Technical drawing and computer-aided design
A technical drawing is not an illustration: it is a document. It has to be readable by another person, in another workshop, ten years from now, and the part must come out exactly the same. All standardization exists to eliminate interpretation.
01A language, not a drawing
A freehand sketch is for thinking. A drawing is for manufacturing, and that is why everything in it is standardized: the sheet formats, the thickness of each line, where each view goes, how the dimensions are written and what information the title block carries. If two people read the same drawing and understand different things, the drawing is badly made.
| Element | Standard | What it sets |
|---|---|---|
| Sheet formats | IRAM 4504 / ISO 216 | A4 (210×297), A3, A2, A1, A0. Each is double the previous one; all keep the same proportion. |
| Scales | IRAM 4505 | Full size (1:1), reduction (1:2, 1:5, 1:10…) and enlargement (2:1, 5:1, 10:1). Dimensions are always the real ones, not the measurements on the paper. |
| Lines | IRAM 4502 | Thick continuous for visible edges, thin for dimensions and axes, dashed for hidden edges, dash-dot for axes of symmetry. |
| Dimensioning | IRAM 4513 | How dimensions are written, where the arrows go and what must not be repeated. |
| Title block | IRAM 4508 | Bottom right: which part it is, who drew it, when, at what scale, what material and what revision. |
You work with an approximate ratio of 1 : 0.5 : 0.25. The thick line (0.5 or 0.7 mm) is the visible outline of the part and is the first thing the eye finds; the thin one carries dimensions, axes and hatching; the dashed one shows what exists but cannot be seen from that side. When a drawing is hard to read, nine times out of ten it is because all the lines have the same thickness.
02Orthographic projections
A three-dimensional part is represented with several flat views, each one looked at perpendicularly. The rule is simple: you draw only the views needed to define the part without ambiguity, and three are almost always enough.
The part is placed between the observer and the projection plane. The view obtained by looking from the left is drawn on the right, and the one from above, below.
It is the IRAM and ISO system: the one used in Argentina and in Europe.
The projection plane is between the observer and the part, as if it were a glass pane: each view ends up on the same side it is viewed from.
It is the ANSI system, used in the United States. A great deal of technical documentation and component datasheets come this way.
By the truncated-cone symbol in the title block. If it is not there, you have to deduce it from the views, and that is where the problems begin: an asymmetric part interpreted in the wrong projection system comes out mirrored. When working with imported documentation—and in electronics that is the norm— it is wise to check before sending anything to be manufactured.
Sections and perspective views
The part is “sliced” with an imaginary plane to show the interior without dashed lines. The cut surface is hatched at 45°. It is indicated with a dash-dot line and two arrows that say which way you are looking.
When a face is inclined, its projection appears distorted. A view looked at perpendicular to that face is added, so it can be dimensioned at true size.
The three axes at 120°, without scale reduction. It does not replace the views—you do not dimension on it—but it helps you understand the part at a glance. It is the one any 3D CAD produces.
03Dimensioning: where manufacturing is decided
It is the part of the drawing that accumulates the most errors, because it is not enough for the dimensions to be there: they have to be where the person making the part needs them.
- Each dimension is given only once, in the view where it is best understood.
- Dimension lines do not cross each other or the extension lines.
- Small dimensions go close to the part and large ones farther out, so they do not intersect.
- Diameters are marked with ⌀ and radii with R; sheet-metal thicknesses, with t or in a note.
- Dimension from real datum surfaces, not from rounded edges: the machinist needs to rest the part against something.
- Tolerances are not optional: 20 mm and 20 ± 0.05 mm are two different parts and two completely different prices.
For a front panel with three cutouts—two ⌀ 16 mm pushbuttons and a rectangular display— the correct approach is:
- Dimension all the cutouts from two reference edges (for example the bottom and the left), not in a chain from one to the next: errors do not accumulate.
- Give “2 × ⌀ 16” only once, with a leader line to one of the holes.
- Give the size of the display window and its position, plus the tolerance: ±0.3 mm if the display is a press fit, ±1 mm if it has a bezel.
- State thickness and material in the title block: “2 mm aluminum sheet”.
That same drawing, with chain dimensions, produces a panel where the last hole is off by two millimeters and the display does not fit.
04From the drawing board to CAD
Computer-aided drawing did not change the rules: it changed how they are produced and modified. What matters is understanding what each tool gains you.
| Tool | What it does | When it is suitable |
|---|---|---|
| CAD 2D (AutoCAD, LibreCAD, QCAD) | Draws exact lines, with layers, blocks and automatic dimensioning | Installation drawings, switchboards, front panels, electrical drawings |
| Parametric 3D CAD (SolidWorks, Fusion 360, FreeCAD, Onshape) | Models the solid and generates the views by itself. Dimensions are parameters: change one and the whole model updates | Parts, enclosures, heat sinks, brackets; anything that will be 3D printed or machined |
| Electronic CAD (KiCad, Eagle, EasyEDA) | Schematic and circuit board, with electrical rules | It is the subject of the next topics: schematics and PCB |
- Layers: separate outlines, dimensions, axes, text and construction lines. It lets you print only what is needed and change line thicknesses all at once.
- Blocks or components: draw once what repeats. If the block changes, all its copies change.
- Constraints and parameters: in parametric 3D, define relationships—this face parallel to that one, this hole centered—instead of fixed coordinates.
- Always draw at 1:1 scale, and apply the scale only when printing. It is the most common initial mistake of people coming from paper.
| Format | What it is for |
|---|---|
| DWG / DXF | 2D drawing. DXF is the interchange format that every program reads. |
| STEP (.stp) | Solid 3D model, interchangeable between different programs. It is the format in which manufacturers publish their components. |
| STL | Triangle mesh for 3D printing. It is neither parametric nor editable: it is the result, not the model. |
| For distributing and printing the finished drawing, so that nobody modifies it. | |
| Gerber | The “drawing” of a printed circuit board, with one layer per file. |
In any real project, the board has to fit inside the enclosure, and its connectors have to line up with the cutouts in the panel. PCB programs export the board as STEP with all its components in 3D: you import it into the mechanical CAD and check, before manufacturing anything, that the heat sink does not hit the cover and that the USB connector sits at the height of its window. That intersection is exactly the work of the Year 7 capstone project.
05In the lab
Choose a real, simple part from the workshop (a square, a bracket, a cover). Measure it with calipers, make a dimensioned freehand sketch and then transfer it to CAD at 1:1 scale, with the three standard views, on an A4 sheet with a title block. Compare the sketch with the final drawing: almost always a dimension is missing.
Exchange drawings between classmates and make—in cardboard or wood—the other person’s part without consulting them. Anything that comes out different is a defect of the drawing, not of the person who interpreted it. It is the best exercise of the topic.
Draw the front panel of an instrument with cutouts for two pushbuttons, a display and a connector. Dimension from two reference edges. Print at 1:1, cut it out and check against the real components that everything matches. It is the cheapest way to find errors before drilling.
Model in 3D a mount for a board, with the holes at the real distance of its standoffs. Then change a single parameter—the width of the board—and check that the entire model updates. Export to STL and, if there is a 3D printer, print it and check the fit.
06Common mistakes
| Symptom | Usual cause |
|---|---|
| The part comes out mirrored | The drawing was in third-angle projection and was read as first-angle (or the other way around). The symbol is missing from the title block. |
| The last hole ends up off position | Chain dimensions: errors accumulate. Dimension from a common reference. |
| The workshop asks for a dimension that “is there” | It is in a view where it cannot be understood, or it has to be deduced by subtracting two dimensions. The drawing must give the dimensions that will be used. |
| It is printed and does not match the part | It was printed with “fit to page” instead of at scale. Always check with a reference dimension on the paper. |
| The drawing is illegible | All lines of the same thickness and text that is too small. The line hierarchy is not decoration. |
| The 3D model breaks when a dimension is changed | Poorly set up constraints: absolute coordinates were used instead of geometric relationships. |
| The part does not fit even though the dimensions match | Tolerance and clearance are missing. Two 20.00 mm parts never fit into each other. |
| The STL comes out with holes or defects | Open mesh: the solid model had loose surfaces. Repair the model, not the STL. |
07Self-assessment
What is the difference between first-angle and third-angle projection?
The position of the projection plane relative to the part and the observer. In first-angle (IRAM/ISO, the one used here) the part is between the observer and the plane, and the views end up on the opposite side from where you look. In third-angle (ANSI) the plane is in the middle and each view ends up on the same side.
A drawing is at 1:5 scale and a dimension reads 250. How big is the part and how big is the drawing?
The part measures 250 mm: dimensions are always real. The drawing measures 250/5 = 50 mm on the paper.
What is the dashed line used for in a drawing?
To represent hidden edges: edges that exist but cannot be seen from that view, such as the bottom of a blind hole.
Why not dimension in a chain?
Because tolerances add up: if each segment can have ±0.2 mm, after five segments the last feature can be off by a millimeter. Dimensioning from a common reference, each dimension carries its own error and nothing more.
What concrete advantage does parametric modeling give?
Dimensions are linked parameters: changing one updates the whole model, the views and the dimensions. In a design that is iterated ten times—the norm—it saves redoing everything each time.
Which format is used to exchange a 3D model between different programs, and which for printing?
STEP to exchange the editable solid; STL for printing, which is only a triangle mesh and no longer keeps the model information.
What minimum information must the title block have?
Part name, scale, projection system, material, units, author, date and revision number. Without a revision, nobody knows whether the drawing they are holding is the latest.
When is an auxiliary view needed?
When a face is inclined relative to the principal planes: in the normal views it appears distorted and cannot be dimensioned at true size.
Why export the board as STEP to the mechanical CAD?
To verify, before manufacturing, that the board fits in the enclosure: that the connectors line up with the panel windows, that tall components do not hit the cover and that the mounting holes match the standoffs.
Why is CAD always drawn at 1:1 scale?
Because the model is the source of all dimensions: you dimension it directly and can measure on it. Scale is a printing decision, applied at the end and it can be different for each sheet.