2.1 Setting Drawing Lines

A technical drawing is a language, and lines are its alphabet. Every line has a fixed appearance (thick or thin, continuous, dashed or chained) and each appearance carries one meaning, so that a technician, a machinist or an inspector reading the drawing understands it in the same way as the person who drew it. Setting drawing lines means learning these standard line types, recognising what each one communicates, and setting them up so that the CAD program draws and plots them with the correct thickness and form. Example: when an automotive technician in a workshop reads a drawing of an engine mounting bracket, the thick outline tells them where the metal is, the dashed lines tell them about holes they cannot see, and the thin chain lines tell them where the centres are.

2.1.1 Recognize standard drawing line conventions

Line conventions are the agreed rules, set out in national and international drafting standards, that link a line's style and thickness to its meaning. Recognising them means being able to look at any line on a drawing and say what it represents and how it should be read. Without them, a dimension line could be mistaken for an edge, or a hidden hole for a visible slot, leading to costly errors in manufacture or repair. The six line types below are the ones every technician must recognise on sight, and each one is later set up as a layer in the CAD program.

Dimension lines

Dimension lines show the size of a feature. They are thin continuous lines with an arrowhead at each end, and the arrowheads touch the extension lines or outlines that mark the limits of the measurement. The dimension figure is written above the line, near its middle, and is read from the bottom or right-hand edge of the sheet. Example: on a drawing of a steel engine mounting bracket, a thin line with arrowheads drawn below the bracket and the figure 120 above it tells the learner that the bracket is 120 mm long.

Points to remember when recognising dimension lines:

• Line quality: They are thin and continuous, so they never compete visually with the thick outline of the object.
• Position: They are drawn outside the view wherever possible, about 10 mm away from the outline, so the shape stays clear.
• Spacing: Parallel dimension lines are spaced evenly, about 6 to 8 mm apart, so figures do not crowd each other.
• Arrowheads: They are small, narrow, solid or open heads of about 3 mm in length, drawn neatly and consistently throughout the drawing.
• Independence: A dimension line is never an extension of an outline or centre line, and an outline is never used as a dimension line.

Hidden detail lines

Hidden detail lines show edges and surfaces that exist but cannot be seen from the viewing direction because they lie behind or inside the object. They are drawn as a series of short, evenly spaced dashes in a thin line (0.25 mm). Example: a 12 mm thick steel mounting plate for a gearbox has a bolt hole, cast-in or drilled, running down through it. In the top view the hole is seen directly as a circle, but in the front view it cannot be seen, so its two sides appear as two parallel lines of short dashes running through the thickness of the plate.

Points to remember when recognising hidden detail lines:

• Dash length: Dashes are about 3 mm long with gaps of about 1 mm, kept equal along the whole line.
• Junctions: Hidden lines meet visible lines or other hidden lines with a dash touching at corners, so the corner looks closed.
• Continuation: Where a hidden line continues in line with a visible line, a gap is left at the junction so the visible edge is not made to look longer.
• Arcs: A hidden arc starts with a dash at its tangent point, so the curve is clearly joined to the straight edge.
• Use with discretion: Hidden lines are used only where they make the drawing clearer. Too many of them confuse the view, and a sectional view is then preferred.

Extension lines

Extension lines (also called projection lines) extend from the edges or features being measured to carry the measurement out to where the dimension line sits. They are thin continuous lines drawn at right angles to the feature being dimensioned. Example: on a drawing of an exhaust flange plate, extension lines run up from the left edge of the plate and from the centre of a hole to a dimension line above, showing the distance of the hole from the edge.

Points to remember when recognising extension lines:

• Gap from the outline: A small gap of about 1 mm is left between the extension line and the feature, so the outline is not confused with the extension line.
• Overrun: The extension line continues about 2 to 3 mm beyond the dimension line, so the arrowhead sits clearly against it.
• Direction: It is normally drawn perpendicular to the length being dimensioned, though oblique extension lines are allowed in special cases for clarity.
• Avoiding crossings: The longer dimensions are placed outside the shorter ones so extension lines do not cross dimension lines unnecessarily.
• Centre lines as extensions: A centre line may be extended beyond the outline and used as an extension line, which saves drawing an extra line.

Section lines

Section lines, also called hatching lines, are thin parallel lines drawn across the surface that has been cut when an object is shown in section. They tell the reader where solid material has been sliced through, and they separate that cut surface from the empty space inside. Example: in a sectional view of a water pump housing, hatching fills the walls of the housing but leaves the central passage clear, so the reader sees at once how thick the walls are and where the coolant flows.

Points to remember when recognising section lines:

• Angle: Hatching is normally drawn at 45° to the main outline or axis of the object.
• Spacing: Lines are evenly spaced, typically 2 to 3 mm apart, and the spacing is kept uniform across the whole area.
• Adjoining parts: In an assembly, neighbouring parts are hatched in opposite directions, or at different spacing, to tell them apart.
• Same part: All the cut areas of one component in one view are hatched in the same direction and spacing.
• Text and dimensions: Hatching is interrupted where a figure or note has to be written inside the hatched area, so that it stays readable.
• Line type: The lines are thin and continuous, and they stop at the visible outline of the cut surface.

Break lines

Break lines show that part of an object has been removed from the drawing, either because it is too long to fit on the sheet or because the shape does not change along its length, or to show a partial view or section. Long breaks are drawn as thin straight lines with occasional zig-zags, and short breaks as thin freehand wavy lines. Example: a 1500 mm steel propeller shaft is drawn with its middle cut out by break lines so it fits on an A3 sheet at a readable scale, while the dimension 1500 still states its full length.

Points to remember when recognising break lines:

• Freehand and ruled forms: Short breaks are drawn freehand as thin wavy lines, and long breaks on large parts use thin ruled lines with zig-zags.
• Round bars and tubes: A solid round bar is broken with an S-shaped curve, and a tube is shown with S-curves on both its outer and inner walls to suggest the hollow.
• Dimensioning: The dimension shows the true length of the object, not the length that is drawn.
• Placement: Break lines are drawn clear of the visible outline and do not coincide with any other line.
• Use only where the shape is uniform: Parts with a constant cross section can be shortened, but features such as holes or steps must never be removed by a break.

Chain

Chain lines are made of long dashes alternating with short dashes. The thin chain line is used for centre lines, lines of symmetry and pitch circles, and the thick chain line is used to mark a special requirement. Example: on a drawing of a wheel hub, a thin chain circle is drawn through the centres of the wheel stud holes (the pitch circle) and thin chain lines cross at the centre of the hub, so the holes can be located and drilled symmetrically.

Points to remember when recognising chain lines:

• Centre lines: A thin chain line marks the axis of a hole, shaft or other round feature and is extended about 3 mm beyond the outline.
• Crossing centres: Centre lines cross at the long dashes, not at the gaps, so the centre point is clearly defined.
• Symmetry: A thin chain line with two short parallel thin lines drawn across each end shows the line of symmetry, and only half of a symmetrical view then needs to be drawn.
• Cutting planes: A thin chain line with thick ends and short thick changes of direction shows where a cutting plane passes through an object, and arrows at the ends show the direction of viewing.
• Thick chain: A thick chain line marks the length or area of a surface that needs a special treatment, such as hardening or a coating.
• Small circles: For very small circles, where a chain line would be crowded, a thin continuous line may be used for the centre line.

Line thickness and priority of coinciding lines

Standard drawing practice uses only two main line thicknesses, a thick line and a thin line, in the ratio of about 2 to 1. Thick lines are normally 0.5 mm wide and thin lines 0.25 mm wide, though the range may be scaled up for larger sheets. Getting the thickness right and keeping it consistent is what makes a drawing readable. When two different lines fall on top of each other, the more important line is drawn and the other is left out. Example: a draughtsperson in an automotive design office draws the outline of an engine mounting plate with 0.5 mm lines and the dimension lines with 0.25 mm lines, so the shape stands out immediately from the dimensioning.

Points to remember when setting line thickness and priority:

• Thick lines: Visible outlines and visible edges are drawn thick and dark, so they are the first thing the eye picks up.
• Thin lines: Hidden lines, dimension lines, extension lines, hatching, centre lines and break lines are drawn thin, so they support the outline without competing with it.
• Line weight in CAD: The line weight is set once for each layer, so every object on that layer plots at the same thickness. For hand sketches only, a soft pencil such as HB is used for thick lines and a harder pencil such as H or 2H for thin lines.
• Uniform lines: A given line type has the same thickness and the same dash and gap lengths along its whole length and across the whole drawing.
• Priority of coinciding lines: A visible outline takes priority over a hidden line, a hidden line over a centre line, and a centre line over dimension or extension lines, so only the most important line is drawn where they would overlap.
• Clean junctions: Lines of the same type meet cleanly at corners, without gaps or overshoots, so the drawing looks neat and accurate.

Setting line types and line weights in CAD

In this unit the conventions above are applied inside the CAD program. Each line type is loaded, placed on its own layer and given a line weight, so that every object drawn on that layer automatically has the correct appearance. This is faster and more consistent than choosing the style of each line by hand. Command names below follow common CAD programs such as AutoCAD and may differ slightly in other software.

Recommended layers for a mechanical drawing:

Procedure for setting line types and line weights:

1. Start a new drawing: Open a new file or template and set the units to millimetres, so that dash lengths and line weights are read in the correct units.
2. Load the line types: Open the line type manager (command LINETYPE), choose Load, and select the types needed, such as HIDDEN or DASHED, CENTER and PHANTOM. Continuous is already available. The phantom type (a long dash followed by two short dashes) is used for cutting planes or alternate positions where the drawing standard in use allows it.
3. Create a layer for each line type: Open the layer manager (command LAYER), create the layers in the table above, and give each one a name, a colour, its line type and its line weight. Using one layer per line type means that changing a layer changes every line on it.
4. Turn on line weight display: Switch on the line weight display (the LWT button on the status bar, or the lineweight settings dialog) so that the difference between 0.50 mm and 0.25 mm lines is visible on screen. This only changes how lines look on screen, while the real weight is applied when plotting.
5. Set the line type scale: Enter LTSCALE and give a value equal to the plotting scale factor. Example: for a drawing plotted at 1:2, LTSCALE = 2, and for a drawing plotted at 1:5, LTSCALE = 5. Then regenerate the drawing (command REGEN). If hidden or chain lines still look continuous, the scale is wrong or the line is too short to show a full dash.
6. Draw on the correct layer: Make the required layer current before drawing and keep colour, line type and line weight set to ByLayer, so objects take their properties from the layer instead of being set one by one.
7. Check overlapping lines: Zoom in where lines coincide, such as a hidden line behind an outline, and make sure the line of higher priority is the one shown. Fix any object on the wrong layer by changing its layer in the properties panel.
8. Plot with the correct line weights: In the plot dialog choose the paper size and plot scale, switch on the option to plot object line weights, preview the sheet, and then plot. Check the printed sheet to confirm that outlines are clearly thicker than the dimension, hatch and centre lines.

Chapter Summary

This chapter covered the fundamental principles of producing geometric drawings, starting with setting drawing lines by recognizing standard line conventions essential for clarity and accuracy. It then examined the practical use of these lines in creating precise drawings. The construction of various types of angles was explored through the application of trigonometry principles to accurately form acute, obtuse, and right angles. The chapter further detailed the creation of basic geometrical forms, including circles, rectangles, triangles, and polygons, all following established standards. Finally, it addressed the development of geometric drawings in different formats, focusing on two-dimensional views, orthographic projections, and isometric representations to provide comprehensive visual communication in technical drawings.