Computer Aided Drawing (CAD) is an essential skill for welding professionals in Kenya, enabling precise geometric representations of welded joints, components, and assemblies. Mastery of drawing lines according to standard conventions ensures clarity and uniformity in technical communication across fabrication workshops, construction sites, and quality control offices. This chapter focuses on setting and using drawing lines correctly to produce accurate geometric drawings that meet professional standards and facilitate efficient welding project execution.

2.1 Setting Drawing Lines

Setting correct drawing lines is fundamental in welding CAD drawings as it dictates how information is visually conveyed. In Kenya's welding industry, communication between designers, fabricators and inspectors must be unambiguous, so understanding line conventions and their application is critical for quality assurance and safety compliance.

2.1.1 Recognize standard drawing line conventions

Drawing line conventions are the standardized rules that govern the types, thicknesses and appearances of lines used in technical drawings. These conventions ensure that welders and engineers interpret drawings consistently, minimizing errors in fabrication. Each line type has one meaning only, so a fabricator reading a drawing for a steel stand can tell at a glance whether a line is an edge, a measurement, a hidden feature or a cutting plane.

Dimension lines

Dimension lines are continuous thin lines that show the extent of a measurement, such as the length of a plate or the diameter of a pipe. They end in arrowheads that touch the extension lines, and the dimension value is written above the line or in a break in it. Example: on a drawing of a steel gate frame for a tea estate, a dimension line placed below the frame carries the value 1800 and tells the fabricator the overall width of the frame in millimetres.

• Line type: Continuous thin line, so that it supports the outline without competing with it.
• Terminations: Arrowheads (or oblique strokes in some styles) at both ends, drawn to the same size throughout the drawing.
• Placement: Drawn outside the part wherever possible, with the shortest dimensions nearest the outline and longer ones further out.
• Spacing: Parallel dimension lines are kept evenly spaced so that figures do not crowd each other.
• Crossing: Dimension lines should not cross other dimension lines, and a centre line or outline must never be used as a dimension line.

Hidden detail lines

Hidden detail lines show edges and features that cannot be seen from the viewing direction because they lie behind or inside another part. They are drawn as dashed lines made of short, evenly spaced dashes. Example: in the side view of a bracket for a hospital bed frame, a dashed line reveals the weld seam and the bolt hole on the far side of the plate, so the welder knows they exist.

• Appearance: Dashed line (hidden edges or features not visible in the current view), with dashes of equal length and equal gaps.
• Typical use in welding: Indicates internal weld seams, concealed joints and the far-side edges of a fillet weld.
• Starting and ending: A dashed line starts and ends with a dash, except where it continues from a continuous line, in which case a gap is left.
• Meeting other lines: Dashed lines meet at corners with dashes touching, so that the corner reads clearly.
• Restraint: Only hidden details that help to understand the part are shown, so that the view does not become cluttered.

Extension lines

Extension lines are continuous thin lines that project from the edges of a feature to carry the dimension away from the outline. They start with a small visible gap from the outline and run slightly beyond the dimension line. Example: on a drawing of a steel water-tank stand for a county water office, extension lines drop from the leg centres to the dimension line that gives the spacing between the legs.

• Line type: Continuous thin line, also called a projection line.
• Gap and overrun: A small gap (about 1 mm) is left between the outline and the start of the line, and the line extends a short distance (about 2 mm) past the dimension line.
• Direction: Drawn at right angles to the feature being dimensioned, except in special cases such as dimensioning a tapered part.
• Crossing: Extension lines should not cross dimension lines where it can be avoided, and may cross each other when necessary.
• From centre lines: A centre line may be extended to act as an extension line when dimensioning the position of a hole or a pipe.

Section lines

Section lines are the thin lines (hatching) that fill the cut surface in a sectional view, together with the cutting plane line that shows where the part was sliced. They reveal internal structure such as weld penetration, wall thickness or the fit-up of a joint. Example: in a school workshop drawing of a flanged pipe joint, hatching on the cut faces of the pipe and flange shows the solid metal, while the cutting plane line on the front view shows where the cut was made.

• Hatching: Continuous thin lines, usually at 45°, evenly spaced and drawn on the cut surface only.
• Different parts: Adjacent parts in a section are hatched in opposite directions or at different spacing so that they can be told apart.
• Spacing: The spacing is chosen according to the size of the area, and it is kept uniform within the same part.
• Cutting plane line: A chain line, thick at its ends and where it changes direction, with arrows showing the viewing direction. It is used for cutting planes or sections, showing where a component is sliced to reveal internal structure for detailed weld inspection.
• Labelling: Cutting planes are labelled with letters (for example A-A), and the resulting section view carries the same letters.

Break lines

Break lines show that part of an object has been removed from the drawing, which allows a long or uniform part to be drawn at a larger scale on a small sheet. The two ends of the shortened part are bounded by break lines, and the true length is given by the dimension. Example: a contractor drawing a long roof truss rafter shows only the two ends, with break lines in the middle and the full length of 6000 written as the dimension.

• Freehand line: A continuous thin freehand wavy line is used for breaks in most parts.
• Straight line with zigzags: A continuous thin straight line with short zigzags is used for long break lines in CAD drawings.
• Round bars and pipes: The break is often drawn as an S-shaped curve or with a partly drawn ellipse to suggest a round section.
• Dimensioning: The dimension is the real length of the part, not the shortened length shown on the sheet.
• Where to use: Only on parts with a uniform cross-section, such as pipes, flats, angles and beams, so that nothing important is removed.

Chain

Chain lines are made of long dashes alternating with short dashes. The chain thin line represents centre lines and axes of symmetry, which are essential for aligning welds and components accurately. Example: on a drawing of a circular flange for a hotel boiler, a chain thin line marks the pitch circle on which the bolt holes lie, and short crossing centre lines mark the hole centres.

• Centre lines: Show the axis of holes, shafts, pipes and cylindrical parts.
• Symmetry lines: Mark the axis of symmetry of a part, so that only one half need be dimensioned in detail.
• Pitch circles: Show the circle on which bolt holes or studs are positioned.
• Drawing detail: The line begins and ends with a long dash and extends a short distance beyond the outline of the feature.
• Small circles: In small circles, centre lines may be drawn as continuous thin lines, because the chain pattern would not be clear.
• Crossing: Centre lines cross at the long dashes, never at the gaps, so that the centre point is clearly marked.

Continuous thick lines for visible outlines

Continuous thick lines are the main lines of a drawing. They represent visible edges and outlines of welded components and depict the shape of the parts. Example: in a drawing of a steel workbench frame for a welding workshop, the thick outline of the tubes stands out against the thin dimension and centre lines around it.

• Visible edges: All edges seen in the view are drawn as thick lines.
• Joint edges: Joint edges are highlighted with thick lines so that the welder sees what must be joined.
• Contrast: The thick line is drawn at about twice the width of the thin line, which lets the outline dominate.
• Continuity: The line is unbroken, except where it must stop to leave a gap for text.
• Priority: Where lines coincide, the visible thick line takes priority over a hidden line, which takes priority over a centre line.

Line thickness and scale

Line thickness tells the reader which lines matter most, and it must stay legible whatever the sheet size or scale. Example: on an A3 print of a gate frame, the outline is drawn at 0.5 mm and the dimension lines at 0.25 mm, so the shape is clear and the figures are still easy to read.

• Variation by function: Visible outlines are thicker to stand out, while dimension and projection lines are thinner.
• Consistency and scale: Thickness is kept consistent within a drawing and adapted to the drawing scale to maintain legibility when printed or viewed digitally.
• Standard series: Line widths are chosen from a standard series (for example 0.25 mm, 0.35 mm, 0.5 mm and 0.7 mm), so that thick lines are about twice as wide as thin lines.
• Same type, same width: Lines of the same type keep the same width in every view of the same drawing.
• Minimum gap: Gaps between separate lines should never be less than twice the width of the thickest line, otherwise they merge when copied or reduced.

Colour and contrast

CAD software allows lines to be shown in colour, but the drawing must still communicate when it reaches the workshop floor as a black and white print. Example: a fabricator in a workshop receives a photocopied drawing of a trailer chassis, and it is readable only because lines are told apart by style and thickness rather than by colour.

• Monochrome practice: Welding drawings are often kept monochrome for ease of printing and photocopying, with differentiation made through line style and thickness instead of colour.
• Agreed conventions: When colour is used, it must follow conventions agreed upon in the project specifications to avoid confusion.
• Layers: On screen, line colours are often assigned to layers (for example outlines, dimensions and centre lines) so that line weights print correctly.
• Contrast with background: Colours chosen must give good contrast with the background, so that the drawing can still be read when printed in black and white.
• Colour key: Where colours carry meaning, a short key on the sheet explains them so that no reader has to guess.

Application of line conventions in welding drawings

Welding drawings combine the ordinary line types with weld symbols, and the lines of the symbol have their own meaning. Example: on a drawing of a pipe stand for a hotel kitchen, a weld symbol sits on a thin reference line joined by an arrow line to the pipe joint, and a centre line locates the weld on the pipe.

• Weld symbols: Weld symbols are placed on specific lines indicating the type and location of welds.
• Reference line and arrow line: The reference line of a weld symbol is drawn as a continuous thin line, with an arrow line that touches the joint.
• Identification line: In ISO 2553 weld symbols, a dashed identification line beside the reference line shows whether the weld is on the arrow side or the other side.
• Centre lines: Centre lines help to position welds on round parts such as pipes, tanks and flanges.
• Tail of the reference line: Where needed, the fork at the end of the reference line carries a note such as the welding process or procedure reference.

Standards compliance

Line conventions are set by standards so that a drawing made in one workshop can be read correctly in another. Example: a fabrication firm sends a drawing of a steel staircase to a client's inspector, and both read the lines the same way because the sheet names the standard used.

• Common standards: Many welding drawings follow ISO 128 or BS 8888 for line conventions.
• International understanding: Compliance ensures drawings are internationally understandable, facilitating export fabrication or inspection collaborations.
• Title block: The standard used should be stated in the title block, so that every reader interprets the lines in the same way.
• Company standards: Company drawing standards may add extra detail, but they must not contradict the national or international standard that the project specifies.
• Current edition: The edition of the standard in force for the project is the one to apply, and any change of edition is recorded on the drawing.

Chapter Summary

This chapter focused on producing geometric drawings by first setting and recognizing standard drawing line conventions essential for clear communication. It emphasized the correct use of various drawing lines to represent different features accurately. The construction of angles was explored through applying trigonometry principles to create acute, obtuse, and right angles precisely. Geometrical forms such as circles, rectangles, triangles, and polygons were developed following established standards to ensure accuracy and consistency. The chapter also covered the development of geometric drawings in different formats, including two-dimensional views, orthographic projections, and isometric drawings. Each type of drawing was presented as a vital skill for representing objects and structures in engineering and design. Overall, the chapter equipped learners with foundational techniques for producing accurate and standardized geometric drawings.