🧭 Why Step 3 Matters #
In PFMEA, Function Analysis translates your PFD and structure into what every station must achieve—with numbers. This is the anchor for Step 4 (Failure Analysis): a function can only “fail” if it’s specified first.
🎯 Objectives of Step 3 #
- Define process functions (verb + object + spec) for each operation and key interface.
- Set measurable requirements/targets with units and tolerances.
- Mark special characteristics (🔺 Safety, 🔧 Regulatory, 🎯 CTQ).
- Specify verification/measurement methods, frequency, and MSA plan.
- Seed the Process Control Plan (method, frequency, records, reaction plan placeholders).
🧩 How to Write a Good Process Function #
Pattern: Do [action] to/for [object] at/within [spec]
Examples:
- “Press impeller to 12.00 ± 0.05 mm with OK force-distance signature.”
- “Tighten housing screws to 4.0 ± 0.3 N·m with angle monitoring.”
- “Verify leak rate ≤ 0.5 ml/min @ 1.5 bar.”
📋 Process Characteristics Matrix (PCM) — Starter (copy to workbook) #
Includes core stations OP01–OP14; add columns for “PFMEA Row ID” and “Reaction Plan” in your sheet. SC: 🔺 Safety, 🔧 Regulatory, 🎯 CTQ.
| Op | Process Function (verb + object + spec) | Requirement / Target | SC | Verification / Gage | Freq | Record / Trace | MSA Need |
|---|---|---|---|---|---|---|---|
| OP01 | Assign serial & verify kit correctness | 100% correct BOM present | – | Scanner + MES kit check | 100% | MES kit result | – |
| OP02 | Press bearings to design depth/alignment | Depth per DWG; align ≤ X mm runout | 🎯 | Servo press depth + LVDT | 100% | Curve/OK to MES | GR&R (depth) |
| OP03 | Prepare shaft (clean/lube) | Cleanliness pass; lube spec ID | – | Visual + check sheet | Per lot | Check sheet | – |
| OP04 | Seat stator with correct orientation | Orientation OK | – | Vision system | 100% | Image/OK to MES | Attribute study |
| OP05 | Press-fit impeller to 12.00 ± 0.05 mm; signature in window | Depth 12.00 ± 0.05 mm; force window X–Y kN | 🎯 | LVDT + force signature | 100% + SPC (30 pcs) | Curve + stats | GR&R (depth) |
| OP06 | Install mechanical seal with correct orientation; maintain cleanliness | Orientation OK; particle ≤ class X; shaft Ra ≤ 0.30 µm | 🔧 | Vision + particle gate; Ra audit | 100% + daily audit | MES + audit log | Ra GR&R (periodic) |
| OP07 | Dispense potting mass/time; cure per profile; maintain ESD < 100 V | Mass ±Δ g; time ±Δ s; cure profile OK; ESD < 100 V | 🎯 | Scale, timer, oven logger; ESD tester | 100% mass/time; cure per lot; ESD per shift | MES + ESD log | Scale GR&R |
| OP08 | Fit connector / make electrical join; verify pull/vision | Pull ≥ spec; features present | – | Pull tester / vision | Sampled/100% (per CTQ) | MES | Pull GR&R |
| OP09 | Tighten screws to 4.0 ± 0.3 N·m with angle window | Torque/angle OK | 🎯 | DC tool with trace + socket ID | 100% | Curve to MES | Tool audit + annual GR&R |
| OP10 | Run pre-leak (air decay/hold) | ≤ X ml/min @ 1.0 bar | 🔧 | Air-decay bench + master | 100% | Value to MES | GR&R (3-point) |
| OP11 | Electrical spin sanity (current/run-up) | I ≤ spec; run-up ≤ spec | – | Electrical stand | 100% | Values to MES | – |
| OP12 | Run final leak & flow/RPM test with recipe lock & temp comp | Leak ≤ 0.5 ml/min @ 1.5 bar; Flow ≥ 35 L/min @ ΔP 20 kPa @ 80% duty | 🔧 / 🔺 | Wet bench; temp probe | 100% | Curves + limits to MES | GR&R (flow/leak) |
| OP13 | Verify labels/visual/pack audit | All OK; label readable & bound to serial | – | Vision/checklist | 100% | MES image/OK | – |
| OP14 | Pack & ship; close traceability | Pallet ID ↔ units; pack audit OK | – | Scanner + audit sheet | 100% | MES closeout | – |
| Interface | Recipe lock for benches/tools | Only approved revs | 🎯 | MES role-based control | 100% | Change log | – |
| Interface | No scan → no progress at OP05/09/12 | Gate enforced | 🎯 | MES gate | 100% | Gate log | – |
Tip: Where you see 100% + SPC, seed your Control Plan with chart type (e.g., X̄–R for depth/torque; I-MR for leak/flow).
🧪 Measurement & MSA Plan — Consolidated View #
| Characteristic | Gage/Method | Study Type | Target | Frequency |
|---|---|---|---|---|
| Press-fit depth | LVDT probe | GR&R (var) | %GRR ≤10%, ndc ≥10 | Annual + post-repair |
| Press signature | Curve match | Correlation/attribute | r ≥ threshold | Quarterly |
| Torque/angle | DC tool | Audit + GR&R | Tool OK; %GRR ≤10% | Daily audit + annual GR&R |
| Leak rate | Air/Wet bench | GR&R (3 points) | Stable bias; %GRR ≤10% | Quarterly + after service |
| Flow | Wet bench + temp comp | GR&R (var) | %GRR ≤10% | Quarterly |
| ESD | Strap/mat tester | Attribute | Pass | Start-of-shift |
🎯 Special Characteristics Mapping (from DFMEA ➜ PFMEA) #
| Product SC (from DFMEA) | Process Control (this lesson) | Where Controlled |
|---|---|---|
| 🔺 Flow capability | Flow test spec + curve storage | OP12 |
| 🔧 Leak integrity | Pre-leak + final leak; seal install method/cleanliness | OP10/12 + OP06 |
| 🎯 Torque joint integrity | Torque spec + trace + socket ID | OP09 |
| 🎯 Impeller retention | Press depth + signature window | OP05 |
| 🎯 ESD safety for PCB | ESD < 100 V; potting/cure discipline | OP07 |
Make sure these SCs are flagged in your PFMEA sheet and later bolded in the Control Plan.
📐 “Mini” Process P-Diagram (optional but helpful) #
| Inputs – Control | Inputs – Noise | Process (Stations) | Desired Outputs | Error States (feed Step 4) |
|---|---|---|---|---|
| MES recipes, tools, WIs | Temp/humidity, material lot spread, operator variation, tool wear | OP05, OP06, OP07, OP09, OP10–12 | Depth in spec; torque in spec; leak/flow within limits; correct orientation | Depth undersize/oversize; torque low/high; leak > limit; flow < target; ESD event |
🧩 Seeding the Control Plan (columns to pre-fill now) #
- Characteristic (from PCM)
- Method / Gage (LVDT, DC tool, benches, vision)
- Sample Size & Frequency (100%, SPC subgroup, audit intervals)
- Records (MES, curve files, check sheets)
- Reaction Plan (placeholder now; will finalize after Step 6)
✅ Outputs of Step 3 #
- ✅ Process functions for each operation and key interface.
- ✅ Measurable requirements/targets with units & tolerances.
- ✅ SC flags set for Control Plan linkage.
- ✅ Verification & MSA plan defined (who/what/when).
- ✅ PCM sheet ready to feed Step 4 (Failure Analysis).
⚠️ Common Pitfalls (and quick fixes) #
| Pitfall | Fix |
|---|---|
| Functions written vaguely (e.g., “assemble impeller”) | Use verb + object + spec with numbers/units. |
| Missing interfaces (recipe lock, gates) | Add interface functions—they often prevent escapes. |
| No link to DFMEA SCs | Import SCs and flag in PCM/Control Plan now. |
| MSA deferred to PPAP | Define MSA plan now; schedule studies. |
| 100% checks without trace | Ensure curve/value storage to MES & serial. |
🔗 Internal Links #
- Next: Lesson 5.4 — Step 4: Failure Analysis (Cause–Mode–Effect chains)
- Templates Hub: Process Characteristics Matrix (Excel), MSA Plan (Excel), Control Plan (shell)
🧠 Pro Tip #
If you can’t measure it, you can’t control it—and you definitely can’t rate D in Step 5. Lock the numbers here, and Step 4–6 will move fast.
