Grade by Grade
A. Design Processes and Related Skills · Design Skills: From Idea to Tested Prototype

Technical Drawings: Orthographic Views, Dimensions and Scale

A1.4 A2.5About 75 minutes
Learning goals
  • I can read and draw the front, top and right side views of an object in third-angle orthographic projection.
  • I can use the correct line types: visible, hidden, centre, dimension and extension lines.
  • I can dimension a drawing in millimetres following basic rules.
  • I can use drawing scales such as 1:2, 1:1 and 2:1, and read a title block.

Words to know

orthographic projection
A way of drawing an object as separate flat views (front, top, side), each looking straight at one face.
third-angle projection
The orthographic layout used in North America: the top view is placed above the front view and the right side view to the right of it.
hidden line
A dashed line that shows an edge you cannot see from that view.
centre line
A thin long-short-long line that marks the centre of a hole or a symmetric part.
dimension
A measurement written on a drawing, with dimension and extension lines showing exactly what is measured.
scale
The ratio between the size on the drawing and the real size; 1:2 means the drawing is half size.
title block
A box on a drawing that lists the title, designer, date, scale, units and drawing number.
Ø
The symbol for diameter.

A drawing anyone can build from

A sketch shows an idea. A technical drawing shows exact sizes and shapes, so a builder in another city (or another country) could make the same part without ever talking to you. Plans for houses, parts for cars, furniture assembly instructions and machine parts are all technical drawings. They are some of the most important industry-related documents (expectation A2.5) you will use.

Remember from Grade 8 math

In Grade 8 you read the top, front and side views of objects, built scale models from them and used scale drawings to find real lengths. Technical drawing adds strict rules: exact alignment, line types, dimensioning and standard scales.

Orthographic views

Imagine the object inside a glass box. Look straight at each face and trace what you see on the glass. Then unfold the box.

In third-angle projection (used in Canada and the United States):

  • The front view is the main view. Choose the side that shows the most about the shape.
  • The top view goes directly above the front view.
  • The right side view goes directly to the right of the front view.

The views line up: the front and top share the width; the front and side share the height; the top and side both show the depth.

TOP VIEWFRONT VIEWRIGHT SIDE VIEW6040301515ALL DIMENSIONS IN mmNOT TO SCALEThird-angle projection:top view above the front,right side view to the right
Orthographic views of an L-shaped bracket. Width lines up between the front and top views, height between the front and side views, and depth (30 mm) appears in both the top and side views. Thin dashed projection lines show the alignment.

Line types

Each kind of line has a meaning, like words in a language:

  • Visible (object) lines: thick and solid; edges you can see.
  • Hidden lines: medium dashes; edges behind a surface.
  • Centre lines: thin, long-short-long; centres of holes and axes of symmetry.
  • Dimension lines: thin with arrowheads, with the measurement written near the middle.
  • Extension lines: thin lines that extend from the object to the dimension line, with a small gap at the object.
Visible (object) linethick, solidHidden linemedium, dashedCentre linethin, long-short-longDimension linethin, arrows at ends802550Ø20 THRU10FRONTSIDE
Left: the four line types you will use most. Right: a plate with a 20 mm hole. The hole is drawn as a circle with centre lines in the front view; in the side view you cannot see it, so it appears as hidden (dashed) lines 20 mm apart. The 25 dimension locates the hole's centre.

Dimensioning rules

  1. Write "ALL DIMENSIONS IN mm" in the title block, then write numbers without units (e.g., 60).
  2. Show each dimension once. Do not repeat it in another view.
  3. Place dimensions outside the object where possible, between views, with smaller dimensions closer to the object and overall dimensions farther out.
  4. Give the overall width, height and depth.
  5. Locate holes by their centres, and give their size as a diameter (Ø).
  6. Avoid dimensioning to hidden lines.

Scale

Large objects are drawn smaller and tiny parts larger. The scale is written drawing : real.

  • 1:1 full size. 1:2 half size. 1:10 one-tenth size. 2:1 twice size.
  • Architects often use 1:50 or 1:100 for floor plans.

The dimensions written on a drawing are always the real sizes, whatever the scale.

Worked example

Working with scale

(a) A table top is 1200 mm long. How long is it on a 1:10 drawing? (b) On a 1:50 floor plan, a wall measures 84 mm. How long is the real wall in metres? (c) A watch gear is 6 mm across. How big is it on a 5:1 drawing?

  1. (a) Divide the real size by 10: 1200 ÷ 10 = 120 mm on the drawing.
  2. (b) Multiply the drawing size by 50: 84 × 50 = 4200 mm = 4.2 m.
  3. (c) Multiply by 5: 6 × 5 = 30 mm on the drawing.
  4. Check: for reductions (1:n) the drawing is smaller than the real thing; for enlargements (n:1) it is bigger.

(a) 120 mm (b) 4.2 m (c) 30 mm

Worked example

Reading a drawing

Use the L-shaped bracket drawing. (a) What is the overall height? (b) How thick is the base? (c) What is the depth? (d) Why is there a line in the top view 15 mm from the left?

  1. (a) The vertical dimension in the front view: 40 mm.
  2. (b) The 15 dimension on the right side view gives the base thickness: 15 mm.
  3. (c) The 30 dimension on the right side view: 30 mm (depth also appears as the height of the top view).
  4. (d) From above, you see two surfaces at different heights: the top of the upright (40 mm high, 15 mm wide) and the top of the base. The line is the edge where they meet, so it is a visible line.

40 mm high, 15 mm base thickness, 30 mm deep; the line in the top view is the step between the upright and the base.

Hands-on activity

Draw three views of a real object

You need: a pencil, an eraser, a ruler, grid paper (5 mm squares if you have it), and a simple solid object from home such as a wooden block, an eraser, a domino, a small box or a LEGO brick.

  1. Measure the object's width, height and depth in millimetres. Record them.
  2. Choose a scale so the views fit on your page (1:1 for small items, 1:2 for larger ones).
  3. Draw the front view near the lower left of the page. Draw the top view directly above and the right side view directly to the right. Use light projection lines to keep them aligned.
  4. Darken visible lines, add hidden lines and centre lines where needed.
  5. Add dimensions following the rules, and a title block: title, your name, date, scale, "ALL DIMENSIONS IN mm".
  6. Swap with a family member: can they tell which object you drew from the drawing alone?

What to look for: views in the correct positions, matching widths/heights/depths between views, correct line types, each dimension shown once, and a complete title block.

Just for fun

Why did the hidden line never win at hide-and-seek?

Everyone could see it was dashing behind something.

Your turn

1

In third-angle projection, where does the top view go?

2

Which two views share the height measurement?

3

What does a dashed line mean on a technical drawing?

4

A 900 mm long shelf is drawn at 1:5. How long is it on the drawing?

5

On a 1:100 site plan, a fence measures 37 mm. How long is the real fence?

6

What does "Ø12" mean?

7

Why should each dimension appear only once?

More practice: warm-up

1

Name the three main views in an orthographic drawing.

2

What kind of line marks the centre of a hole?

3

What does a 1:1 scale mean?

4

Where do units go on a metric technical drawing?

5

What line type shows visible edges?

More practice: on your own

1

A part is 18 mm long. How long is it on a 2:1 drawing? On a 1:2 drawing?

2

A cylinder (like a can) stands upright. Describe its front view and top view.

3

A box has a hole drilled through it from front to back. In which view does the hole appear as hidden lines: the front view or the top view? Explain.

4

Why do drawings show real dimensions even when the scale is 1:10?

5

A drawing shows a part 64 mm long on paper; the real part is 320 mm. What is the scale?

Challenge

1

Sketch (describe) the three views of a step block: a 40 mm wide, 30 mm deep block with two steps, each 20 mm wide; the left step is 30 mm high and the right step is 15 mm high.

2

A floor plan is 1:50. A room is 4.5 m by 3.6 m. What size is it on the plan? Will it fit on a page with a 200 mm × 150 mm drawing area?

3

Explain why a builder in Germany might misread a Canadian drawing if no projection symbol is shown.

Check your understanding

Quiz

1.

In third-angle projection, the right side view is placed...

2.

Which line type shows an edge that cannot be seen?

3.

A 2400 mm board drawn at 1:20 is how long on paper?

4.

Which views both show the depth of an object?

5.

What does "Ø25" on a drawing mean?

6.

Which is a correct dimensioning rule?

For parents and teachers

Big idea: Expectation A1.4 (communicating design ideas with industry terminology) and A2.5 (using industry-related documents to guide creation) both rely on technical drawings, the universal language of trades and engineering.

How to help at home:

  • Look at furniture assembly instructions or a house plan together. Find the views, dimensions and scale.
  • Hold up a cereal box or a mug and ask, "What would the top view look like?"

Common difficulties:

  • Views in the wrong places. Top above front, right side to the right.
  • Views not lined up. Light projection lines fix this.
  • Scale confusion. Real ÷ n for 1:n drawings; real × n for n:1 drawings. Dimensions written on the drawing are always real sizes.

Grade 10 connection: students who enjoy this can explore computer-aided design (CAD) in technological design or communications technology courses.

Safety: paper-and-pencil only.