5.4 Computer-Aided Drawing (CAD) & Engineering Graphics

5.4 Computer-Aided Drawing (CAD) & Engineering Graphics

GEAS lists Computer-Aided Drawing (CAD) as a practical skill area. For ECEs, CAD means producing and reading the schematic diagrams, PCB layouts, and mechanical drawings that communicate a design to fabricators and regulators.

1. Engineering Drawing Fundamentals

Engineering drawings use standardized projections and conventions:

• Orthographic projection: representing a 3-D object in 2-D by projecting views onto mutually perpendicular planes. The standard multiview set is front, top, and right side.
• Isometric projection: a single pictorial view with the three principal axes 120 degrees apart, preserving scale along each axis.
• First-angle vs third-angle projection: first-angle places the view on the far side of the object (common in Europe/ISO); third-angle places it on the near side (common in the US and the Philippines). The choice is signaled by a projection symbol in the title block.
• Scale: ratio of drawn size to actual size (1:1 full, 1:2 half, 2:1 double).
• Line conventions: visible (continuous thick), hidden (dashed), center (chain dashed), dimension/extension (continuous thin), phantom (long-dash double-dash) for alternate positions.

2. Dimensioning & Tolerancing

• Dimension lines show measurements; extension lines offset from the object; leaders point to features.
• Tolerances specify allowed variation, e.g., $25.00 \pm 0.05\text{ mm}$.
• Limits: the maximum and minimum acceptable sizes.
• Fits: clearance, transition, and interference fits between mating parts; the ISO hole/shaft-basis system standardizes fit classes (e.g., H7/g6).

Worked Example: Tolerance Stack on a Bearing Fit

A shaft is dimensioned $20.00 \pm 0.02\text{ mm}$ (max $20.02$, min $19.98$) and its bearing bore $20.10 \pm 0.03\text{ mm}$ (max $20.13$, min $20.07$). The nominal clearance is $20.10 - 20.00 = 0.10\text{ mm}$. The minimum clearance occurs with the largest shaft and smallest bore: $20.07 - 20.02 = 0.05\text{ mm}$. The maximum clearance occurs with the smallest shaft and largest bore: $20.13 - 19.98 = 0.15\text{ mm}$. The clearance therefore ranges from $0.05\text{ mm}$ to $0.15\text{ mm}$.

3. CAD Software Environment

Typical 2-D/3-D CAD packages (AutoCAD, SolidWorks, KiCad for ECE schematics and PCBs, Altium) share a common environment:

Snapping Precision

Object snaps (OSNAP) force the cursor to geometrically significant points: ENDpoint, MIDpoint, CENter, INTersection, PERpendicular, TANgent, QUAdrant, NODe. Snapping is essential to producing closed, dimensionally accurate drawings; freehand "eyeball" placement creates gaps and overlaps that fail fabrication.

4. ECE-Specific Drawing Standards

Electronics drawings follow specialized standards distinct from mechanical drafting:

• Schematic diagrams (ANSI/IEEE 315 and IEC 60617 symbols): show logical electrical connections; component values and reference designators (R1, C2, U3) are labeled.
• PCB layout (IPC-2221, IPC-A-600): copper traces, pads, vias, silkscreen, and solder mask, controlled by design rules (trace width, clearance, annular ring).
• Wiring/connection diagrams: physical wiring harnesses.
• Block diagrams: functional system decomposition (used in ECE system design and RA 9292 scope documentation).
• Philippine Electrical Code (PEC) drawings: lighting/power layouts and single-line diagrams, relevant where ECE work interfaces with electrical installation.

Title Block & Sheet Sizes

Every engineering drawing carries a title block in the lower-right corner identifying the drawing number, title, scale, projection symbol, general tolerances, material, the drawn-by / checked-by / approved-by names, the date, and the revision history. Standard sheet sizes follow the ISO A-series (A0 through A4) or ANSI sizes (A through E); the Philippines commonly uses ISO A-sizes for technical drawings.

First-Angle vs Third-Angle Projection, Made Concrete

The distinction is purely where the right-side (or top) view is placed relative to the front view, but it is the single most common source of reading errors on a drawing. In third-angle (US/Philippine) practice, the top view is placed above the front view and the right-side view to the right of the front view — that is, each view is placed on the side of the object nearest the viewer, as if you unfolded the glass box around the object toward yourself. In first-angle (European/ISO) practice, the views are mirrored in placement: the top view appears below the front view and the right-side view to the left, because each view is projected onto the plane on the far side of the object and then unfolded. The trapezoidal projection symbol in the title block (a truncated cone drawn in both conventions) is the authoritative signal; always confirm it before interpreting an unfamiliar drawing. Getting this backwards flips the meaning of holes, pockets, and tapers, which is a classic fabrication error.

Dimensioning Rules and Why Tolerancing Matters

Good dimensioning follows a few rules that GEAS tests implicitly: dimension to visible (not hidden) outlines wherever possible; place dimensions outside the object outline; do not duplicate a dimension already implied by another view; stagger parallel dimension lines for legibility; and dimension from a common datum or baseline for accuracy. Tolerancing exists because no manufactured feature lands exactly on its nominal value; the tolerance band sets the functional limit. A clearance fit (hole larger than shaft) allows relative motion, a transition fit is borderline, and an interference fit (shaft larger than hole) requires pressing the parts together and holds by friction or elastic grip. The worked bearing example earlier showed how shaft and hole tolerances combine through worst-case stacking to give a clearance range. In PCB work the analogous idea is the design rule: minimum trace width, minimum clearance to adjacent copper, and minimum annular ring around a via must all be met by the fabricator's capability, or the board will not manufacture reliably.

Putting CAD Into ECE Practice

For an ECE, the CAD workflow typically moves from schematic capture (logical connectivity, reference designators, part values) to netlist extraction to PCB layout (placing footprints, routing copper under design-rule constraints, generating Gerber and drill files). Modern EDA tools (KiCad, Altium Designer, Cadence OrCAD) embed electrical rules — unrouted nets, unconnected pins, clearance violations — that mechanical CAD does not, so the drawing discipline is coupled to circuit correctness. A clean schematic that follows IEEE 315/IEC 60617 symbols and consistent reference designators is readable by any other engineer and is the legal record of the design; RA 9292 places professional responsibility on the signing ECE for plans and specifications, so drawing standards are an ethics issue, not just a craft one.

5. Summary