The performance of a CNC cutting tool strongly depends on what it’s made of. Tool materials must be harder than the workpiece material and capable of withstanding the heat and forces of cutting. Different tool materials offer trade-offs in hardness, toughness, heat resistance and cost.
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This article includes:
- High-speed steel (HSS)
- Carbide (Tungsten Carbide)
- Ceramics and cermets
- Cubic Boron Nitride (CBN)
- Polycrystalline Diamond (PCD)
High-speed steel (HSS)
Here we overview the common tool materials – high-speed steel, carbide, ceramics, PCD (and CBN) – and discuss when to use each, as well as the beneficial role of tool coatings like TiN or TiAlN. High-speed steel (HSS): HSS is a tool steel alloy containing elements such as tungsten, molybdenum and chromium. It was the workhorse material for cutting tools throughout much of the 20th century and is still widely used for drills, taps, reamers and some milling cutters.
HSS is tougher and less brittle than harder tool materials and it can withstand reasonable cutting temperatures (hence “high-speed” – it can be used at higher speeds than carbon steel tools, due to its ability to retain hardness when hot). HSS is a popular tool material known for its ability to retain hardness at high temperatures. It’s versatile and can be used for various applications like milling, drilling and turning.
Advantages
Advantages of HSS include its toughness (it can endure interrupted cuts or less rigid setups without chipping) and its relative affordability. It can also be ground to sharp edges easily and even re-sharpened multiple times.
Typically in lower speed operations, on older or smaller machines that can’t achieve the high speeds that carbide demands, or in situations requiring a very sharp cutting edge (HSS can be sharper than sintered carbide). For example, many job shops use HSS twist drills for general drilling of mild steel or aluminium because they are inexpensive and get the job done for moderate holes – or HSS reamers to finish holes with fine surface finish.
Some HSS tools are cobalt HSS (e.g. M35 or M42 grade) which contain cobalt for improved hot hardness. These “HSS Co” tools are used for tougher materials or slightly higher speeds.
Downsides
However, HSS loses its hardness if overheated and wears out faster, so in production environments it has largely been superseded by carbides and coatings. Still, for hobbyist or light industrial use and for tools like taps and dies (where some flexibility is needed), HSS remains common.
Carbide (Tungsten Carbide)
Carbide (tungsten carbide): Carbide tools are made of tungsten carbide particles cemented in a cobalt binder (often called cemented carbide). Carbide is significantly harder than HSS and can retain a sharp cutting edge at much higher temperatures, enabling much higher cutting speeds. Carbide tools are formed by binding tungsten carbide particles with a metal (usually cobalt). They’re known for their extreme hardness and ability to maintain sharp edges… with high wear resistance, suitable for high-speed machining.
In modern CNC machining, carbide has become the dominant tool material for milling and turning tools. Solid carbide end mills and drills can operate at spindle speeds and feed rates several times greater than HSS, greatly improving productivity. Carbide inserts on turning tools similarly allow cutting of hard metals and continuous use without quick dulling.
Advantages
Carbide has very high hardness (~90 HRA) which translates to excellent wear resistance (tools stay sharp longer), hot hardness (cutting edge remains hard even at red-hot temperatures, ~800°C or more, so you can cut faster or dry without coolant in some cases) and decent toughness for the harder grades.
Carbide tools often yield a better surface finish too, because they resist deformation and stay sharp. In UK manufacturing, carbide tooling is ubiquitous – from automotive factories using carbide drills and reamers in engine production, to precision subcontractors using carbide end mills for intricate milling.
Downsides
Carbide’s main downside is brittleness – it will chip or break if it encounters too much shock or is flexed. Thus, it performs best in rigid machines/setup and in continuous cuts. In applications like heavy interrupted cuts or on less stable machines, HSS or tougher insert grades might perform better to avoid chipping.
Carbide also is more expensive per tool than HSS (but often cheaper in the long run due to tool life).
When to use carbide
Essentially for production machining of metals – steel, cast iron, nonferrous – whenever the machine capability allows.
Carbide inserts come in different grades (with varying grain sizes and binder content) which balance hardness vs toughness and different coatings (discussed below) further boost performance.
Ceramics and cermets
Ceramic cutting tools are non-metallic, often made from aluminium oxide (Al₂O₃) or silicon nitride (Si₃N₄) based materials, sometimes with added carbides or other phases. Ceramic tools can be even harder than carbide and withstand even higher temperatures, but are also more brittle. They are typically used in the form of ceramic inserts in turning or milling. Ceramic tools are generally used for high-speed machining of hardened metals.
There are different types:
- Alumina-based ceramics (white or pink in colour often) which excel at continuous cutting of hardened steel or cast iron at very high surface speeds (they can handle heat but need a stable cut),
- Silicon nitride ceramics (usually black or gray) which are tougher and often used for roughing cast iron or superalloys. For example, in automotive production, ceramic inserts are used to turn cast iron brake discs at extremely high speeds (far beyond what carbide could handle) – the ceramic’s heat resistance allows cutting dry at, say, 1000+ m/min cutting speed, shearing the cast iron which actually helps because cast iron machining can generate heat that softens the material in the cutting zone, aiding the process.
Advantages
High heat resistance, suitable for high-speed operations and reduced wear in certain applications.
Downsides
Ceramic tools can also machine hardened steels (~60 HRC) in a process called “hard turning,” replacing grinding in some cases, as long as the cut is continuous (ceramics don’t like interrupted cuts on hard materials – they may fracture).
Another category related to ceramics are cermets (ceramic-metal) which typically involve titanium carbide/nitride with binders – these are used for finishing cuts at high speeds with very good surface finish on steels (they leave a very smooth surface and have good wear resistance, but can be brittle).
- Brittleness: Ceramics, while extremely hard and heat-resistant, are also very brittle. They are prone to chipping or fracturing, especially in interrupted cuts, impact-prone operations, or on less rigid machines.
- Limited toughness: Unlike materials such as high-speed steel or tougher grades of carbide, ceramic tools lack toughness, meaning they can’t absorb vibration or deflection well. This makes them unsuitable for unstable setups or roughing irregular parts.
- Poor performance in interrupted cuts: Ceramic tools require a stable, continuous cut to avoid fracture. Interrupted cutting (e.g. milling with gaps or turning parts with keyways) risks tool failure.
- Specialised application range: While ceramics excel at high-speed machining of hardened materials and cast irons, they aren’t suitable for general-purpose machining or softer, ductile materials like aluminium or plastics.
- Higher cost and risk: Though long-lasting under ideal conditions, ceramic inserts can be expensive and are not forgiving if misapplied – a small error in setup can result in rapid breakage and lost productivity.
- Cermets also share some brittleness: While they offer excellent surface finishes and wear resistance, cermets are also brittle compared to carbide and don’t tolerate heavy or interrupted cuts well.
Cubic Boron Nitride (CBN):
CBN is a synthetic superhard material, the second hardest after diamond. It comes typically as either tipped inserts or as solid CBN inserts. CBN is perfect for machining hardened steels and cast iron. CBN is second only to diamond in terms of hardness, effective for machining hardened steels and cast irons, with exceptional thermal stability and wear resistance.
You’ll find CBN inserts used in applications like hard turning (e.g. finish turning of a hardened gearbox gear at 58 HRC) or precision boring of hardened bearing seats. CBN can handle the heat and hardness where even carbide would wear out almost instantly. It’s also used for some cast irons where carbide gets chemical wear. CBN is expensive, so it’s usually reserved for when you truly need it (hardened materials).
A common use-case is, rather than grinding a hardened steel part, a CBN insert on a CNC lathe can turn it to final size and finish, reducing processing steps.
Advantages
Extreme hardness: CBN is the second hardest material after diamond, making it ideal for machining extremely hard materials, especially hardened steels and cast irons.
Exceptional thermal stability: CBN retains its hardness and cutting performance at very high temperatures, far beyond what carbide can handle. This allows for high-speed dry machining of hardened components without coolant.
Outstanding wear resistance: It offers a long tool life when used in the right applications, maintaining edge sharpness and part quality even in abrasive environments.
Improved productivity: CBN can replace grinding in some finishing operations – for example, hard turning with CBN can achieve tight tolerances and smooth finishes on hardened steel, reducing time and equipment needs.
Chemical resistance: CBN is resistant to chemical wear that affects carbide when machining certain cast irons and hard alloys.
Disadvantages
High cost: CBN tools are significantly more expensive than carbide or ceramic alternatives, both per insert and per edge.
Limited to hard materials: CBN is not suitable for soft materials (like aluminium or mild steel), as its advantages only apply when machining hardened ferrous metals.
Brittleness: Like ceramics, CBN is brittle and sensitive to shock. Interrupted cuts, poor setups, or unstable machines can cause chipping or breakage.
Niche applications: CBN’s high performance only justifies the cost in specialised or high-volume applications involving hardened steels (typically >45 HRC) or certain cast irons.
Not regrindable: Many CBN tools (especially tipped inserts) are single-use or difficult to recondition, so cost per part must be carefully justified.
Polycrystalline Diamond (PCD)
PCD tools are made from synthetic diamond particles sintered together, usually as a thin layer on a carbide tool or as a segment on an insert. Diamond is the hardest material, giving PCD tools extreme wear resistance – they can maintain a sharp edge for a very long time when cutting abrasive materials. PCD cutting tools are known for their exceptional wear resistance, making them ideal for machining composites. They offer long-lasting edges, which help counter surface finish issues and reduce the risk of delamination (in composites).
PCD is most often used for non-ferrous materials and non-metals: for example, machining aluminium alloys (especially those with high silicon content), graphite, fibreglass or carbon fibre composites, plastics and wood-based materials.
In these materials, PCD tools can last many times longer than carbide. A classic example: when machining abrasive carbon fibre composite panels for aerospace, a carbide tool might dull after a few parts, whereas a PCD tool can cut many more parts with consistent quality.
Another example is high-volume machining of aluminium engine components – PCD tipped milling cutters and reamers are often used because they can hold a sharp edge and tight tolerances for tens of thousands of holes or surfaces, whereas carbide would wear and size out more quickly. PCD provides an ultra-sharp edge which results in excellent surface finish on soft materials (like a mirror finish on aluminium) and avoids built-up edge due to its low affinity with those materials.
Important
PCD cannot be used on steel or other ferrous metals at high speeds – the carbon in the diamond can diffuse into the iron (or it simply gets very hot and can graphitize). Essentially, diamond has a chemical incompatibility with steel at cutting temperatures. So PCD is reserved for non-ferrous use. UK industries using PCD include automotive (for aluminium alloy wheels, engine blocks, etc.), aerospace (for composite components) and woodworking (PCD-tipped router bits for large production runs). Companies like Element Six (with operations in the UK) produce the PCD compacts that tool makers integrate into cutters. In summary, PCD tools offer extremely hard, long-lasting cutting edges for the right materials, albeit at a high cost per tool.
When to use each material:
- Use HSS for low to medium cutting speeds, in easier-to-cut materials or when you need a tough tool for interrupted cuts or a very sharp edge; common for drills, taps, small mills, especially in one-off jobs or on machines that can’t achieve carbide’s potential.
- Use carbide for the majority of production machining in metals – it’s the first choice for tooling on CNC mills and lathes for anything from mild steel to superalloys (with proper grade selection) because of its speed and wear advantages.
- Use ceramics/CBN for extremely hard or heat-intensive conditions – e.g. hard turning of hardened steel (CBN), high-speed dry cutting of cast iron or heat-resistant alloys (ceramics). These are more niche and often for very high volume or specialized processes, as they require stable conditions.
- Use PCD/diamond for non-ferrous, highly abrasive materials and when ultra-fine finish is needed on those materials – e.g. trimming composite parts, high-volume aluminium machining, etc.
Many cutting tool manufacturers offer hybrid or composite tools as well, like carbide tools with PCD tips (for example, a carbide end mill with PCD teeth for routing CFRP). The “etc.” in materials could also include things like coated carbides (we consider coatings separately next), or newer developments like carbide-enhanced HSS (powder metallurgy HSS) or even steel-bodied tools with indexable carbide/ceramic inserts, which combine a tough body with hard cutting edges.
Tool coatings
In addition to base material, coatings on tools play a huge role in modern machining. A coating is a thin hard layer (a few microns or less) applied to the surface of the tool, typically by PVD or CVD processes. Coatings can dramatically improve a tool’s performance by increasing surface hardness, reducing friction and providing thermal protection.
As a result, coated tools generally cut faster and last longer than uncoated ones. Some common coatings and their benefits:
1. TiN (Titanium Nitride)
TiN is a golden-coloured coating often seen on drill bits and milling cutters. It is a general-purpose hard coating. Benefits: increases surface hardness and wear resistance and it has a relatively high oxidation temperature (~600°C) which helps at higher cutting temps. It also provides a low friction coefficient, reducing cutting forces a bit. TiN is often considered an “entry” coating – it’s been around a long time and is used on HSS and carbide tools alike. It can roughly triple tool life compared to uncoated in many cases.
For example, TiN-coated HSS drills can drill more holes before dulling. It’s good for a broad range of materials (steels, stainless, cast iron, etc.) but newer coatings outperform it in specific areas.
2. TiCN (Titanium Carbo-Nitride)
TiCN is usually bluish-grey. It’s harder than TiN (the carbon addition increases hardness) and also has lower friction. It’s often used on taps, end mills and drills to boost wear resistance, especially in tougher steels or where a bit more tool life is needed over TiN.
However, TiCN has a slightly lower max operating temperature (~400°C) than TiN, so it’s not ideal for very high-temp cutting, but great for medium-speed operations requiring durability.
3. TiAlN / AlTiN (Titanium aluminium Nitride)
There are a few variants (TiAlN, AlTiN, etc., with different aluminium content). These coatings are usually dark purple or black. TiAlN family coatings excel at high-temperature performance. TiAlN stands out and is particularly adept at handling high-temperature operations, often seen in dry machining. Advantages: superb heat resistance, it forms a protective Al₂O₃ layer at the hot tool tip which insulates the tool and it’s excellent for high-performance applications.
In practical terms, TiAlN-coated tools can often run at higher speeds or without coolant because the coating prevents the tool from softening and reduces crater wear. It’s very common on solid carbide end mills used for steel and stainless steel at high speeds. AlTiN is a similar variant often optimized for even higher aluminium content, giving even better heat resistance.
These coatings generally require the heat to form that protective oxide, so they really shine in high-speed machining of steels, nickel alloys, etc. Under lower temperature conditions, TiAlN is still good, but one might prefer TiCN or others for lower cutting speeds. Many “high-performance” end mills marketed to aerospace or mould machining have AlTiN coatings to enable extreme machining parameters.
4. CrN (Chromium Nitride) and ZrN (Zirconium Nitride)
These are sometimes used for aluminium machining because they prevent built-up edge (aluminium sticking to the tool) while not having aluminium in the coating (TiAlN’s aluminium oxide can sometimes stick with aluminium workpiece – although often TiAlN works fine on aluminium, but ZrN is known as a great coating for aluminium specifically). ZrN is a goldish colour similar to TiN but paler.
5. Diamond coatings
Not to be confused with PCD (which is a bulk diamond segment on a tool), diamond-coated tools are typically carbide tools with a thin layer of polycrystalline diamond film deposited on them (often by CVD). These coatings are extremely hard and give amazing wear life when cutting abrasive materials like graphite, composites, or even abrasive aluminium alloys. They basically approach the performance of PCD, though the coating can be thinner and less durable than a solid PCD tip.
Diamond coatings are used a lot in machining graphite electrodes (for EDM), where tool wear needs to be minimal for accuracy. Also in cutting carbon fibre or PCB boards where the abrasiveness would destroy normal carbide quickly.
They are not used on steel because the diamond would react (just like PCD). A variant is DLC (Diamond-Like Carbon) coatings which are amorphous carbon coatings – these are very slick and hard, often used for aluminium machining to prevent edge build-up and give long life (though not as hard as crystalline diamond).
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