Line types and line weights in technical drawings (ISO 128)
Whether a drawing is easy to read depends a lot on its lines. At a glance a machinist must tell the part's outline from a hidden hole and from a mere dimension line. ISO 128 standardises line types and widths so drawings read the same everywhere. The simple rule: visible edges are thick continuous lines; almost everything else is thin, with thick lines about twice the width of thin ones.
Common line types
| Line type | Looks like | Used for |
|---|---|---|
| Continuous wide | ──────── (thick) | Visible edges and outlines; the visible crest of threads |
| Continuous narrow | ──────── (thin) | Dimension and extension lines, hatching, leaders, thread roots, bend lines on flat patterns |
| Dashed narrow | ─ ─ ─ ─ | Hidden edges that cannot be seen in that view |
| Long-dash dotted narrow (centre line) | ─── · ─── | Centre lines, axes of symmetry, hole centres, pitch circles (PCD) |
| Long-dash dotted wide | ─── · ─── (thick) | Ends of cutting planes; areas with special requirements such as hardened surfaces |
| Long-dash double-dotted narrow | ─── · · ─── | Adjacent parts, extreme positions of moving parts, shapes before forming |
| Freehand / zigzag narrow | 〰 or ─╱╲─ | Break lines, showing a view shortened or partly cut |
Where several lines fall in the same place, draw the most important one. The usual order is: visible edge → hidden edge → cutting plane → centre line → dimension line.
Standard line weights
ISO 128 defines the widths 0.13, 0.18, 0.25, 0.35, 0.5, 0.7, 1, 1.4 and 2 mm. Each step is about ×√2 — the same ratio as the A paper sizes — so enlarging or reducing between A sizes keeps the widths in the series. A sheet usually uses just one pair:
| Paper / job | Thick | Thin | Lettering (common minimum) |
|---|---|---|---|
| A4–A3, general part drawings | 0.5 mm | 0.25 mm | 3.5 mm |
| A2–A0, or drawings with little detail | 0.7 mm | 0.35 mm | 5 mm |
| Very detailed drawings on A4 | 0.35 mm | 0.18 mm | 2.5 mm |
Line weight only matters when printing or exporting PDF. A DXF sent for laser cutting ignores it — the cutter always follows the centre of the line.
Setting it up in DXCAD2D Online
- Make layers by purpose — for example
0for outlines,HIDDEN,CENTER,DIM— in the Layers section of the side panel. - Choose the layer to draw on, then set the line type (dashed, centre, phantom and so on) and line weight (0.13–2.00 mm) in the side panel. The choice applies to what you draw next.
- For things already drawn, select them first, then change the type or weight; only the selection changes.
- If dashes look too dense or too sparse for the size of the part, adjust the dash scale: larger = longer dashes.
- Let hole centre lines extend just past the circle, about 2–3 mm — not so far that they clutter the drawing.
- Export a PDF and zoom in to check that thick and thin lines are clearly different before printing.
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Open DXCAD2D OnlineCommon mistakes
- Every line the same width. The drawing looks flat and outlines are hard to tell from dimensions.
- Dashed lines used as centre lines. The machinist reads them as hidden edges, not axes.
- Dimension lines as thick as outlines. The numbers get lost in the drawing.
- Every hidden edge drawn dashed. Complex parts become unreadable; use a section instead — see section views and hatching.
- Colour instead of line type. Meaning is lost on black-and-white prints and photocopies. Colour helps on screen, but the meaning must be in line type and weight.
For dimension layout, read ISO dimensioning basics; for paper, see drawing paper sizes.