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Practice
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Interviewers hand you a print, not a definition. These drills work the same muscle.
Drill 01 · Position at MMC
How much tolerance do you really have?
The print calls position Ø0.2 (M) | A | B | C. The hole comes off the machine at LMC, Ø10.2. What positional tolerance is actually available?
✓ Ø0.4. With an Ⓜ modifier, tolerance grows one-for-one as the feature departs from MMC. Departure = 10.2 − 10.0 = 0.2 bonus, so 0.2 + 0.2 = Ø0.4. This is why MMC callouts cut scrap — and why functional gauges work. Full bank: LMC, virtual condition, gauge sizing.
Drill 02 · Form vs orientation
One of these callouts is illegal. Which?
Flatness and parallelism, side by side. Which callout can never be valid as written?
✓ (b). Flatness is a form control — the surface against itself, two parallel planes, no datum reference allowed. Parallelism is orientation and requires a datum — which also makes (d) suspicious on a real print, but (b) is illegal by definition. Bonus: parallelism automatically limits flatness within its zone.
Drill 03 · Runout on a shaft
Pass or fail this shaft?
Circular runout 0.1 | A on the large diameter. At one cross-section, a full revolution reads TIR 0.08. Verdict?
✓ Pass. Circular runout is checked per circular element: full indicator reading over one revolution must be ≤ 0.1 — and 0.08 qualifies. It's a composite of form + coaxiality, and a dial indicator is exactly the right tool. (Total runout would sweep the whole surface; concentricity is a different, stricter animal — in the full bank.)
Stack-ups · Worst-case vs RSS
The 60-second gap check
Parts B and C sit inside housing A: A = 50 ±0.10, B = 20 ±0.05, C = 29.8 ±0.05. Find the gap.
Gap = A − (B + C) = 0.2 nominal. Worst case: ±(0.10 + 0.05 + 0.05) = ±0.20 → gap = 0.00 to 0.40. Zero margin at minimum — say that out loud; it's what they're listening for. RSS: √(0.10² + 0.05² + 0.05²) = ±0.122 → gap ≈ 0.08 to 0.32.
Why they ask: worst-case for safety and 100% interchangeability; RSS when volume + process control justify a statistical argument. Knowing when each applies is the senior signal.
Follow-up: keep A and B as-is, set C's nominal to 29.7. What's the loosest tolerance on C that still guarantees a 0.05 minimum gap, worst case?
Nominal gap = 50 − (20 + 29.7) = 0.3. Worst-case minimum: 0.3 − (0.10 + 0.05 + tC) ≥ 0.05 → tC ≤ ±0.10. Loosening C from ±0.05 to ±0.10 might halve its machining cost — tolerance allocation is a design decision.
Why they ask: real stack-up questions run backwards — from a functional requirement to the tolerances you're allowed to buy.
The five freeze points
MICRO-SCRIPTS FOR THE WORST 10 SECONDS
Interviews aren’t lost on knowledge. They’re lost in five predictable moments when your mouth goes ahead of your brain. Each one has a script — not to fake it, but to buy your brain the two seconds it needs.
1
The drawing slides across
A print appears and your mind empties.
SCRIPT Orient out loud, datums first: “A is the mounting face, B locates, C clocks…” Narrating the reading is the answer — they’re grading your process, not your speed.
2
“Walk me through your part”
You start describing geometry and feel it going nowhere.
SCRIPT Fixed order, every time: function → constraints → the three decisions that mattered → what you’d change today. Never open with what it looks like.
3
The on-the-spot hand calc
“Roughly size that bolt for me.” Numbers evaporate.
SCRIPT State the model before any math: “I’ll treat it as a cantilever, worst case at the wall, then check shear.” The setup earns most of the points — arithmetic is a formality.
4
“What would you do differently?”
Instinct says defend the design. Instinct is wrong.
SCRIPT Keep one real regret loaded, with its fix: “I’d pull the datum scheme apart — we measured that part three different ways.” “Nothing” is an instant red flag.
5
The blank
It happens to everyone. What you do in the next three seconds decides the round.
SCRIPT Buy the beat with a frame: “Let me think about the load path for a second.” Structured silence reads as senior. Fast noise reads as panic.
Say this, not that
INTERVIEWERS GRADE IN PHRASES
Six sentence-pairs that separate the senior answer from its junior twin — pulled straight from the model answers above.
JUNIOR“I’d use a stronger steel so it deflects less.”
SENIOR“Strength won’t help — deflection is a stiffness problem. E doesn’t change with heat treat; the geometry has to.”
JUNIOR“Torque it to spec and it’s fine.”
SENIOR“Torque is a friction-corrupted proxy for preload — for a critical joint I’d go torque-plus-angle or measure stretch.”
JUNIOR“I tightened the tolerances to be safe.”
SENIOR“I toleranced what the stack-up said function needs, and let everything else float at process capability.”
JUNIOR“Just add the tolerances up.”
SENIOR“Worst case here — it’s a safety interface. I’d only argue RSS with volume and process control behind it.”
JUNIOR“The test setup was probably off.”
SENIOR“I was wrong — the data showed it. Here’s the root cause and the check I added so that class of error can’t recur.”
JUNIOR“I’d have to look that up.”
SENIOR“It depends — let me tell you on what: load direction, duty cycle, and whether we control the interface.”
Company loop guides
ARCHETYPE PREVIEWS — 6 FULL GUIDES AT LAUNCH
Loops cluster into patterns. Know which pattern you're walking into and nothing surprises you. Full guides name rounds, timing, interviewer roles, and the questions each round favors.
Space & Launch
SpaceX-style archetype
1
ScreenResume walk + rapid fundamentals; expect a hand-calc ("size this bolt") on the call.
2
Take-home design challenge~1 week CAD package: part + drawing + trade rationale. Graded on judgment, not surfacing.
3
Onsite deep-diveThey pull threads on YOUR project two levels deep. Every decision needs a "because."
4
Culture + barOwnership, pace, "why here." Bring questions that show you read the hardware.
Full guide: challenge rubric, common failure modes, timeline.