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.
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?
Clearance is the gap between parts that fit together. Tolerance is how far a real size can be from the drawn size. A typical FDM printer squashes each line of plastic outwards a little, so holes print about 0.2 mm smaller and pegs about 0.1 mm bigger. (Yours may differ: that's what a test card is for.)
printed clearance
fit
under 0
won't go in
0 to 0.15 mm
press fit (tight, stays put)
0.15 to 0.4 mm
sliding fit
over 0.4 mm
loose (wobbles)
Predict: Read the problem and decide what you think before you try anything. A wrong guess is fine: it shows you what to look for.
Try: Try it on the board and compare the result with your prediction.
Investigate: Work out why it happens, one step at a time.
Modify: Change the problem on purpose and see what happens.
Practise: A few on your own, to check you have it.
Make: Make your own problem using what you have learned. Expect a few tries; that is normal.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
PredictRead the problem and decide what you think before you try anything. A wrong guess is fine: it shows you what to look for.
Same size
TryTry it on the board and compare the result with your prediction.
Try: make it slide
Try it yourselfYour edits are saved in this browser.
Make the hole bigger, a little at a time, until the peg slides in.
Use the board. This ticks when you get there.
InvestigateWork out why it happens, one step at a time.
Worked example: design the hole
Worked exampleBefore each step, say what you think comes next.
peg drawn 10→printed 10.1Pegs print about 0.1 mm bigger.
want clearance 0.3A sliding fit.
hole printed=10.1+0.3=10.4The hole must come out this big.
hole drawn=10.4+0.2=10.6Holes print 0.2 mm smaller, so draw it bigger.
Now check it with the numbers.
A 10 mm peg, a sliding fit with 0.3 mm of clearance after printing. What size should the hole be drawn (mm)?
Sam's lid peg won't go in its hole. Click the line where Sam's reasoning first goes wrong.
ModifyChange the problem on purpose and see what happens.
Vary: change one thing
Try it yourselfYour edits are saved in this browser.
A press fit for a 10 mm peg (it should stay put without glue).
Use the board. This ticks when you get there.
A 6 mm peg with a sliding fit.
Use the board. This ticks when you get there.
Modify TypicalChange the problem on purpose and see what happens.
Print a test card
PractiseA few on your own, to check you have it.
Fits
A 12 mm peg, a press fit with 0.1 mm of clearance after printing. Hole drawn (mm)?
MakeMake your own problem using what you have learned. Expect a few tries; that is normal.
A lid peg
Try it yourselfYour edits are saved in this browser.
Need a hint?
The peg is 8 mm. Work out the hole for 0.2 and for 0.3 mm of clearance.
Another hint
8.1 + 0.2 + 0.2 = 8.5; 8.1 + 0.3 + 0.2 = 8.6.
Another hint
Set the hole between 8.5 and 8.6.
A lid with an 8 mm peg should lift off easily but not rattle: 0.2 to 0.3 mm of clearance after printing.
Use the board. This ticks when you get there.
For teachers
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. About 40 minutes.
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.
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
A student adds only the clearance. Ask: What does the printer do to the hole, and to the peg? Fix: Add 0.2 for the hole and 0.1 for the peg too.
Stretch and support
Stretch: Design a snap-fit lid: a small ridge on the peg and a groove in the hole.
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.
Now you can
Explain why same-size parts don't fit when printed.
Work out a hole size for a sliding or press fit.
Calibrate a design to our printer with a test card.