Cut and dried

Improved techniques for high-performance tooling manufacture are expanding the range of materials that can be cut with a single product. Martin Oakham reports.

For as long as cutting tools have existed, we have been faced with the problem of making a material that is both hard and durable.

The Holy Grail for cutting-tool manufacturers is to offer a single material that meets all customer demands for higher productivity, longer effective cutting times and the best performance across the widest range of cutting conditions.

It may seem an unrealistic goal but it is one tooling developers are moving closer towards. This is further complicated by the fact that metallurgists are also developing new materials to withstand greater temperature, featuring higher strength-to-weight ratios, and that tool geometry and stiffness play an equally important role as construction.

All tooling manufacturers will tell you that the the job setup, machine build and tool holding system is as important to performance as the construction itself. In short, the maximum designed cutting potential is only achieved if you have the right tool material/grade, the right tool geometry, quality tool holders, a solidly built machine tool with suitable jigs and/or work holding solution.

Most cutting applications can be effectively dealt with using sintered micrograin carbides. The right combination of carbide insert composition, coating materials, layer sequence and selection of the appropriate coating technology makes it possible to substantially increase metal-cutting productivity without sacrificing insert wear resistance.

Two basic coating techniques are now available — chemical vapour deposition (CVD) and physical vapour deposition (PVD). CVD coatings are generally used where applications demand good edge wear resistance. However, the process is less chemically resistant and cannot withstand high temperatures. PVD coatings are chosen where maximum edge-line resistance is required. Typically, modern insert coatings use layers of MTCVD-TiCN, AL203 and TiN, seasoned with intermediate, thin layers to achieve excellent adhesion and desired crystal structures. In particular, the AL203 exists in many different crystal modifications and grain structures with different properties.

While much of the science involved in developing PVD coatings remains secret, the end results are very public. For example, WNT’s HCF 1130 turning grade is one of the leading solutions for machining steel, High Speed Steels (HSS) materials and chrome-nickel steels. In this application area the grade operates with high process stability, resulting in a highly cost-effective solution. Similarly, HCF 3110 delivers a viable carbide alternative to ceramics when machining cast iron, particularly in roughing operations.

Walter, an established world leader in solid carbide and coated tooling, claims to be the first company to successfully develop a PVD aluminium oxide coated carbide insert on a commercial basis. The result is tooling that exhibits excellent tribochemical characteristics and is able to accommodate a wide range of metal-cutting operations, particularly difficult-to-machine nickel-based alloys, titanium alloys as stainless steels.

Walter claims the inserts are capable of running for double the usual cutting time — and therefore able to increase productivity by up to 100 per cent, and permit high-speed machining at speeds normally associated with CVD coated tooling. Until now, aluminium oxide has never been successfully applied to carbide using the lower-temperature, PVD coating system. Traditionally, aluminium oxide (AL203) could only be applied with the CVD process at high temperature — effective for plain carbon materials but compromised when machining chrome or nickel alloys due to loss of cutting edge definition and high cutting temperatures.

Sandvik Coromant has invested significant resources developing new, improved coated insert grades. In the vanguard is the GC4225 grade — now the world’s largest selling carbide grade offering very high-performance, multi-functional steel machining capabilities for both turning and milling operations.

This was rapidly followed by GC4220, GC1030 and, most recently — for release this spring — GC4230 and GC4235 that will be available in all insert styles in both the positive CoroTurn type and the negative T Max P type inserts.

In mixed-production, low-volume machine shops where the emphasis is on greater flexibility, the GC4225 grade has the capability for mixed production machining with small batches of different components and materials. This capability results in fewer insert changes, reduced downtime and the potential for tool rationalisation.

There are also two main groups of so-called advanced, ‘superhard’ materials used in today’s cutting tools; namely ceramics and diamond-based materials. Their uses in metal cutting are geared around their ability to hold high hardness at greatly elevated temperatures.

Ceramics have high hardness but relatively low strength and toughness properties as well as low thermal conductivity, making them prone to thermal cracking if used with coolant. Their excellent wear characteristics, however, make them ideal for finish turning, boring and grooving of cast iron. In certain cases under the correct conditions aluminium oxide ceramics are used in high-speed roughing of steels at speeds of up to 500m/min, although this is not always successful due to unpredictable insert failure.

Cubic boron nitride (CBN), and poly-crystalline diamond (PCD) are the most commonly used diamond-based tooling materials. CBN is the diamond-structure crystal form of boron nitride. Cutting inserts are formed with a varying percentage content of CBN with a binder material, such as cobalt or titanium carbo-nitrides. The high tenacity and extreme hardness of these materials make CBN an excellent medium where the inherent weakness of ceramic is problematic, such as components with small radius requirements, intermittent cutting or where positive rake inserts are required. The other major application for CBN is in the high-speed machining of cast iron.

PCD is a synthetic, single-crystal diamond and is best suited to high-speed machining non-ferrous metals such as aluminium, aluminium/silicon alloys, zinc and manganese alloys as well as highly abrasive non-metallic materials such as graphite, epoxy resin and phenolic fibreboards. Tool life when machining components such as aluminium engine blocks is extremely high, even at cutting speeds as high as 3,000m/min. This advantage over typically coated carbides falls neatly in line with current trends in the increased use of aluminium in vehicle manufacture.

However, this increasing use of aluminium has also hastened the development of thin-film diamond-coated carbide cutting tools. Kennametal, for example, offers diamond-coated carbides with a wear resistance comparable to PCD, while also providing multiple insert edges and the ability to support complex chip control geometries.

From WNT’s perspective ceramic cutting materials will remain niche products with a relatively narrow application area, such as cars. ‘We are finding that the development of carbides is diminishing the advantages that ceramics once had,’ said Adrian Fitts. ‘This is compounded by the time required to develop the practical functionality of ceramic materials, an area that depends, to a much greater extent than carbide, on the detailed knowledge.’

However, WNT predicts that polycrystalline cubic boron nitride and CBN will have a bigger role in metal cutting due to the ‘sandwich’ method of manufacture. This allows inserts to be pressed on both sides, simplifying the manufacturing process considerably.

For multi-functional metalcutting including ramping, helical milling, shoulder milling and peck drilling, Daishowa Seiki of Japan has expanded its FullCut end mill range. FullCut mills have spiral chip pockets for high rigidity, and dry cutting is normally recommended. But through-holes are provided for where a built-up edge dictates the use of soluble oil coolant.