---
title: "Design to a dimension"
subtitle: "Design and print · Designing to size · Lesson 1"
---

# Design to a dimension

**Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}}

> **Key question:** Your Arduino needs a case. You print a box 'about the right size', and it's 2 mm too small. Plastic is cheap; your time and the printer's aren't. How do you get it right first time?


:::teacher-note

**In one sentence:** Students find a box's inside size from its outside and walls, design a case for an Uno from the part outwards (part, gap, wall), meet Sam's one-wall slip, change the wall thickness, build the case in Tinkercad 3D by typed dimensions, and find the smallest case that fits.

**Students should be able to say:** I can work out a box's inside from its outside and walls. I can design a case from the part outwards. I can set exact sizes in Tinkercad.

**Width:** the unmarked parts are the floor every student does (predict, run, investigate and make). Typical adds the modify practice; wider adds the real-world contexts. They add contexts, not difficulty. Students work in the online tutorial alongside this sheet.

:::


## Predict


:::keep-together

**A box is 40 mm wide on the outside, with 2 mm walls. How wide is the inside?**

☐ 38 mm

☐ 42 mm

☐ 40 mm

☐ 36 mm

My prediction, and why:

{{lines-3}}

:::


:::teacher-note

**What happens:** 36 mm: 40 − 2 − 2. There is a wall on the left and one on the right.

:::


## Run

Try it on the board in the tutorial. Was your prediction right? What was different?

{{lines-2}}

## Worked example: a case for an Uno


:::keep-together

### From the part outwards

1. $\text{Uno width} = 68.6$ — Start from what goes inside.
2. $68.6 + 2 \times 1 = 70.6$ — A 1 mm gap on each side: the inside width.
3. $70.6 + 2 \times 2 = 74.6$ — A 2 mm wall on each side: the outside width.
4. $74.6 \to 75$ — Round up to a size you can set (never down).

Now the depth.

**The Uno is 53.4 mm deep. With a 1 mm gap and a 2 mm wall each side, how deep is the case outside (mm)?**

Answer: {{field-15}}

:::


:::teacher-note

**Answer:** 59.4

53.4 + 2 × 1 + 2 × 2 = 59.4 mm.

:::


:::keep-together

### Find the mistake

**Sam's 57 mm part won't go in. Click the line where Sam's working first goes wrong.**

```
 1  The box is 60 mm wide on the outside.
 2  The walls are 2 mm thick.
 3  So the inside is 60 − 2 = 58 mm wide.
 4  A 57 mm part will fit with 1 mm to spare.
```

Line: {{field-15}}

Why is it wrong?

{{lines-2}}

:::


:::teacher-note

**Answer:** line 3

Line 3: there is a wall on each side, so the inside is 60 − 2 − 2 = 56 mm. The 57 mm part is 1 mm too big.

:::


:::typical

## Vary: change one thing


**The case.** 

**Thicker walls.** 

Work in the tutorial. Plan first: what will you change, and how will you test it?

{{box-4}}

:::


:::typical

## Build it in Tinkercad


Work in the tutorial. Plan first: what will you change, and how will you test it?

{{box-4}}

:::


## Sizes


:::keep-together

### Inside

**A box is 80 mm outside with 2.5 mm walls. Inside width (mm)?**

Answer: {{field-15}}

:::


:::teacher-note

**Answer:** 75

80 − 2.5 − 2.5 = 75 mm.

:::


:::keep-together

### Outside

**A 30 mm part, a 0.5 mm gap each side, 2 mm walls. Outside width (mm)?**

Answer: {{field-15}}

:::


:::teacher-note

**Answer:** 35

30 + 2 × 0.5 + 2 × 2 = 35 mm.

:::


## Make: the smallest case


Work in the tutorial. Plan first: what will you change, and how will you test it?

{{box-4}}

## Exit ticket

:::keep-together

**A part is 50 mm wide. With a 1 mm gap and 3 mm walls, how wide is the case outside?**

{{lines-3}}

:::


:::teacher-note

**Answer:** 50 + 2 × 1 + 2 × 3 = 58 mm.

:::