Making the grade

, its main components, the impact a coating can have and what needs to be considered when creating a new design.

A key part of any indexable insert is its grade. Dormer Pramet's recently launched M8340 PVD milling grade was a landmark development within the company's overall insert programme and is set to be a central figure for future R&D projects.

The development of the new M8340 grade was the culmination of a two year programme involving significant investment. An extensive period of planning preceded the project, which included numerous new experimental procedures, a thorough evaluation process and involved more than 20 technicians.

The result was one of our most successful development projects in the last ten years and the M8340 is now a leading grade used to support most general milling applications, particularly when machining steels, stainless steels and under certain conditions cast iron. It has become our ‘go to' grade for customers.

Key qualities of versatility and durability within an insert are driven by the composition of its grade. As there are so many operations possible within milling (face, shoulder, slot, plunge, progressive and ramp milling to name but a few), it is important to have a grade which can be used across as many applications as possible.

The focus when establishing a new grade is to determine what the main aims are and ultimately what the cost will be to customers.

Increasing reliability

When Dormer Pramet began the development of the M8340 the aim was to create a grade which would support both wet and dry milling with a specific requirement to increase reliability of the cutting edge, minimise cracking and improve performance when using coolant.

Created under the Pramet indexable range, the M8340 would support our existing 8215 and 8230 milling grades to offer a complete programme of options for the end user. Reviewing the capabilities of these existing grades offers an insight into their structure and how they differ from what was required from the M8340.

We began by looking at the basic element of the insert, the substrate. This is made of cemented carbide – a powdery metallurgical material made up of a combination of tungsten carbide (WC) particles and metallic cobalt (Co).

Varying the type, size and proportion of particles within the substrate can significantly influence the make-up and ability of the insert. For example, Pramet's 8215 grade has a low cobalt content offering high hardness and considerable toughness, more suited to high cutting speeds with a focus on performance. The 8230 has a medium cobalt content offering a balance of hardness and toughness.

M8340 focused on the other end of the spectrum, having a high cobalt content providing the highest toughness of all Pramet's grades with considerable hardness, more suitable to unfavourable cutting conditions and reliability.

The amount of cobalt used, mixed with additional company specific materials, can make every substrate unique to the next and provides a core foundation to deliver a successful performance from the insert.

Coating benefits

However, it is the addition of the coating which can make the most significant difference to the composition of a grade and give it the additional benefit of being durable in tough machining conditions. The coating protects the insert from abrasion, corrosion and heat – all of which impact on the life of the cutting tool.

Coated cemented carbide constitutes up to 85% of all inserts since it was introduced to the industry more than 40 years ago.

Key to adding a coating is to make sure the most suitable material is used to combine it with the substrate ensuring a perfect match. The order of applying the coating, its thickness and the processes involved, are all variables which impact on overall performance.

In the case of the M8340 grade, a Physical Vapour Deposition (PVD) coating was added. This is achieved by evaporating a metal after it reacts with nitrogen to create a hard nitride coating on the cutting tool surface. This is performed at the relatively low temperature of 400-600°C.

PVD coatings are a popular way of providing wear resistance while its compressive stresses also add edge toughness and comb crack resistance – all required to combat against the grade's hardness. It is ideally suited to deal with instability and high chip evacuation from the cutting area.

Recommended for tough, sharp cutting edges and appropriate for a wide range of general applications for drilling, grooving, threading and milling, PVD is also used for finishing applications.

Modern coatings are combinations of sequenced layers and/or lamellar coatings which have numerous thin layers in the nanometre range, making it even harder.

Pramet's M8340 substrate is surrounded by nano-layers of TiN (titanium nitride) and TiAlN (titanium aluminium nitride) for increased thermal crack and wear resistance. It then has another layer of TiN. Titanium nitride has all-round properties to provide wear resistance and is often a golden colour, while titanium aluminium nitride has high hardness in combination with oxidation resistance, which improves overall wear resistance.

Layer on layer

To use multiple layers of different coating materials enables us to combine all the benefits each one offers. It is this combination of the type H sub-micron grain size of the substrate and the multi-layer PVD coating which enhances wear resistance – in particular, preventing the initiation and propagation of thermal cracks and in turn increasing tool life.

Following the development of the M8340, an extensive evaluation period commenced both at our production facility in Sumperk in the Czech Republic and at selected sales units around the world. Tests were completed with and without coolant against an existing grade and similar ones from competitors. It was clear at the outset that the M8340 was the grade we had been aiming for.

It achieved on average a 50% improvement in performance compared to previous grades; an increase in life span between 30-60% and an increase in productivity by 30% with improved reliability.

Thermal crack initiation and propagation was minimised and while it was designed for operation with coolant, it also achieved greater performance in dry cutting conditions.

The success of the trials led to the M8340 being released into the market in November 2014 as part of a wide range of products being launched and it was made available across almost 150 inserts within Pramet's indexable programme. The M8340 grade features in the new Dormer Pramet End User Nett Price Brochure.

DORMER PRAMET
www.dormerpramet.com