CAD drawing: from 2D drafting to the parametric 3D model and the drawing sheet
2D CAD concepts, parametric 3D modelling with sketches and constraints, assemblies, creating drawings, DWG, STEP and STL formats, and mechanical-electronic CAD collaboration.
Today almost nobody draws by hand any more: design is done on the computer with CAD (Computer Aided Design). The rules of the previous lessons all still apply, because the end result is still a standard drawing that goes to the workshop. What changes is how you get there: first you build a model, then the drawing is derived from it. In this lesson we look at how a modern CAD workflow works, from 2D drawing to the parametric 3D model and on to the drawing sheet.
2D CAD
2D programs such as AutoCAD, DraftSight or LibreCAD are the electronic drawing board: you draw lines, arcs and circles with exact coordinates. Key concepts:
- Coordinates: absolute (
from the origin), relative (offset from the last point) or polar (distance and angle). You always draw at 1:1 with the real dimensions: the scale is chosen only when printing. - Object snaps: exact snapping to endpoints, midpoints, centres, intersections, tangents. This is how the program carries out geometric constructions.
- Layers: each line type lives on a layer with its own colour, linetype and weight (outlines, centre lines, hidden lines, dimensions, hatching). Layers can be hidden, locked or printed separately.
- Blocks: reusable groups of objects, such as the title block, a standard screw or the roughness symbol. Editing the definition updates every copy.
- Model space and paper space: in model space you draw the part at full size; in paper space (the layout) you lay out the sheet with the title block and one or more viewports showing the model at the required scale.
- Associative dimensions: they are tied to the geometry and update automatically when the part changes.
Parametric 3D modelling
Programs such as SolidWorks, Inventor, Fusion 360, Solid Edge, CATIA or the free FreeCAD work differently. The part is built as a solid, with a sequence of features recorded in a tree:
- draw a 2D sketch on a plane;
- constrain it with geometric constraints (horizontal, parallel, tangent, concentric, equal) and dimensions, until it is fully defined;
- turn it into a solid with an operation: extrude, revolve about an axis, sweep along a path, loft between several profiles;
- add further features: standard holes (threaded too), fillets, chamfers, shells, circular or linear patterns.
The key word is parametric: every dimension is an editable parameter. If the plate must become 120 mm wide instead of 100, you change the number and the whole model rebuilds, holes, fillets and drawings included. Dimensions can also be linked by equations, for example “the hole is always at half the width”.
Assemblies
Parts are put together in an assembly with mates: coincident faces, a pin concentric with a hole, a fixed distance. The program checks interference, that is volumes where two parts overlap, simulates motion and computes mass, centre of gravity and moments of inertia. The bill of materials (the list of all parts with quantities and codes) is generated automatically.
From model to drawing
Creating the drawing is almost automatic: choose the sheet size and title block, drag the model onto the sheet, and the program generates the orthographic views with the chosen method (first or third angle), the sections once you mark the cutting plane, the enlarged details and the axonometric views. Hidden lines, centre lines and hatching follow the set styles.
What the program does not do on its own is the design part: deciding which views are really needed, which dimensions to give and from which datums, where tolerances and roughness are needed. Automatic dimensioning usually produces a drawing full of redundant or non-functional dimensions, and it must be reviewed by hand with the criteria of the lesson on dimensioning.
Exchange formats
| Format | Content | Typical use |
|---|---|---|
| DWG, DXF | 2D vector drawing | exchanging drawings, laser and plasma cutting, printed circuit board outlines |
| STEP (.stp) | exact solid 3D model | exchange between different CAD programs, electronic component models |
| STL | triangulated surface | 3D printing |
| printable drawing | sending to the workshop and archiving |
Mechanical and electronic CAD together
An electronic product is also a mechanical object. PCB design programs such as KiCad or Altium export the 3D model of the assembled board in STEP format, with component models supplied by the manufacturers. The mechanical designer imports it into the enclosure assembly and checks that the connectors line up with the openings, that the heat sink does not touch the capacitors and that there is room for the cables. The board outline and the position of the fixing holes travel the other way, often as DXF. Finding an interference on screen takes a few minutes; finding it on the first prototype can cost weeks.
The example linked to this subject, the renovation of a trullo drawn with SketchUp and AutoCAD, shows the same workflow in architecture: survey, 3D model and 2D drawings derived from the model.
