard height. Small prints are smaller than the normal prints

and are used when space is limited. They may vary in height from 1.5 to 2.5 mm. It is often used in revision blocks

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and part lists or BOM. Plot height in large prints can vary from 5 to 10 mm (0.188” to 0.375”). They are used for

headers, view names, titles, labels, and numbers in title blocks. For large-sized sheets, text height of 0.175–0.25” (5–

6 mm) are common; but may be as high as 0.375 (10 mm). Text height for zone letters and numbers is usually larger

than those for dimensions or tolerances. Un-crowded text (high aspect ratio) is easy to read but needs more space

than crowded text (small aspect ratio). Some companies may prefer crowded text to un-crowded, however, clean and

easy to read annotations should be the goal. It is good practice to find out what the convention is in your company

and stick to it! The design drafter must choose a plot size that is legible and comfortable when hard copies are made.

Small plot sizes tend to be “hard” on the eyes and should normally be avoided.

In CAD situations, there are two aspects of text height: plot size and screen size. The plot size is the actual text height

value on a printed or plotted document. ANSI recommended plot size for small sized drawings is 3 mm (0.125”). The

screen text size in CAD is the text display size on the monitor screen. This may be different from the plot size if a

drawing is not full scale in the default workspace of a CAD package. In this case, a screen factor must be applied to

the desired plot size for comfortable reading or viewing on the screen. The screen text height is the plot size times the

screen factor in reduction scaling where the image plot size is smaller than the image design size. The screen text size

is the plot size divided by the screen factor in enlargement scaling where the image plot size is larger than the image

design size. The ANSI standard plot or print text height of 3 mm (1/8”) works well with A4-size (Metric) or A-size

(English) sheet. For other sheet sizes, some adjustment in text height may be necessary for comfortable reading of

prints.

1.7 Linestyles

Linestyle describes the visual appearance of lines on papers and monitor screens. Drafting uses different linestyles

and symbols to describe object models, especially in describing details of 3D graphics in 2D space. Good line quality

is essential for accurate communication of drawings. CAD linestyles are crisp, consistent, clear and different

lineweight and colors can be assigned to them. Their dashes have consistent spacing and constant width. Fig. 1.9

shows some line styles.

Fig. 1.9: Linestyles

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There are two fundamental linestyles; namely continuous (solid) and broken lines. Continuous lines have no gaps but

broken lines do. Continuous line variants include visible (object), construction, extension, and border lines. These

lines are distinguished by thickness or width. ANSI recommends two line weights of thick and thin with the thick

twice the line weight of the thin. Thick lines have width greater than 0.3mm and thin lines have width of 0.3mm or

less. Visible and border lines are thick while guidelines, construction, and extension lines are thin. Broken lines have

visible gaps between consecutive line segments. The length of dash lines can vary from 3 mm to 10 mm (1/8” - 3/8”)

and the gap can vary from 1.5 mm to 3 mm (1/16”–1/8”). Thickness of lines and length of dashes mentioned here are

best for A-size sheet.

Visible (object) lines are thick continuous (solid) lines that represent visible edges or outlines of object. Straight

edges are formed where two planes intersect. Curved edges arise from curved faces and surfaces. Visible lines should

be crisp and black with thickness of 0.40 mm, 0.50 mm or 0.60 mm, depending and sheet size, but ANSI

recommended thickness of visible line is 0.60 mm.

Hidden lines are thin dashed lines representing edges that are within the object or behind some features and so are

not directly seen from a view direction. The edges are known to be physically present in an object. Hidden lines

generally have dash length of 3 mm (1/8”) and a gap of 1 mm (1/32”) but can vary with sheet size or drawings. The

gap is about a quarter of the dash length. Hidden lines should start or end at visible or other hidden lines. No gap is

allowed between hidden and visible lines.

Centerlines are thin broken lines of alternating long and short strokes separated by a gap. Centerline is used to show

and locate centers of circles and arcs and to represent lines of symmetry and paths of motion in objects. Center lines

should cross visible lines with 3 mm or more beyond them. The gap and short stroke are of equal length. The short

stroke is about a quarter of the long stroke which is about 10 mm long.

Dimension lines are continuous thin lines used to indicate the value of a dimension. A dimension line has three

elements: the dimension value, the terminator, and the stem. The stem is the thin line that ends with the terminators at

both ends. The terminator may be arrows (usually filled), slashes, or filled circles. The dimension value may be

placed on top of the stem or at a broken portion of the stem.

Extension lines are a pair of continuous thin lines used to establish the extent of a dimension. The extension line

references a point on a feature with a small gap (1.5 mm minimum) between the point and the beginning of the

extension line. They are used in conjunction with dimension lines and slightly extend beyond the dimension lines

about 3 mm. Extension and dimension lines are always perpendicular.

Phantom lines are thin dashed lines used to identify alternative positions of moving paths, adjacent positions of

related paths, or repetitive details. A phantom line consists of a long dash, two short dashes, and gaps between the

dashes. Gaps are about 3 mm long but can vary.

Cutting plane lines are used to indicate the position and direction of view for cutting planes placed on an object

model to create section views. They are also used to indicate auxiliary view plane and direction. Cutting plane lines

are either thick phantom or hidden lines with arrow heads that are normal to the main lines. The arrows point in the

view directions. The long dash is about five times the short dash. The short dash and gap are of equal length. Gaps

are about 3 mm long but can vary.

Section (hatch) lines are thin inclined lines used to identify a solid material cut through by a section plane. They

form a pattern on the section affected. Section assembly drawings often have components of different materials in the

section plane. The deferent materials are distinguished by using different angles for section lines in the section.

Section line angles normally vary between 15o to 75

o.

Break lines can be either thin or thick. Long breaks are thin while short breaks are thick. They are used to show that

some portion of an object is left out. Short break line is used for small areas of interest and allows greater details to

be shown. Long break lines are used when space needs to be saved in representing very long objects. Usually, the

middle portion of the object is broken off or the portion without additional information is left out.

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Stitch lines consist of a series of dots and are also called dot lines. They may be used as projection lines or guidelines

in grid papers used for freehand sketching.

Precedence of Linestyles When lines of different styles overlap or coincide in a view, some take precedence. Generally, lines of thicker weight

take precedence over others of thinner weight. Visible lines take precedence over all other linestyles. The following

order of precedence is generally accepted: visible, hidden, cutting plane, centerline, break line, dimension and

extension lines, and hatch line. If more than one linestyles coincide in a view, then the rule of precedence must be

applied. 1.8 Applying Linestyles Fig. 1.10 shows a drawing view with several linestyles used in representation. The visible, hidden and centerline

styles are perhaps the most frequently used in drawings. Though CAD has highly simplified linestyle creation and

placements, attention should be paid to the placement of center lines. This is because when the length of the

horizontal and vertical center lines are unequal over a circle or arc, the center mark for the circle or arc will appear

unequal. This does not give a neat appearance in a drawing. One way to fix this is to draw the center lines across the

circle or arc diameters. Then scale the center lines with a scale factor slightly more than 1.0, say 1.25 or 1.5.

Fig. 1.11 shows the use of centerlines and center marks. Note that centerlines must not terminate on visible lines.

They should extend beyond visible lines at least 3 mm. The center marks may be used in place of centerlines in

circles or arcs of small radii or when overcrowding of line types may be a problem. This is due to concern about

drawing clarity and readability, a top priority in graphic communication. Conventions and standards must be applied

to ensure unambiguous communication. Center marks are easy and fast to apply to drawings in CAD systems.

Fig. 1.10: Drawing view with different linestyles Fig. 1.11: Use of centerline and center mark

Linestyle mistakes used to be quite common with board drafting. However, CAD has largely eliminated these

because the coding of the CAD software can implement consistent and accurate line weight, line crossing, and

display. But in freehand and instrument sketches, efforts must be made to avoid these errors.

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1.9 Modern Drafting Technology

Up till the 1970s engineers and draftsmen use the drawing board for making and modifying drawings on

paper with ink or pencil. Computer design drafting (CDD) was born in 1980s with the gradual introduction

of computer as drafting and design tools. Since then, the drawing board has been practically replaced by

computers and CAD packages. Computers of different sizes and capabilities can be used in design and

drafting. As the prize of computer hardware components dropped and the processors became more

powerful in the 1980s, CAD software began to migrate into microcomputers, desktops and laptops.

a) Desktop Computer b) Laptop Computer

Fig. 1.12: Common computers used for CDD (Google images)

a) Ink jet printer b) Laser jet printer

Fig. 1.13: Common prints used for CDD (Google images)

a) Roll large format printer b) Very large format printer

Fig. 1.14: Common plotters (Google images)

1.10 Summary Technical documents consist of images and annotations. The images are constructed from lines and curves while

annotations are composed from characters and symbols. Some standards and convention apply to both lines,

characters and symbols in drafting. These are defined in the universal technical language called draughting in this

textbook. A technical drawing is a 2D graphic model created using drafting skills. Drafting skills consist of a

knowledge base, drawing, and annotation skills. The knowledge base deals with the theory of drafting and

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incorporates algebra, geometry, trigonometry, manufacturing technologies, draughting, etc. The drawing skills deal

with the mechanics of using the hand and available drawing tools like instruments, computers, etc., in creating

graphic entities. The annotation skills deal with the addition of symbols and textual information to drawing views for

proper and complete documentation. Hard or paper copies of drawings and models are printed or plotted on standard

paper sizes in both Metric and English systems. Drawing views and annotations must be thoughtfully placed on

standard paper sheets.

The two fundamental linestyles are continuous (solid) and broken lines. Continuous lines have no gaps but broken

lines do. Continuous linestyle variants include visible (object), construction, extension, and border lines. Broken

linestyles include hidden, center, phantom lines, etc. Lines can also be distinguished by thickness or width. Thin lines

have line thickness of 0.3 mm or less and thick lines have line thickness more than 0.3 mm. Linestyle mistakes have

been largely eliminated in CAD environments, however, in sketches, efforts must be made to avoid these errors.

Chapter Review Questions

1. Define the terms draughting and drafting as used in this textbook.

2. Define the terms conventions and standards.

3. State the principles for the creation of good technical drawings.

4. What are the meanings of the acronyms ANSI and ISO?

5. What ANSI standard deals with drafting?

6. Which section of ANSI drafting standard is concerned with dimensioning and tolerancing?

7. What measurement units are found or used in drafting?

8. List the first three standard paper sizes in metric system.

9. List the first three standard paper sizes in English system.

10. What are the size specifications of A- and A4-sheets?

11. What information are often shown in a title block?

12. Define zoning as used in drawing sheets.

13. What is annotation? Describe lettering.

14. What are the two fundamental types of linestyles?

15. List 3 examples of each fundamental type of linestyles.

16. What are the types of line thickness mentioned in this chapter?

17. Distinguish between visible and hidden linestyles. When are they used in drawings?

18. When are phantom lines used in drawings?

19. Where are center lines used in drawings?

20. Can center lines end at visible lines?

21. When can you replace center lines with center marks?

Exercises

Sketch the following linestyes:

1. Visible line

2. Hidden line

3. Centerline

4. Phantom line

Sketch two circles: one big and the other small. Show center lines on the big circle and center marks on the

small circle.