In any workshop's tool drawer, materials twenty years apart in technology sit side by side. Knowing what each one brings avoids two expensive mistakes: using cheap tooling where it cannot hold up, and paying for high-performance tooling the machine cannot exploit.
High-speed steel (HSS)
It is steel alloyed with tungsten, molybdenum, chromium and vanadium, hardened to a high hardness. It holds up to around 550–600 °C before it loses its edge, which limits cutting speed to low values: of the order of 20–40 m/min in steel.
Its advantage is toughness. HSS bends before it breaks, which in practice means it forgives. It forgives a machine with backlash, a badly clamped part, an abrupt entry, an interrupted cut. It is still the right choice for:
- Old or less rigid machines, and manual mills.
- Small-diameter drills, where carbide's brittleness is a liability.
- Taps, outside serious production.
- One-off parts where machine time is not what matters.
HSS-Co (with 5–8% cobalt, sometimes marked HSSE) takes more heat and is the version to reach for in stainless if you have to work with high-speed steel.
Carbide
It is not steel: it is a composite, tungsten carbide grains sintered with cobalt as the binder. It is far harder than HSS and takes 900–1,000 °C, which allows cutting speeds five times higher or more.
The price it pays is brittleness. Carbide does not bend: it chips. It needs a rigid machine, good workholding and steady parameters. On a machine that vibrates, a carbide cutter lasts less than an HSS one, and costs five times as much.
The cobalt content sets the character: more cobalt is tougher and less hard (roughing, interrupted cutting); less cobalt is harder and more brittle (finishing, abrasive materials). It is why manufacturers have dozens of grades of what looks at first glance like just "carbide".
In solid milling — solid end mills — carbide is now the standard. In turning it is almost always used as an indexable insert, which is cheaper per edge and lets you change without losing the tool's setting.
The ones above it
- Cermet: titanium carbide based. Very good on fine finishing in steel, poor toughness. A narrow niche.
- Ceramic: takes more than 1,200 °C and allows enormous speeds, but tolerates neither impact nor, in many cases, coolant. Its territory is grey cast iron and nickel superalloys (Inconel), almost always dry.
- CBN (cubic boron nitride): the second hardest material there is. It is the material of hard machining: steels hardened above 45 HRC, where it replaces grinding and saves a whole operation.
- PCD (polycrystalline diamond): unbeatable in aluminium, copper, graphite and composites. It cannot be used on steel: at cutting temperature the carbon in the diamond reacts with the iron and the tool is consumed.
The coatings
A coating is a layer a few microns thick deposited on the carbide. It does not change the core of the tool; it changes what happens at the contact surface, and that is where the life of the edge is decided.
- TiN (gold, the classic): hard and general purpose, moderate temperature resistance. Today it is almost always the basic option.
- TiCN (blue-grey): harder than TiN and very abrasion resistant. Good in low-alloy steel and cast iron.
- TiAlN / AlTiN (dark violet or black): the current workhorse. As it heats it forms a surface layer of aluminium oxide that thermally insulates the edge, so it works better the hotter it runs. It is the choice for steel at high speed and for dry or MQL machining.
- AlCrN: the next step on, more stable still at temperature. Widely used in stainless and in hard steels.
- DLC (diamond-like carbon): a very low coefficient of friction. Made for materials that stick: aluminium, copper, plastics.
- CVD diamond: a real diamond layer. For graphite, fibre composites and high-silicon aluminium. Never on steel.
Two details almost nobody mentions: a coating does not fix an unsuitable geometry — a coated four-flute cutter is still a bad idea in aluminium — and a coated tool usually has a slightly less keen edge than the same tool uncoated, because the layer rounds the edge by a few microns. On very fine aluminium finishing that matters.
How to decide in two minutes
A mental order that works:
- What is the machine like? If it has backlash or chatters, HSS or carbide with a tough geometry. There is no point buying performance the machine cannot use; the causes of vibration are in vibration and chatter.
- What material? Aluminium and non-ferrous, sharp geometry and DLC or PCD. Steel, carbide with TiAlN. Stainless, AlCrN and a keen edge. Hardened, CBN.
- How many parts? One part, whatever tool you already have. A thousand parts, the tool that cuts the cycle time, because the machine time saved pays any difference in tool price.
- Continuous or interrupted cut? If there are slots, holes or entries and exits, raise the toughness and lower your speed expectations.
If you are putting work out to a supplier, this explains why two quotes can differ so much: a shop with a modern tool crib and a rigid machine does in twenty minutes what another takes an hour over. In the directory you can see which processes and materials each shop declares, and the glossary defines the terms used in this article.
Need a part machined? The directory of Spanish machine shops lists more than 700 workshops across Spain with their processes, materials and certifications verified, filterable by province. And if you are learning, the CNC Path takes you from G-code to your first part.