CNC 加工不良：診断と予防ガイド (2026)

The 10 most common CNC defects

Based on production data from JLYPT’s last 12 months, these 10 defects account for over 90% of all non-conformance reports:

Root-cause decision tree

When a defect appears, work through this decision tree before randomly changing parameters. Most defects have 2–3 plausible root causes; eliminating each in order saves time:

1. 1

   Is it dimensional or cosmetic?

   Dimensional → check tool offsets, work coordinates, thermal state. Cosmetic → check tool wear, feeds, coolant.

2. 2

   Did it appear suddenly or gradually?

   Sudden = tool break, programming change, fixture shift. Gradual = tool wear, thermal drift, parameter drift.

3. 3

   On every part or random?

   Every part = systematic (programming, fixturing, machine). Random = tool wear cycles, raw material variation, operator inconsistency.

4. 4

   In one feature or many?

   One feature = specific tool or operation. Many features = global issue (machine, fixture, thermal).

5. 5

   Now isolate the cause

   Once you have these four answers, the root cause is usually one of 2–3 specific things. Check those first.

Layer 1: Design-stage prevention (catches ~50% of all defects)

The single highest-leverage moment for preventing defects is during design review. Issues caught here cost nothing; the same issues caught at shipping cost the entire batch.

• Add internal corner radii ≥ 1 mm. Sharp corners need small tools that break, chatter, or wear quickly. Each tool change is a chance for inconsistency.
• Avoid pocket depths > 4× tool diameter. Deep pockets cause tool deflection and chatter — both leading to dimensional inaccuracy.
• Equalise material removal across faces. Asymmetric removal causes warping. If 90% of the material comes off one side, expect bow.
• Avoid wall thickness changes > 2:1. Sudden thickness changes cause stress concentrations during cooling. Taper transitions over 3× the thickness.
• Specify minimum wall thickness ≥ 1 mm metals, ≥ 1.5 mm plastics. Thinner walls deflect during cutting.
• Use standard fastener sizes. Custom thread sizes need custom taps — more chance of breakage and stripped threads.
• Mark critical features clearly. Inspection time scales with the number of toleranced features. Star the 5–10 that actually matter.
• Tolerance only what matters. ±0.1 mm default is achievable on every CNC machine. Tighter tolerances drive scrap rate up.
• Provide a 3D STEP file alongside 2D drawings. Eliminates interpretation errors that cause "part to drawing but not to intent" defects.

Layer 2: Material-stage prevention (catches ~15%)

Layer 3: Process-stage prevention (catches ~30%)

Once the part is on the machine, defects come from tooling, parameters, fixturing or operator practice. Each has a standard mitigation:

Inspection that catches defects before shipping

Even with strong design and process prevention, some defects escape. The inspection strategy decides whether they ship or get caught:

1. 1

   First Article Inspection (FAI)

   Comprehensive inspection of part #1 of every production run. Verifies the program produces a part matching the drawing. Catches programming and fixturing errors before the rest of the batch is made.

2. 2

   In-process gauging at critical features

   On-machine probing or off-machine micrometer checks at programmed checkpoints during the run. Catches drift early.

3. 3

   Statistical Process Control (SPC) on production batches

   Sample N parts every X units, log key dimensions, watch for trends. Cheap to run, catches systematic issues before they go out of control.

4. 4

   Final 100% inspection on critical features

   Pass-fail check of every critical dimension on every part before packaging. The "you-shall-not-pass" gate.

5. 5

   Outgoing audit by independent inspector

   For high-stakes work (aerospace, medical), a separate inspector audits a final sample. Catches systematic issues missed by production QC.

When defects ship anyway — what to do

1. Document immediately with photos and measurements. Frame each part the same way; record the actual measured value vs the drawing spec.
2. Quarantine the affected lot. Don’t use any of the parts until cause is determined. Otherwise good parts and bad parts mix and you can’t recover.
3. Send the NCR to the supplier within 48 hours. Late reports are easier to dispute. Within 48 hours, the supplier’s production records are still warm.
4. Request a corrective action report (CAR). A real supplier provides root-cause analysis, immediate containment, and long-term corrective action — not just a refund.
5. Decide: rework, replace or refund. Each has cost trade-offs. Rework is fastest if defect is minor and rework yield is high. Replacement is cleanest. Refund is appropriate when the project can’t wait.
6. Track supplier performance. Single defects happen. Repeated defects from the same supplier on different orders mean the supplier’s quality system is broken — switch.