The pressure is on to cut out cutting fluids. They represent health and environmental hazards and are, some would say, a source of unnecessary expense. In Europe, particularly Germany, the cost of treating used cutting fluids, as well as the level of the associated eco-taxes, is rising.
Recent statistics from Germany indicate that costs associated with cutting fluids amount to between 7.5% and 17% of all production costs. The largest component of these costs, 40-50%, relates to the central coolant system, says Eckehard Kalhofer from the Institute for Production Engineering and Machine Tools at the Darmstadt University of Technology.
The obvious solution is to machine `dry’. But this is easier said than done, not least because the fluid is responsible for keeping the workpiece cool during the cutting process. It also lubricates the process and helps remove swarf.
Tests, however, have shown that dry machining is not impossible. Some production applications are already operating. According to Kalhofer, dry machining is used at Heidelberger Druckmaschinen for most milling operations and many drilling ones. At DaimlerChrysler’s Kassel plant, he says, most turning operations on axles are performed dry too.
Not all types of machining operation lend themselves equally well to a dry process, Kalhofer warns. The best combination of process and material is the milling of cast iron or steel, where the absence of coolant actually increases tool life by more than 20%.
It has, therefore, become common practice to use dry milling in many applications, particularly for high-speed machining using carbide tooling.
More problematic processes
Other processes, however, are not so easy to perform dry. Deep-hole drilling is highly problematic because of the difficulty of removing swarf. And aluminium alloys are difficult to mill dry since the fine swarf particles heat up and tend to stick to the cutting tool, which reduces tool life considerably.
At the Darmstadt Institute, studies on dry machining have produced some of the answers. Temperature problems can, for example, be solved by using aggressive cutter geometry and by applying a large feed per tooth.
This, Kalhofer explains, reduces the frictional warming created per volume of swarf generated and increases the amount of heat removed in the swarf.
It also helps to use a tool coated with a high-temperature resistant coating such as a titanium derivative. For aluminium, a diamond tool reduces the chip-adhesion tendency, as do amorphous carbon coatings.
For the swarf problem, simple solutions such as blowing it away with compressed air or removing it by suction are easy to incorporate into a machine tool.
Nevertheless, the pioneers of dry machining have begun to accept that some cutting applications will never be completely `dried up’. They are proposing another approach: minimum lubrication.
`A minimum lubrication system can, with a very small amount of lubricant, significantly reduce friction and swarf adhesion,’ Kalhofer says. He says that with just 20ml/h of lubricant, a very effective result is obtained. The main problems relating to the use of cutting fluids are eliminated, particularly the costly central coolant system.
French machine tool company Renault Automation is working on both dry machining and minimum lubrication. Its Urane high-speed machining centre has been designed to minimise the problems generated when cutting fluids are eliminated and is easily fitted with the equipment necessary for minimum lubrication cutting.
`Because of the swarf problem, we have designed the Urane machining area with a maximum of vertical surfaces and minimum of horizontal ones,’ explains Claude Fioroni, R&D manager at Renault Automation.
Where horizontal surfaces are unavoidable, they are fitted with mechanical scraping devices which are activated by the rotation of the turntable supporting the workpiece and its pallet.
Dealing with swarf
The swarf is collected in a vibrating hopper which also acts as a means of isolating the cutting area, where heat builds up, from the machine structure, whose temperature must remain stable if machining accuracy is to be maintained.
According to Fioroni, these measures are sufficient for the dry machining of the external surfaces of a part. But they are not adequate for internal operations such as drilling, tapping and boring.
This has led to the development of a minimum lubrication system which is fitted on the Urane as an option. Renault Automation describes it as a micro-pulverisation system.
It is fitted to the electric spindle, which has been specially modified for the operation, and it creates a cloud of microscopic oil droplets which is sprayed out onto the cutting area through the centre of the tool.
`This is a highly innovative solution which enables the swarf to be removed more easily and which prevents particles adhering to the cutting tool,’ Fioroni says. `We have tested it with a great variety of tools on a wide range of hole diameters. We have drilled sequences of five or six thousand holes with it and the results are very encouraging.’
The car company Renault is also assessing the feasibility of machining aluminium cylinder heads with a minimum lubrication system. According to Antonio Vaz, engineer at Renault’s mechanical engineering department, tests have proved that the process works.
`The next stage will be to show that the process is economically feasible,’ he says. `The modification of machining centres to operate semi-dry (by adding micro-pulverisation and swarf suction devices) might be more costly than the savings achieved by eliminating the central coolant systems,’ he says.
To establish the economics of the minimum lubrication process, Renault envisages starting full-scale tests at one of its engine production facilities over the next few months.
`The main objective is to verify cutting times and tool costs,’ says Vaz. `We will have to produce – dry – several tens of thousands of good components which can be subsequently put into the production cycle. Only then will we have the confidence that this innovation can perform successfully on an industrial scale.’
Dry solutions pay off
As for Fioroni, he has no doubts that dry – or minimum lubricant – machining does pay off in the right situations. `It depends on the application and the country, because of differing regulations regarding the disposal of fluid. I have seen applications which gave a 15-20% saving in operating costs. It is the maintenance of the central cutting fluid system which costs a lot,’ he says.
An example is French foundry Montupet, where dry machining is commonplace. Its use is so sought-after that the capability for dry machining has even become one of the key criteria in the machine tool selection process.
`We prefer to machine without cutting fluids because it simplifies the machine shop installation and we don’t have to bother about ecological constraints, which can be quite onerous,’ says Raymondo Valenti, maintenance engineer.
High-volume special machines constitute the bulk of Montupet’s machining facility at its plant in Nogent-sur-Oise, not far from Paris, but the company has recently invested in its first NC machine, a Urane high-speed machining centre, which was chosen for its flexibility, its productivity and, of course, its ability to machine dry. The Urane machine is used for rough milling three faces of an aluminium cylinder head, at a maximum rate of 1,500 heads a day, without the use of cutting fluids.
The rough milling operations produce a large amount of swarf. The Urane machine has been fitted with an additional air nozzle which blows the swarf away from the cutting area and, in particular, prevents it from getting between the cutting tool and its holder during a tool change.
Since starting up the machine more than a year ago, Montupet has achieved good results and also significantly improved tool life. `A set of polycrystalline diamond inserts used to last for 500 cylinder heads. Today that figure exceeds 20,000,’ Valenti says.
It just proves that, in the right circumstances, dry machining can be feasible.