Cutting-edge calculations: diamond fever or hard-metal reality?

Diamonds are among the best friends of innovative machiners of lightweight materials. They are running head-to-head with hard-metals, which are likewise numbered among the friends of lightweight material machiners.

The broad bandwidth of present-day lightweight materials also necessitates a concomitantly broad choice of cutter materials. In the estimation of Dr Steffen Reich, head of Research and Development at GFE – The Society for Production Technology and Development in Schmalkalden, besides the use of “classical” cutter materials for lightweight construction jobs, like polycrystalline diamond PCD) or monocrystalline diamond (MCD), “the use of coated (not diamond-coated) hard-metals can be expected to gain in importance. This is because the unsurpassed universality of hard-metal enables almost any desired combination of materials to be machined. In addition, there are definite advantages, too, in terms of procurement costs.”

The use of monocrystalline diamonds, says Dr Reich, “will continue to be restricted to the ‘upmarket niche’ of creating high-gloss surfaces; for this, there is as yet no real alternative. Use in further areas that can also be covered by other cutter materials cannot be anticipated, due to the high costs involved.” PCD and CVD thick-layer diamonds coated using chemical vapour deposition definitely constitute competing cutter materials in some categories of application. The use of PCD tends to lie mostly in the field of roughing operations, whereas the CVD thick-layer diamond is typically utilised in the finishing pass. The expert from Schmalkalden, however, emphasises that not all lightweight materials can be machined with diamond, whatever the specific form involved: “Other cutter materials, like coated hard-metal, are required here.”

The question of a longer useful lifetime, can “be answered neither categorically nor seriously," he adds, since the answer will depend on the application and the type of cutter material involved: “In the case of purely abrasive wear and tear, of course, the harder diamond will have advantages over hard-metal. However, if other wear mechanisms are involved (diffusion, adhesion) or even dominant, then the advantage may also lie with hard-metal.”

As in other fields of machining, too, the choice of tools should not be governed by the useful lifetime, but by the production costs. This means it may be quite sensible to use a tool that in comparison to a competing product exhibits a shorter useful lifetime, but, for example, enables production costs to be downsized thanks to higher machining parameters.

When machining carbon-fibre-reinforced plastic components and special materials, such as graphite, diamond coatings have meanwhile become a generally accepted option. They are here competing in some applications with PCD in particular. Further improvements in regard to morphology in the edge sharpness can be expected in the future. Unfortunately, says Reich, “the problem of decoating has not yet been solved. Also, other carbon coatings are likewise gaining in importance. The main focus here is on machining materials with a higher tendency towards adhesion.”

One option of steadily increasing importance for expanding the range of applications for cutter materials is laser machining of cubic-crystalline boron nitride (CBN) and diamond materials. “By incorporating chip groove geometries and cutting edge preparations, the range of applications for the cutter materials can be expanded. The restrictions applying up to a few years ago, especially in regard to chip groove geometries, have been overcome. This particularly benefits the process-dependability of the tools concerned, and compensates for existing deficits in competition with other cutter materials, such as diamond-coated hard-metal,” explains Dr Reich.

Hard-metal for small diameters

In the view of Dr Stefan Sattel, head of Research and Development at Gühring KG, Albstadt, hard-metal tools are primarily used for diameters of less than 10 mm: “With hard-metal, complex geometries for drilling and milling operations can be implemented without any problems even for the small diameters.” For efficient machining of hybrid structures, such as combinations of carbon-fibre-reinforced plastics, aluminium, titanium and VA-steel, he adds, full-hard-metal tools are likewise indispensable.

PCD/CBN tools, by contrast, promise stable processes in series manufacturing operations. “Thanks to the sharp cutters, the fibres can be neatly severed. Abrasive composite materials, too, can be machined without delamination. What’s more, high cutting speeds can be achieved,’’ he explains.

“Both cutter materials provide exceptionally good characteristics for metal-cutting jobs, and, depending on the application concerned, have different advantages. In this context, the use of self-sharpening geometries opens up entirely new options for machining lightweight materials,’’ says Dr Sattel. In order to achieve maximised tool lifetimes, he adds, diamond coatings in conjunction with hard-metal tools are absolutely essential: “There is a definite trend towards nano-structured layers for maximum protection against wear and tear.”

Geometry optimization

Peter Büttler, director Business Development at Komet Schweiz AG and responsible for the lightweight construction operations of the Komet Group in Besigheim, clarifies: “The principal focuses of development work lie in geometry optimisation for an enormous variety of different machining jobs and materials.” Diamond tools (PCD) should always be used under maximally stable machining conditions. Uncoated or in some cases also diamond-coated tools are a preferred choice for manual or robot-controlled jobs. Compared to hard-metal, MCD or PCD “are the significantly harder materials, and perform substantially better in the durability test.”

For S?nke Lange, key account manager Aircraft at Kromi Logistik AG, Hamburg, it is “not always the supposedly best tool that is also the most sensible choice in terms of cost-effciency”. The multiplicity of coatings featuring light-metal and fibre composite materials, he adds, also necessitates different machining strategies, in order to prevent delamination, for instance. Depending on whether the machining process involved is controlled manually, semiautomatically or fully automatically, entirely disparate requirements will be entailed for the machining tool. Monocrystalline diamond will fnd suitable applications here, as will polycrystalline or coated tools.

How long a tool will remain sufficiently sharp is essentially decided by the process itself: “Replacement of a blunt tool during an ongoing process is generally counter-indicated for financial reasons. In the case of near-series processes, there is for carbon-fibre-reinforced materials accordingly a trend towards diamond-coated full-hard-metal tools, since they achieve a higher level of cost-effciency in the abrasive materials concerned.”

Heinz Peter Boost, head of Product Management at Seco Tools GmbH, Erkrath, has a succinct answer to the question of diamond or hard-metal: “Diamond cutters score heavily in terms of enormous useful lifetimes, which in series production, particularly, are of crucial importance. Hard-metal tools based on inserts, both uncoated and polished and also coated, provide the user with an interesting alternative when the costs per cutter are a major consideration.”

And the useful lifetimes? Here, too, there comes an unequivocal answer: “Diamond stays sharp for longer, hard-metal stays affordable for longer. Whenever stringent requirements for toughness apply for the process concerned, and unfavourable machining conditions are involved, then coated, tough hard-metal types may sometimes remain sharp for longer. This has been evidenced by recent studies conducted under the aegis of universities, in which uncoated polished inserts were used in comparison to coated and diamond-studded ones. Here, the coated hard-metal inserts scored better by reason of the longest useful lifetimes.”