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What materials are commonly available for custom CNC machining parts production?
The short answer
Aluminium 6061 and 7075, stainless 303, 304 and 316, carbon and alloy steel, titanium Ti-6Al-4V, brass C360 and copper C110 cover most metal work; POM, nylon, PEEK and ABS cover most plastic work. Material and machinability together drive 35 to 60 percent of part cost.
Machinability, not price per kilo, drives the part cost
Two materials can cost the same per kilogram and produce part prices three times apart, because the part price is set by how long the cutter stays in the material, how often the tool is replaced, and how much scrap the process generates. That is what a machinability index measures, and it is why material choice and machining cost cannot be separated.
The reference point in almost every published index is free-cutting brass C360, rated 100, which is the easiest common metal to cut: chips break cleanly, tool life is long and the surface comes off smooth. Aluminium 6061 sits near 90, aluminium 7075 near 70 because its higher strength slows the cut, stainless 304 around 45 and 316L around 40 because austenitic stainless work-hardens as it is cut, and titanium Ti-6Al-4V around 22 because it generates heat, wears tools and springs back. On the plastic side, POM is roughly comparable with brass at about 95, nylon around 80, and PEEK about 55. Broad indices put anything below 40 out of 100 in the band that carries an extra 75 to 150 percent of machining cost over a free-cutting grade.
Two practical consequences follow. First, material selection alone accounts for up to 35 to 60 percent of total manufacturing cost and cycle time, which is why the material decision is a cost decision rather than a specification formality. Second, a lower-grade but adequate material is often the largest single saving available on a part, larger than any negotiation on machine rate.
Six families, and the grade inside each that is usually right
Published capability data is usually organised around a small number of families, and within each family there is a default grade that suits most work. Starting from the family and then refining to a grade is faster and more reliable than trying to compare twenty materials at once.
| Family | Typical grades | Tensile strength | Machinability | Where it fits |
|---|---|---|---|---|
| Aluminium | 6061-T6, 7075-T6 | 310 MPa, 572 MPa | Excellent, good | Enclosures, brackets, frames, prototypes |
| Stainless steel | 303, 304, 316, 17-4 PH | 515 to 1,170 MPa | Fair to moderate | Food, medical, marine, chemical duty |
| Carbon and alloy steel | 1045, 4140, 4340 | 570 to 650 MPa | Good to fair | Shafts, gears, jigs, structural parts |
| Titanium | Grade 2, Ti-6Al-4V | 344 MPa, 950 MPa | Poor | Aerospace, medical, corrosive service |
| Copper alloys | C360 brass, C110 copper | 385 MPa, 220 MPa | Excellent, fair | Fittings, connectors, bus bars, heat sinks |
| Engineering plastics | POM, nylon, PEEK, ABS | 70 to 100 MPa | Good to excellent | Insulators, gears, bushings, wear parts |
The metals, grade by grade
Aluminium 6061-T6 is the default for most machined metal work: 310 MPa tensile, excellent machinability, good corrosion resistance when anodised and the cheapest route to a precise part. It is soft, so thin walls deflect and deep threads strip, and both are design constraints rather than machining problems. Aluminium 7075-T6 is nearly twice as strong at 572 MPa and is the aerospace standard for high-load parts, but it costs about 30 to 40 percent more, welds poorly, is less corrosion resistant and anodises to a duller, streakier finish. The rule is to default to 6061 and move to 7075 only when the mechanical load demands it.
Stainless steel is a family of trade-offs rather than one material. Type 303 is the free-machining grade and is the right choice for high-volume fittings and fasteners, Type 304 is the general corrosion-resistance grade for food equipment and welded assemblies at 515 MPa, Type 316 raises chemical and marine resistance at 579 MPa and roughly one extra cost tier, and 17-4 PH is precipitation hardening and reaches 1,170 MPa, which puts it in aerospace and oil and gas work. All of them work-harden, so the cutting strategy must take a real depth of cut rather than rubbing the surface, and all of them need coated carbide tooling.
Carbon and alloy steels are the value option while corrosion is controlled by coating. Grade 1045 is a straightforward medium-carbon shaft and gear steel, and 4140 and 4340 add alloying for toughness and wear resistance at the cost of slower cutting and heavier tooling. Titanium is the opposite end of the value spectrum: Ti-6Al-4V combines 950 MPa tensile with a density of 4.43 grams per cubic centimetre and excellent corrosion resistance, and in exchange it conducts heat poorly, work-hardens quickly and demands low cutting speeds, which is why titanium parts cost several times an equivalent aluminium part. Copper alloys split by function: C360 brass is the reference free-cutting material and is used for fittings, connectors and decorative work, while C110 copper is specified for electrical and thermal conductivity and machines gummily, raising a burr that has to be controlled.
The plastics question, and when PEEK is justified
Engineering plastics machine cleanly and are the right answer when the part must insulate, run without lubrication, resist chemicals or stay light. POM is the most machinable plastic, with high stiffness, low friction and excellent dimensional stability, which makes it the default for gears, bushings and fixtures. Nylon 6/6 is tougher and more impact resistant than POM but absorbs moisture and swells, so tight tolerances on a nylon part are a risk rather than a specification. ABS is an economical general-purpose material for housings and prototypes with no load duty.
PEEK is the material most often over-specified. It is genuinely necessary when the part sees sustained heat above roughly 150 degrees Celsius, aggressive chemicals, or a medical or semiconductor environment that requires the combination, and it holds up to about 250 degrees Celsius continuously. Outside those cases it costs roughly twenty to fifty times the material cost of a sane alternative, and it is harder to machine than POM because it needs sharp tooling and controlled temperatures. Plastics also move more than metals with moisture and heat, so the correct approach is to tolerance only the features whose function needs it.
The four questions that decide the material
The published selection guides converge on the same sequence of questions, and answering them in order avoids most specification errors. The first yes normally settles the family, and the grade then follows from cost and availability.
- Will it see weather, salt, chemicals or food contact? If yes, the family is 316 stainless, titanium or PEEK. If no, aluminium or steel is still on the table.
- Is weight critical, or is load the driver? For weight, 7075 aluminium is the inexpensive answer and titanium the best strength-to-weight answer. If neither matters, any metal fits.
- Must it conduct electricity, or must it insulate? Conductivity points to copper, brass or aluminium and often overrides everything else for electrical parts; insulation points to a plastic.
- What is the cheapest grade that passes the first three? That is usually 6061 aluminium, 1045 steel plated, or POM, and it is the answer unless a functional requirement rules it out.
Over-specifying is the most common material mistake
Three failure patterns show up repeatedly. The first is specifying titanium or PEEK for a part whose actual duty is indoor and light, which multiplies cost for a property nothing in the application uses. The second is combining a high-grade material with an ultra-tight tolerance because both feel like quality: every extra decimal place of tolerance adds inspection time and scrap risk, and the two cost premiums compound rather than overlap. The third is ignoring geometry limits that the material imposes: thin walls in aluminium deflect, deep threads strip, long slender parts deflect under cutting force above a length-to-diameter ratio of roughly 4 to 1, and a material that is hard to cut does not become easier because the drawing calls for it.
A practical rule is to write the function next to the material on the drawing. A note such as 316L because the part sees washdown chemicals is worth more to the supplier than a bare grade callout, because it lets the engineer confirm the real requirement rather than guess at it.
What to send with a material callout
Send the STEP model and a toleranced drawing, the material grade and its condition, the quantity with an annual forecast, the finish per surface, and a note naming the environment and the functional features. Those five items let an engineer confirm whether the specified grade is sufficient, whether a cheaper grade would do the same job, and which cutting strategy the material requires before a price is fixed. See CNC machining for how each family behaves in the cut, surface finishing for what can be applied after machining, and injection moulding when the same part in plastic at volume belongs in a mould rather than on a machine.
Scope and sources. Machinability ratings, tensile strengths, cost tiers and the share of part cost driven by material come from a CNC material selection guide (material choice influencing 35 to 60 percent of total cost and cycle time, aluminium 6061-T6 at 310 MPa and 9 out of 10 machinability, 7075-T6 at 572 MPa and 8 out of 10, stainless 303 at 620 MPa and 8 out of 10, 304 at 515 MPa and 6 out of 10, 316 at 579 MPa and 5 out of 10, 17-4 PH at 1,170 MPa, titanium Grade 2 at 344 MPa and 3 out of 10, Ti-6Al-4V at 950 MPa and 2 out of 10, brass C360 at 385 MPa and 10 out of 10, copper C110 at 220 MPa, PEEK at 100 MPa, POM at 70 MPa and nylon at 80 MPa, with grades below 40 out of 100 carrying a 75 to 150 percent machining premium, aluminium cutting 30 to 50 percent faster than steel, and each extra decimal place of tolerance adding 20 to 40 percent to cost). A second machinability scale expressed against brass as the 100 reference, plus grade-level detail, comes from a CNC milling material guide (aluminium 6061-T6 at 90 percent, 7075-T6 at 70 percent, stainless 304 at 45 percent, 316L at 40 percent, Ti-6Al-4V at 22 percent, brass C360 as the 100 percent reference, POM at 95 percent and PEEK at 55 percent, with 7075 costing about 30 to 40 percent more than 6061 and PEEK holding continuous service to about 250 degrees Celsius). Selection sequence, cost tiers and the PEEK rule of thumb come from a material selection matrix (a five-question sequence led by environment, then weight, then conductivity, then cost, with PEEK justified only above roughly 150 degrees Celsius or in aggressive or clean-room service, where it otherwise costs about twenty to fifty times a sane alternative). Tensile, yield, hardness and density values used in the grade notes also draw on a precision machined parts material table (6061-T6 at 310 MPa and 2.70 grams per cubic centimetre, 7075-T6 at 570 MPa, 1045 at 570 to 650 MPa, 304 at 520 to 620 MPa, 316 at 515 to 620 MPa, brass C360 at 400 to 450 MPa, bronze C932 at 275 to 345 MPa, copper C110 at 210 to 240 MPa and Ti-6Al-4V at 900 to 1,000 MPa and 4.43 grams per cubic centimetre, with a length-to-diameter limit of about 4 to 1). Figures are planning ranges and typical published values, not a quotation and not a certification of any mill lot; confirm the grade, condition and paperwork against your own specification.
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