Technical drawing is essential for automotive engineering technology professionals in Kenya to communicate design ideas, manufacture components, and ensure precise assembly. Producing plane geometry drawings requires understanding the fundamental types of lines used to convey different information clearly and unambiguously. Accurate use of lines following established standards enhances drawing readability and facilitates quality control in automotive workshops and manufacturing plants. This chapter focuses on the types of lines used in technical drawings, their characteristics, and their correct application in automotive engineering contexts.

2.1 Types of lines in drawings

In automotive engineering technical drawings, different line types represent various features such as outlines, hidden details, centre axes, and cutting planes. Each line type has specific visual conventions governed by standards such as BS 8888 and ISO 128, which Kenyan professionals must follow to maintain consistency and clarity. Understanding these lines allows technicians, engineers, and fabricators to interpret drawings correctly and avoid costly errors during vehicle repair or component fabrication.

2.1.1 Boarder lines

Boarder lines define the outer edges of a drawing sheet and separate the drawing area from the surrounding space. They provide a neat frame and establish the maximum drawing size, ensuring all elements fit within the designated area for easy handling and filing.

Characteristics of boarder lines

• Thickness: Boarder lines are the thickest lines on the drawing, typically between 0.7 mm and 1.0 mm wide, making them clearly distinguishable.
• Continuity: These lines are continuous and unbroken to form a complete rectangle around the drawing area.
• Colour and Contrast: Printed in solid black to maintain high contrast against the white sheet, aiding visibility.
• Positioning: They run along the edges of the drawing sheet, leaving a margin of about 10 mm inside the sheet border.
• Standard Compliance: Conform to BS 8888 and ISO 128 guidelines for sheet layout and framing.

Purpose of boarder lines

• Defining Drawing Limits: Clearly mark the boundary within which all drawing details must be confined.
• Facilitating Handling: Provide a sturdy frame for mounting or filing drawings without damaging critical details.
• Aiding Reproduction: Ensure consistent cropping during photocopying or scanning processes.
• Supporting Title Block: Create a reference frame for positioning the title block and revision table.
• Improving Aesthetics: Give the drawing a professional and organized appearance.

Drawing boarder lines procedure

1. Select a 0.7 mm or 1.0 mm technical pen or pencil grade suitable for thick lines.
2. Mark the drawing sheet edges, maintaining a 10 mm margin from the sheet border.
3. Using a ruler and T-square, draw continuous straight lines along all four edges.
4. Ensure corners meet precisely at right angles to form a perfect rectangle.
5. Double-check line thickness and continuity after drawing.
6. Leave space inside the boarder for title block and notes.

2.1.2 Faint continuous lines

Faint continuous lines, also known as construction or guide lines, are used during the initial stages of drawing to establish geometry without dominating the final image. They help in setting out shapes, angles, and proportions for automotive parts before final detailing.

Characteristics of faint continuous lines

• Line Thickness: Usually between 0.13 mm and 0.18 mm, much thinner than object lines.
• Continuity: Solid and unbroken but lighter to differentiate from primary outlines.
• Visibility: Visible enough to guide drawing but intended to be erased or ignored in final prints.
• Colour: Typically grey or light black when drawn manually; may be lighter in printed CAD drawings.
• Standard Reference: Follow ISO 128 recommendations for construction lines.

Uses of faint continuous lines in automotive drawings

• Setting Reference Points: Marking intersections, centres, and alignment guides.
• Drawing Auxiliary Geometry: Constructing tangents, arcs, and circles to define shapes.
• Dimensioning Support: Positioning measurement lines without cluttering the main drawing.
• Layout Planning: Assisting in arranging multiple components on one sheet.
• Design Iteration: Allowing adjustments before committing to permanent lines.

Techniques for drawing faint continuous lines

1. Use a hard pencil (e.g., 2H or 4H) or light CAD layer for faint lines.
2. Draw lightly with minimal pressure to avoid deep indentations.
3. Employ a ruler or template for straight lines and curves.
4. Keep lines continuous but thin and faint to avoid confusion.
5. Erase carefully after final lines are drawn, if manual drawing.
6. In CAD, assign faint line layers and control visibility settings.

2.1.3 Broken lines

Broken lines represent hidden edges or features not visible from the current viewing angle in automotive component drawings. They are essential for showing internal details such as holes, slots, or recesses that affect assembly or function.

Characteristics of broken lines

• Line Thickness: Medium thickness, typically 0.35 mm to 0.5 mm.
• Pattern: Consist of a series of short dashes spaced evenly, with a dash length about 3 mm and space 1.5 mm.
• Continuity: Non-continuous with regular breaks to indicate hidden features.
• Colour: Solid black for clarity against the white background.
• Standard Specification: Comply with BS 8888 and ISO 128 for hidden detail representation.

Purpose of broken lines

• Indicating Hidden Features: Show edges or surfaces obscured in the current view.
• Assisting Assembly: Reveal internal cut-outs, holes, or recesses critical for fitting parts.
• Supporting Inspection: Help quality controllers verify hidden dimensions.
• Clarifying Complex Shapes: Provide a complete understanding of three-dimensional parts.
• Facilitating Communication: Prevent misinterpretation by clearly differentiating visible and hidden details.

Drawing broken lines procedure

1. Select a 0.35 mm or 0.5 mm pen or pencil for medium thickness.
2. Using a ruler, mark dash lengths approximately 3 mm with 1.5 mm gaps.
3. Draw dashed lines parallel to the hidden edge or feature.
4. Maintain consistent dash and gap lengths throughout the line.
5. Confirm alignment with visible edges for accuracy.
6. Avoid overusing broken lines to prevent drawing clutter.

2.1.4 Chain lines

Chain lines are used to indicate special features such as centre lines of symmetrical parts, paths of motion, or cutting planes in sectional views. They consist of alternating long and short dashes, conveying specific meanings in automotive drawings.

Characteristics of chain lines

• Line Thickness: Medium, generally 0.35 mm to 0.5 mm.
• Pattern: Alternating long dashes (about 6 mm) and short dashes (about 1.5 mm).
• Continuity: Continuous with repeating dash pattern.
• Colour: Solid black for visibility.
• Standard Guidelines: Defined by ISO 128 for indicating special features.

Applications of chain lines in automotive drawings

• Cutting Planes: Mark sections where the component is "cut" to show internal details.
• Paths of Motion: Indicate movement trajectories of parts like pistons or linkages.
• Symmetry Indication: Show axes of symmetry for balanced components.
• Limit Lines: Define boundaries for machining or assembly tolerances.
• Special Features: Highlight areas requiring particular attention or treatment.

Drawing chain lines step-by-step

1. Choose a 0.35 mm or 0.5 mm pen or pencil for consistent thickness.
2. Draw a long dash approximately 6 mm in length.
3. Follow with a short dash of about 1.5 mm.
4. Repeat this alternating pattern continuously along the required path.
5. Ensure line segments are straight and evenly spaced using a ruler.
6. Verify that chain lines do not overlap other line types to avoid confusion.

2.1.5 Centre lines

Centre lines mark the axes of symmetry for circular or symmetrical parts, such as wheels, shafts, or brake discs in automotive engineering. They assist in dimensioning and locating features relative to a central reference.

Characteristics of centre lines

• Line Thickness: Thin, generally 0.18 mm to 0.25 mm.
• Pattern: Alternating short and long dashes, similar to chain lines but thinner.
• Continuity: Continuous and uninterrupted along the axis.
• Colour: Solid black to ensure visibility without overpowering other lines.
• Standard Compliance: Follow BS 8888 and ISO 128 for centre line representation.

Functions of centre lines in automotive drawings

• Identifying Symmetry: Highlight the central axis of shafts, cylinders, and holes.
• Locating Features: Provide reference points for drilling, machining, or assembly.
• Supporting Dimensioning: Serve as baselines for radial and axial measurements.
• Assisting Alignment: Guide assembly of components along a common axis.
• Clarifying Geometry: Distinguish symmetrical features from irregular shapes.

Drawing centre lines procedure

1. Use a technical pen or pencil with 0.18 mm to 0.25 mm thickness.
2. Draw a long dash approximately 6 mm long.
3. Follow with a short dash of about 1.5 mm.
4. Repeat the pattern continuously along the axis of symmetry.
5. Ensure the line passes exactly through the centre points of circles or symmetrical features.
6. Avoid intersecting centre lines with thick object lines to maintain clarity.

2.1.6 Cutting lines

Cutting lines indicate the location where a component or assembly is imagined to be sliced to reveal internal features in sectional views. These lines are crucial for understanding complex automotive parts such as engine blocks and transmission housings.

Characteristics of cutting lines

• Line Thickness: Medium, usually 0.35 mm to 0.5 mm.
• Pattern: Chain line style with alternating long and short dashes.
• Arrowheads: Ends are marked with arrowheads pointing toward the viewing direction.
• Lettering: Often labelled with letters (e.g., A-A) to identify the section.
• Colour: Solid black for prominence.

Purpose of cutting lines in automotive drawings

• Defining Section Planes: Show where the part is "cut" to produce sectional views.
• Clarifying Internal Details: Reveal hidden internal features like cooling channels or bolt holes.
• Supporting Manufacturing: Guide machining and inspection by illustrating cross-sections.
• Improving Communication: Help engineers and technicians understand complex assemblies.
• Facilitating Quality Control: Enable precise measurement of internal features during inspections.

Procedure for drawing cutting lines

1. Select a pen or pencil with 0.35 mm or 0.5 mm thickness.
2. Draw a chain line with alternating long (6 mm) and short (1.5 mm) dashes.
3. Add arrowheads at each end pointing in the direction of the sectional view.
4. Place section identification letters near the arrows (e.g., A-A).
5. Ensure the line crosses the component at the precise location of the intended section.
6. Check that cutting lines are distinct and do not overlap other line types.

Chapter Summary

This chapter covered the essential types of lines used in technical drawings, including border lines, faint continuous lines, broken lines, chain lines, centre lines, and cutting lines, each serving a specific purpose in conveying information clearly. It then detailed the step-by-step construction of various angles such as acute, right, reflex, obtuse, and straight angles, emphasizing accuracy and adherence to standards. The bisection of these angles was explained to facilitate precise division in geometric constructions. Measuring angles accurately using appropriate tools was also discussed for all angle types. The chapter further explored the construction of fundamental plane geometric forms including triangles, quadrilaterals, polygons, as well as circles and tangents, highlighting the methods for drawing each shape correctly. Finally, it addressed the creation of different scales, plane, diagonal, and those used for reducing and enlarging drawings, enabling precise representation of objects at various sizes. Mastery of these topics is crucial for producing accurate and standardized plane geometry drawings in technical fields.