Drawings are the shared language of engineers, electricians and mechanics. This beginner course teaches you to read them and to make simple ones. You will read orthographic and isometric views, line types, scales and title blocks; interpret dimensions and tolerances; read electrical schematics and wiring diagrams, common mechanical and GD&T symbols, and site and floor plans. Then you will sketch a part by hand in three views, draw and dimension it in free 2D CAD software, and extrude a simple bracket in a free 3D tool. All hands-on work uses paper and a ruler or free software (FreeCAD, LibreCAD or the Onshape free plan). It builds on the measurement and geometry from Math for Tech & STEM.

Read orthographic views, scales, dimensions, tolerances, schematics, GD&T callouts and floor plans, then sketch and model parts in free CAD tools.

Technical Drawing & CAD Basics

## Why it matters

A photo of a part shows what it looks like; a technical drawing shows exactly what to make. To do that on flat paper, drawings use standard ways of projecting a 3D object into 2D. If you can read them, you can build, install, inspect or repair from a print. If you misread them, you can drill a hole on the wrong face.

## Orthographic projection

**Orthographic projection** shows an object as a set of flat views, each looking straight at one face, with lines of sight parallel and perpendicular to that face. Each view shows only two of the three dimensions:

| View | Shows | Hides |
|---|---|---|
| Front | width and height | depth |
| Top | width and depth | height |
| Right side | depth and height | width |

Most parts need three views; simple parts (a washer, a shaft) may need only one or two plus a note. The **front view** is chosen to show the most characteristic shape of the part, not necessarily the side that faces you in use.

### The views line up

Views share dimensions, so they are aligned:

- The **top view** sits directly above the front view and has the same width.
- The **right side view** sits directly to the right of the front view and has the same height.
- Depth appears in both the top and side views.

You can project features from one view to another with light construction lines. If a hole appears in the front view, find it in the top view directly above.

### Third-angle and first-angle projection

There are two conventions:

- **Third-angle projection** (used in the US and Canada, ASME standards): top view above the front, right view on the right.
- **First-angle projection** (used in much of Europe and Asia, ISO standards): the top view goes **below** the front, and the right side view goes on the **left**.

The title block shows which is used with a small symbol of a truncated cone. Always check it, especially on prints from overseas suppliers. Misreading the projection mirrors the part.

## Hidden and centre lines

Features you cannot see in a view, such as a hole through the back, are drawn as **hidden lines** (short dashes). The axis of a hole or cylinder is shown with a **centre line** (long and short dashes). Lesson 2 covers all line types.

## Isometric (pictorial) views

An **isometric drawing** shows the object in one picture, tilted so that three faces are visible. The three axes are drawn 120 degrees apart: vertical edges stay vertical, and width and depth edges run at **30 degrees** from horizontal. In an isometric drawing, lengths along the three axes are drawn at true scale, so you can measure them.

Isometric views help people visualise a part. They are often shown in a corner of a print or in assembly instructions, but the orthographic views carry the dimensions.

## Worked example: an L-shaped block

Picture a block 60 mm wide, 40 mm deep and 30 mm tall, with a 30 mm wide by 15 mm tall notch cut out of the top right along its full depth, leaving an L shape when seen from the front. A 10 mm hole goes straight down through the low (right) part.

- **Front view:** an L outline 60 wide and 30 tall; the step is 15 mm down, 30 mm from the right. The hole, which runs vertically, appears as two hidden lines with a centre line between them, in the right half.
- **Top view (above the front):** a 60 × 40 rectangle with a line across it 30 mm from the right edge (the step edge). The hole is a circle in the right half, with crossing centre lines.
- **Right side view (right of the front):** a 40 × 30 rectangle with a visible line 15 mm down where the step is seen edge-on. The hole shows as hidden lines.

Check: the top and front views both span 60 mm wide; the front and side views are both 30 mm tall; the top and side views are both 40 mm deep.

## How to read views quickly

1. Find the front view (usually the largest, bottom left in third-angle).
2. Check the projection symbol.
3. Take one feature at a time and find it in every view.
4. Build the 3D shape in your head, or sketch a quick isometric.
5. Confirm hidden lines match features you expect.

## Common mistakes

- Assuming third-angle on a first-angle drawing.
- Reading one view alone. A circle in the top view could be a hole, a boss or a cylinder; the other views tell you which.
- Ignoring hidden lines.
- Measuring an isometric drawing for dimensions. Use the dimensioned views.

## Recap

- Orthographic views: front (width × height), top (width × depth), side (depth × height), aligned with each other.
- Third-angle (US) puts the top above and the right side on the right; first-angle reverses them. Check the symbol.
- Isometric drawings show one 3D picture with axes at 30 degrees from horizontal.


Read the top, front and side views of a part and relate them to an isometric picture.

Orthographic and isometric views

Orthographic Drawing - Simplified

Isometric Drawing - Simplified

## Why it matters

Every line on a print means something specific: an edge you can see, an edge you cannot, an axis, a dimension, a cut. The scale tells you how the paper relates to the real object, and the title block tells you which part, which revision and whose responsibility it is. Building from an out-of-date revision or misreading a scale is one of the most expensive mistakes on a job.

## The alphabet of lines

Standards (ASME Y14.2 in the US, ISO 128 internationally) define line types by **thickness** and **pattern**:

| Line | Looks like | Means |
|---|---|---|
| Visible (object) | thick, continuous | edges you can see |
| Hidden | medium, short dashes | edges behind the surface |
| Centre | thin, long-short-long | axes of holes, cylinders, symmetry |
| Dimension | thin, with arrowheads, number in the middle | a measured distance |
| Extension | thin, continuous, with a small gap from the part | extends a feature out to a dimension |
| Leader | thin, angled, with an arrow or dot | points a note or callout at a feature |
| Cutting plane | thick, long dashes with arrows, letters at the ends | where a section view is cut |
| Section (hatch) | thin, evenly spaced diagonal lines | solid material cut in a section view |
| Phantom | thin, long-short-short-long | alternate positions, adjacent parts |
| Break | freehand or zigzag | a long part shortened on the drawing |

The thick-thin contrast is deliberate: the eye picks out the part's outline first.

## Section views

When interior features are complex, the drawing shows a **section view**: imagine cutting the part along the **cutting-plane line**, removing the piece toward the arrows, and looking at what is left. The view is labelled, for example **SECTION A-A**, and the cut surfaces are hatched.

## Scale

**Scale** relates drawing size to real size.

- **Metric and mechanical scales** are ratios: **1:1** full size, **1:2** half size (1 mm on paper = 2 mm real), **2:1** twice size for small parts.
- **Architectural scales** (US building plans) are written as paper inches per real foot: **1/4" = 1'-0"** means a quarter inch on the paper represents one foot (a ratio of 1:48). 1/8" = 1'-0" is 1:96.
- **Engineering (civil) scales** use one inch to a number of feet: **1" = 20'** (1:240), **1" = 50'**, common on site plans.

### Worked example: reading a scaled distance

A floor plan is drawn at 1/4" = 1'-0". A wall measures 3 1/2 inches on the print.

Real length = 3.5 in ÷ 0.25 in per foot = **14 feet**.

A site plan at 1" = 20' shows a driveway 2.3 inches long: 2.3 × 20 = **46 feet**.

### Do not scale the drawing

Printing, copying and PDF zoom change the size. That is why many title blocks say **"DO NOT SCALE DRAWING"**: use the written dimensions. Scaling is fine for a rough estimate; if a dimension is missing, ask for it rather than measuring the paper. A **graphic (bar) scale** printed on the sheet stays correct when the print is resized, so it is safer than the ratio for estimates.

## The title block

Usually in the lower-right corner. Typical fields:

- **Title** (part or project name) and **drawing number**.
- **Revision** letter or number, plus a **revision block** listing what changed, when and who approved it.
- **Scale**, **sheet** (for example "2 of 5"), **size** (A, B, C, D or A4, A3...).
- **Units** (mm or inches) and **general tolerances** (lesson 3).
- **Projection symbol** (first- or third-angle).
- **Material** and **finish** for mechanical parts.
- **Drawn by, checked by, approved by**, with dates; on construction drawings, the engineer's or architect's seal where required.

### Worked example: checking a print before you start

You are handed drawing 4410-202, revision C, sheet 1 of 2, scale 1:2, units mm, third-angle. Before you cut anything: confirm revision C is the current revision on the project's document list, find sheet 2, note that 1:2 means the part is twice the size you see, and note the general tolerance block.

## Common mistakes

- Working from an old revision.
- Mixing up 1:2 and 2:1.
- Reading a 1/8" scale plan with the 1/4" side of the scale ruler.
- Missing a second sheet.
- Ignoring the units box (a 25.4 mm hole and a 1.000 in hole are the same; 25.4 inches is not).

## Recap

- Line types tell you visible, hidden, centre, dimension, cutting plane and more; thick lines are outlines.
- Scales: ratios (1:2), architectural (1/4" = 1'-0"), engineering (1" = 20').
- Use written dimensions, not a ruler on the paper.
- The title block gives the number, revision, scale, units, projection and approvals.


Read line types, drawing scales and the information in a title block.

Line types, scale, and title blocks

Blueprint Line Types Explained | Visible, Hidden, Center, Section and Dimension Lines

How to Read and Use an Engineer's Scale for Beginners

## Why it matters

No part is made exactly to size. A machine, a saw or a concrete form always varies a little. **Tolerances** say how much variation is acceptable, and dimensions say what the target is and where it is measured from. Get them right and parts from different shops fit together; read them wrong and a shaft will not go into its bearing.

## Size and location dimensions

- **Size dimensions** give how big a feature is: length, width, diameter, radius, depth.
- **Location dimensions** give where a feature is, measured from an edge, a centre line or a datum.

### Common symbols

| Symbol | Meaning | Example |
|---|---|---|
| Ø | diameter | Ø10 |
| R | radius | R5 |
| □ | square | □20 |
| ⌴ | counterbore | ⌴ Ø12 ↧ 6 |
| ⌵ | countersink | ⌵ Ø12 × 90° |
| ↧ | depth | ↧ 15 |
| THRU | goes all the way through | Ø8 THRU |
| 4X | repeated four times | 4X Ø6 |
| ( ) | reference dimension, for information only | (120) |

## Tolerance formats

1. **Limit dimensions:** both limits shown, larger on top. `25.05 / 25.00` means any size from 25.00 to 25.05 mm is acceptable.
2. **Bilateral (plus/minus):** `25.00 ±0.05` means 24.95 to 25.05. Unequal versions exist: `25.00 +0.10 / -0.05`.
3. **Unilateral:** variation in one direction only: `25.00 +0.05 / 0`.
4. **General tolerances in the title block:** dimensions with no tolerance of their own use the block. Example (mm):

| Decimal places | Tolerance |
|---|---|
| X | ±0.5 |
| X.X | ±0.2 |
| X.XX | ±0.05 |
| Angles | ±0.5° |

So on that drawing `40` means 39.5 to 40.5, while `40.00` means 39.95 to 40.05. **The number of decimal places carries meaning.** On inch drawings the same idea applies (.X, .XX, .XXX).

The **tolerance** itself is the total allowed variation: for `25.00 ±0.05` the tolerance is 0.10 mm.

## Chain versus baseline dimensioning

How dimensions are arranged changes how errors add up.

- **Chain dimensioning:** each feature is dimensioned from the previous one. Tolerances **stack up**.
- **Baseline dimensioning:** every feature is dimensioned from one common edge or datum. Each location keeps its own tolerance.

### Worked example: tolerance stack-up

Three holes are chain-dimensioned from the left edge: 20 ±0.1, then 30 ±0.1, then 30 ±0.1.

- Nominal distance from the edge to the third hole = 20 + 30 + 30 = 80 mm.
- Worst-case variation = 0.1 + 0.1 + 0.1 = ±0.3 mm, so the third hole could be anywhere from 79.7 to 80.3 mm.

If the third hole were baseline-dimensioned at 80 ±0.1 from the edge, it would stay within 79.9 to 80.1. Choose the scheme that controls what matters to the fit.

## Fits between parts

When a shaft goes into a hole, the **fit** depends on both tolerances:

- **Clearance fit:** the shaft is always smaller than the hole (slides or rotates).
- **Interference (press) fit:** the shaft is always larger (pressed in, stays put).
- **Transition fit:** could be either.

Worked example: hole 20.00 to 20.05, shaft 19.95 to 19.98. Smallest clearance = 20.00 - 19.98 = 0.02 mm; largest = 20.05 - 19.95 = 0.10 mm. Always positive, so it is a clearance fit. Standard fit systems such as ISO 286 (for example H7/g6) are tabulated so designers do not reinvent them.

## Dimensioning rules you will see followed

- Each feature is dimensioned **once**; no repeated or conflicting dimensions.
- Dimensions are placed on the view that shows the feature's true shape.
- Dimensions are not taken to hidden lines where avoidable.
- Extension lines do not touch the part; a small gap remains.
- Units are stated once in the title block, not after every number.

## Common mistakes

- Treating `40` and `40.00` as the same.
- Adding up chain dimensions without the stack-up.
- Reading `4X Ø6` as one hole.
- Using a reference dimension (in brackets) to inspect a part.
- Confusing the tolerance (total band) with the plus/minus value.

## Recap

- Size dimensions say how big; location dimensions say where.
- Tolerances: limit, bilateral, unilateral, or the title-block general tolerance (decimal places matter).
- Chain dimensions stack tolerances; baseline dimensions do not.
- Fits: clearance, interference, transition, worked out from the hole and shaft limits.


Interpret size and location dimensions, tolerance formats and tolerance stack-up.

Dimensions and tolerances

Why Engineering Tolerances Matter (And How to Use Them)

Dimensioning Rules in Engineering Drawings

Read orthographic and isometric views, line types, scales, title blocks, dimensions and tolerances.

Reading Drawings

## Why it matters

Electricians install from drawings, mechanics troubleshoot machines from them, and electrical engineers design with them. The same circuit can be drawn several ways, each for a different job. Knowing which kind of drawing you are holding and what its symbols mean is the first step in any electrical task.

> Reading diagrams is safe; working on circuits is not. Troubleshooting energised equipment requires training, qualification and your employer's procedures (see Workplace Safety & Tools).

## Four kinds of electrical drawing

| Drawing | Shows | Used for |
|---|---|---|
| **Schematic** | how components connect electrically, using symbols, laid out for clarity, not physical position | understanding and troubleshooting a circuit |
| **Wiring (connection) diagram** | physical layout: terminals, wire colours and numbers, where each wire lands | building or rewiring a panel or device |
| **Ladder diagram** | control logic between two vertical power rails, one rung per circuit | motor controls, relays, PLC logic |
| **One-line (single-line) diagram** | power distribution with one line per circuit, regardless of phases | services, switchboards, transformers |

Building plans also have **electrical floor plans** with symbols for receptacles, switches and lights (lesson 6).

## Common schematic symbols

Symbols follow standards such as IEEE 315 / ANSI Y32.2 (US) and IEC 60617 (international), so the same part can look different on a European print.

- **Battery / DC source:** long and short parallel lines; the long line is positive.
- **AC source:** a circle with a sine wave.
- **Resistor:** a zigzag (US) or a small rectangle (IEC).
- **Capacitor:** two parallel plates; one curved for polarised types.
- **Inductor / coil:** a series of loops.
- **Diode:** a triangle pointing to a bar (current flows in the direction of the triangle); an **LED** adds two arrows pointing out.
- **Switch:** a line hinged away from a contact point. **Normally open (NO)** is drawn open; **normally closed (NC)** is drawn closed.
- **Push button:** a bridge contact with a plunger; NO and NC versions.
- **Fuse:** a rectangle with a line through it, or an S-curve. **Circuit breaker:** a curved contact.
- **Lamp:** a circle with an X.
- **Motor:** a circle with M.
- **Transformer:** two coils side by side with lines (core) between them.
- **Ground / earth:** a set of decreasing horizontal lines. Chassis ground and signal ground have their own variants.
- **Relay:** a coil (often a circle or rectangle with a label like CR1) plus contacts elsewhere on the drawing with the **same label**.

### Wires and junctions

A **dot** where lines meet means the wires are connected. Lines that simply cross with no dot are **not** connected. (Some older drawings use a small hump to show a crossing.)

### Labels and reference designators

Each component has a **reference designator**: R1, C3, F2, M1, CR1, S1. Wires often have **wire numbers** that match the labels on the real wires, which is how you trace a schematic onto a panel.

### Shelf state

Schematics show contacts in their **de-energised, at-rest ("shelf") state**. A relay's NO contact is drawn open even though it closes when the coil is powered.

## Worked example: a simple DC circuit

A 12 V battery's positive terminal goes through fuse **F1** (5 A), then switch **S1**, then lamp **L1**, and back to the negative terminal.

Read it as a loop: + → F1 → S1 → L1 → −. With S1 closed, current flows and the lamp lights. If L1 is dark with S1 closed, the likely suspects in order are: blown F1, faulty S1, failed lamp, open wiring. The schematic tells you where to test, with the right meter and under the right procedure.

## Worked example: a start/stop ladder rung

Two vertical rails, L1 (left) and L2 (right). One rung, from left to right:

`L1 — [STOP: NC push button] — [START: NO push button] — (M coil) — L2`

with an **M auxiliary NO contact** drawn in parallel around the START button.

- Press START: current flows through STOP (closed) and START to the motor starter coil M. The motor runs.
- The M auxiliary contact closes and "seals in" the circuit, so the motor keeps running when you release START.
- Press STOP: the NC contact opens, M drops out, and the seal-in contact opens.

This three-wire control circuit appears in almost every industrial machine and is a classic exam and interview question.

## Common mistakes

- Treating a schematic as a map of where parts physically sit.
- Reading crossing lines as connected.
- Forgetting that contacts are shown in shelf state.
- Missing that a relay's coil and contacts can be on different pages; follow the label.

## Recap

- Schematic = logic; wiring diagram = physical connections; ladder = control logic between rails; one-line = power distribution.
- Know the core symbols; dots mean connected; contacts are drawn de-energised.
- Trace circuits as loops from source to load and back.


Read common electrical symbols and trace a simple schematic, wiring diagram and ladder diagram.

Electrical symbols and schematics

How to Read a Schematic

Schematic Diagrams & Symbols, Electrical Circuits - Resistors, Capacitors, Inductors, Diodes, & LEDs

## Why it matters

Plus/minus tolerances control size, but they do a poor job of controlling **form** (is it flat?), **orientation** (is it square?) and **location** (is the hole where it should be relative to the surfaces that matter?). **Geometric Dimensioning and Tolerancing (GD&T)** solves this with a compact symbolic language. Mechanical engineers, machinists, inspectors and maintenance mechanics all meet it on prints.

In the US the standard is **ASME Y14.5** (current edition 2018); internationally, ISO GPS standards such as ISO 1101 cover similar ground with some differences. Check which one the title block references.

## The feature control frame

A GD&T requirement is written in a **feature control frame**, a row of boxes attached to the feature:

`[ ⌖ | Ø0.2 Ⓜ | A | B | C ]`

Read it left to right:

1. **Geometric characteristic symbol** (here ⌖, position).
2. **Tolerance zone**: Ø means the zone is a cylinder 0.2 mm in diameter; no Ø means a zone between two parallel lines or planes.
3. **Material condition modifier** (optional): Ⓜ maximum material condition (MMC), Ⓛ least material condition (LMC). No modifier means "regardless of feature size".
4. **Datum references** in order of priority: primary A, secondary B, tertiary C.

"The axis of this hole must lie within a 0.2 mm diameter cylinder at MMC, located relative to datums A, B and C."

## The characteristics, by group

| Group | Characteristics | Datum needed? |
|---|---|---|
| Form | straightness, flatness, circularity, cylindricity | no |
| Orientation | perpendicularity ⊥, parallelism ∥, angularity ∠ | yes |
| Location | position ⌖ | yes |
| Profile | profile of a line, profile of a surface | usually |
| Runout | circular runout ↗, total runout | yes |

ASME Y14.5-2018 lists 12 characteristics; older editions also had concentricity and symmetry, which you will still see on older prints.

## Datums

A **datum** is a theoretically exact reference: a plane, axis or point. The **datum feature** is the real surface on the part that establishes it, marked with a **datum feature symbol** (a letter in a box attached to a triangle on the surface, for example A).

The order in the frame matters. A typical part sits on datum A (the primary plane, three points of contact), is pushed against B (two points), and then against C (one point): the **3-2-1 principle**. Inspection fixtures copy this setup.

## Basic dimensions

Dimensions in a **rectangular box** are **basic**: theoretically exact values with no tolerance of their own. The GD&T frame supplies the tolerance. A hole located by boxed 25 and 40 from datums B and C, with a position tolerance of Ø0.2, must have its axis inside a 0.2 mm cylinder centred at exactly (25, 40).

## Worked example: bonus tolerance at MMC

A hole is sized Ø10.0 to Ø10.2 with position Ø0.2 Ⓜ to A, B, C.

- For a hole, **MMC** is the smallest size (most material left): Ø10.0.
- At MMC, the position tolerance is Ø0.2.
- If the hole is made at Ø10.1, it is 0.1 away from MMC, and that departure is added as **bonus tolerance**: Ø0.2 + 0.1 = **Ø0.3**.
- At the largest size, Ø10.2, the position tolerance becomes Ø0.4.

This makes sense functionally: a bigger hole can be slightly further out of place and a bolt will still fit. That is why Ⓜ is common on clearance holes.

## Other mechanical symbols

- **Threads:** metric `M8×1.25-6H` (8 mm nominal, 1.25 mm pitch, tolerance class 6H for an internal thread); unified inch `1/4-20 UNC-2B` (1/4 in, 20 threads per inch, coarse, class 2B internal). Threads are usually drawn simplified, with a dashed or thin line for the thread depth.
- **Surface texture:** a check-mark symbol with a value, for example Ra 1.6 (μm), meaning average roughness. A bar across the top means material must be removed by machining.
- **Welding symbols (AWS A2.4):** a reference line with an arrow; symbols below the line apply to the arrow side, above the line to the other side. A triangle is a fillet weld.
- **Chamfer:** `2 × 45°`. **Fillet/round:** `R3`.
- **Section and detail views:** a circle labelled DETAIL B, scaled up, for small features.

## Common mistakes

- Reading the datum order as alphabetical rather than as written in the frame.
- Forgetting that boxed (basic) dimensions have no plus/minus.
- Treating the Ø in the frame as the hole size; it describes the shape of the tolerance zone.
- Applying bonus tolerance when there is no Ⓜ modifier.
- Mixing ASME and ISO rules on one print.

## Official source

- ASME Y14.5 Dimensioning and Tolerancing: https://www.asme.org/codes-standards/find-codes-standards/y14-5-dimensioning-tolerancing

Checked on 2026-10-23.

## Recap

- A feature control frame reads: characteristic, zone (Ø = cylindrical), modifier, datums in priority order.
- Groups: form, orientation, location, profile, runout.
- Basic dimensions are exact; the frame gives the tolerance.
- At MMC, a hole gets bonus tolerance as it grows from its smallest size.


Recognise common GD&T callouts, datums, threads and surface finish symbols on mechanical drawings.

Mechanical and GD&T symbols

What is GD&T? A Simple Explanation

Understanding GD&T

## Why it matters

Buildings and civil works are built from sets of drawings, sometimes hundreds of sheets. Electricians lay out boxes and runs from floor plans, civil engineers design grading and utilities on site plans, and mechanics and engineers use them to find equipment rooms and access routes. Reading the set quickly, and knowing which sheet to open, is a core site skill.

## How a drawing set is organised

Sheets are grouped by discipline, usually with a letter prefix (the US National CAD Standard uses this convention):

| Prefix | Discipline |
|---|---|
| G | general (cover, index, code summary) |
| C | civil (site, grading, utilities) |
| L | landscape |
| S | structural |
| A | architectural |
| M | mechanical (HVAC) |
| P | plumbing |
| E | electrical |
| FP | fire protection |

A sheet like **E-201** is electrical, floor plans series, sheet 1. Start with the **sheet index** on the cover and the **general notes and legend** sheets, which define the symbols used in that set.

## Floor plans

A **floor plan** is a horizontal section: imagine cutting the building about 4 feet (1.2 m) above the floor and looking down.

- **Walls:** thick lines (cut), often with hatching or wall-type tags that refer to a wall schedule.
- **Doors:** a gap in the wall with a thin arc showing the swing, and a door tag that refers to a door schedule.
- **Windows:** thin lines within the wall, with window tags.
- **Stairs:** a series of lines with an arrow and **UP** or **DN**; a break line where the cut passes through.
- **Room names and numbers.**
- **Dimension strings:** usually the overall dimension on the outside, then openings and walls inside it. Check the general notes for whether dimensions are to the face of stud, the centre of walls or the face of finish.
- **Grid lines:** lettered and numbered column lines in bubbles (A, B, C... and 1, 2, 3...) for locating everything: "the panel is at C-4".
- **North arrow** and **scale** (often 1/8" = 1'-0" or 1/4" = 1'-0").
- **Section and detail markers:** a circle split in two, with the detail number on top and the sheet number below. "3 over A-501" means detail 3 on sheet A-501.

### Electrical floor plan symbols (typical)

- **Duplex receptacle:** a circle with two short parallel lines; **GFCI** written next to it for ground-fault protected outlets.
- **Switch:** an **S**; **S3** is a three-way switch.
- **Ceiling light:** a circle, often with a fixture letter that refers to the lighting fixture schedule.
- **Panelboard:** a rectangle, often filled, with its name (LP-1).
- **Home runs:** an arrow with a circuit number to the panel.

Always check that set's legend: symbols vary between firms.

## Site plans

A **site plan** shows the building in its surroundings, at an engineering scale such as 1" = 20'.

- **Property lines** with bearings and distances (for example N 45°12' E, 150.00').
- **Setbacks** and **easements** (areas where you may not build, or where utilities have rights).
- **Existing contours** (dashed) and **proposed contours** (solid), with elevations.
- **Spot elevations** at corners, curbs and drains, and the **finished floor elevation (FFE)** of the building.
- **Utilities:** water, sanitary sewer, storm drainage, gas, electric and telecom lines, with structures such as manholes and catch basins.
- **Paving, parking, sidewalks**, and the **benchmark** that elevations refer to.
- **North arrow**, scale, and legend.

### Worked example: slope from contours

Two proposed contours, 101.0 and 100.0 (1 ft interval), are 25 ft apart measured on the plan.

Slope = rise / run = 1.0 / 25 = 0.04 = **4%** falling away from the building. Steeper ground shows contours closer together.

### Worked example: finding a detail

The site plan shows a catch basin with a marker "5 / C-501". Open sheet C-501 and find detail 5, which shows the basin's construction.

## Reading order for a new set

1. Cover sheet and index.
2. Legends, abbreviations and general notes for your discipline.
3. Site plan to understand the building's position, access and utilities.
4. Floor plans for your discipline, using grid lines to locate work.
5. Sections and details referenced from the plans.
6. Schedules (doors, fixtures, panels, equipment).
7. Revision clouds and addenda: check you have the latest issue.

## Common mistakes

- Using symbols from memory instead of the set's legend.
- Missing a revision cloud (a scalloped outline marking a change).
- Confusing existing and proposed contours.
- Scaling a dimension that is written elsewhere (or one that was resized when printed).

## Recap

- Sets are organised by discipline prefix; start with the index and legends.
- Floor plan = horizontal cut at about 4 ft: walls, doors, windows, stairs, grids, dimensions, section and detail markers.
- Site plan: property lines, setbacks, contours, elevations, utilities; slope = rise / run.


Read the main features of an architectural floor plan and a civil site plan.

Site and floor plans

How to Read a Site Plan - The Basics

How to Read House Plans – Beginner’s Guide (Explained Simply)

Read electrical schematics and wiring diagrams, mechanical and GD&T callouts, and site and floor plans.

Discipline Symbols

Measure the object's overall width, depth and height with a tape or ruler and write them down with units.

Measure two or three features (a hole, a notch, a step) and their locations from an edge.

Choose the front view and lightly block in the front, top (above) and right side (to the right) views on grid paper, keeping them aligned.

Project each feature between views with light lines, then add hidden lines and centre lines.

Darken the visible lines so they are clearly thicker than hidden and centre lines.

Add the key dimensions, each feature once, on the view that shows it best.

On a separate area or sheet, sketch an isometric view of the object, boxing it in first and drawing any holes as ellipses.

Add a small title block: part name, your name, date, units, NOT TO SCALE and third-angle projection.

Show your sketch to someone else and ask them to describe the object without seeing it. Note anything they got wrong.

## Why it matters

Even in a world of CAD, the fastest way to explain a part, a bracket you need, or a conduit route is a quick sketch. Engineers sketch in design meetings, electricians sketch as-built changes, and mechanics sketch a worn part before it goes to the machine shop. A clear sketch with the right views and a few key dimensions can save hours of back-and-forth.

## What you need

- Pencil (a mechanical 0.5 mm works well) and an eraser.
- **Grid paper** (5 mm or 1/4 in squares) for orthographic sketches, and **isometric grid paper** if you have it (free printable sheets exist online).
- A ruler for checking, though good technical sketches are freehand.
- The part itself, or a picture of it, and a tape or caliper to measure it.

## Freehand line technique

- Draw lines with your arm, not just your wrist. Mark the start and end points first, then sketch light strokes between them.
- Pull lines toward you or across, rotating the paper if needed.
- Sketch **light construction lines** first; darken the final outline at the end.
- Use **line weight** to communicate: dark, thick visible lines; medium dashed hidden lines; thin centre and dimension lines.

## Proportion over precision

A sketch does not need to be to scale, but proportions should be right. Use the grid: if the part is 60 mm by 40 mm, make it 12 squares by 8 squares on 5 mm paper. Estimate features by halves and quarters of a known length.

## Sketching three views: step by step

Use third-angle projection (US convention) unless told otherwise.

1. **Choose the front view**: the face that shows the most shape.
2. **Block in the front view** as a light rectangle using the overall width and height.
3. **Block in the top view** directly above, the same width, using the depth for its height. Leave a gap of a few squares.
4. **Block in the right side view** to the right of the front, the same height, using the depth for its width.
5. **Project features between views** with light horizontal and vertical lines. A step in the front view appears as a line in the top and side views.
6. **Add circles and arcs**: sketch the centre lines first, mark the radius on each line, then join the marks.
7. **Add hidden lines** for features you cannot see.
8. **Darken visible lines**, then add centre lines.
9. **Add key dimensions** with extension and dimension lines, once each, on the view that shows the feature best.
10. **Add a title block**: part name, your name, date, units, "NOT TO SCALE" if it is not.

## Sketching an isometric view

1. Draw three light axes from one corner: one vertical, two at 30 degrees from horizontal (follow the isometric grid).
2. Mark the overall width, depth and height along the axes and box in the part.
3. Cut away notches and steps from the box.
4. For holes, sketch **ellipses** on the faces (a circle in isometric is an ellipse). Box the ellipse first in a rhombus.
5. Darken the visible edges only. Isometric sketches usually leave out hidden lines.

## Worked example: a small angle bracket

A bracket is an L shape seen from the front: 50 mm wide base, 40 mm tall upright, 5 mm thick, 30 mm deep. Two Ø6 holes go through the base, 12 mm in from each end.

- **Front view (12 × 8 squares at 5 mm, close enough):** the L outline. The base holes appear as hidden lines.
- **Top view:** a 50 × 30 rectangle with a line 5 mm from the left end where the upright rises, and two circles with centre lines on the base.
- **Side view:** a 30 × 40 rectangle with a line 5 mm above the bottom for the base thickness.
- **Dimensions:** overall 50, 40 and 30; thickness 5; hole locations 12 from each end, centred in depth; 2X Ø6 THRU.

## Common mistakes

- Starting with details before blocking in the overall size.
- Views that do not line up, so features cannot be projected.
- All lines the same weight.
- Dimensioning the same feature in two views.
- Forgetting units.

## Recap

- Sketch light, then darken; use line weights to separate visible, hidden and centre lines.
- Block in all three views first, aligned, then project features across.
- Use grid paper for proportion; circles in isometric become ellipses.
- Add key dimensions once each and a simple title block.

## Try it

The **Do** tab walks you through sketching a real object from your home in three views and an isometric view.


Sketch a simple part in three orthographic views and an isometric view using paper and a ruler.

Sketching by hand

Manual Drafting: Orthographic Sketching on Grid Paper

Sketching Multiviews and Dimensions

Download and install LibreCAD from https://librecad.org/ or FreeCAD from https://www.freecad.org/ (official sites only).

Create a new drawing and set the units to millimetres.

Create layers for outline, centre lines and dimensions with different line types or weights (in FreeCAD Sketcher, skip layers and use construction lines for centre lines).

Draw the 120 x 80 mm outline starting at the origin, using typed coordinates or lengths, not the mouse alone.

Draw four 8 mm holes 10 mm in from each edge and a 30 mm hole in the centre. In FreeCAD, constrain the sketch until it reports fully constrained.

Add dimensions: overall size, hole locations, 4X Ø8 THRU and Ø30 THRU.

Add a simple title block with part name, your name, date, units, scale and projection.

Save the file (DXF or FCStd) with a revision letter in the name and export a PDF.

Open the PDF and check every dimension against the reading. Fix any mistakes.

## Why it matters

Almost every engineering and trade drawing you will see is made in CAD (computer-aided design). Industry tools such as AutoCAD, SolidWorks, Revit or Inventor are expensive, but the core ideas (precise coordinates, layers, snaps, dimensions, title blocks) are the same in free software. Learning them now means your first week with a commercial package is about where the buttons are, not how CAD thinks.

## Free tools for 2D drawing

- **LibreCAD**: free, open-source 2D CAD for Windows, macOS and Linux. Closest in feel to classic AutoCAD 2D drafting. Saves DXF, which most CAD tools open.
- **FreeCAD**: free, open-source 3D modeller with a **Sketcher** workbench for constrained 2D profiles and a **TechDraw** workbench for making 2D drawings from 3D models. Better if you want to continue into 3D (lesson 9).
- **Onshape free plan**: browser-based, no install. Free-plan documents are **public**, so never put school, employer or client designs in it.

Download only from the official sites: https://librecad.org/ and https://www.freecad.org/. Checked on 2026-10-23.

## How CAD differs from drawing on paper

1. **You draw at full size.** A 120 mm plate is drawn 120 mm long. Scale is applied only when printing or placing the drawing on a sheet.
2. **You enter exact values.** Type coordinates or lengths instead of eyeballing. LibreCAD accepts absolute coordinates like `0,0`, relative ones like `@120,0`, and polar ones like `@50<90` (50 units at 90 degrees).
3. **Snaps** lock the cursor to endpoints, midpoints, centres and intersections, so lines meet exactly.
4. **Layers** separate content (outline, hidden, centre, dimensions, title block), each with its own line type, colour and weight. Turn layers on and off to simplify.
5. **Dimensions are associative**: they measure the geometry, so if you edit the geometry the number updates. Never type over a dimension value to "fix" it.
6. **Constraints** (FreeCAD Sketcher, Onshape) hold relationships such as horizontal, equal, tangent and fixed lengths. A fully constrained sketch cannot change accidentally.

## Worked example: a mounting plate in LibreCAD

Target: a 120 × 80 mm plate with four Ø8 holes, each 10 mm in from both edges, and a 30 mm diameter centre hole.

1. **Set units:** Options > Current drawing preferences > Units: millimetres.
2. **Create layers:** `outline` (continuous, 0.5 mm), `centre` (CENTER line type, 0.25 mm), `dims` (0.25 mm).
3. **Draw the outline** on `outline` with the Line tool: `0,0`, then `@120,0`, `@0,80`, `@-120,0`, then close back to `0,0`. (Or use the rectangle tool from `0,0` to `120,80`.)
4. **Draw the corner holes** with the circle (centre, radius) tool: centres at `10,10`, `110,10`, `110,70`, `10,70`, radius 4.
5. **Draw the centre hole** at `60,40`, radius 15.
6. **Add centre lines** on the `centre` layer through each hole.
7. **Dimension** on the `dims` layer: overall 120 and 80; hole locations 10 from each edge (or baseline dimensions from the lower-left corner); `4X Ø8 THRU`; `Ø30 THRU`.
8. **Add a title block**: a rectangle in the corner with part name, your name, date, units (mm), scale (1:1), third-angle symbol or note, and general tolerances.
9. **Save** as DXF and **export or print to PDF**.

In FreeCAD the same plate is a Sketcher profile: draw a rectangle and five circles, then apply constraints (horizontal/vertical distance, radius, symmetric) until the sketch turns fully constrained (FreeCAD shows it in a different colour and reports zero degrees of freedom).

## Good CAD habits from day one

- Start at the origin and keep it meaningful (a corner or a datum).
- Use snaps; never guess a line onto another.
- Put everything on the right layer.
- Save often and use clear file names with revision letters (`plate-120x80_revA.dxf`).
- Check the PDF at 100% before sending it.

## Common mistakes

- Drawing in inches when the part is in mm (or the reverse); set units first.
- Lines that look joined but overlap or leave tiny gaps (zoom in and check, or use snaps).
- Typing over dimension text.
- Putting dimensions and geometry on the same layer.
- Uploading private designs to a public free-plan workspace.

## Recap

- Draw at full size with exact coordinates, snaps and layers; scale only at print time.
- Dimensions are associative; constraints lock design intent.
- Free tools: LibreCAD for 2D, FreeCAD Sketcher/TechDraw, Onshape free plan (public documents).

## Try it

In the **Do** tab, install a free tool and draw the mounting plate from the worked example.


Draw and dimension a simple plate in free 2D CAD software and export it as a PDF.

Your first 2D CAD drawing

LibreCAD - Lesson 01 - Introduction, Download, Install, Interface, Panels, Toolbar, Customize, Reset

FreeCAD Beginner Tutorial : Sketch

Create a free Onshape account (no payment needed) or install FreeCAD from https://www.freecad.org/, and start a new part in millimetres.

Sketch the L profile on the front plane starting at the origin, add the dimensions 50, 40 and 5, and constrain it fully.

Extrude (Pad) the profile 30 mm, symmetric about the sketch plane.

Sketch two 6 mm circles on the top face of the base, 12 mm from each end and centred in depth, and cut them through all (or use the Hole feature).

Add an R3 fillet to the inside corner of the L.

Rename the features in the tree so another person can understand the model.

Change the upright height from 40 to 50 mm in the first sketch and confirm the holes and fillet update without errors. Change it back.

Export the part as STEP, and take a screenshot of the model with the feature tree visible.

## Why it matters

Mechanical engineers design almost everything in 3D, civil and electrical engineers increasingly use 3D models (BIM, equipment layouts, cable trays), and maintenance teams use models to order parts and plan repairs. A 3D model is a single source of truth: drawings, 3D prints, CNC programs and assembly checks all come from it.

## Parametric, feature-based modelling

Most mechanical CAD tools (SolidWorks, Inventor, Fusion, Onshape, FreeCAD Part Design) work the same way:

1. **Sketch** a 2D profile on a plane, fully constrained with dimensions.
2. **Create a feature** from it: **extrude** (FreeCAD calls it **Pad**), **revolve**, **sweep** or **loft** to add material, or **cut** (FreeCAD: **Pocket**) to remove it.
3. **Add more features** on faces of the solid: holes, fillets, chamfers, patterns.
4. The **feature tree** (history) records every step in order. Change a dimension early in the tree and everything after updates. That is what **parametric** means.

**Design intent** is the idea behind it: model the part the way it should change. If the holes must always stay 12 mm from the ends, dimension them from the ends, not from each other.

## Free 3D tools

- **Onshape free plan**: runs in a browser, saves automatically, good tutorials. Documents on the free plan are **public**, so keep private designs out.
- **FreeCAD**: free and open source, installs locally. Use the **Part Design** workbench for feature-based solids.

Official sites: https://www.onshape.com/en/products/free and https://www.freecad.org/. Checked on 2026-10-23.

## Worked example: the angle bracket in 3D

The bracket from lesson 7: an L 50 mm wide, 40 mm tall, 5 mm thick, 30 mm deep, with two Ø6 holes through the base.

**Plan the model:** one L-shaped profile extruded 30 mm, then holes, then fillets. Three or four features.

1. **New part** in millimetres.
2. **Sketch on the front plane.** Draw the L profile with the line tool starting at the origin: 50 along the bottom, up 5, back 45, up 35, back 5, down 40 to close. Add dimensions 50, 40 and 5 (twice) and constrain it fully; the corner sits on the origin.
3. **Extrude (Pad) 30 mm.** Use a **symmetric** extrude (15 mm each side of the sketch plane) if you want the front plane to be a plane of symmetry; it keeps later features centred.
4. **Holes.** Sketch on the top face of the base: two circles Ø6 with centres 12 mm from each end and centred in depth (a symmetric constraint or 15 mm from the edge). Extrude-cut **through all** (FreeCAD: Pocket, type "Through all"), or use the dedicated **Hole** feature, which records the hole as a hole for drawings.
5. **Fillet** the inside corner of the L with R3 to make it stronger and easier to manufacture.
6. **Check:** measure the model with the measure tool, rename features ("L profile", "Base holes", "Inner fillet") and set a material if the tool supports it to see the mass.
7. **Change test:** edit the first sketch and change the height from 40 to 50. The holes and fillet should update without errors. If something breaks, your references or constraints need fixing.

## From model to drawing and file

- Make a 2D drawing from the model (Onshape: Drawing; FreeCAD: TechDraw) with front, top, side and isometric views; the dimensions come from the model.
- Export **STEP** to share with other CAD tools (the universal format for solids) and **STL** or **3MF** for 3D printing.

## Common mistakes

- Under-constrained sketches that shift when you edit later.
- Building everything in one complicated sketch instead of simple features.
- Dimensioning features from each other instead of from the edges or datums that matter.
- Referencing edges that disappear when an earlier feature changes, causing errors downstream.
- Sharing a native file when the other side needs STEP.

## Recap

- Sketch, constrain, then extrude/revolve or cut; add holes and fillets as features.
- The feature tree makes the model parametric: change early dimensions and the rest updates.
- Capture design intent with good references and constraints.
- Export STEP for CAD exchange and STL/3MF for printing.

## Try it

In the **Do** tab, model the bracket in Onshape's free plan or FreeCAD and test a design change.


Model a simple bracket in a free 3D CAD tool by sketching, extruding and adding holes and fillets.

Intro to 3D modelling

Beginner Tutorial 1/5 - Onshape 3D CAD - Creating Sketches and Objects

FreeCAD Sketch to 3D Solid (Pad + Constraints) [FreeCAD Tutorial 2026 - Part 1]

Sketch a part by hand, then draw it in free 2D CAD and model it in a free 3D tool.

Technical Drawing & CAD Basics

About this course

Course content

Reading Drawings

Discipline Symbols

CAD Hands-On

Where this course leads

Next course on the path

Careers that use it