How to choose the right material for a machined part

Complete Guide · Material Selection How to choose the right material for a machined part The material is the biggest single decision in a machined part — it sets strength, weight, corrosion, thermal behavior, finish and cost before a single cutter touches the block. This guide gives you a repeatable selection framework, the grade tables, and the cost and finish decisions that follow. Request a quoteStart with the framework A selection framework that takes five minutes Resist the urge to reach for the metal you know. Work through five questions in order, and the material usually picks itself: What loads will it carry? This sets the strength floor. A bracket holding 50 kg needs more than a cosmetic bezel. How much may it weigh? Weight-sensitive (drones, wearables, robotics) pushes toward aluminum, magnesium or plastic. What environment will it live in? Salt water, chemicals, or autoclave sterilization each eliminate whole material families. What does it touch? A mating bearing, a medical implant, a food-contact surface — each imposes its own constraint. What volume and budget? At very high volume, a plastic that can be molded beats a metal that must be machined. Only after these five do you compare the grade tables below. Starting from a specific alloy and working backward is how you end up paying for titanium when 6061 would have done. The properties that actually decide a part, in shop terms The five-question framework works because each question maps to a real, measurable property. Before the grade tables, it is worth saying exactly what those properties mean once the part is on a machine and in the field — because the textbook definition and the buyer's definition are not always the same thing. Yield strength is the stress at which the part takes a permanent set. For a structural bracket it is the number that matters most; a part can be well under ultimate tensile strength and still be useless if it has bent a tenth of a millimeter and will not seat. Tensile strength is where it finally breaks, useful for safety-factor math but secondary to yield for most machined components. Hardness decides wear and indentation resistance, and it also tells you how the material will cut. A material that is hard in service is hard on tooling; that is the hidden link between the property sheet and your machining quote. Elongation is the percentage the material stretches before fracture. High elongation means toughness and the ability to absorb shock without cracking; low elongation means a brittle part that fails without warning. A 304 stainless at 40% elongation is far more forgiving than a 17-4PH at 10%, even though the 17-4PH is "stronger." Thermal conductivity is the property that quietly decides whether a heatsink works and whether a part distorts in machining. Aluminum moves heat roughly ten times better than stainless; that single fact is why enclosures and heat spreaders are aluminum and why stainless is a poor choice for them. Coefficient of thermal expansion (CTE) is the one buyers forget. When you bolt aluminum (CTE about 23 µm/m·°C) to steel (about 17), they grow and shrink at different rates across a temperature swing, and a joint that is tight at 20 °C can loosen at 60 °C. Dissimilar-metal joints are a design decision, not an afterthought. Corrosion resistance is the property that eliminates whole families, and it is never a single number — it is a relationship between the alloy and the specific chemical it meets. Chloride (salt spray, de-icing, sweat) attacks 304 far more readily than 316; crevice corrosion hides where two surfaces trap liquid; and galvanic corrosion occurs when two dissimilar metals sit in an electrolyte. The data sheet lists a rating; the environment writes the verdict. None of these properties is "good" in isolation. The job is to name the two or three that govern your part and let everything else go. That is the whole discipline of selection, and it is why a generic "use stainless" instruction on a drawing is a red flag — it tells you the designer never decided which corner of the triangle the part lives in. The metal map, at a glance FamilySignature strengthTypical useMachinability AluminumLight, stiff for its weightEnclosures, brackets, heatsinks, roboticsExcellent Stainless steelCorrosion + strengthMedical, food, marine, shaftsModerate TitaniumBest strength-to-weightAerospace, medical implants, racingDifficult Brass / copperConductivity + machinabilityConnectors, fittings, RF shieldsExcellent MagnesiumLightest structural metalCamera bodies, handheld devicesGood (with care) Nickel alloysHigh-temperature strengthTurbine, exhaust, downholeDifficult Aluminum in depth: 6061, 7075, 2024, 5052 and 5083 Aluminum is the default machined metal for a reason: it is light, it cuts fast, and it anodizes into a durable, attractive finish. But "aluminum" is not one material — the 6xxx, 7xxx, 2xxx and 5xxx series behave very differently, and the difference shows up in the shop and in the field. The figures below are representative mill values for the common tempers; confirm the exact numbers against the mill certificate for your lot. Grade (temper)Tensile (MPa)Yield (MPa)Density (g/cm³)Elongation (%)Thermal cond. (W/m·K)Relative machinability 6061-T63102762.7012167Very good 7075-T65725032.8111130Good (moderate) 2024-T34833452.7818121Fair (poor corrosion) 5052-H322281932.6812138Good 5083-H1112901452.6616117Fair 6061-T6 is the all-rounder and the right answer for the majority of machined parts. Its yield of 276 MPa is plenty for enclosures, fixtures and brackets; its thermal conductivity (about 167 W/m·K) is the reason it doubles as a heatsink; and it anodizes to a hard, even, attractive coat. It is also one of the easiest alloys to machine, which keeps the cutting cost down. 7075-T6 trades some of that for roughly 80% more yield strength (about 503 MPa) at only a small weight penalty (2.81 g/cm³). That is why it is the aerospace and performance-vehicle default. The cost is threefold: it is not weldable (you cannot repair it by welding the way you can 6061), it is more susceptible to stress corrosion in thick sections, and it is a notch-sensitive, moderate-machinability alloy that needs sharper tooling and slower feeds than 6061. 2024-T3 is the older aerospace structural alloy. It is strong and tough, but its copper content makes it corrode badly in unprotected service, which is why it is almost always clad or painted. For most buyers it is a specialist choice; if you are not building an airframe spar, 7075 or 6061 will serve you better. 5052 and 5083 are the 5xxx series — work-hardening, non-heat-treatable, and highly corrosion-resistant. They are more common as sheet than as bar stock, but when a part needs seawater resistance (5083 is a marine and cryogenic favorite) or a formed-and-machined hybrid, they earn their place. They machine a little less freely than 6061 and do not anodize to quite the same hard coat, so finish expectations should be set accordingly. Two practical notes for the buyer. First, the "T" temper matters as much as the number — 6061-O (annealed) is soft and formable but weak, while 6061-T6 is the strong, machinable default; always specify the temper, not just "6061." Second, anodizing builds on the surface: Type III hard coat can add 25–100 µm, and you must account for that growth in the drawing, or the first-article will be out of tolerance by the coat thickness. Aluminum grades, and when to use each Aluminum is the default machined metal because it cuts fast, anodizes beautifully and carries most everyday loads. The grades matter more than most buyers realize: GradeBest forNotes 6061General structural, enclosures, fixturesThe all-rounder: strong, weldable, anodizes well 7075High strength-to-weight, aerospaceStronger than 6061 but not weldable; costs more 2024Aerospace structureHigh strength, poor corrosion resistance without cladding 5052Sheet, marine, corrosionExcellent corrosion resistance; forms well 5083Marine, cryogenicStrong, seawater-resistant For most machined parts the answer is 6061; for a part that must be as light and strong as possible, 7075. The full aluminum guide. Steel in depth: 1018, 4140, 304, 316 and 17-4PH Steel is the broadest family a buyer will touch, and the mistake is to treat "steel" as one thing. Carbon steel, alloy steel and stainless are different materials with different cost, machinability and corrosion behavior. The numbers below are representative; heat treatment moves the strength of 4140 and 17-4PH dramatically, so always pair the grade with its condition. Grade (condition)Tensile (MPa)Yield (MPa)Density (g/cm³)Elongation (%)Notes 1018 (cold drawn)4403707.8715Cheap, easy to machine, low hardenability 4140 (annealed)6554157.8525Tough alloy steel; hardens when treated 4140 (hardened)~1570~13807.85~10High strength after quench & temper 304 (annealed)5152058.0040General stainless; high toughness 316 (annealed)5792908.0050Marine grade; chloride resistant 17-4PH (H900)107010007.7510Precipitation-hardening; very strong 1018 is the entry-level carbon steel: inexpensive, readily available as bar and plate, and one of the easier steels to machine. It will not harden to any depth, so it is for low-stress brackets, jigs and non-wear parts where cost dominates. Its density of 7.87 g/cm³ means it is roughly three times heavier than the aluminum it often replaces. 4140 is the alloy steel workhorse for shafts, pins and load-bearing parts. In the annealed condition it machines well and is tough; after quench-and-temper it jumps to very high strength while staying tougher than a through-hardened carbon steel. The buyer's job is to specify the condition, because "4140" unqualified tells the shop nothing about the final hardness. 304 is the general-purpose stainless. Note the shape of its properties: tensile 515 MPa but yield only 205 MPa, and elongation a generous 40%. It stretches and work-hardens rather than snapping, which is exactly why it is so formable and why it can be annoying to machine (the work-hardening layer blunts tools if you take too light a cut). It resists most atmospheres but pits in chloride. 316 adds molybdenum, which is the single change that earns the "marine grade" label — it resists chloride pitting that would destroy 304. The cost is a higher material price and slightly worse machinability. For anything that sees salt, sweat or de-icing fluid, 316 is the floor. 17-4PH is a precipitation-hardening stainless that reaches 1000 MPa yield in the H900 condition. That puts it in the strength league of heat-treated alloy steel while keeping stainless corrosion resistance. It is the answer for shafts and structural parts where both strength and corrosion matter. The penalty is a difficult, work-hardening cut and a heat-treat step, which is why it is the most expensive of this group to turn into a part. Across all of these, the buyer should remember that steel's machinability is good-to-moderate, its raw cost is low, but its weight and cutting time are the real drivers. A 17-4PH shaft can cost more to machine than a titanium one of the same geometry, because both cut slowly and both work-harden. Stainless grades, and why they cost more Stainless is chosen for corrosion or strength, and paid for with slower cutting and higher tool wear. The four you will actually spec: 303 — the free-machining grade; the easy choice for turned parts that need corrosion resistance. 304 — the general-purpose stainless; slightly harder to machine than 303 but more corrosion-resistant. 316 — marine and chemical grade; adds molybdenum for chloride resistance. 17-4PH — precipitation-hardening; very high strength for shafts and structural parts. If you only need corrosion resistance and the part is turned, specify 303 and save machining cost. The full stainless guide. Titanium in depth: Grade 2, Grade 5 and Grade 23 Titanium earns its premium only when weight or biocompatibility is a hard requirement, and the three grades a buyer actually meets each play a different role. The property profile is what makes it special: strength close to hardened steel at about 60% of the weight, and a passive oxide film that shrugs off body fluid and seawater alike. GradeTensile (MPa)Yield (MPa)Density (g/cm³)Elongation (%)Why you pick it Grade 2 (CP)3452754.5120Corrosion, formability, low stress Grade 5 (Ti-6Al-4V)9508804.4314Strength-to-weight; the workhorse Grade 23 (ELI)9008304.4314Medical implant; extra-low interstitial Grade 2 is commercially pure titanium — not an alloy, just high-purity metal with oxygen content that sets its strength. At 275 MPa yield it is the softest of the three, but it is the most formable and the most corrosion-resistant, which is why it appears in chemical plant, heat exchangers and non-load-bearing medical parts. It is also the easiest titanium to machine, which is relative: it is still far harder on tooling than aluminum or steel. Grade 5 / Ti-6Al-4V is the alloy that built the titanium reputation. Its yield of 880 MPa at 4.43 g/cm³ gives a strength-to-weight ratio no steel can match, and it is the default for aerospace structure, racing components and load-bearing implants. The machining reality is the catch: titanium is a poor conductor of heat (about 6.7 W/m·K), so the heat from cutting stays at the tool edge instead of spreading into the chip, the tool wears fast, and you run low feeds with copious coolant. That is the entire reason a Grade 5 part costs several times an aluminum one. Grade 23 (ELI) is Grade 5 with extra-low interstitial content — the iron, oxygen and hydrogen held to tighter limits for fracture toughness and biocompatibility. It is the medical-implant grade. If your part goes inside a body, this is the one to specify, and you should expect full medical-grade traceability on the material. A note on heat treatment that affects lead time: stress-relieving Grade 5 needs around 788 °C. A shop whose in-house ovens top out near 500 °C (as is common for aluminum low-temperature ovens) cannot do that step and must subcontract it. Ask whether titanium stress-relief is done in-house or outsourced — it changes both the lead time and the chain of custody on your certs. Titanium: when the premium is worth it Titanium delivers the best strength-to-weight of any machinable metal and is biocompatible, which is why it owns medical implants and high-end aerospace. It is also slow to machine and hot to cut, so its cost per part is several times aluminum’s. Grades: Grade 2 (commercially pure, for corrosion and forming), Grade 5 / Ti-6Al-4V (the workhorse alloy), and Grade 23 (ELI, for medical implants). Specify titanium when weight or biocompatibility is a hard requirement — not as a default. The full titanium guide. Brass, copper and bronze in depth Where electrical or thermal conductivity is the point of the part, you leave the structural metals behind. Brass and copper are defined by their conductivity and their machinability, and the grades you will actually see line up on a simple trade: more conductivity means worse machinability. AlloyTensile (MPa)Yield (MPa)Density (g/cm³)Electrical cond. (%IACS)Thermal cond. (W/m·K)Machinability Brass C3603702008.50~35115Excellent (100) Brass C2603901308.53~28100Good Copper C110210338.96~101391Poor (gummy) Bronze C9322401258.84~1250Good Brass C360 (free-cutting) is the benchmark the whole industry measures machinability against — it is arbitrarily set to 100. It chips cleanly, holds tight tolerances, and takes a fine finish, which is why it dominates turned connectors, fittings and RF shields. Its conductivity (~35% IACS) is enough for most electrical contacts while staying easy to cut. The raw material is the cost driver, not the machining. Copper C110 is near-pure copper and the conductivity king: about 100% IACS electrically and roughly 391 W/m·K thermally. That is why it is the busbar and heatsink-insert metal. The price is machinability — pure copper is gummy, smears under the tool and builds up on the edge, so it demands sharp tools, positive rake and often a squeegee or chip-breaker geometry. If you need maximum conductivity, it is worth the trouble; if you only need "good enough," brass is cheaper to make. Bronze trades some conductivity for wear resistance and is the traditional choice for bushings, bearings and gears that run against steel shafts. It is a good machining alloy and a long-wearing one. The buyer's rule: pick brass when you want easy, cheap, conductive parts; pick copper when conductivity is the spec; pick bronze when the part wears against something. All three are far more expensive per kilo than aluminum, but all three cut so well that the machining cost stays modest. Brass, copper and the conductivity family Where electrical or thermal conductivity matters, you leave aluminum and steel behind. Brass C360 is the classic: superb machinability and good conductivity, for connectors and fittings. Copper C110 is near-pure copper for maximum conductivity but is gummy to machine. Bronze adds wear resistance for bushings and bearings. The cost is mostly in the raw material, not the machining. The full brass and copper guide. Magnesium in depth: AZ31B and fire-safe machining Magnesium is the lightest structural metal you can machine — at 1.78 g/cm³ it is about a third lighter than aluminum and a quarter the weight of steel for the same volume. That single property is why it shows up in camera bodies, drone frames and handheld electronics where every gram is paid for in flight time or handling. But magnesium has one property that dominates the conversation: as fine chips and dust it is flammable, and as a powder it can be explosive. GradeTensile (MPa)Yield (MPa)Density (g/cm³)Elongation (%)Thermal cond. (W/m·K)Machinability AZ31B2301301.781296Good (with care) AZ31B is the workhorse magnesium alloy — aluminum and zinc as hardeners, decent strength for its weight (yield about 130 MPa), and good thermal conductivity. It machines well: it is soft, cuts cleanly and does not work-harden, so feeds can be high. The catch is entirely about chip management. Magnesium ignites around 630 °C, and once a chip pile or slurry catches, ordinary water makes it worse (it reacts with water to release hydrogen). The safe shop runs flood coolant or mist to carry chips away and keep them below ignition temperature, keeps the table clear of accumulation, uses the right extinguishing media (Class D powder, not water), and ventilates to avoid a flammable-hydrogen pocket. The question to put to any supplier is blunt: "How do you control the fire risk during magnesium machining?" A real answer names chip handling, coolant strategy and extinguisher type. A blank look is a reason to walk away. Magnesium is absolutely machinable and safe in competent hands — it is just not a material for a shop learning on your job. On finishing, magnesium does not anodize the way aluminum does; it takes a conversion coating (typically a chromate or the newer trivalent treatments) for corrosion protection, and it can be painted or powder-coated. Set that expectation before quoting, because "anodize the magnesium part" is a finish the material cannot take. Magnesium, and the question you must ask Magnesium is the lightest structural metal — about a third lighter than aluminum — which is why it lives in camera bodies and handheld electronics. But magnesium is flammable as fine chips and powder, so the question to ask a shop is “how do you control the fire risk during machining?” A shop that answers with a real procedure (chip control, extinguishing media, ventilation) is safe to use; a blank look is not. The full magnesium guide. Engineering plastics in depth: PEEK, PEI/ULTEM, POM, nylon and more Plastic wins where weight, cost, electrical isolation or chemical resistance matter more than strength. The machining set spans a huge range, from cheap cosmetic ABS to PEEK that survives under the hood and in the autoclave. The numbers below are representative for the solid stock; filled grades (glass, carbon) run stronger but machine differently. PolymerTensile (MPa)Density (g/cm³)Continuous use (°C)Thermal cond. (W/m·K)CTE (µm/m·°C)Why it POM (Delrin)691.41~1000.3185Gears, bearings; low friction Nylon (PA66)801.14~1000.2480Wear parts; tough PC651.20~1150.2065Transparent, impact ABS401.05~800.1790Cheap, cosmetic PMMA701.18~800.1970Clear, optical PEI (ULTEM 1000)1051.27~1700.2247High temp, flame, FST PEEK~1001.32~2600.2547Harsh chem, medical, 260°C POM (Delrin) is the machinist's friend: it cuts cleanly, holds tight tolerance, has low friction and good dimensional stability. It is the default for gears, bushings and precision parts that slide. Its weakness is a high CTE (about 85 µm/m·°C) and sensitivity to hot water and strong acids, so it is not a chemical or steam part. Nylon (PA66) is tougher and slightly stronger but absorbs moisture from the air, which changes its dimensions and properties over time. For a wear part that lives dry, it is excellent; for a precision part with a tight tolerance that must hold for years, you must account for the moisture uptake or pick something more stable. PC, ABS and PMMA are the cosmetic and enclosure plastics. PC is the clear, impact-resistant one; PMMA is the optical-clear one; ABS is the cheap, easy, paint-ready one. None of the three likes high heat or aggressive solvents, and all three machine with care to avoid melting at the edge. PEI (ULTEM) is the step up: a high-temperature amorphous polymer that holds ~170 °C continuously, is inherently flame-retarded, and meets the strict FST (fire, smoke, toxicity) rules for aircraft interiors. It machines like a soft metal if you control heat, and it is the go-to for aerospace and semiconductor-tooling parts that must not burn or outgas. PEEK is the top of the machinable-plastic range: continuous use near 260 °C, outstanding chemical resistance, and biocompatibility that puts it in spinal cages and dental tools. It is expensive stock and it springs back from the cutter, so it needs sharp tools and rigid setups, but where metal would corrode or overheat, PEEK quietly does the job. It is the plastic that genuinely replaces metal in harsh service — at a plastic's weight and a metal's price. The crossover question for every plastic here is volume: below a few thousand parts, machining beats paying for an injection mold; above it, molding wins on unit cost. Pick the polymer for the environment, then pick the process for the quantity. Engineering plastics: when metal is the wrong answer Plastic wins where weight, cost, electrical isolation or chemical resistance matter more than strength. The machining set: PolymerBest forNotes POM (Delrin)Gears, bearings, precision partsMachines beautifully, low friction NylonWear parts, bushingsTough, absorbs moisture (dims change) PCTransparent housings, lensesImpact-resistant, clear PEEKHigh-temperature, chemical, medicalPremium; replaces metal in harsh environments ABS / PMMACosmetic covers, panelsCheap, easy to machine, cosmetic The crossover question is volume: below a few thousand parts, machining plastic beats paying for an injection mold; above it, molding wins. The full plastics guide. Surface treatment: the finish is a function, not a flourish Finish is not decoration — it is corrosion resistance, hardness, conductivity and appearance, all in one decision: Anodizing (Type II / Type III). Aluminum only; adds a hard, decorative oxide layer. Type III “hard” anodizing is thicker and more wear-resistant. Plating. Nickel, chrome, zinc and others for corrosion, conductivity or appearance on steel and copper alloys. Passivation. Stainless only; strips free iron and strengthens the passive oxide film. Powder coating. A durable, colored polymer layer for enclosures and brackets. Brushing / bead blasting / laser marking. Cosmetic and identification treatments. Finish choice must be locked before quoting, because it adds a process step, lead time and cost — and, for anodizing, changes a dimension by a few microns. Our finishing capabilities. How each grade actually takes a finish The finish table in the framework gives the yes/no, but the buyer needs the nuance because "yes" hides conditions that change cost and tolerance. The behavior below is what shows up on a first article. Anodizing is aluminum-only, and even within aluminum the result varies. 6061 takes a consistent, attractive Type II and Type III coat; 7075 anodizes darker and a little less evenly because of its alloying; 2024 anodizes poorly and is usually clad or painted instead. Type III hard coat grows the dimension by 25–100 µm depending on the spec, so the machine shop must cut the part undersize to leave room — that is a drawing note, not a surprise at inspection. Passivation is stainless-only and is not optional for critical parts. Machining leaves free iron smeared on the surface; passivation (citric or nitric) dissolves it and lets the chromium form its protective film. A stainless part that skips passivation can rust in service despite being "stainless." Plating — zinc, nickel, chrome, silver, gold — works on most metals but with caveats. Steel and copper alloys plate readily; aluminum needs a zincate pre-treatment or the plate will not adhere; titanium plates only with special preparation. Plating adds a measurable layer (microns to tens of microns) and, for threaded or fitted features, that layer must be allowed for or the part binds. Powder coat and paint need a substrate that survives the cure temperature (typically 160–200 °C) and a surface that holds the film. Magnesium takes conversion coat then paint; plastics take only low-temperature powder or liquid paint rated for the polymer. A powder coat that looks right but was cured above the substrate's limit is a latent failure. Titanium "anodizing" is the trap: it is an oxide colorization driven by voltage, not the structural hard coat aluminum gets. It is cosmetic only. If you need wear resistance on titanium, you specify a different process (e.g., nitride or DLC), not anodize. Lock the finish against the material before the quote, and write the coating thickness into the tolerance block. A finish you did not approve, substituted to make the part feasible, is a risk you inherit. What drives material cost For a sourcing engineer. Two separate costs hide inside “material”: the raw stock and the machinability premium. Copper and titanium are expensive to buy; stainless and titanium are expensive to cut (slow feeds, frequent tool changes); aluminum is cheap on both. This is why the cheapest material on paper is not always the cheapest part — a 7075 part can cost more than a stainless one if the geometry is unforgiving. Ask for the material cost and the machining cost separated, and the picture clears. Raw stock cost versus machining cost: a worked model The single most useful thing a buyer can ask for is the split between material cost and machining cost, because the two move in opposite directions for some alloys. Here is the shape of it, expressed as relative indices rather than live prices (which move with the LME and the alloy surcharge): MaterialRaw stock costMachining costWhere the money goes Aluminum 6061LowLowCheap on both — the default Aluminum 7075ModerateLow–moderateMaterial up, cutting similar Steel 1018LowLow–moderateCheap stock, easy cut Stainless 303ModerateModerateMaterial up, free-cutting helps Stainless 316HighModerate–highBoth sides up 17-4PHHighHighHard cut + heat treat Titanium Gr5Very highVery highBoth sides high Brass C360HighLowPay for stock, save on cutting PEEKVery highModerateStock dominates Two patterns to notice. First, brass and copper are expensive to buy but cheap to cut — so a conductivity part in brass is often cheaper than you fear once you separate the costs. Second, titanium and 17-4PH are expensive on both sides, which is the entire reason a titanium bracket can cost four-to-six times the aluminum version of the same drawing. The geometry is the multiplier. A part with deep pockets, thin walls or long reach tools pays the machining premium many times over, because cutting time and tool wear scale with the difficulty, not the volume of metal. This is why "the cheapest material on paper is not always the cheapest part" is not a slogan — it is arithmetic. Ask for the split, and ask what the machining cost would be in your second-choice material; a good shop will show you both. Mechanical properties, decoded When a drawing calls out a material, it is shorthand for a set of properties. The ones that matter for choosing a machined part material: Yield strength. The stress at which the material permanently deforms — the real “will it bend” number. Hardness. Resistance to indentation and wear — matters for bearing and sliding surfaces. Elongation. How much it stretches before breaking — a measure of toughness, not just strength. Thermal conductivity. How well it moves heat — the deciding property for heatsinks and enclosures. Corrosion resistance. How it holds up in its environment — often the property that eliminates whole families. You rarely need all of these for one part. Name the two or three that matter for the function, and the material selection collapses to a short list. This is why the five-question framework above works: it is filtering by the properties that matter, not by brand familiarity. The full property table, side by side The single most useful selection page is one where every candidate sits in the same rows, so the trade-offs are visible at a glance. The values below are representative mill/technical data for common machinable grades; always confirm against your mill certificate and the applicable specification, because tempers and sources vary. MaterialDensity (g/cm³)Tensile (MPa)Yield (MPa)Modulus (GPa)Elong. (%)Thermal cond. (W/m·K)CTE (µm/m·°C) Al 6061-T62.70310276691216723.6 Al 7075-T62.81572503721113023.2 Steel 10187.87440370205155211.7 Steel 4140 (HT)7.8515701380190104312.3 SS 3048.00515205193401617.2 SS 3168.00579290193501616.0 17-4PH H9007.7510701000196101811.0 Ti Gr24.5134527510520168.6 Ti Gr54.43950880114146.78.6 Brass C3608.50370200973511520.3 Copper C1108.96210331174039116.9 Mg AZ31B1.7823013045129626.0 PEEK1.32100953.6200.2547 PEI (ULTEM)1.271051003.3600.2247 Read the density column first — it is the weight you will actually ship and hold. Then read strength divided by density to see why titanium wins aerospace: Gr5 at 880 MPa yield over 4.43 g/cm³ beats 7075's 503 over 2.81, and crushes steel on a per-kilo basis even though steel's absolute numbers look bigger. Then read thermal conductivity to see why aluminum and copper are heatsink metals and stainless is not. The table does not choose for you, but it ends the argument about which metal is "stronger" — strength without weight and environment is just a number on a page. Machinability, and why it changes the price For a manufacturing engineer. Machinability is how fast and cleanly a material can be cut, and it drives cost independently of raw material price. The rough ranking, best to worst: MaterialMachinabilityCost implication Brass (C360)Best-in-classCheap to machine, moderate material cost Aluminum (6061)ExcellentCheap on both Carbon / alloy steelGoodModerate Stainless (303)Good (free-machining)Higher material + slower cutting Stainless (316 / 17-4PH)ModerateSlower, more tool wear TitaniumDifficultHighest machining premium Nickel alloysDifficultHighest machining premium The lesson: the cheapest raw material is not always the cheapest part. A 17-4PH part can cost more than a titanium one if the geometry is unforgiving. Ask for material cost and machining cost separated, and the true price picture appears. What drives machining cost. Machinability ratings and the cutting reality The machinability ranking above is useful, but a buyer who understands why a material cuts the way it does can write a drawing that costs less. Machinability is conventionally indexed against free-cutting brass C360 = 100. MaterialRelative machinabilityWhat you feel in the shop Brass C360100Chips break cleanly; high feeds Aluminum 6061~60–70Fast, but built-up edge if dull Steel 1018~78Easy, predictable Stainless 303~70Free-cutting sulfur grade Stainless 304~45Work-hardens; needs firm cut Stainless 316~35Work-hardens more; gummy 17-4PH~25Hard, work-hardens, tool wear Titanium Gr5~20–25Low heat transfer; slow, cool Copper C110~20Smears; needs sharp tools The pattern is that work-hardening alloys (304, 316, 17-4PH) punish a timid cut: take too light a pass and you skate on the hardened surface, dull the tool and make it worse. They want a positive, firm cut with sharp edges. Titanium's enemy is heat — its low thermal conductivity keeps the cutting heat at the tool tip, so you slow the feed, flood the coolant and change tools often. None of this is a reason to avoid the material; it is a reason the machining cost is what it is, and a reason to ask the shop to separate it from the stock cost. One more buyer lever: tolerances and finishes that the function does not need quietly inflate the machining cost. A feature held to ±0.01 mm costs more than one held to ±0.05 mm, and a cosmetic surface grind costs more than an as-milled one. Tighten only what the part actually requires. A quick-reference: if the requirement is… To compress everything above into a lookup: If you need…Reach for Light, stiff, cheap, anodizableAluminum 6061 Lightest possible structureMagnesium (with a fire-safety process) Highest strength-to-weightAluminum 7075 or Titanium Gr5 Corrosion in salt / chemicalStainless 316 Biocompatible implantTitanium Gr23 ELI Electrical / thermal conductivityCopper C110 or Brass C360 High-temperature strengthNickel alloy Weight, cost or isolation over strengthPOM, nylon, or PEEK Start from the requirement, not from a metal you like. When the requirement is still ambiguous, that is exactly when to send the drawing and environment to a partner and ask for the recommendation — a good shop will name the cheapest material that meets the spec, not the most expensive one it happens to stock. Corrosion and environment fit: matching material to where it lives Corrosion is the property that eliminates whole families, and it is never a single number — it is the alloy meeting the specific chemical. Mapping the common environments to the safe materials keeps a part alive for its service life. EnvironmentSafe choicesAvoid Indoor, dryAnything; 6061, 1018None (cost is the driver) Occasional moisture6061 (anodized), 304Plain carbon steel uncoated Salt / chloride316, Titanium, 5083304, 2024, carbon steel Chemicals / solvents316, PEEK, PEIAluminum, nylon, ABS Body fluid / medicalTitanium Gr23, 316L, PEEKCarbon steel, 6061 High temperature (>200°C)PEEK, PEI, nickel alloyMost plastics, leaded brass Food contact304/316, PEEKUncertified alloys Two failure modes buyers miss. Crevice corrosion hides where two surfaces trap a film of liquid — a bolted joint, a blinded hole — and eats 304 that would survive open air. Galvanic corrosion occurs when two dissimilar metals sit in an electrolyte: the more active metal (magnesium, then aluminum, then steel, then stainless, then titanium at the noble end) sacrifices itself. A 6061 bracket bolted to a steel frame in a wet environment will corrode at the joint unless you insulate it or pick a closer pair. The environment writes the verdict; the data sheet only lists the rating. Certifications, traceability and why they matter For a quality lead. For regulated parts — medical, automotive, aerospace — the material is not “aluminum”, it is “6061-T6, heat number XYZ, with a mill certificate.” Ask for material certificates, lot traceability, and, where required, RoHS or medical-grade documentation. We verify incoming alloy by XRF against the mill cert, so a mixed or substituted lot is caught before it becomes your finished part. How we verify. On the floor the checks are concrete rather than aspirational. Every BOM line is cross-checked against the drawing and the purchase spec before release, so a 6061 callout cannot silently become 5052. Critical alloys are bought through a cross-checked second source with a matching mill test certificate, and each incoming lot is confirmed by XRF / PMI before it reaches a machine. For aerospace programs we run FAI per AS9102 so the first article is dimensionally signed off before the run begins. Material certification, XRF verification and lot traceability For any regulated or safety-relevant part, the material claim has to be provable, not promised. The chain that makes it real runs from the mill to your receiving dock, and a buyer should know what each link is supposed to contain. The mill certificate (MTC / CoC). The smelter or mill issues a certificate stating the heat number, the grade, the chemical composition and the mechanical properties for that specific lot. It is the root of trust. A part spec that says only "6061" is not traceable; "6061-T6, heat lot A2471" is. XRF verification at incoming. Handheld X-ray fluorescence (XRF) spectroscopy reads the alloy chemistry on the spot, non-destructively. The point is to catch a substituted or mixed lot before it is cut — a bar that is 6063 sold as 6061, or a "304" that is actually 303 or a lower grade, shows up immediately in the spectrum. This is the practical check that turns a paper cert into a verified one, and it is why a shop with an XRF on the floor is materially safer than one that trusts the paperwork. Lot traceability. The certified lot must stay linked to your parts through cutting, finishing and shipping — material source, which heat, which machine, which operator, which inspection result, which batch. If a field failure happens, traceability is what lets you find and quarantine the rest of the lot instead of recalling everything. What our plant actually holds. The quality system is certified to ISO 9001 (certificate 116024/A/0001/UK/En), IATF 16949 (131941/A/0001/SM/En) and ISO 14001 (116024/B/0001/UK/En), all issued by URS and valid through 2027. The IATF 16949 scope is important to state honestly: it covers the manufacture of CNC metal machining parts but excludes product design under Clause 8.3 — this shop executes your design, it does not take design responsibility for the part. For automotive programs that distinction is exactly what your audit will check. A real SPC example. That is a concrete demonstration of process capability, not a marketing claim, and the same method is what backs a PPAP submission. Material and finish compatibility, in one matrix The single most common finish mistake is specifying a finish the material cannot take. Anodizing is aluminum-only; passivation is stainless-only; plating works on most metals but not always well; powder coat needs a substrate that survives the cure. The matrix below is the lookup that prevents the wasted step: MaterialAnodizePassivatePlatePowder coat AluminumYes (Type II / III)NoLimitedYes Stainless steelNoYesYesYes Carbon / alloy steelNoNoYes (Zn / Ni / Cr)Yes Brass / copperNoNoYes (Ni / Au / Ag)Yes TitaniumNo (color oxide only)NoLimitedYes MagnesiumNo (conversion coat)NoLimitedYes Engineering plasticsNoNoNoLimited Two notes buyers miss: titanium can be “anodized,” but the process is an oxide colorization, not the structural hard coat aluminum gets; and magnesium takes a conversion coating, not anodizing. If your drawing calls for anodize on steel, the shop will either reject it or substitute — and a substitution you did not approve is a risk. Lock the finish against the material before quoting. The full finishing guide. The cost–weight–strength triangle, quantified The triangle is easier to reason about with real specific-strength and specific-stiffness numbers. "Specific" means the property divided by density — the honest comparison when weight is in play. MaterialYield / density (MPa·cm³/g)Stiffness / density (GPa·cm³/g)Where it sits Al 6061-T610225.6Cheap, light, moderate Al 7075-T617925.6Light, strong Steel 10184726.1Cheap, heavy Steel 4140 (HT)17624.2Strong, heavy SS 3042624.1Corrosion, heavy 17-4PH H90012925.3Strong + corrosion, heavy Ti Gr519925.7Best strength-to-weight Mg AZ31B7325.3Lightest, lower strength The table exposes the myth that "steel is strongest." On absolute numbers, hardened 4140 crushes 6061. On a per-kilo basis — which is what an aircraft, a drone or a wearable actually pays for — 7075 and Gr5 pull ahead, and Gr5 leads. Stiffness per kilo is roughly equal across the metals (around 25), which is why "make it lighter" usually means a lighter alloy at similar rigidity, not a different shape. Magnesium is the lightest but at lower specific strength, so it earns its place when absolute mass beats absolute load. When you know which axis your part lives on, the material short-list is short. The cost–weight–strength triangle Most material decisions are a three-way trade between cost, weight and strength, and you rarely win on all three. The triangle is a faster lens than a spec sheet: Cost wins, strength and weight secondary. Carbon or alloy steel, or 6061 aluminum — the default for brackets, frames and enclosures where mass is not critical. Weight wins, cost secondary. Aluminum 7075, magnesium, or titanium at the extreme end — aerospace, wearables, drones, where every gram is paid for in range or handling. Strength wins, weight and cost secondary. 17-4PH, titanium, or a forged steel — structural and load-bearing parts where failure is not an option. Notice that “strength wins” and “cost wins”– pull opposite directions — which is why the five-question framework at the top works: it forces you to name which corner of the triangle the part actually lives in, instead of picking a hero material and hoping. Common material-selection mistakes Specifying from familiarity. Reaching for 304 stainless because it is known, when 6061 would have done the job for a third of the cost and weight. Ignoring machinability. Designing thin features in titanium or 17-4PH, where slow cutting and tool wear multiply the part cost. Forgetting the finish. Calling for anodize on a non-aluminum, or passivation on a part that will be plated — a rework loop that costs a week. Over-buying certification. Paying for full aerospace traceability on a part that only needs a mill cert, or vice versa and failing audit. Treating “aluminum” as one material. 6061 and 7075 are different animals; so are cast and wrought alloys. Name the grade. A worked example: one bracket in three materials Take a 100 mm structural bracket carrying a 30 kg load, machined three ways: 6061 aluminum. Light, cheap to cut, anodizes well. The sensible default; lowest total part cost. 304 stainless. Heavier, two-to-three times the machining cost, but corrosion-resistant without coating. Worth it only if the environment demands it. Ti-6Al-4V. Half the weight of stainless, but four-to-six times the part cost. Justified only where weight is the binding constraint. The point is not the exact numbers — it is that the right answer changes completely with the environment and the volume, and a partner that shows you the three side by side (with machining cost separated) is doing the selection work for you. What drives machining cost. Weight and cost tradeoffs, with real numbers The bracket example is easier to defend with the actual mass and cost shape. Take a 100 mm bracket as a solid-ish machined block of roughly 150 cm³ of stock removed to a ~80 g finished part — the point is the ratio, not the exact part. MaterialFinished mass (relative)Raw stock costMachining costTotal part cost (relative) 6061 aluminum1.0×LowLow1.0× (baseline) 304 stainless~3.0×Moderate2–3×~2.5× Ti-6Al-4V~1.6×Very high4–6×~5× Mg AZ31B~0.65×HighLow–moderate~1.5× The mass column tracks density (aluminum 2.70, steel 8.00, titanium 4.43, magnesium 1.78 g/cm³), so a steel part is about three times the aluminum mass and a magnesium part about two-thirds. The cost column is where the surprise lives: titanium's part cost is driven mostly by machining, not by the ~1.6× mass, because slow cutting and tool wear dominate. Magnesium is lighter than aluminum and not much harder to cut, so it can undercut aluminum on total cost once stock price is set aside — but its stock is pricier and its fire-safe handling adds a process premium. The takeaway for the buyer: weight and cost move together only loosely; the machining difficulty is the wild card, and the only way to see it is to ask for the split. RoHS, REACH and recyclability For regulated or consumer markets, material choice is also a compliance question. RoHS restricts lead, cadmium and other substances in electronics; REACH covers a longer list of registered chemicals in the EU; and WEEE pushes recyclability and recovery. The practical buyer checks three things: that the alloy and any plating are RoHS / REACH-compliant for the target market, that the material certificates state compliance, and that the finish does not trap the part in a non-recyclable composite. A shop that cannot produce compliant material documentation is a non-starter for EU or North American consumer electronics. Our certification and documentation. How to read a data sheet A data sheet is where a material claim becomes a number. The values that actually decide a machined part: Property6061-T6 Al304 SSTi-6Al-4VBrass C360 Tensile strength (MPa)310515950370 Yield strength (MPa)276205880200 Density (g/cm³)2.708.004.438.50 Elongation (%)12401435 Modulus (GPa)6919311497 Read these together, not in isolation. Note that 304 stainless has higher tensile than 6061 but lower yield — it stretches more before it gives. Titanium’s strength is spectacular per unit weight (divide strength by density), which is exactly why it wins aerospace. Brass trades strength for machinability and conductivity. And watch the coefficient of thermal expansion when mating dissimilar metals: aluminum and steel expand at different rates over temperature cycles, which can loosen a joint. The data sheet does not choose for you, but it ends the argument about which metal is “stronger.” How we verify incoming material. Reading a supplier's capability against your spec The material is only half the decision; the shop has to hold the tolerance the material allows. Knowing what good equipment and what capability numbers look like lets you judge a quote instead of trusting it. Tolerance. A competent CNC shop holds ±0.005 mm on milled and turned features and ±0.002 mm on ground or wire-EDM features as a routine capability. If your drawing demands tighter than that, ask how it is held — grinding, lapping or EDM — because that step is where the cost and lead time live. Machine mix. A versatile plant running 80+ CNC machines — 50+ three-axis, 20+ four-axis and 20+ five-axis mills, 25+ turning centers, 7+ precision grinders — on brands such as Mazak, Brother, TSUGAMI and Sodick can take a part from prototype to volume on one floor. Five-axis matters for complex geometry: it cuts a contoured part in fewer setups, which protects tolerance and shortens lead time. Turn-mill combinations do a shaft in one chucking, which protects concentricity. Verification kit. The inspection side is what makes the material claim real. A coordinate measuring machine (CMM) for form and position, vision measuring for small features, a roughness tester for surface finish, a hardness tester for heat-treated parts, a salt-spray chamber for corrosion validation, and an XRF spectrometer for alloy verification together are the difference between "we think it's 6061" and "we confirmed it's 6061, heat lot A2471." Process capability. For production, ask for the Cpk on your critical characteristics. The floor to look for is Cpk ≥ 1.33 on general characteristics and ≥ 1.67 on critical ones; this shows what that looks like in a real run. A shop that cannot state a Cpk target is not running a capable process — it is hoping. Lead time and MOQ. No minimum order quantity (MOQ of one piece) means you can prototype without committing to volume, and tiered lead times — about 3 days for prototype, about 7 days for small batch, about 30 days for mass production with finishing — let you plan the program. Treat the exact day as variable with complexity; the tier is the signal. The material selection checklist Before you release a drawing, run this: Named the two or three properties that matter — strength, weight, corrosion, conductivity? Picked the grade, not just the family? Locked the finish against the material? Checked machinability and the machining-cost implication? Confirmed certification and traceability for the target market? If all five are answered, the material is decided on function, not habit — which is the whole point of the framework above. The expanded buyer's checklist The five-item list above is the minimum. Before release, add these and the part is genuinely specified rather than hoped: Named the temper and condition. "6061" is not a spec; "6061-T6" is. "4140" unqualified says nothing about hardness — state annealed or hardened. Set the tolerance to what the function needs. Tighten only critical features; every ±0.01 mm you do not need is cost you are paying. Allowed for the finish in the dimensions. Anodize and plate add microns; write the coating allowance into the tolerance block so the first article passes. Checked the environment against the alloy. Salt, chemical, body fluid, temperature — each one can eliminate a family you were about to specify. Accounted for dissimilar-metal joints. If you bolt two metals together in a wet environment, plan insulation or a closer pair to avoid galvanic corrosion. Split the quote. Material cost and machining cost separated, so you see the true price picture. Required the cert chain. Mill certificate with heat number, XRF verification on incoming, and lot traceability through to shipping — especially for medical, automotive or aerospace. Confirmed the shop's capability claim. Ask the Cpk floor (1.33 general / 1.67 critical) and the equipment that holds your tolerance. Run that, and the material is decided on evidence, not habit — which is the entire point of the framework at the top of this guide. Material myths that cost buyers money “Stronger is always better.” Stronger usually means harder to machine, heavier and more expensive. The right material meets the spec with margin, not maximal strength. “Stainless never rusts.” It resists corrosion, but free iron left from machining or a harsh chloride environment still attacks it. Passivation is not optional for critical parts. “Titanium is the best metal.” Only when weight or biocompatibility is binding. Elsewhere it is an expensive habit. “One grade fits all.” 6061 and 7075, cast and wrought, are different materials with different cost and behavior. Name the grade. Frequently asked questions What is the best metal for CNC machining?Aluminum 6061, for most parts: it is light, strong enough for everyday loads, cuts fast and anodizes well. Reach for 7075 for strength-to-weight, stainless for corrosion, titanium for weight-critical or medical, brass for conductivity. How do I choose between aluminum and stainless?Ask what the environment is. If it will see moisture, salt or chemicals, choose stainless. If weight, cost or thermal conductivity matters more, choose aluminum. When in doubt, 6061 aluminum and 303/304 stainless are the two safe defaults. Can you machine plastic instead of metal?Yes — POM, nylon, PC, ABS, PMMA and PEEK. Below a few thousand parts, machining plastic beats paying for a mold; above that, injection molding usually wins on unit cost. What is hard anodizing?Type III anodizing — a thicker, harder aluminum oxide layer than standard Type II, for wear and corrosion resistance. It adds lead time and changes the dimension slightly, so it must be locked before quoting. Do you provide material certificates?Yes. We verify incoming alloy by XRF against the mill certificate and can provide certificates and lot traceability for regulated programs. Why does my titanium part cost so much more?Titanium is expensive to buy and slow to cut — low feeds, high heat, frequent tool wear. The premium is the material and machinability together, not markup. What is the difference between 6061 and 7075 aluminum?6061-T6 yields about 276 MPa and is the easy-machining, weldable, anodizable all-rounder for most parts. 7075-T6 yields about 503 MPa — roughly 80% stronger — but is not weldable, is notch-sensitive, and costs more to buy and cut. Pick 7075 only when strength-to-weight is the binding constraint. Which stainless should I use near salt water?316, not 304. The molybdenum in 316 resists chloride pitting that destroys 304 in salt spray, de-icing fluid or sweat. For critical marine structure, also consider 5083 aluminum or titanium, both of which shrug off seawater. Is 17-4PH stainless worth the cost?Only when you need both high strength and corrosion resistance in one part — shafts, structural brackets, valve bodies. At about 1000 MPa yield in the H900 condition it rivals heat-treated alloy steel while staying stainless, but it is the most expensive stainless to machine because it work-hardens. Can you machine magnesium safely?Yes, with a documented fire-safety process: flood or mist coolant to keep chips below ignition, no chip accumulation, Class D extinguishing media (not water), and ventilation. Ask the shop to describe exactly that before you send magnesium work — magnesium is the lightest structural metal but flammable as fine chips. What plastic replaces metal in harsh service?PEEK for chemical and high-temperature (continuous ~260 °C) duty and medical implants; PEI (ULTEM) for high-temperature, flame-safe aerospace and semiconductor tooling to about 170 °C. Both are expensive stock and need rigid setups, but they survive where aluminum would corrode or soften. How do I avoid a finish that the material cannot take?Lock the finish against the material before quoting. Anodize is aluminum-only (titanium "anodize" is cosmetic color only); passivation is stainless-only; plating needs pretreatment on aluminum and special prep on titanium; magnesium takes a conversion coat, not anodize. Writing the coating thickness into the tolerance block prevents a failed first article. What Incoterm should I use for China-made parts?FOB is the common balanced choice — you control ocean freight and import clearance. EXW gives the lowest unit price but the most work. DDP delivers duty-paid to your door but embeds the tariff in the quote and puts import compliance on the seller. Always state "Incoterms 2020." Will a China-made part be hit by US tariffs?Likely. US imports from China carry base MFN plus Section 301 (7.5%–25% by HTS) and, for steel or aluminum articles, Section 232 (25% / 10%) regardless of source. There is no US–China FTA to remove it, so budget the full stack and ask the supplier for the proposed HTS and any add-on duty in writing. What tolerance can a CNC shop hold?A routine capability is ±0.005 mm milled or turned and ±0.002 mm ground or wire-EDM. Tighter than that needs a specified process (grind, lap or EDM) and shows up in cost and lead time. Always state the tolerance the function requires and no tighter. How do you prove the alloy is what you claim?Mill certificate with heat number, verified by handheld XRF on incoming material, with lot traceability through cutting, finishing and shipping. For production, ask for the Cpk on critical characteristics — the floor is 1.33 general and 1.67 critical, demonstrated by real SPC data. Sources & further reading ASM Handbook series — alloys and properties reference (ASM International) MatWeb — material property database ISO 9001:2015 (ISO) IATF 16949:2016 (IATF Global Oversight) Not sure which material to spec?Send the drawing and the environment it will live in to [email protected] — we will recommend the cheapest material that meets the requirement, with a cost comparison.Request a quote Related articlesThe Complete Guide to CNC Machining in ChinaAluminum Machining Guide: Grades and FinishesSurface Finishing Guide