---
marp: true
theme: default
html: true
---
# Parts that fit
Design and print · Designing to size
---
## Today
**We are learning to** drawn the same size, a printed peg won't go in its hole: printers make holes smaller and pegs bigger. Plan a clearance: about 0.2–0.4 mm for a sliding fit, less for a tight press fit. Every printer differs, so calibrate with a test card.
- I can explain why same-size parts don't fit when printed.
- I can work out a hole size for a sliding or press fit.
- I can calibrate a design to our printer with a test card.
---
## Why it matters
> The lid you designed is exactly the size of the box's opening. You print both, and the lid won't go on. Nothing was measured wrongly. What happened?
---
💬 Discussion
## Predict
**A 10 mm peg and a 10 mm hole are printed. The peg…**
- Fits perfectly
- Won't go in
- Fits, but only one way round
- Slides in loosely
---
✍️ Activity
## Try it
{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fengineering%2Fcad%2Ffit%2Fstep-2&code=widget%3A+fab%0Amode%3A+fit%0Apeg%3A+10%0Ahole%3A+10%0A&fresh=1}}
[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/engineering/cad/fit/?full=step-2#step-2)
---
✅ Check
## What happened?
* It won't go in: the hole prints at about 9.8 mm and the peg at about 10.1 mm.
---
💬 Discussion
## Work it out
Now check it with the numbers.
$$\text{peg drawn } 10 \to \text{printed } 10.1$$
1) $\text{want clearance } 0.3$ — A sliding fit.
2) $\text{hole printed} = 10.1 + 0.3 = 10.4$ — The hole must come out this big.
3) $\text{hole drawn} = 10.4 + 0.2 = 10.6$ — Holes print 0.2 mm smaller, so draw it bigger.
---
✅ Check
## Work it out
**A 10 mm peg, a sliding fit with 0.3 mm of clearance after printing. What size should the hole be drawn (mm)?**
* Answer: 10.6
---
✅ Check
## Find the mistake
**Sam's lid peg won't go in its hole. Click the line where Sam's reasoning first goes wrong.**
```
1 The lid's peg is 10 mm across.
2 So the hole in the box should be 10 mm across too.
3 Same size means a perfect fit.
4 Sam prints both and the peg won't go in.
```
* Answer: line 2
---
✍️ Activity
## Vary: change one thing
- **Press fit:**
- **A 6 mm peg:**
[Open this step](https://studio.learningwaypoints.com/tutorials/engineering/cad/fit/?full=step-4#step-4)
---
✍️ Activity
## Print a test card
---
✅ Check
## Will it go in?
**A 5 mm peg, hole drawn 5.2 mm. Printed clearance, and fit?**
- −0.1 mm: won't go in
- 0.5 mm: loose
- 0.2 mm: sliding fit
- 0.1 mm: press fit
* Answer: −0.1 mm: won't go in
---
✅ Check
## Press fit
**A 12 mm peg, a press fit with 0.1 mm of clearance after printing. Hole drawn (mm)?**
* Answer: 12.4
---
✍️ Activity
## Make: a lid peg
[Open this step](https://studio.learningwaypoints.com/tutorials/engineering/cad/fit/?full=step-7#step-7)
---
✅ Check
## Exit ticket
**Why won't a 10 mm printed peg go in a 10 mm printed hole?**
* The printer makes holes smaller and pegs bigger, so the printed hole is smaller than the printed peg.