Very high temperatures are generated in the cutting zone during machining. Coolant lubricants reduce friction, provide cooling (but may also cause destructive thermal shocks) and support the removal of chips. Back at the turn of the millennium, some experts predicted dry machining would make a breakthrough as a substituting process. It is now time to assess the current position. In which areas have this process, or Minimum Quantity Lubrication (MQL) (as quasi-dry machining) become established and to what extent?
Particular relevance for series machining
“The main area of application for minimum quantity lubrication lies in the machining of prototype parts as found in large-scale series production in the automotive industry - especially of powertrains. The parts range from the cylinder head and engine block to the crankshaft or camshaft, connecting rod, gearbox and wheel carrier, etc,” says Jürgen Keppler in Technical Sales at bielomatik Leuze GmbH + Co KG in Neuffen, Germany. The BadenWürttemberg-based engineering company is a recognised specialist in the development and manufacture of high-quality MQL systems. “Further industrial applications include the machining of cubic components and mechanical engineering castings such as fittings, pump housings or valves. It is a great advantage in the aviation industry, too, if complex components are not flooded with emulsion.”
The expert estimates that MQL is used for machining about 15% of new large-series components, rising to 70% for deep hole drilling in crankshafts, for example. “MQL will continue to grow in the other application areas I just mentioned,” says Keppler, convinced. “The upswing in MQL machining predicted some 20 years ago has mainly occurred in the automotive sector. Here, the advantages of MQL could be fully exploited in the machining of cast and forged parts. The high quantities involved also allowed the related R&D work to be carried out. New application areas will also arise from the forthcoming changes in E-mobility and Additive Manufacturing. The great advantage of MQL lies in the cost savings to be made in resources such as oil, water and energy.” Further advantages are dry workpieces, no carry-over of emulsion & contamination in the production bays, and prevention of associated health risks. “The constant further development of materials and applications is placing new demands on machining processes and thus on MQL systems. This will certainly result in interesting solutions,” says Keppler.
What do the scientists say?
“Thanks to modern cutting materials, dry machining has been introduced in almost all areas of machining production. Increasing cost pressure, but also energy consumption and ecological aspects are leading to a renaissance of these technologies,” says Dr Ivan Iovkov, Head of Cutting Technology at the Institute of Machining Technology ISF of the Technical University of Dortmund, Germany. “Dry machining is not only used in conventional milling or turning, there are also efforts to minimise or completely avoid the use of coolant lubricants in complex processes such as deep drilling and gear hobbing. However, the cutting processes and the technology still need to be adapted in certain ways,” he added. Dry machining tends to be more common in big companies, which process larger quantities, than in smaller companies which specialise in varying types of high-precision and complex components.
“I think, there will be both dry and wet machining in the future,” he predicts. “We need to take a holistic view of production when deciding whether dry machining makes sense or whether it will involve disproportionately high process adaptation costs. The continuous further development of MQL device technology and coatings, the increasing accuracy of the machine park, but also digitalisation – for example through inprocess sensor monitoring of relevant variables – will make it possible in the future to carry out more and more dry or MQL machining under robust practical conditions.”
Over twenty years ago, some experts predicted a triumphant future for dry machining. In which areas have dry machining and Minimum Quantity Lubrication become established?
Gear hobbing now exclusively dry
“The advantages of MQL over wet machining lie in the lower costs for the cooling lubricant and the elimination of chemical additives in the lubricant. This results in cost advantages, and also fewer adverse health effects for the employees,” says Dr Hartmuth Müller, Head of Technology & Innovation at Klingelnberg GmbH’s Hückeswagen production site. The mechanical engineering company, headquartered in Zurich (Switzerland), employs around 1,300 people worldwide and specialises in bevel and spur gear technology.
“The principle behind MQL is based on wetting the rake face. This is easy to achieve with internally cooled tools, while it is not feasible with the complicated tool geometries of gear cutting tools. For this reason, Klingelnberg relies exclusively on dry machining for gear hobbing. We only use MQL when machining cast’s components for use in engineering.” Tungsten carbide bar cutting tools are deployed for bevel gear milling. For cost reasons, tungsten carbide tools are not widely used for spur gear milling; dry-cut PM-HSS hobs are generally used for this purpose. The subsequent hard finishing of gear wheels is carried out – whenever possible – by grinding, using oil as coolant lubricant.
“Dry machining has definitely triumphed in gear manufacturing, as predicted,” says Dr Müller. “Thanks to developments in cutting materials and coatings, cutting speeds are up to five times higher than they were with wet machining 20 years ago. Dry machining has not only led to a considerable increase in productivity, but also to cleaner production.” At EMO Hannover, Klingelnberg will demonstrate its highly proficient ranges for the gearing industry, including the development and manufacture of machines for gear production, precision measuring centres for all kinds of rotationally symmetric objects and the manufacture of custom-made, high-precision gear components.
Cost-efficient and environment-friendly
Hartmetall-Werkzeugfabrik Paul Horn GmbH from Tübingen is one of the leading tool manufacturers. “MQL has replaced conventional coolant lubricants in some machining processes,” says Lothar Horn, Managing Director. “Cooling and lubricating using minimum quantity lubrication offer users a number of advantages. This has led to MQL being used in many machining operations, particularly in series production. This type of cooling reduces the high costs in the maintenance, preparation and disposal of conventional coolant lubricants. In addition, it eliminates the energy costs for the high-pressure pumps and feed pumps required for returning lubricants to the collection basin. In addition, the government is imposing stricter requirements on coolant lubricants with regard to hazardous ingredients. In spite of the advantages of MQL I just mentioned, conventional coolant lubricants are indispensable in production facilities. Many processes – such as drilling, machining of high-temperature alloys, or internal machining with high chip removal rates – require an adequate supply of conventional cooling lubricant.” At EMO Hannover 2019, the tool specialists will be showcasing their extensive turning, milling, slotting and reaming portfolios as well as their ranges of ultra-hard cutting materials and special tools, etc.
Planning dry machining correctly
“In Grob machines, dry or MQL machining is used for processes with a geometrically defined cutting edge such as drilling, milling or roughing,” reports Jochen Nahl, CSO of Grob-Werke GmbH & Co KG from Mindelheim. “Typical workpieces are frame structure or chassis parts, combustion engine and drive train components made of aluminium or grey cast iron, and now also turbocharger housings made of highly heat-resistant cast steel. Wet machining is still very much part of the DNA of many companies, yet the share of MQL machines has already increased to 12% in recent years and is continuing to rise, especially for the machining of frame structure and chassis parts.” The family-owned company has a total workforce of around 6,900 employees worldwide and has been a pioneer in the construction of highly innovative production and automation systems for more than 90 years. “The advantages of MQL machining speak for themselves,” continues Jochen Nahl. “Production line investment costs can be lowered and the effort involved in cleaning the workpieces can be reduced significantly. In addition, MQL uses around a quarter less energy compared to wet machining.”
Although dry machining is not currently being deployed to the extent that was expected, it is clear that industry is gradually accepting it. “If the benefits are to be reaped, any switch from wet to dry machining should be based on an intelligent strategy and implemented with a competent machine supplier as a partner,” recommends CSO Nahl. “Modern companies are already benefitting from recent positive developments such as temperature compensation, the use of higher air pressure levels, and modular systems that reduce manufacturing costs and avoid unnecessary overengineering. The global player, with headquarters in Mindelheim (Bavaria), will showcase its current portfolio, conduct live machining demonstrations and present other technical highlights at this year’s EMO in Hannover.
Conclusion and outlook
It is too soon to speak of conventional machining processes being comprehensively replaced by dry machining or minimum quantity lubrication, as wet machining (involving larger quantities of coolant lubricant) still accounts for an estimated 85% of machining. Nevertheless, dry processes are conquering more and more areas, both in the general machining sector and above all in special areas.
Wet machining involving larger quantities of cooling lubricant is still used for the vast majority of machining processes. Nevertheless, dry machining and minimum quantity lubrication are on the advance, especially in special areas such as gear hobbing.
In addition to technological aspects (such as workpiece materials and machine performance), potential cost savings for energy and systems play an important role in selecting the appropriate process. In addition, health aspects, sustainability and stricter legal requirements are being accorded ever greater weight as factors within the overall assessment. Technological advances and detailed knowledge of the cutting edge processes will offer more and more possibilities in the future for safe and (in many cases also) highly productive dry machining and MQL.
At EMO Hannover 2019, trade visitors will find indepth information, suggestions and help in making the right choice between wet and dry machining and MQL for their own applications.