Cutting tool materials and coatings: choosing carbide grades, inserts and coatings that pay

A typical machining workshop spends a surprisingly small share of its costs on cutting tools, often a few per cent of the cost of a machined part. Yet tool choice has a large effect on the other costs: cycle time, set-up time, scrap, surface finish, machine utilisation and how often an operator must stop the machine. A tool that costs twice as much but lets the machine cut a third faster, or runs three times as long between changes, can reduce the cost per part substantially. Conversely, a cheap tool chosen by habit can quietly limit a whole production line.

The range of tool materials, grades, geometries and coatings is large and constantly evolving, and tool catalogues can be overwhelming. Underneath the detail, however, the logic is consistent. Different tool materials trade hardness and wear resistance against toughness. Coatings add wear and heat resistance at the surface. Standard systems classify workpiece materials and describe insert shapes. And the right choice is the one that gives the lowest total cost per good part on a particular machine, not the lowest price per tool.

This article explains, in general terms, the main cutting tool materials, the ISO workpiece material groups used to select grades, how indexable insert designations work, what coatings do, how tools wear and how to choose and manage tools on the basis of cost per part. It is general information. Tool suppliers’ recommendations should guide choices for specific materials, operations and machines.

What a cutting tool material must do

At the cutting edge, a tool faces high temperatures, pressures, abrasion and sometimes impact. A good tool material needs:

• Hot hardness: retaining hardness at the high temperatures generated in cutting;
• Wear resistance: resisting abrasion and chemical attack from the workpiece;
• Toughness: resisting chipping and fracture, especially in interrupted cuts; and
• Chemical stability: not reacting with the workpiece at cutting temperatures.

No material excels at all of these. Harder, more wear-resistant materials tend to be more brittle. Tougher materials wear faster. Tool selection is a series of trade-offs matched to the job.

The main tool materials

Cemented carbide deserves more detail, because it dominates modern machining. It consists of hard tungsten carbide grains bonded with a metal binder, usually cobalt. Finer grains and less binder generally give harder, more wear-resistant grades; more binder gives tougher grades that better resist chipping. Other carbides are sometimes added to improve hot hardness and resistance to cratering.

PCD is unsuitable for ferrous materials because carbon from the diamond reacts with iron at cutting temperatures, causing rapid wear. That is why diamond tools excel in aluminium but not in steel.

ISO material groups

Tool suppliers commonly classify workpiece materials into six groups, each identified by a letter and a colour code:

• P (blue): steels, including carbon and low-alloy steels;
• M (yellow): stainless steels;
• K (red): cast irons;
• N (green): non-ferrous metals such as aluminium and copper alloys;
• S (brown): heat-resistant superalloys and titanium; and
• H (grey): hardened steels and other hard materials.

Within each group, grades are often given an application number, such as P10 or P30. Lower numbers indicate grades suited to higher speeds and lighter, more continuous cuts, emphasising wear resistance. Higher numbers indicate tougher grades for heavier, interrupted or less stable cuts. Choosing the right group first, then the right position within it, is the starting point for grade selection.

Reading an indexable insert code

Indexable inserts are described using a standard code that captures their geometry. A common turning insert such as CNMG 120408 can be read as:

Suppliers add their own codes for chipbreaker geometry and grade. The nose radius affects both strength and finish: larger radii are stronger and can produce good finishes at higher feeds, but increase cutting forces and the tendency to chatter on slender parts. The chipbreaker controls chip formation and must suit the feed and depth of cut. Negative inserts are strong and economical, offering more edges; positive inserts cut more freely, suiting lighter cuts, slender parts and gummy materials.

What coatings do

Most carbide tools today are coated. Coatings are thin, hard layers applied to the tool surface that improve wear resistance, act as a heat barrier and reduce friction and adhesion.

Two broad coating methods are used:

• Chemical vapour deposition (CVD) produces relatively thick, very wear-resistant coatings, often including layers of titanium carbonitride and aluminium oxide. CVD coatings are common for turning steels and cast irons at higher speeds.
• Physical vapour deposition (PVD) produces thinner coatings that preserve sharper cutting edges, suiting milling, drilling, finishing, stainless steels and superalloys. Common PVD coatings include titanium nitride, titanium aluminium nitride and aluminium chromium nitride.

For aluminium and other non-ferrous materials, very smooth or uncoated polished tools, or diamond-like carbon coatings, help prevent material sticking to the edge.

How tools wear

Recognising wear patterns helps diagnose problems:

• Flank wear: gradual abrasion of the clearance face, the normal, predictable form of wear.
• Crater wear: a hollow on the rake face caused by heat and chemical reaction, common at high speeds in steel.
• Notch wear: localised wear at the depth-of-cut line, often from work-hardened surfaces or scale.
• Built-up edge: workpiece material welding to the edge, common at low speeds in soft, gummy materials.
• Chipping and fracture: from impact, vibration, excessive feed or a grade that is too brittle.
• Thermal cracking: cracks perpendicular to the edge from repeated heating and cooling in interrupted cuts.
• Plastic deformation: the edge softening and deforming under excessive heat and pressure.

Each pattern suggests a response: a tougher grade for chipping, a more wear-resistant grade or lower speed for rapid flank wear, a higher speed or sharper geometry for built-up edge, and so on.

Tool life and cutting speed

Tool life falls as cutting speed rises, usually steeply. A classic relationship, Taylor’s tool life equation, expresses tool life as a power function of cutting speed. Its practical message is that small increases in speed can significantly reduce tool life, while small reductions can extend it greatly. The economic optimum balances machine time against tool cost and the time spent changing tools.

Coolant and tool life

Cutting fluids interact with tool materials in ways that are easy to overlook. Flood coolant removes heat, flushes chips and lubricates the cut, which generally benefits HSS tools, drilling, tapping and many operations in stainless steel and aluminium. Through-tool coolant is especially valuable for drilling and deep features, because it delivers fluid exactly where heat and chips are generated.

Carbide and ceramic tools are more sensitive to thermal shock. In interrupted cuts such as face milling, intermittent cooling can cause rapid heating and cooling of the edge, leading to thermal cracks. For that reason, some milling operations in steel are run dry or with air blast or minimum-quantity lubrication, following the tool supplier’s guidance. Ceramics in particular are usually run dry.

Coolant condition matters as much as coolant choice. Wrong concentration, contamination with tramp oil and bacterial growth reduce lubrication, increase corrosion and can create health issues for operators. A simple routine of measuring concentration, skimming oil and monitoring fluid condition protects both tools and people.

Choosing tools on cost per part

The useful measure is total cost per good part, which includes:

• machine and labour time per part, driven largely by cutting speed and feed;
• tool cost per part, the cost per edge divided by parts per edge;
• tool change time per part;
• scrap and rework linked to tool performance; and
• the effect on finish, accuracy and downstream operations.

Because machine time usually costs far more than tooling, a premium tool that allows faster cutting or longer, more predictable life often reduces total cost. The designing parts that are economical to machine article explains how machine time drives the cost of machined parts.

Managing tools well

• Standardise. A preferred range of inserts, grades and solid tools simplifies stock, programming and setting.
• Record performance. Track parts per edge or tool life for important jobs, so changes can be judged by evidence.
• Trial properly. Test a new tool on the same job, machine and conditions as the current one, and compare cost per part, not tool price.
• Control stock. Keep enough tools to avoid stoppages, with a simple reorder system.
• Consider regrinding. Solid carbide drills and end mills can often be reground economically.
• Use suppliers’ expertise. Tool suppliers’ application engineers can often recommend grades and geometries and support trials.

A worked example

This is an illustrative example. The figures are hypothetical.

A small manufacturer turns 316 stainless steel shafts, about 1,800 a month. It has used a general-purpose steel-grade insert costing $18 with four usable edges. Each edge lasts about 25 parts, and edges often chip, occasionally scrapping a part.

Tool cost per part is $18 ÷ 4 edges ÷ 25 parts, or $0.18. Each edge change takes about 3 minutes; at a charge-out rate of $90 an hour, that is $4.50 per change, or $0.18 per part. Tooling plus changes therefore cost about $0.36 per part.

The supplier recommends an M-group grade with a PVD coating and a sharper, positive chipbreaker suited to stainless steel, costing $24 per insert with four edges. In a trial, each edge lasts about 70 parts, chipping stops and the cutting speed can be increased slightly, reducing cycle time by about 10%.

The new tool cost per part is $24 ÷ 4 ÷ 70, or about $0.086, and change cost per part is $4.50 ÷ 70, or about $0.064, for a total of about $0.15 per part, compared with $0.36. At 1,800 shafts a month, that saves about $380 a month on tooling and changes alone, before counting the shorter cycle time and the scrap that no longer occurs. The more expensive insert is clearly cheaper per part.

How this applies to a small Australian business

• Measure cost per part, not price per tool.
• Start with the ISO material group and the operation, then refine with supplier advice.
• Read the wear. Wear patterns tell you what to change.
• Standardise and record. A preferred tool list and simple life records make improvement possible.
• Trial fairly. Compare tools under the same conditions over enough parts to be meaningful.
• Use supplier support. Application engineers can save weeks of trial and error.

Common mistakes

• Buying on insert price rather than total cost per part.
• Using one general grade for every material.
• Ignoring wear patterns and simply changing tools more often.
• Running too fast or too slow without understanding the effect on tool life and built-up edge.
• Using diamond tools on steel or other mismatches of tool and workpiece chemistry.
• Uncontrolled tool variety, which complicates programming and stock.

Frequently asked questions

Is coated carbide always better than uncoated? Usually for steels, cast irons and stainless steels. For aluminium and some non-ferrous materials, sharp uncoated or specially coated tools may perform better because they resist material sticking.

When should we use HSS instead of carbide? For less rigid machines, manual work, some taps and form tools, and situations where toughness and ease of resharpening matter more than speed.

Questions to ask

• What is our total cost per part for the jobs that use the most tooling?
• Are we using grades matched to each material group?
• What do the worn tools from our main jobs show?
• How many different inserts and grades do we stock, and could we standardise?
• When did we last trial a better tool on a high-volume job?

Bringing it together

Cutting tool materials span a range from tough high-speed steel to extremely hard CBN and PCD, with cemented carbide at the centre of modern machining. ISO material groups guide grade selection, insert codes describe geometry, and coatings add wear and heat resistance. Tools wear in recognisable ways that point to the right response. Above all, tool choice should be judged by total cost per good part, which usually means that the right tool, matched to the material and operation and proven in a fair trial, pays for itself many times over.

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Source: KEVOS editorial notes. Examples and figures in this article are illustrations. Always follow tool suppliers’ recommendations for specific materials, operations and machines.