Students see that a same-size peg and hole won't fit once printed, open the hole up to a sliding fit, work out a hole size from the peg, the printer's effect and the clearance wanted, design a press fit and a different peg size, print a tolerance test card, and design a lid peg's hole.
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Students will be able to
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.
Timing
Predict and Try 5 min · Worked example 10 min · Vary 6 min · Test card (print runs while you continue) · Practise and Make 10 min
Before class
The board's printer effect (holes −0.2 mm, pegs +0.1 mm) is typical of a 0.4 mm nozzle FDM printer, not exact. Print a tolerance test card on each school printer once and pin up the results: that replaces the board's numbers. Small holes print proportionally worse than large ones; vertical holes worse than horizontal.
Running the lesson
Predict: pass round a printed peg and hole drawn the same size, if you have one.
Worked example: two corrections, two lines.
Test card: start the print early in the lesson.
The hard part (on purpose)
The two printer corrections (hole −0.2, peg +0.1) blur. Write them as two lines every time.
When students get stuck
What you will see
A question to ask
The fix
A student adds only the clearance.
What does the printer do to the hole, and to the peg?
Add 0.2 for the hole and 0.1 for the peg too.
Answers to the checks
same size: A 10 mm peg and a 10 mm hole are printed. The peg… It won't go in: the hole prints at about 9.8 mm and the peg at about 10.1 mm.
Try: make it slide: Make the hole bigger, a little at a time, until the peg slides in.
Around 10.5 to 10.7 mm drawn: 0.2 to 0.4 mm of clearance after printing.
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)? 10.6 printed peg 10.1, so the printed hole must be 10.4; drawn 10.4 + 0.2 = 10.6 mm.
Find the mistake: Sam's lid peg won't go in its hole. Click the line where Sam's reasoning first goes wrong. line 2 Line 2: the hole needs a planned clearance. Drawn the same, the printed hole is smaller than the printed peg.
Press fit: A press fit for a 10 mm peg (it should stay put without glue).
A hole drawn about 10.35–10.4 mm: 0.05–0.1 mm of clearance.
A 6 mm peg: A 6 mm peg with a sliding fit.
About 6.5–6.7 mm: the same clearance added to a different peg.
Will it go in?: A 5 mm peg, hole drawn 5.2 mm. Printed clearance, and fit? −0.1 mm: won't go in Hole prints 5.0, peg 5.1: −0.1 mm.
Press fit: A 12 mm peg, a press fit with 0.1 mm of clearance after printing. Hole drawn (mm)? 12.4 Peg prints 12.1; hole must print 12.2; draw 12.4.
a lid peg: A lid with an 8 mm peg should lift off easily but not rattle: 0.2 to 0.3 mm of clearance after printing.
A hole drawn 8.5 to 8.6 mm.
Solutions
One possible solution for each task. Other correct programs are fine.
Vary: change one thing › Press fit
Open task: any program that does what the step asks.
Vary: change one thing › A 6 mm peg
Open task: any program that does what the step asks.
a lid peg
Open task: any program that does what the step asks.
Stretch and support
Stretch: Design a snap-fit lid: a small ridge on the peg and a groove in the hole.
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.
What each step is for
same size: Committing to what happens with a same-size peg and hole tests the new move before the printer's effect is shown.
Try: make it slide: Opening the hole up on the board settles the prediction and shows how much clearance a sliding fit needs.
Worked example: design the hole: Working the hole size from the peg, the printer's effect and the clearance wanted models the move. Sam's 'same size fits' is planted after the method.
Vary: change one thing: A press fit needs less clearance, and a different peg size needs the same added clearance, not the same hole.
Print a test card: A tolerance test card turns the board's typical numbers into your printer's real ones.
fits: Fit sizes both ways with the same printer model.
a lid peg: Designing a hole for a given peg and clearance range applies the whole calculation.