What Materials Can Be CNC Machined?
Almost any solid, stable stock can be cut. The real work is picking the grade that survives the part's load, environment and cost target. This guide covers five material groups, the machinability trade-offs inside each one, and the cases where a grade is the wrong choice.
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The rule that decides whether a material can be CNC machined
A material can be CNC machined when it can be clamped without distorting and cut without melting, smearing or shattering. That is the whole test. Hardness alone does not disqualify a grade: hardened tool steel cuts fine on a rigid machine with the right insert. Softness does not guarantee easy cutting either, because gummy plastics and pure copper grab the tool and tear instead of shearing.
Three properties drive the decision. Thermal conductivity sets how fast heat leaves the cut zone, which is why aluminium tolerates high speeds and titanium does not. Ductility sets whether the chip breaks or smears, which separates 303 stainless from 304. Abrasiveness sets tool wear, which is the hidden cost in glass-filled nylon, carbon fibre and beryllium copper.
If a grade sits outside the window on any of these, it is still possible to cut, just slower and with more tool changes. That slower cut shows up in the price, not in a rejection notice. When we quote a job we state which of the three is driving the cycle time, so you can decide whether a different grade solves a cost problem or just moves it.
There is one hard boundary. Loose, friable or highly flexible stock will not hold tolerance. Foams crush under the vise, thin elastomers deflect away from the cutter, and green-state ceramics chip at the edges. Those parts belong in a molding or additive process, not on a mill.
Aluminium alloys: the default for most machined parts
Aluminium is the first choice for a large share of CNC work because it cuts fast and holds tight tolerance. 6061-T6 is the general-purpose grade: weldable, corrosion resistant, and stable enough for ±0.005 mm features on a rigid machine. 7075 offers roughly twice the yield strength of 6061 and machines to a better finish, which is why it shows up in aerospace brackets and racing components.
The trade-off is cost and corrosion. 7075 costs more per kilogram and its copper content makes it less corrosion resistant than 6061, so exposed marine parts should stay on 6061 or 5052. 2024 machines well and takes high fatigue loads, but it needs a protective finish. Anodizing is the usual answer, and it works on all of these grades.
Some alloys are made for casting, not cutting. ADC12 is a die-casting alloy, so we use it when a program is moving from cast prototypes to production. For thin walls below 1 mm, keep to 6061 and expect to slow the finishing pass.
Watch the heat treatment state. 6061-T6 machines cleanly; the same alloy in annealed condition gums up drills and produces a poor finish. If your drawing does not call out temper, we will ask before cutting.
Stainless and tool steels: where grade choice changes the outcome
Stainless steel is where grade selection has the largest effect on both price and quality. 303 is the free-machining grade, and it produces short chips and a good finish at high feed rates. 304 costs less per kilogram but work-hardens quickly, so a rubbing tool will harden the surface and dull on the next pass. 316 and 316L add molybdenum for chloride resistance and are the standard for medical and food-contact parts.
For wear parts, 17-4PH (SUS630) gives high strength after heat treatment and still machines in the annealed state. 440C takes a mirror polish and holds an edge, which suits shafts and valve components. 420 and 431 sit between the two on hardness and cost.
Carbon and alloy steels follow a similar logic. 1018 and 1045 machine easily and suit fixtures and general hardware. 4130, 4140 and 4340 are the chromoly grades: tough, heat-treatable, and used in high-load structures. They machine best in the normalized or annealed state, then go out for hardening.
Hardened stock changes the setup, not the feasibility. Above roughly 45 HRC we switch to ceramic or CBN tooling, take lighter depths of cut and accept longer cycle times on one of the 16 simultaneous 5-axis centers.
Copper, brass and titanium: conductivity against strength
Copper and brass cover two different jobs. C101 and C110 are the high-conductivity grades for busbars and heat sinks, but pure copper is gummy and needs sharp, polished tooling to avoid smearing. C36000 free-cutting brass is the opposite: fast, clean and dimensionally stable, which makes it the practical choice for fittings, connectors and small valve bodies.
Beryllium copper is a special case. It combines high strength with good conductivity, and it machines to a fine finish, but the dust is a health hazard. We machine it wet with containment and treat the chips as controlled waste. That handling is part of the cost, so only specify it when the electrical and mechanical requirements genuinely need it.
Titanium is where the physics gets difficult. TA1 and TA2 are commercially pure and reasonably machinable. TC4 (Ti-6Al-4V) has low thermal conductivity, so heat stays in the cut and the tool edge heats up. Cutting speeds drop to a fraction of aluminium, coolant flow must be high, and tool life is short.
The payoff is strength-to-weight ratio and corrosion resistance. If an aluminium part would work with a slightly thicker section, titanium is usually the more expensive path. Choose it when weight is fixed and the load is not.
Nickel alloys and magnesium: two ends of the risk scale
Inconel and similar nickel-based alloys are used for high-temperature service: turbine parts, exhaust components, heat-treatment fixtures. They work-harden severely, so a tool that rubs instead of cutting will destroy the surface in one pass. Speeds stay low, depths of cut stay shallow, and the part is often machined from a near-net blank to limit how much material gets removed.
Magnesium AZ31B and AZ91D are the lightest structural metals we machine. They cut fast and leave a good finish. The risk is fire: fine magnesium chips ignite easily, so the machine runs with a dedicated chip-management plan and no accumulated fines. We do not run magnesium jobs casually, and we will tell you when the part geometry makes the risk not worth it.
Both groups sit at the edges of the machinability window. Neither is a routine choice, and both are justified by the service environment rather than by the drawing alone.
If your application only needs moderate heat resistance, a 400-series stainless or a heat-treated alloy steel will usually cost less and machine in a fraction of the time.
Engineering plastics: where the failure mode is thermal, not mechanical
Plastics machine on the same equipment but fail differently. There is no chip to break; instead the material heats, softens and smears onto the tool. Sharp geometry and generous coolant or air blast matter more than spindle speed. POM and ABS are the forgiving grades and are used for housings, jigs and functional prototypes.
PC and PMMA are stiffer and optically clear respectively, but both are notch sensitive and can craze around a drilled hole if the feed is too aggressive. PMMA needs a finishing pass with a polished cutter to reach optical clarity. PA absorbs moisture, so dimensions shift after machining unless the stock is dried and the part is measured in a controlled state.
PEEK holds strength at high temperature and resists chemicals, which is why it appears in medical and semiconductor parts. It is also expensive and abrasive to tooling, so it is specified when the environment demands it. Carbon fibre reinforced grades wear carbide quickly; we plan for polycrystalline diamond tooling on longer runs.
Wall thickness drives everything in plastics. Sections below 1 mm deflect under clamping pressure, so we design soft jaws or vacuum fixturing before quoting rather than after the first parts fail inspection.
How material choice changes tolerance, finish and lead time
Material choice sets the achievable tolerance band before the machine does. Aluminium, brass and free-machining stainless hold ±0.005 mm on critical features in our shop. Titanium and nickel alloys hold the same band, but only with more passes and more inspection, because thermal growth and tool wear move the cut over the run. Plastics hold tolerance only when temperature and moisture are controlled during and after machining.
Surface finish follows the same pattern. We can reach Ra 0.2–0.8 μm on aluminium and brass with a finishing pass. On 304 or Inconel, Ra 0.8–1.6 μm is the realistic target without extra operations. As-machined finish on most grades lands at Ra 1.6–3.2 μm, which is fine for brackets and housings.
Lead time is less sensitive than most buyers expect, because we hold all five groups in stock. Quotation and DFM analysis come back within 12 hours, production can start within 24 hours, and parts ship in 3–5 days. The material that changes the schedule is the one we have to order, not the one that is hard to cut.
One habit saves money: pick the grade from the service conditions first, then let us optimize the cut. Choosing a harder grade than the part needs raises tool cost, cycle time and inspection effort without improving the part.
Machinability and best use by material group
Typical values from our production floor. Ranges vary with geometry and feature count.
| Material group | Relative machinability | Typical finish | Best suited to |
|---|---|---|---|
| Aluminium 6061-T6 | Excellent | Ra 0.2–0.8 μm | Brackets, housings, heat sinks |
| Aluminium 7075 | Very good | Ra 0.2–0.8 μm | High-strength aerospace and racing parts |
| Stainless 303 | Very good | Ra 0.8–1.6 μm | Shafts, fittings, fasteners |
| Stainless 304 / 316L | Moderate | Ra 0.8–1.6 μm | Medical, food-contact, marine parts |
| Alloy steel 4140 | Good when annealed | Ra 0.8–1.6 μm | High-load structural and tooling parts |
| Brass C36000 | Excellent | Ra 0.2–0.8 μm | Connectors, valve bodies, small fittings |
| Copper C110 | Fair | Ra 0.8–1.6 μm | Busbars and thermal components |
| Titanium TC4 | Difficult | Ra 0.8–1.6 μm | Weight-critical aerospace parts |
| Inconel | Difficult | Ra 1.6–3.2 μm | High-temperature turbine and exhaust parts |
| POM / ABS | Very good | Ra 0.8–1.6 μm | Functional prototypes, jigs, housings |
| PEEK | Moderate | Ra 0.8–1.6 μm | Medical and semiconductor components |
| Carbon fibre composite | Difficult | Ra 1.6–3.2 μm | Stiff, lightweight structural parts |
Which material to choose
If the part carries load in a normal environment, choose 6061-T6 or 4140 and spend the savings on tolerance. Choose titanium, Inconel or PEEK only when weight, temperature or chemical exposure leaves no alternative, because you pay for them in cycle time and tooling.
Material questions engineers ask
Can hardened steel be CNC machined?
Yes, within limits. Above roughly 45 HRC we use ceramic or CBN tooling, take shallow depths of cut and accept a longer cycle. Below that, carbide handles most tool steel and 440C without special setup.
In most programs it is cheaper to machine in the annealed state and send the part for heat treatment afterward, then grind only the critical surfaces. That sequence avoids hard-machining the whole geometry.
Why is 304 stainless harder to machine than 303?
303 contains sulfur, which makes the chip break cleanly and lowers cutting forces. 304 has no such addition, so it work-hardens under the tool. If the tool rubs rather than shears, the surface gets harder and the next pass is worse.
The result is a shorter tool life and a slower feed rate, not an impossible cut. If your part does not need 304's corrosion performance, 303 is the more economical grade.
Can CNC machines cut carbon fibre and other composites?
Yes, but the tooling plan changes. Carbon fibre is abrasive and wears carbide quickly, so longer runs move to diamond tooling. Dust must be extracted at the cut, both for worker safety and to stop the abrasive dust from reaching the machine ways.
Delamination is the main quality risk. We use sharp tooling and controlled feeds to keep the top and bottom plies from lifting, and the achievable finish is usually Ra 1.6–3.2 μm.
Do you machine magnesium?
Yes, AZ31B and AZ91D are both within our capability, but magnesium runs under a dedicated chip-management plan because fine chips ignite easily.
We will review your geometry first. If the part has deep pockets that trap fines, or if the application does not really need the weight saving, we will say so before quoting.
How close can you hold tolerance across different materials?
On aluminium, brass and free-machining stainless we hold ±0.005 mm ( ±0.0002 in ) on critical features.
Titanium, nickel alloys and engineering plastics can reach the same band, but they need more passes, temperature control and in-process checking. We confirm the achievable tolerance for your specific geometry during DFM review, before the quote is final.
What happens if I do not specify a material grade?
We come back with a question rather than an assumption. Grade and temper change machinability, corrosion behavior and heat treatment, so guessing risks a part that fits but fails in service.
If you have no preference, we will propose a grade based on the load, environment and finish you describe, and note the alternatives in the DFM analysis.
Send your drawing and material callout
Upload your files and we will return a quotation with free DFM analysis within 12 hours, including a machinability note on your chosen grade.
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More machining notes from the shop floor
We publish setup notes, tooling trials and inspection data from the factory floor.
