CNC Machining Materials: A Machinability Reference by Family
Ask a machinist which material is “best” and you will get a question back: best at what? Aluminium cuts beautifully and is light but soft. Steel is strong but cuts a fraction as fast. Titanium is the aerospace darling and the shop floor’s nightmare in one material. Plastics machine easily until the day they melt, flex or string. There is no best material — there are trade-offs, and the trade-offs are surprisingly consistent once you understand why a material is easy or hard to cut.
That “why” is machinability. This guide is the family-by-family reference of the materials topic: it explains how to read a machinability rating without being misled, the physical reasons materials behave so differently under a cutter, the main families of metals and engineering plastics you will actually machine, and how to turn a part’s requirements into a shortlist. It is deliberately the data companion to our guide to setting machining parameters — that page covers the formulas and the method, this one covers the materials the formulas are applied to. If you need the wider process picture first, start with what CNC machining is. Term definitions are in the CNC glossary.
What a machinability rating actually means
Walk onto any shop floor and you will hear a material described as having a machinability “rating” — aluminium around 270, mild steel around 70, titanium in the low twenties. It sounds like a material property, like hardness or density. It is not. It is a relative index, and misreading it as an absolute number causes most of the confusion around this topic.
The convention across cutting-tool and engineering references is to assign free-machining steel a baseline of 100, and express every other material as a percentage of how the two behave under identical tooling and tool-life conditions. A rating above 100 means the material is easier to machine than that baseline — you can cut it faster for the same tool life. A rating below 100 means it is harder. So aluminium at 200-plus is not “twice as good” in any engineering sense; it simply machines at roughly double the speed of the baseline steel before the tool wears the same amount.
Three caveats matter before you use any number:
- Ratings describe one condition, not the material. Most published figures assume the annealed (softest practical) state. Heat treatment is the single biggest swing: the same steel that rates near 70 annealed can fall by roughly half once hardened. Always ask what condition the rating assumes.
- Ratings disagree between sources. Different toolmakers, charts and texts quote different figures for the same grade because the number depends on the tooling assumed, the test and the cutting conditions. Treat any published value as a band and a starting direction, never as a precise constant.
- Ratings assume typical tooling. They are comparative, not absolute speeds — the underlying numbers move with the tool. Carbide cuts several times faster than high-speed steel, and coatings shift the practical window again, but the ranking between materials survives: a carbide cutter does not make titanium easier relative to aluminium, it just makes both faster.
The practical way to use a rating: it tells you the direction the parameters must move compared with a material you already know. A rating of 70 means start meaningfully slower than your free-machining baseline; a rating of 270 means the ceiling is usually your spindle, not the material. The precise numbers still come from the toolmaker’s data and your own tuning — see the parameters guide for that method.
Why materials are easy or hard: the physical levers
Hardness is the least useful predictor of difficulty. What actually decides how a material behaves under a cutter is a handful of physical behaviours, and the same few levers explain almost every family:
Heat — where it goes. When a tool cuts, the energy of the cut becomes heat, and that heat goes somewhere: into the chip, into the work, or into the tool. Materials that conduct heat well (aluminium, copper, mild steel) carry most of it away in the chip, so the cutting edge stays cool and can run fast. Materials that conduct heat poorly (titanium conducts a small fraction of what steel does; nickel alloys worse still) leave the heat at the cutting edge — the tool burns while the material stays cool. This single lever is why “light” titanium and “tough” nickel are forced to run so slowly: it is a fight about heat, not hardness.
Chip form. Easy materials produce chips that break and clear cleanly. Difficult ones produce the pathologies — long stringy birds’-nests (many steels and plastics), torn and ragged edges (austenitic stainless), or fine dust that means the tool is rubbing instead of cutting. Chip form decides whether the tool keeps cutting or spends its time re-cutting its own swarf.
Work hardening. Some materials — stainless steels, nickel alloys, and to a degree titanium — harden at the surface the moment it is cut, deformed or merely rubbed. A tool that dwells, rubs or loses its edge in one of these materials instantly hardens the very surface it must next cut, and each pass gets harder than the last. This is why the rule for these alloys is keep the edge engaged with a real chip at all times.
Chemical affinity and built-up edge. Soft, gummy materials such as aluminium and mild steel have an annoying habit: fragments of the work weld onto the cutting edge and form a built-up edge that changes the effective geometry and ruins finish. Titanium and some nickel alloys go further and gall — they weld to the tool under pressure and heat. This is why edge preparation, positive rake and the right coating matter as much as the speed.
Abrasiveness. A few materials are just hard on tools regardless of cutting speed: glass-filled plastics, some composites, and hardened or high-carbide tool steels wear the edge mechanically. For these, tool material and coating choice — not speed — is the primary lever.
Keep these five levers in mind and the table below stops being a list of arbitrary numbers and becomes predictable. Aluminium is easy because it conducts heat away and cuts in clean chips — its only vice is gumminess. Stainless is hard because it work-hardens and holds heat. Titanium combines low conductivity, springiness and reactivity. Nickel takes stainless’s bad habits and multiplies them.
The metal families, hardest to easiest
The table gives approximate relative machinability bands for the families you will meet, on the free-machining-steel-equals-100 convention and in the annealed or as-supplied condition. Treat them as bands, not constants — and remember the index orders the direction of your parameters, it does not set them.
| Material family (typical grades) | Machinability* | Cutting character | What fights you |
|---|---|---|---|
| Aluminium alloys (6061, 7075, 2011) | 200–300 | Very fast | Gumminess and built-up edge at the wrong edges |
| Free-cutting brass & copper alloys (C360) | 200–300 | Very fast | Almost nothing — the easiest material class overall |
| Free-machining steel (1215, 12L14) | 130–170 | Fast | The baseline — deliberately easy |
| Mild / low-carbon steel (1018, 1020, A36) | 60–80 | Moderate | Built-up edge at low speeds; stringy chips |
| Medium-carbon & alloy steel (1045, 4140, 4340, annealed) | 40–70 | Moderate | Higher strength; heat-treatment distortion if hardened after |
| Tool steel (O1, A2, D2) | 25–50 | Slow | Hardness and abrasion; cut annealed, heat-treat after |
| Stainless steel (303 free-cut; 304, 316; 17-4PH) | 35–80 | Low–moderate | Work hardening, heat retention, torn chips |
| Titanium (commercially pure; Ti-6Al-4V) | 20–35 | Very slow | Low conductivity, springiness, chemical reactivity |
| Nickel superalloys (Inconel 718, Hastelloy) | 10–20 | Very slow | Hot strength, extreme work hardening |
*Approximate index relative to free-machining steel = 100, annealed/as-supplied. Published values vary by source and condition; read as bands.
The grades within each family are worth knowing, because they are not interchangeable:
Aluminium. 6061 is the universal default — excellent machinability, moderate strength, corrosion resistance, cheap to machine. 7075 is meaningfully stronger for aerospace-style brackets at the price of finish and anodising behaviour. 2011 and 2024 lean further toward cutting speed at the price of corrosion resistance or strength. All of them machine fast; the differences are engineering, not difficulty.
Steel. 12L14 and 1215 are the free-cutting grades for turning and light milling — the closest steel comes to aluminium. 1018 is the everyday low-carbon structural choice. 1045 and the alloys (4140, 4340) bring real strength and toughness, and are usually machined annealed then heat-treated — which is where the distortion risk enters. Tool steels (O1, A2, D2, and above) are bought for wear resistance and cut in the soft state whenever possible.
Stainless. The 300-series austenitic grades (304, 316) are the work-hardening, heat-retaining ones that punish rubbing. 303 is the free-machining variant, with a small sulphur addition that makes it far friendlier — the standard choice for stainless turned parts. 17-4PH and other precipitation-hardened grades are strong and corrosion-resistant and get harder to machine as they are aged. When a drawing says “stainless” without a grade, the machining question is which stainless.
Titanium and nickel. Grade 2 commercially pure titanium is the approachable end — good corrosion resistance, noticeably easier than the alloys. Ti-6Al-4V (Grade 5) is the aerospace workhorse and the difficult one: slow speeds, rigid setups, high-pressure coolant and constant engagement are the price of its properties. The nickel superalloys such as Inconel 718 exist to hold strength at red heat, and they extract the full penalty for it in machining — the hardest common materials a shop will meet.
The engineering plastics and non-metals
Plastics are not “cheap metal.” They have their own physics — low melting points, elastic deflection under load, stringy or powdery chips, and sensitivity to clamping force — and their own hierarchy. On the machinability scale most of them sit somewhere friendly, provided the tool is sharp and the heat is low enough that the material never melts or smears.
| Material | What it is for | What to watch |
|---|---|---|
| Acetal (POM/Delrin) | Precision gears, bushings, general parts | The “aluminium of plastics”: stiff, stable, slippery, machines beautifully — the default engineering plastic |
| Nylon (PA) | Wear parts, gears | Absorbs moisture — parts can move after machining; machine and store dry for tight work |
| ABS | Enclosures, prototypes, consumer parts | Cheap and easy; low melting point, so keep speeds sensible |
| Polycarbonate (PC) | Impact, transparent parts | Stress-cracks easily; needs dry material and sharp tools |
| Acrylic (PMMA) | Optical, display parts | Brittle — chips out at edges on exit; polish-cut the final pass |
| PTFE (Teflon) | Seals, low-friction, chemical | Floppy and gummy — deflects under the tool; climb-cut and hold it well |
| PEEK | High-performance metal replacement | Excellent heat and chemical resistance; premium cost, cuts cleanly when held rigidly |
| PEI (Ultem) | Electrical, aerospace interiors | Stiff and heat-resistant; good machining, higher cost |
| Glass-filled / G10 / FR4 | Boards, insulators, structural laminates | The abrasive exception — wears tools fast whatever the speed |
| UHMW / PP / HDPE | Cheap chemical-resistant parts | Soft and slippery — easy to smear; sharp tooling and light cuts |
The machining rules for plastics are consistent. Use sharp tools with positive rake — a dull edge melts instead of cuts. Control heat, because most engineering plastics soften far below the temperatures a metal happily ignores; for thin or large sections that may mean coolant or reduced speeds rather than flood-and-forget. And respect that plastics flex: they deflect away from the cutter, so they need full workholding support and light finishing passes, or the part moves and the dimensions wander. Clamp them hard enough to hold, gently enough not to crush.
Choosing a material for your part
Machinability is only half the decision. The other half is whether the material does the job — and the honest order is to let the function pick the shortlist, then let machinability and cost pick within it:
- Strength-to-weight: aluminium 7075, or titanium when the requirement is extreme.
- Corrosion resistance: 316 stainless, aluminium, commercially pure titanium.
- High temperature: the nickel alloys and titanium among metals; PEEK and PEI among plastics.
- Wear resistance: hardened or alloy steel; nylon and PEEK on the plastic side.
- Low friction: brass, or the bearing plastics — acetal, nylon, PTFE.
- Electrical / thermal insulation: the engineering plastics generally.
- Food or medical contact: 316 stainless, or PEEK for a metal-free alternative.
Cost enters last, and it is worth being clear-eyed about the pattern because it is consistent: for most of these families, machining cost scales with difficulty, not with raw material price. Aluminium and acetal are cheap to machine because they cut fast and easy. Titanium and the nickel alloys are expensive to machine because they cut slowly and wear tools — the material price is real but the machining penalty usually dominates. Stainless costs more to machine than mild steel for the same reason. This is why material selection is inseparable from the machine-buying question — the capability floor (spindle character, rigidity) is set by your hardest material, which is exactly the lens our guide to choosing a machine uses — and why it shows up as a cost driver in the cost guide.
Two defaults are worth internalising because they are right so often: when nothing in the application demands otherwise, 6061 aluminium is the default metal — it is easy to machine, light, corrosion-resistant and modestly priced. When nothing demands otherwise on the plastic side, acetal is the default — it is dimensionally stable, slippery, and cuts like a dream. Every “which material should I use” question should start from one of those two and move only when a real requirement justifies it.
From machinability to parameters
Once you know your material’s difficulty, the direction of every parameter follows — and that is where this guide hands off. High-machinability materials let you chase material removal rate; low ones force slower cutting speeds, disciplined engagement and constant cutting. The machining parameters guide takes the material’s character and turns it into spindle speed, feed and depth of cut, with the recipes for roughing, finishing, thin walls and difficult alloys. Read the two together: this page tells you which end of the difficulty scale you are on, that page tells you what to do about it.
Frequently asked questions
What does a machinability rating above or below 100 mean? Free-machining steel is set to 100 as the baseline. Above 100 means easier to machine than that baseline — you can run faster for equal tool life (aluminium is in the 200s); below 100 means harder (titanium is in the 20s). It is a relative index, not an absolute property: it varies with heat-treatment condition, tooling and source, so treat any figure as a band that sets your starting direction, not a constant.
Which material is easiest to CNC machine? Aluminium alloys and the free-cutting brasses are the easiest metals — high machinability, fast cutting, easy chips — with 6061 the default because it balances that ease with strength and corrosion resistance. Among plastics, acetal (POM) is the friendliest engineering plastic. The “easiest” material is always a trade: you are choosing ease, and giving up whatever strength or behaviour the harder material would have bought.
Why is titanium hard to machine if it is lighter than steel? Because the difficulty was never about weight. Titanium conducts heat very poorly, so the cutting heat stays at the tool edge instead of leaving in the chip; it is springy, so it deflects; and it is chemically reactive, so it wants to weld to the tool. The result is slow speeds and aggressive tool wear — the difficulty is thermal and chemical, not hardness.
Aluminium or steel for my part? If strength and stiffness at low weight matter and the part is a bracket, housing or enclosure, aluminium is usually right. If the part carries real load, must be hard or wear-resistant, or needs the stiffness of steel at thicknesses where aluminium would flex, steel is right. And if you are genuinely unsure which family your parts need, the machine choice follows the material — the machine-selection guide starts from exactly this question.
Why do different charts give different ratings for the same material? Because a machinability figure bundles assumptions — the tool material, the cutting condition, the material’s heat-treatment state — and references differ in all of them. This is normal and expected. Use ratings to rank materials against each other and to set a starting direction, and get your actual numbers from the cutting-tool maker for the specific tool, then tune from the chips. Our parameters guide is the method for that tuning.
Bottom line
Machinability is not a scoreboard of good and bad materials; it is a relative measure of how a material behaves under a cutter, driven by a few physical levers — where the heat goes, how the chips form, whether the material work-hardens, how it reacts with the tool, and how abrasive it is. Learn to see those levers and the whole difficulty spectrum becomes predictable, from free-machining aluminium at one end to the nickel superalloys at the other, with the engineering plastics on their own axis governed by heat and flexibility. Let function pick your shortlist, let machinability order it, and default to 6061 aluminium or acetal until a requirement justifies otherwise. Then hand the material’s character to the parameters method and let the chips confirm it.
This guide is part of the CNC Media guides library — a neutral reference for the shop floor, deliberately free of prices and of any single supplier’s material to push. What matters is the framework, and the framework does not go stale.