{ "slug": "thermal-expansion-coefficients", "title": "Thermal Expansion Coefficients for CNC Materials (CTE)", "description": "CTE for 22 CNC materials in µm/m·°C with the temperature range each value covers, plus worked examples of what the differences do to fits and inspection.", "lang": "en", "tables": [ { "caption": "Coefficient of thermal expansion (CTE) by material and temperature range", "columns": [ "Material (grade)", "CTE (µm/m·°C)", "Temperature range", "Engineering significance", "Source" ], "rows": [ { "Material (grade)": "Aluminum 1060 (commercially pure)", "CTE (µm/m·°C)": "23.6", "Temperature range": "20–100 °C", "Engineering significance": "Highest CTE among common CNC metals; busbar and heat-sink designs must allow expansion gaps", "Source": "ASM Handbook Vol. 2 via matmake.com" }, { "Material (grade)": "Aluminum 6061-T6", "CTE (µm/m·°C)": "23.6", "Temperature range": "20–100 °C", "Engineering significance": "Most machined aluminum alloy; CTE is temper-independent (O, T4, T6 and T651 all 23.6)", "Source": "ASM Handbook Vol. 2, 10th ed.; MIL-HDBK-5J" }, { "Material (grade)": "Aluminum 6061-T6", "CTE (µm/m·°C)": "24.6", "Temperature range": "20–200 °C", "Engineering significance": "Elevated-temperature mean rises ~4%; use a range-specific CTE above 150 °C", "Source": "MIL-HDBK-5J; ASM Handbook Vol. 2" }, { "Material (grade)": "Aluminum 7075-T6", "CTE (µm/m·°C)": "23.6", "Temperature range": "20–100 °C", "Engineering significance": "Aerospace grade; same CTE as 6061 despite twice the yield strength — simplifies mixed-alloy fixture design", "Source": "matmake.com; arcuscnc.com" }, { "Material (grade)": "Copper C11000 (ETP)", "CTE (µm/m·°C)": "17.0", "Temperature range": "20–100 °C", "Engineering significance": "Busbar and connector material; CTE mismatch vs aluminum (23.6) drives joint fatigue in bimetallic lugs", "Source": "fabdigit.com (C11000-O properties)" }, { "Material (grade)": "Copper C11000 (ETP)", "CTE (µm/m·°C)": "17.0", "Temperature range": "20–300 °C", "Engineering significance": "Mean CTE over a wide range; manufacturer datasheet value", "Source": "jtcopper.com C11000 datasheet" }, { "Material (grade)": "Brass C36000 (free-cutting)", "CTE (µm/m·°C)": "20.5", "Temperature range": "20–100 °C (approx.)", "Engineering significance": "Highest-machinability copper alloy; CTE between copper and aluminum — relevant for brass inserts in aluminum housings", "Source": "mechcodex.com C36000 thermal table" }, { "Material (grade)": "Carbon steel AISI 1045 (annealed)", "CTE (µm/m·°C)": "11.6", "Temperature range": "0–100 °C", "Engineering significance": "Baseline shaft and gear steel; CTE about half of aluminum — Al-to-steel bolted joints develop 0.12 mm of differential per 100 mm at ΔT = 100 °C", "Source": "theworldmaterial.com AISI 1045 physical properties" }, { "Material (grade)": "Carbon steel AISI 1045 (annealed)", "CTE (µm/m·°C)": "12.3", "Temperature range": "0–200 °C", "Engineering significance": "Elevated-temperature mean; rises with the temperature range", "Source": "theworldmaterial.com AISI 1045" }, { "Material (grade)": "Alloy steel AISI 4140 (normalized)", "CTE (µm/m·°C)": "12.3", "Temperature range": "20–100 °C", "Engineering significance": "High-strength shaft and axle steel; CTE similar to 1045 — interchangeable in thermal-stress calculations", "Source": "ASM Handbook Vol. 1; metalzenith.com" }, { "Material (grade)": "Stainless steel 304/304L", "CTE (µm/m·°C)": "17.3", "Temperature range": "0–100 °C", "Engineering significance": "Austenitic stainless expands ~44% more than carbon steel; critical for heat-exchanger tube-to-tubesheet joints", "Source": "jnalloys.com CTE comparison table" }, { "Material (grade)": "Stainless steel 316/316L", "CTE (µm/m·°C)": "15.9", "Temperature range": "0–100 °C", "Engineering significance": "Lower CTE than 304 despite the similar austenitic structure (molybdenum addition); preferred for marine piping expansion design", "Source": "jnalloys.com CTE comparison table" }, { "Material (grade)": "Stainless steel 17-4PH (H900)", "CTE (µm/m·°C)": "10.8", "Temperature range": "0–100 °C", "Engineering significance": "Martensitic PH steel; CTE closest to carbon steel among stainless grades — ideal for precision aerospace fittings mating with steel structures", "Source": "xometry.com; alloy-materials.com (20–100 °C)" }, { "Material (grade)": "Titanium Ti-6Al-4V (Grade 5)", "CTE (µm/m·°C)": "8.6", "Temperature range": "20–100 °C", "Engineering significance": "Lowest CTE of common structural metals; good match to CFRP (0–2) in aerospace hybrid joints", "Source": "arcuscnc.com CTE comparison" }, { "Material (grade)": "Titanium Ti-6Al-4V (Grade 5)", "CTE (µm/m·°C)": "8.8", "Temperature range": "20–100 °C (approx.)", "Engineering significance": "Alternate published value; the difference sits within measurement uncertainty (±2%)", "Source": "mechcodex.com Ti-6Al-4V properties" }, { "Material (grade)": "Tool steel P20 (pre-hardened, 28–32 HRC)", "CTE (µm/m·°C)": "11.7", "Temperature range": "20–100 °C", "Engineering significance": "Mould base steel; CTE must match the insert steels to avoid thermal stress cracking in injection moulds during thermal cycling", "Source": "ASM Handbook Vol. 1; firstmold.com (P20 context)" }, { "Material (grade)": "Tool steel H13 (hot-work, 44–52 HRC)", "CTE (µm/m·°C)": "12.0", "Temperature range": "20–200 °C", "Engineering significance": "Die-casting and forging die steel; runs at 500–650 °C surface temperature, so the elevated-range CTE matters for die insert fits", "Source": "ASM Handbook Vol. 1; metalzenith.com" }, { "Material (grade)": "Delrin / POM (acetal homopolymer)", "CTE (µm/m·°C)": "110", "Temperature range": "23–60 °C", "Engineering significance": "About 5× steel; plastic-to-metal press fits loosen as temperature rises — design the interference at the maximum operating temperature", "Source": "firstmold.com; DuPont Delrin 500P datasheet" }, { "Material (grade)": "Nylon 6/6 (unfilled, dry-as-molded)", "CTE (µm/m·°C)": "80", "Temperature range": "23–60 °C", "Engineering significance": "Moisture absorption adds hygroscopic expansion on top of the thermal term; 50% RH raises the effective dimensional change ~0.2%", "Source": "DuPont Zytel 101L datasheet; ASM Handbook Vol. 21" }, { "Material (grade)": "PEEK (unfilled, e.g. Victrex 450G)", "CTE (µm/m·°C)": "47", "Temperature range": "23–100 °C", "Engineering significance": "Lowest CTE among unreinforced engineering plastics; approaches aluminum at elevated temperature — viable for metal replacement in thermal-cycling environments", "Source": "makerstage.com (citing Victrex); criterionprecision.com (45–55)" }, { "Material (grade)": "PEEK (30% glass-filled, GF30)", "CTE (µm/m·°C)": "27", "Temperature range": "23–100 °C", "Engineering significance": "Glass reinforcement cuts CTE ~43%; chosen when dimensional stability under thermal cycling is critical", "Source": "makerstage.com PEEK grade comparison (Victrex)" }, { "Material (grade)": "ABS (general purpose, unfilled)", "CTE (µm/m·°C)": "90", "Temperature range": "23–60 °C", "Engineering significance": "High CTE limits use in precision assemblies; acceptable for consumer enclosures with generous clearance", "Source": "firstmold.com; Cycolac/Terlux datasheets" } ] }, { "caption": "When the Difference Becomes a Dimension", "columns": [ "Case", "Calculation", "Result", "Design response" ], "rows": [ { "Case": "6061-T6 housing (CTE 23.6) bolted to a 1045 steel base (CTE 11.6), 200 mm bolt pattern, ΔT = 50 °C", "Calculation": "ΔL(Al) = 23.6 × 10⁻⁶ × 200 × 50; ΔL(steel) = 11.6 × 10⁻⁶ × 200 × 50", "Result": "0.236 mm vs 0.116 mm — 0.120 mm differential", "Design response": "With only 0.1 mm of bolt clearance the joint binds at ΔT = 50 °C. Use slotted holes, clearance ≥ 0.3 mm or compliant washers; for precision fixtures, build aluminum-to-aluminum or steel-to-steel." }, { "Case": "Steel structure (CTE 12.0) bolted directly to a CFRP panel (CTE ~1.0 in the fibre direction), 300 mm span, ΔT = 80 °C", "Calculation": "(12.0 − 1.0) × 10⁻⁶ × 300 × 80", "Result": "0.264 mm relative slip per cycle", "Design response": "A Ti-6Al-4V insert (CTE 8.6) cuts the mismatch to 7.6 µm/m·°C on both sides and reduces differential movement ~30% — the reason titanium fittings dominate composite-joint aerospace designs." }, { "Case": "Delrin bushing (CTE 110) press-fitted into a steel housing (CTE 11.6): 25 mm OD, 0.05 mm interference at 20 °C, service at 80 °C", "Calculation": "ΔL(POM) = 110 × 10⁻⁶ × 25 × 60; ΔL(steel) = 11.6 × 10⁻⁶ × 25 × 60", "Result": "0.165 mm vs 0.017 mm — 0.148 mm of interference lost (−0.098 mm, i.e. clearance)", "Design response": "The press fit becomes a slip fit at 80 °C. Calculate interference at maximum operating temperature, retain POM mechanically (snap rings, knurling), or switch to PEEK (CTE 47) or glass-filled PEEK (CTE 27)." }, { "Case": "A 500 mm 6061 part machined at 30 °C shop temperature, inspected at 20 °C (ISO 1 reference temperature)", "Calculation": "23.6 × 10⁻⁶ × 500 × 10", "Result": "0.118 mm — reads 500.000 mm at 30 °C, 499.882 mm at 20 °C", "Design response": "For ±0.05 mm work that error is 2.4× the tolerance band: soak parts to 20 °C ± 1 °C before final inspection, or apply in-process compensation." }, { "Case": "P20 mould base (CTE 11.7) carrying an H13 core insert (CTE 12.0), 100 mm insert diameter, ΔT = 60 °C, 0.02 mm designed clearance", "Calculation": "12.0 × 10⁻⁶ × 100 × 60; 11.7 × 10⁻⁶ × 100 × 60", "Result": "0.072 mm vs 0.070 mm — clearance grows only 0.002 mm", "Design response": "P20 to H13 is thermally compatible (0.3 µm/m·°C mismatch). A beryllium-copper insert (CTE 16.6) against P20 loses 0.029 mm of clearance at the same ΔT — design BeCu inserts with a growth allowance." } ] } ] }