A hard turning research project has been completed at Cranfield University in collaboration with Cranfield Precision - Division of Landis Lund, undertaken as part of an EU Framework 5 project (ULTRAFLEX). The main automotive partner companies were the research organisations of FIAT (CRF), Renault (Regienov) and Nissan (NTCE-UK).
The objective project was to investigate the process for the machining of finished alloy steel surfaces with optical quality, in the order of 10nm Ra, for automotive transmission components such as the drive surface of a continuous variable transmission (CVT) shaft.
From a review of work to date, average roughness achieved on precision machine tools is in the range of several tens of microns.
However, work undertaken by Knuefermann (1), on the ULTRAFLEX project, using a high precision lathe of high dynamic stiffness (DeltaTurn 40) developed by Cranfield Precision, showed that it was possible to obtain 10nm Ra surfaces for short lengths by hard turning.
Achieving these surfaces consistently over longer cuts proved to be very difficult, although surfaces of 20nm Ra were achieved consistently over long periods. The main factors affecting the surface finish were tool defects, machine vibration, and the cutting parameters.
In spite of these difficulties Knuefermann concluded that hard turning was highly deterministic when using an ultra stiff hard turning machine such as the DeltaTurn 40. He developed a new numerical model for surface roughness generation that showed that surface roughness and topography could be predicted very precisely assuming one has appropriate knowledge of the process disturbances.
For example, as well as the standard cutting parameters, other necessary inputs are defects on the tool cutting edge, such as their height (or depth) and width. These affect surface roughness more than any other factor at medium feeds, and at low feeds machine vibration also contributes significantly to surface roughness.
Extremely good correlation with extensive cutting trials was obtained when these inputs were included in the model.
In order for ultra precision turning to be exploited outside the research laboratory, several developments are necessary. These include an improvement of the quality of the edges of hard turning tools, to a level that currently is commercially available only for monocrystalline diamond tools.
In addition, tool defects need to be reduced considerably and it would be extremely beneficial if the manufacturers of the cutting tools specified the defect level so that this data could be used to predict the machined surface roughness.
It is also apparent that for ultra precision applications, the dynamic properties of current hard turning machines need to be improved significantly.
High dynamic stiffness, which requires appropriate damping properties, is required to avoid the tendency of the cutting tools to chatter at low feed rates.
Manufacturers and users of less precise machine tools may also use results of this work - such as information regarding the edge effect and machine vibration on surface roughness.
Machine/tool 'fingerprints' can be generated from Knuefermann's work which plot the expected surface roughness against feed,. These can be applied to ensure that the machine achieves its maximum potential.
Reference (1): M M W Knuefermann, Machining Surfaces of Optical Quality by Hard Turning, PhD Thesis, Cranfield University, Nov 2003.(end)
