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.