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Session 10: The Mismatch Activity + Debrief

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Session 10

The Mismatch Activity

Last Session

You calculated min and max for one part at a time.

Today: what happens when two parts, each with their own tolerance, have to fit together?

Worst-Case Fit

Peg spec: 0.250 in ± 0.005 in (min 0.245, max 0.255)

Hole spec: 0.250 in ± 0.005 in (min 0.245, max 0.255)

Worst-Case Fit

Worst case: the peg comes out at its max (0.255), the hole comes out at its min (0.245).

The peg is now bigger than the hole — even though both parts individually passed their own spec.

Both Parts "Passed." They Still Don't Fit.

A spec sheet check on each part, alone, doesn't guarantee the assembly works.

This Isn't Bad Luck

Look at those two specs again. The peg and the hole were given the same nominal — 0.250 in.

So the peg is too big any time it lands above whatever the hole landed at. That's not a rare edge case — it's close to a coin flip.

The Fix: Clearance

Engineers don't hope the worst case doesn't happen. They design so that even the worst case still fits.

Make the hole's nominal bigger than the peg's. The gap you deliberately leave is called clearance.

The Fix: Clearance

Peg: 0.245 in ± 0.005 (max 0.250)

Hole: 0.260 in ± 0.005 (min 0.255)

Biggest possible peg: 0.250. Smallest possible hole: 0.255. It fits — every time, not most of the time.

Today's Activity: Fit Test

  1. Measure both parts of your pair
  2. Predict whether they'll fit
  3. Physically test the fit
  4. Diagnose any mismatch against your numbers

Two Things to Remember

Trust your measured result over the spec sheet's assumption.

The spec tells you what should happen in theory. Your caliper and your physical test tell you what actually did.

Two Things to Remember

A mismatch isn't a failed activity.

A pair that doesn't fit, diagnosed clearly, teaches the exact same lesson as a pair that does. Explaining why matters more than which outcome you got.

Next Time

Session 11: Measurement & Data Collection Lab

Repeated-trial measurement, logged like real data — the exact skill the launcher project runs on later.

'F' → Fullscreen

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Build

Step 1: Measure Your Pair

Using your Session 6 caliper technique, measure both parts of your assigned pair. At least 2 trials each.

PartTrial 1Trial 2
Part A
Part B

Step 2: Predict

Based on your measurements (and each part's spec, if your instructor provided one), predict: will these two parts fit together cleanly, fit tight, or not fit at all? Write your prediction down before testing.

Step 3: Physical Fit Test

Actually assemble the pair. Record what really happened.

Step 4: Diagnose

If your prediction didn't match reality, use your measured numbers to explain why — not just "it didn't fit."

Whether it fit or not, answer this with your actual numbers: was the hole bigger than the peg, and by how much? That difference is the clearance you actually got.

Step 5: Respec It

Your pair was specced with the same nominal for both parts, which is why mismatches happen. Rewrite both specs — a nominal and a ± for the peg and for the hole — so that even the biggest allowed peg still fits the smallest allowed hole.

Show the two worst-case numbers that prove it works.

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Checkpoint

Before you submit, confirm:

  • You measured both parts, not just one
  • You wrote your prediction down before the physical test, not after
  • Your recorded result is honest, whether or not it matches your prediction
  • Your respec in Step 5 works at worst case — biggest allowed peg, smallest allowed hole — not just at nominal
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Reflection

Answer these on your own. Reflections are individual, even when the rest of the session was team work.

  1. A classmate says their pair fit because “both parts came out really close to nominal.” Is that actually the reason a pair fits? Use your own numbers to argue for or against.
  2. This exact idea — trust your measured data over a theoretical prediction — comes back later, during the launcher project's calibration phase. Why do you think an empirically-tested model might beat a theoretical prediction there too?
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Submit

Submit your measurements, prediction, fit test result, and reflection.

Activity Complete