{ "slug": "inspection-methods", "title": "CNC Inspection Methods Compared: CMM, NDT and PMI", "description": "Seventeen measurement and NDT methods compared by geometric capability, accuracy, cost tier, cycle time and qualification, with CMM and UT worked examples.", "lang": "en", "tables": [ { "caption": "Inspection methods compared (17 methods; cost tier is editorial)", "columns": [ "Method", "Measures (GD&T scope)", "Accuracy / range", "Cost tier", "Cycle per part", "Destructive?", "Operator qualification", "Detects", "Standard", "Source" ], "rows": [ { "Method": "CMM, bridge (touch-trigger / scanning)", "Measures (GD&T scope)": "size, position, flatness, roundness, perpendicularity, parallelism, concentricity, profile — full 3D GD&T", "Accuracy / range": "MPE ±(1.7–2.5 + L/250–300) µm; high-end ±(0.6 + L/1000) µm; to ~1 m class", "Cost tier": "5", "Cycle per part": "3–15 min", "Destructive?": "no", "Operator qualification": "ISO 10360 calibration report + GD&T-literate measurer", "Detects": "any geometric deviation; probing cannot find internal voids", "Standard": "ISO 10360-2; ASME B89.4.1", "Source": "S1–S5" }, { "Method": "Blue-light / laser 3D scanning", "Measures (GD&T scope)": "surface profile, 3D best-fit, full-field deviation map", "Accuracy / range": "point 0.017–0.05 mm; volumetric 0.02–0.1 mm", "Cost tier": "4", "Cycle per part": "1–4 min (scan + align)", "Destructive?": "no", "Operator qualification": "moderate; mesh/alignment skill", "Detects": "form deviation over the whole surface; line-of-sight limits undercuts; internal cavities unreachable", "Standard": "VDI/VDE 2634 Part 3", "Source": "S9, S10" }, { "Method": "Profile projector (optical comparator)", "Measures (GD&T scope)": "2D profile, radius, angle, pitch, 2D true position, edges", "Accuracy / range": "±(1.5–2.5 + L/100) µm; magnification ±0.1%; stage 203 × 102 mm typ.", "Cost tier": "2", "Cycle per part": "15 s–2 min", "Destructive?": "no", "Operator qualification": "low; parallax/edge training", "Detects": "2D contour deviation, burrs, thread profile; no depth or roughness", "Standard": "Mitutoyo PJ/PH specs", "Source": "S6–S8" }, { "Method": "Vision measuring machine (VMM)", "Measures (GD&T scope)": "2D size, position, small features, high feature count", "Accuracy / range": "±(0.8–1.5) µm", "Cost tier": "3", "Cycle per part": "10–60 s auto", "Destructive?": "no", "Operator qualification": "low–moderate; program once, run operator", "Detects": "2D geometry on small/flat parts; edge-defined features", "Standard": "Mitutoyo QV specs", "Source": "S6" }, { "Method": "Calipers", "Measures (GD&T scope)": "linear size (OD/ID/depth/step) quick check", "Accuracy / range": "±0.02–0.04 mm", "Cost tier": "1", "Cycle per part": "seconds", "Destructive?": "no", "Operator qualification": "none (shop-floor universal)", "Detects": "coarse size error; not for GD&T", "Standard": "ISO 13385-1 class", "Source": "S6" }, { "Method": "Outside micrometer", "Measures (GD&T scope)": "diameter, thickness", "Accuracy / range": "±0.001–0.003 mm", "Cost tier": "1", "Cycle per part": "seconds", "Destructive?": "no", "Operator qualification": "low", "Detects": "diameter/thickness deviation; lapped faces needed", "Standard": "ISO 3611", "Source": "S6" }, { "Method": "Height gauge on surface plate", "Measures (GD&T scope)": "height steps, 1D comparative, scribing", "Accuracy / range": "±0.003–0.008 mm", "Cost tier": "1", "Cycle per part": "< 1 min", "Destructive?": "no", "Operator qualification": "low", "Detects": "height/step error; runout with lever attachment", "Standard": "Mitutoyo indicator class", "Source": "S6" }, { "Method": "Surface roughness tester (Ra/Rz stylus)", "Measures (GD&T scope)": "Ra, Rz, Rt per ISO 4287", "Accuracy / range": "±0.5–2% of reading; resolution 0.001 µm; a 2 µm tip cannot resolve features below about 4 µm", "Cost tier": "1", "Cycle per part": "30 s per trace", "Destructive?": "contact but non-damaging", "Operator qualification": "low; cutoff/filter training", "Detects": "surface texture deviation; not form", "Standard": "ISO 4287; JIS B 0601", "Source": "S6" }, { "Method": "Hardness — Rockwell (HRB/HRC)", "Measures (GD&T scope)": "bulk hardness (strength / heat-treat proxy)", "Accuracy / range": "±0.5 HRC bench class; loads 60/100/150 kgf", "Cost tier": "1", "Cycle per part": "< 1 min per indent", "Destructive?": "small indent remains (semi-destructive to cosmetic surface)", "Operator qualification": "low; ASTM E18 procedure training", "Detects": "wrong heat-treat condition; soft spots; missing case", "Standard": "ASTM E18 / ISO 6508", "Source": "S15, S16" }, { "Method": "Hardness — Brinell (HB)", "Measures (GD&T scope)": "bulk hardness on rough/large parts", "Accuracy / range": "10 mm WC ball, 500/3000 kgf, 15 s; best on castings/forgings", "Cost tier": "1", "Cycle per part": "about 1 min", "Destructive?": "visible indent (larger than HRC)", "Operator qualification": "low", "Detects": "grade mix-up vs expected HB; Q&T condition", "Standard": "ASTM E10 / ISO 6506", "Source": "S15, S16" }, { "Method": "Hardness — Vickers/Knoop micro (HV/HK)", "Measures (GD&T scope)": "case depth (CHD), thin sections, coating cross-section", "Accuracy / range": "loads 10 gf–10 kgf; lab method", "Cost tier": "2", "Cycle per part": "sample-prep dependent (minutes)", "Destructive?": "cut + polish section — destructive to the part", "Operator qualification": "moderate (metallurgical prep)", "Detects": "case depth out of spec; decarburization", "Standard": "ASTM E92/E384; ISO 6507", "Source": "S15, S16" }, { "Method": "Ultrasonic testing (UT, incl. PAUT/TOFD)", "Measures (GD&T scope)": "internal flaws, wall thickness, lack of fusion", "Accuracy / range": "1–5 MHz; penetration 100–300 mm; TOFD depth ±0.5 mm on 25 mm wall; finds tight cracks < 0.1 mm", "Cost tier": "3", "Cycle per part": "minutes per weld/joint", "Destructive?": "no", "Operator qualification": "ASNT Level II per SNT-TC-1A / CP-189 / ISO 9712", "Detects": "internal cracks, voids, LOF, inclusions, thickness loss", "Standard": "ASTM E428/E317; EN 1714; ASME Sec V; AMS 2632", "Source": "S11, S12" }, { "Method": "Magnetic particle testing (MT)", "Measures (GD&T scope)": "surface and near-surface cracks in ferromagnetic parts", "Accuracy / range": "cracks below the 1.6 mm class band (A903 MT column); dry/wet, visible/fluorescent", "Cost tier": "1", "Cycle per part": "minutes", "Destructive?": "no (may need demagnetize/clean)", "Operator qualification": "ASNT Level I/II; NAS 410 for aerospace", "Detects": "grinding, fatigue and quench cracks; seams", "Standard": "ASTM E709; E1444 (aero); ISO 9934", "Source": "S13, S14, S24" }, { "Method": "Liquid penetrant testing (PT)", "Measures (GD&T scope)": "surface-opening cracks, any material incl. non-ferrous", "Accuracy / range": "pairs with MT on castings (MT below 1.6 mm, PT complementary); fluorescent more sensitive than visible", "Cost tier": "1", "Cycle per part": "15–60 min (dwell incl.)", "Destructive?": "no (residue cleanable)", "Operator qualification": "ASNT Level I/II", "Detects": "surface cracks, porosity openings, laps — incl. aluminium and austenitic stainless", "Standard": "ASTM E165/E1417", "Source": "S14" }, { "Method": "Radiographic testing (RT / X-ray)", "Measures (GD&T scope)": "internal voids, porosity, inclusions, weld penetration", "Accuracy / range": "volume defects by density contrast; thickness limit set by energy (typ. ≤ 100 mm steel; Co-60/linac beyond)", "Cost tier": "4", "Cycle per part": "10–30 min per exposure", "Destructive?": "no (radiation hazard to operator, not part)", "Operator qualification": "ASNT Level II; radiation-safety license (site dependent)", "Detects": "porosity, shrinkage, slag, LOF (planar defects angle-sensitive)", "Standard": "ASTM E94; EN 17635; ASME Sec V", "Source": "S11" }, { "Method": "Salt-spray cabinet (NSS)", "Measures (GD&T scope)": "corrosion resistance of coatings", "Accuracy / range": "5% NaCl, 35 °C per ASTM B117 / ISO 9227; screening only", "Cost tier": "2", "Cycle per part": "24–1000+ h cabinet (parallel batch)", "Destructive?": "yes for the tested coupon (exposure degrades the finish)", "Operator qualification": "low; chamber operation", "Detects": "coating failures: white/red rust onset, blister, creep", "Standard": "ASTM B117; ISO 9227", "Source": "batch1 T3 table (caliber cross-reference)" }, { "Method": "PMI — OES spark / XRF / helium leak test", "Measures (GD&T scope)": "chemical composition (OES all elements incl. C; XRF no C); hermeticity", "Accuracy / range": "OES 0.01% class (carbon weak — LECO combustion better); XRF portable non-destructive; helium MSD 5×10⁻¹²–5×10⁻¹³ Pa·m³/s, response < 1 s", "Cost tier": "OES/XRF 2; helium 3", "Cycle per part": "OES/XRF seconds; helium minutes", "Destructive?": "OES leaves a spark mark (slight); XRF and helium non-destructive", "Operator qualification": "OES/XRF instrument training + PMI procedure; helium vacuum training", "Detects": "grade mix-up, off-spec incoming chemistry, leak rate over limit", "Standard": "ASTM E415/E1086/E572; API RP 578; helium: vendor nameplate", "Source": "S20–S23" } ] }, { "caption": "What CMM accuracy do I need to verify a ±0.02 mm position callout?", "columns": [ "Inspection purpose", "Primary method", "Quick screen", "Disabled / ineffective", "Accuracy anchor" ], "rows": [ { "Inspection purpose": "Size + full GD&T (position/profile/flatness)", "Primary method": "CMM", "Quick screen": "blue-light scan (full profile fields)", "Disabled / ineffective": "calipers cannot evaluate GD&T", "Accuracy anchor": "MPE ±(1.7 + L/300) µm" }, { "Inspection purpose": "Small 2D profile / thin deformable parts", "Primary method": "projector / VMM", "Quick screen": "—", "Disabled / ineffective": "contact probes deform thin walls", "Accuracy anchor": "±(1.5 + L/100) µm" }, { "Inspection purpose": "Surface roughness Ra/Rz", "Primary method": "stylus roughness tester", "Quick screen": "—", "Disabled / ineffective": "CMM and projector cannot measure roughness", "Accuracy anchor": "±0.5–2% of reading" }, { "Inspection purpose": "Heat-treat condition check", "Primary method": "Rockwell HRC (batch sampling)", "Quick screen": "Brinell (rough large parts)", "Disabled / ineffective": "hardness alone cannot identify the grade", "Accuracy anchor": "±0.5 HRC" }, { "Inspection purpose": "Case depth (carburized/nitrided)", "Primary method": "Vickers micro section", "Quick screen": "—", "Disabled / ineffective": "macro hardness (HRC/HB) cannot see the layer", "Accuracy anchor": "CHD per ISO 2639" }, { "Inspection purpose": "Internal flaws (cracks/voids/LOF)", "Primary method": "UT incl. PAUT/TOFD", "Quick screen": "RT (best on volume defects)", "Disabled / ineffective": "MT/PT only see surface openings", "Accuracy anchor": "TOFD depth ±0.5 mm per 25 mm" }, { "Inspection purpose": "Surface cracks (ferromagnetic)", "Primary method": "MT", "Quick screen": "PT (only option for non-ferromagnetic)", "Disabled / ineffective": "UT near-field masks the surface", "Accuracy anchor": "MT below the 1.6 mm class" }, { "Inspection purpose": "Coating corrosion comparison", "Primary method": "NSS salt spray", "Quick screen": "—", "Disabled / ineffective": "cabinet hours ≠ outdoor life (ISO 10683)", "Accuracy anchor": "endpoint must state white vs red rust" }, { "Inspection purpose": "Grade / mix-up verification (incoming)", "Primary method": "OES (carbon matters) or XRF (no carbon)", "Quick screen": "spark test (shop qualitative)", "Disabled / ineffective": "XRF cannot see carbon — cannot separate 304 from 304L", "Accuracy anchor": "OES 0.01% class" }, { "Inspection purpose": "Hermeticity", "Primary method": "helium mass spectrometry", "Quick screen": "pressure decay (coarse leaks)", "Disabled / ineffective": "bubble method only locates gross leaks", "Accuracy anchor": "5×10⁻¹² Pa·m³/s class" } ] }, { "caption": "Can a caliper check GD&T true position?", "columns": [ "Hand tool", "Accuracy", "What it honestly resolves" ], "rows": [ { "Hand tool": "Calipers", "Accuracy": "±0.02–0.04 mm", "What it honestly resolves": "linear OD/ID/depth/step quick check — no GD&T" }, { "Hand tool": "Outside micrometer", "Accuracy": "±0.001–0.003 mm", "What it honestly resolves": "diameter and thickness" }, { "Hand tool": "Height gauge", "Accuracy": "±0.003–0.008 mm", "What it honestly resolves": "height steps; runout with a lever attachment" }, { "Hand tool": "Stylus roughness tester", "Accuracy": "±0.5–2% of reading", "What it honestly resolves": "Ra/Rz only — not form, not size" } ] }, { "caption": "How long does a full-FAI CMM inspection take per part?", "columns": [ "Timing / cost anchor", "Value", "Status" ], "rows": [ { "Timing / cost anchor": "Bridge CMM cycle", "Value": "3–15 min per part", "Status": "published range" }, { "Timing / cost anchor": "Landing-gear bushing case: 7 radii R0.15 ± 0.02 + stepped bore", "Value": "14 min per part", "Status": "source case" }, { "Timing / cost anchor": "Projector pre-screen", "Value": "under 90 s per part", "Status": "published" }, { "Timing / cost anchor": "CMM queue-time reduction from pre-screening", "Value": "scheduling-dependent (editorial estimate)", "Status": "no source figure — deliberate replacement of an unsourced claim" }, { "Timing / cost anchor": "CMM equipment anchor", "Value": "$215K class", "Status": "TCO table" }, { "Timing / cost anchor": "Scanner equipment anchor", "Value": "$142K class", "Status": "TCO table" }, { "Timing / cost anchor": "Projector equipment anchor", "Value": "$48.5K class", "Status": "TCO table" } ] }, { "caption": "Can blue-light scanning replace my CMM for GD&T sign-off?", "columns": [ "Scanner class", "Stated accuracy", "Note" ], "rows": [ { "Scanner class": "GOM Scan 1", "Stated accuracy": "0.017 mm single point", "Note": "VDI/VDE 2634 Part 3 metering standard" }, { "Scanner class": "ATOS Q", "Stated accuracy": "0.02 mm class", "Note": "current-generation rated value" }, { "Scanner class": "Handheld laser", "Stated accuracy": "0.02 mm class", "Note": "imported industrial tier" }, { "Scanner class": "ATOS (2010 lab data)", "Stated accuracy": "0.002–0.02 mm, single frame 1 s", "Note": "older wide band kept for reference" } ] }, { "caption": "How accurate is a shop-floor Rockwell test?", "columns": [ "Method", "Division of work", "Standard" ], "rows": [ { "Method": "Rockwell (HRB/HRC)", "Division of work": "batch hardness verification of heat-treated parts", "Standard": "ASTM E18 / ISO 6508" }, { "Method": "Brinell (HB)", "Division of work": "rough, large parts: castings and forgings", "Standard": "ASTM E10 / ISO 6506" }, { "Method": "Vickers/Knoop micro (HV/HK)", "Division of work": "case depth (CHD), thin sections, coatings — destructive section", "Standard": "ASTM E92/E384 / ISO 6507" } ] }, { "caption": "How deep can ultrasonic testing find a crack?", "columns": [ "Parameter", "Value", "Source" ], "rows": [ { "Parameter": "Frequency band", "Value": "1–5 MHz conventional", "Source": "S11" }, { "Parameter": "Penetration in steel", "Value": "100–300 mm", "Source": "S11" }, { "Parameter": "TOFD depth sizing", "Value": "±0.5 mm on a 25 mm wall", "Source": "S11" }, { "Parameter": "Tight crack detection", "Value": "below 0.1 mm", "Source": "S11" }, { "Parameter": "Thin-material spec", "Value": "0.5–12.7 mm at 2.0–10.0 MHz", "Source": "S12" }, { "Parameter": "Wall-thickness formula", "Value": "5,850 m/s × 3.4 µs ÷ 2 = 9.94 mm", "Source": "S11" }, { "Parameter": "Qualification", "Value": "ASNT Level II (SNT-TC-1A / CP-189 / ISO 9712)", "Source": "S24" } ] }, { "caption": "Why did my buyer reject magnetic particle on the aluminum bracket?", "columns": [ "Method", "Covers", "Source" ], "rows": [ { "Method": "MT", "Covers": "ferromagnetic parts; detects cracks below the 1.6 mm class band", "Source": "S13, S14" }, { "Method": "PT", "Covers": "any material; complementary above 1.6 mm on castings — a division of labor, not a conflict", "Source": "S14" } ] }, { "caption": "What can XRF verify on incoming material?", "columns": [ "PMI tool", "Strength", "Limit" ], "rows": [ { "PMI tool": "XRF", "Strength": "portable, non-destructive, seconds per reading", "Limit": "cannot detect carbon" }, { "PMI tool": "Spark OES", "Strength": "all elements incl. carbon at 0.01% class", "Limit": "leaves a small spark mark; bench instrument" }, { "PMI tool": "LIBS", "Strength": "handheld; fast screening", "Limit": "lower precision than OES for certification" }, { "PMI tool": "Carbon (LEEC combustion)", "Strength": "accurate carbon when OES is marginal", "Limit": "sample destruction" }, { "PMI tool": "Helium MSD leak test", "Strength": "5×10⁻¹²–5×10⁻¹³ Pa·m³/s, response under 1 s", "Limit": "nameplate sensitivity, not a method guarantee" } ] }, { "caption": "How sensitive is a helium leak test?", "columns": [ "Caliber", "Sensitivity", "How to read it" ], "rows": [ { "Caliber": "Instrument nameplate (MSD)", "Sensitivity": "5×10⁻¹²–5×10⁻¹³ Pa·m³/s, response < 1 s", "How to read it": "vendor high-end class (S22, UGC-sourced; logged as nameplate value)" }, { "Caliber": "Conservative method-level", "Sensitivity": "10⁻⁹ Pa·m³/s order", "How to read it": "S23; may describe the whole rig rather than the detector" }, { "Caliber": "Pressure decay / bubble", "Sensitivity": "orders of magnitude coarser", "How to read it": "volume- and temperature-dependent; no fixed floor (declared gap)" } ] }, { "caption": "Do salt-spray hours prove outdoor life?", "columns": [ "Parameter", "Value" ], "rows": [ { "Parameter": "Electrolyte", "Value": "5% NaCl solution" }, { "Parameter": "Cabinet temperature", "Value": "35 °C" }, { "Parameter": "Solution pH", "Value": "6.5–7.2" }, { "Parameter": "Collection rate", "Value": "1–2 mL per 80 cm² per hour" }, { "Parameter": "Meaning", "Value": "screening comparison — not outdoor service life" } ] }, { "caption": "What reports prove my supplier's inspection capability?", "columns": [ "Scenario", "Dimensional evidence", "Material / process evidence", "Deliverable" ], "rows": [ { "Scenario": "AS9102 full FAI (new part number)", "Dimensional evidence": "CMM report on all characteristics + ballooned drawing (Form 3)", "Material / process evidence": "MTR/CoC + HT/plating/NDT certificates (Form 2)", "Deliverable": "Form 1 + 2 + 3 + ballooned drawing" }, { "Scenario": "AS9102 delta FAI (revision / machine / supplier change)", "Dimensional evidence": "re-test affected characteristics only", "Material / process evidence": "certificates for the changed processes only", "Deliverable": "delta Form 3 referencing the original FAIR" }, { "Scenario": "PPAP Level 3 (automotive)", "Dimensional evidence": "dimensional report + MSA / gauge R&R", "Material / process evidence": "material / performance test reports", "Deliverable": "PSW + 18-element package, Cpk ≥ 1.67 on key characteristics" }, { "Scenario": "Buyer capability audit", "Dimensional evidence": "annual ISO 10360 CMM calibration certificate + 17025-traceable gauge list", "Material / process evidence": "ASNT Level II certificates + approved special-process vendors", "Deliverable": "one package covers most audits" } ] }, { "caption": "When does a part need a full FAI again?", "columns": [ "Trigger", "FAI scope" ], "rows": [ { "Trigger": "New part number", "FAI scope": "full FAI" }, { "Trigger": "Design change affecting fit, form or function", "FAI scope": "full FAI (or delta per agreement)" }, { "Trigger": "Process or supplier change", "FAI scope": "delta FAI on affected characteristics" }, { "Trigger": "Production lapse over two years", "FAI scope": "full or delta FAI per AS9102 Rev C — confirm which with the customer" }, { "Trigger": "Revision", "FAI scope": "AS9102 Rev C, published 2023-06" } ] }, { "caption": "Is a 0.092 mm bore-position reading really out of tolerance at Ø0.05?", "columns": [ "Step", "Item", "Value", "Note" ], "rows": [ { "Step": "1", "Item": "CMM MPE at L = 200 mm", "Value": "±(1.9 + 200/300) ≈ ±2.57 µm", "Note": "Zeiss Contura-class specification (S4/S5)" }, { "Step": "2", "Item": "Combined standard uncertainty u_c (k=1)", "Value": "√(6² + 4² + 5² + 3²) = √86 ≈ 9.3 µm", "Note": "probe 6 + scale 4 + thermal 5 + setup 3, root-sum-of-squares (S3 budget, values in µm)" }, { "Step": "3", "Item": "Expanded uncertainty U (k=2)", "Value": "≈ ±0.018 mm", "Note": "2 × u_c, per the source's own U(k=2) = ±0.018 mm caliber" }, { "Step": "4", "Item": "Verdict", "Value": "0.092 − 0.018 = 0.074 mm > 0.025 mm", "Note": "out of tolerance stands under both calibers — argument only matters near the boundary (ISO 14253-1)" }, { "Step": "5", "Item": "Fitting requirement", "Value": "≥ 16 points, least-squares cylinder; 3-point method amplifies 4.3×", "Note": "S3 verification practice" } ] }, { "caption": "How do I prove a Ø120 mm 4140 Q&T shaft has no internal cracks?", "columns": [ "Step", "Item", "Value" ], "rows": [ { "Step": "1", "Item": "Method elimination", "Value": "MT/PT surface-only; RT needs high energy, insensitive to planar flaws" }, { "Step": "2", "Item": "Frequency selection", "Value": "2–5 MHz (fine-grained forging); 0.5–12.7 mm thin stock per AMS 2632 at 2.0–10.0 MHz" }, { "Step": "3", "Item": "Wall-thickness check", "Value": "5,850 m/s × 3.4 µs ÷ 2 = 9.94 mm" }, { "Step": "4", "Item": "Acceptance caliber", "Value": "crack-like (planar) indications evaluated per ASME Sec V; pressure-boundary forgings typically zero tolerance" }, { "Step": "5", "Item": "Personnel", "Value": "ASNT Level II; report bound to the day's calibration status" } ] } ] }