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# Design to a dimension
Design and print · Designing to size
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## Today
**We are learning to** a box has a wall on every side, so its inside is the outside minus two walls. To fit something inside, start from the thing: its size, plus a gap each side, plus a wall each side, gives the outside.
- 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.
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## Why it matters
> 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?
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💬 Discussion
## Predict
**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
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✍️ Activity
## Try it
{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fengineering%2Fcad%2Fdimension%2Fstep-2&code=widget%3A+fab%0Amode%3A+box%0Awidth%3A+60%0Adepth%3A+50%0Awall%3A+2%0Aobject%3A+none%0A&fresh=1}}
[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/engineering/cad/dimension/?full=step-2#step-2)
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✅ Check
## What happened?
* 36 mm: 40 − 2 − 2. There is a wall on the left and one on the right.
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💬 Discussion
## From the part outwards
Now the depth.
$$\text{Uno width} = 68.6$$
1) $68.6 + 2 \times 1 = 70.6$ — A 1 mm gap on each side: the inside width.
2) $70.6 + 2 \times 2 = 74.6$ — A 2 mm wall on each side: the outside width.
3) $74.6 \to 75$ — Round up to a size you can set (never down).
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✅ Check
## From the part outwards
**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: 59.4
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✅ Check
## 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.
```
* Answer: line 3
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✍️ Activity
## Vary: change one thing
- **The case:**
- **Thicker walls:**
[Open this step](https://studio.learningwaypoints.com/tutorials/engineering/cad/dimension/?full=step-4#step-4)
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✍️ Activity
## Build it in Tinkercad
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✅ Check
## Inside
**A box is 80 mm outside with 2.5 mm walls. Inside width (mm)?**
* Answer: 75
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✅ Check
## Outside
**A 30 mm part, a 0.5 mm gap each side, 2 mm walls. Outside width (mm)?**
* Answer: 35
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✍️ Activity
## Make: the smallest case
[Open this step](https://studio.learningwaypoints.com/tutorials/engineering/cad/dimension/?full=step-7#step-7)
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✅ Check
## Exit ticket
**A part is 50 mm wide. With a 1 mm gap and 3 mm walls, how wide is the case outside?**
* 50 + 2 × 1 + 2 × 3 = 58 mm.