Cutting edge

Cutting inserts are the modern alternative to high-speed steel tools for metal cutting. Used for milling and turning, they take the guesswork out of planning how long a tool will last. A stock of inserts will give a calculable performance, a range of cutting edges and, in recent years, the potential to last longer with […]

Cutting inserts are the modern alternative to high-speed steel tools for metal cutting. Used for milling and turning, they take the guesswork out of planning how long a tool will last. A stock of inserts will give a calculable performance, a range of cutting edges and, in recent years, the potential to last longer with high-tech new coatings.

In the past 15 years, they have taken at least half the market. Made of tungsten carbide and conventionally coated with titanium nitride, cutting inserts have replaced high-speed steel in most tasks except drilling. Inserts are conventionally too big to be fitted to the small edges of drills.

The factors driving the use of these are duration and speed. A manufacturer can save money by making tools last longer, or work faster. In both cases, inserts need to be harder, and this is where development effort is concentrated.

Cutting inserts have to face onerous conditions. Friction between the cutting edge and the workpiece can generate temperatures of 1000iC.

Mass market expansion has led insert manufacturers to develop products that suit more closely the material being machined. And behind closed doors of research labs, engineers are experimenting with exciting new materials and techniques which could push productivity and tool life into new realms.

Typically, inserts break because they have worn out from friction from the workpiece. In addition, chips of waste metal collect around the insert and try to pull away the coating.

Pressures to increase speed and feed rates tempt staff to push their inserts to the maximum, but every time one breaks, the machine must be stopped and refitted.

Improved performance requires harder inserts which in turn can be achieved in three ways: a more robust base substrate of tungsten carbide; harder surface coatings; and sophisticated cutting edges that remove waste chippings quickly. Cutting tool manufacturers have tried to improve all three.

For years, the big tooling manufacturers – Sandvik Coromant, Kennametal Hertel and Seco Tools – have offered one type of insert designed to machine steel. Stainless steel or cast iron components had to be machined using inserts designed for steel and only slightly modified in geometry or coating to cater for the former’s stickiness and the latter’s abrasiveness.

`During the recession, to reduce inventories, people wanted one grade that could cut all metals’ says Les Thornes, a specialist in milling tools at Sandvik. `People have now got to be more competitive so we try to offer the best solution with two or three grades.’

From last year, users could buy inserts with the substrate specified for the material in hand, making inserts last longer and reducing machining problems.

Martin Pollard, product support specialist at Sandvik in Halesowen, says: `Nodular cast iron can’t have a thick coating on its insert, because it pulls the coating away. But grey cast iron has to have a thick coating because of the effect of abrasive wear.

`But thick coatings can’t be applied to sharp edges because they will come off.’

Sandvik is soon to bring out a line of inserts for cast iron to follow the stainless steel versions it put out last September.

Increased feed rates

Sterling Tubes, the Walsall-based stainless steel tube manufacturer which is part of Sandvik, has used Sandvik’s new tools in stainless steel and seen rapid improvements in productivity. In its manufacture of stainless steel billets to prepare for extrusion, it has found the feed rates in machining stainless steel 28% faster, and the insert lasting 50% longer.

One insert now lasts the length of machining time for one component, instead of only 75%. It means that the machine only stops once, after machining the complete component, instead of having to stop and replace the insert part way through.

`With our previous equipment, if you increased the feed rate, the insert would break,’ says Andy White, Sterling’s extrusion production manager. `The new inserts give us extra life, and we now use the extra time to work on other areas.’

Manufacturers harden the substrate by using a finer grade of tungsten carbide. Held together in a matrix of cobalt, this gives a better bond and greater rigidity, and can machine at higher speeds for longer.

A bigger problem is making the coating of titanium nitride attach more effectively to the insert.

Development work on coating techniques has improved the bonding. Manufacturers are experimenting with chemical vapour deposition (CVD), a coating technique which has been around for 15 years. In this process the coating material is deposited on the substrate from a plasma at temperatures of around 1000oC.

A more recent development is moderate temperature CVD, which takes place at lower temperatures.

Physical vapour deposition (PVD), requires lower temperatures and a less complicated process.

American insert manufacturer, Kennametal Hertel, has invested more than others in PVD to enhance coating technologies for the specialist metals: aluminium, stainless steel and nickel alloys.

Bernard North, the firm’s director for materials and process development, says PVD has evolved a lot over the past 10 years: `As people understand more about the parameters, they produce higher quality coatings.’

Despite the fact that stainless steel users only provide around 20% of the market for inserts, compared with around 40% each for iron and steel, inserts for stainless steel break much more quickly, so the investment is worthwhile.

The alternative to new coating techniques is exotic new coating materials. Diamond, ceramics and cubic boron nitride have each been used to harden cutting tools. Synthetic diamond is twice as hard as titanium nitride and cuts soft materials such as aluminium and copper at high speeds.

But a new substance – molybdenum disulphide (MoS2) – could in some circumstances replace even these. Conventionally known as a lubricant, it is being developed by Teer Coatings in Kidderminster as an insert.

MoS2 is ideal for machining stainless steel and other soft metals at high speed because it has very low friction – a 25th of titanium nitride and a fifth of diamond. This means the ideal application for it would be in lubricant-free and high temperature environments. And it is half the price of diamond.

It does not react chemically with the metal of the workpiece, unlike diamond which, being a form of carbon reacts with the carbon present in steel alloys.

Teer Coatings is still developing MoS2. It is very soft, and so far only machines aluminium alloys and some stainless steels. But some companies are trying it. General Motors in the US plans to evaluate MoS2 in its research labs.

But few companies in the UK are tempted to try it. `They want cast iron guarantees that it’s going to work,’ says Keith Laing, Teer Coatings’ production manager.

`We need to build up a body of test results and develop it with our customers, but MoS2 is working in more and more applications.’

Synthetic diamond as a cutting edge is also starting to be upgraded, however, with breakthroughs in coating application techniques.

Conventional diamond tips are developed by manufacturing polycrystalline diamonds which are broken off to form small tips, and placed on the end of cutting tools.

Diamond tips are used conventionally to machine aluminium and copper and the aluminium super alloys common in the nuclear industry. They are extremely hard, and are brought in for machining at very high speeds.

The drawback is that the tip starts to burn at around 700iC and cannot be used on the higher temperature metals, such as cast iron. It also has a tendency to chip off.

But newly-developed techniques allow deposition of diamond coatings at moderate temperatures from hydrocarbons such as methane in a plasma state, and they should last longer and eventually be able to machine steel.

`If manufacturers get the process right, these diamond tips are going to make a huge difference,’ says Richard Dewes, research fellow at the University of Birmingham’s School of manufacturing and mechanical engineering. `For now, it is still on trial.’