Cranfield University's production trials have revealed potential costs saving using High Efficiency Deep Grinding - or HEDG - technology in the plunge grinding of automotive crankshafts.

Cranfield University's School Of Industrial and Manufacturing Science, Cranfield, UK, has employed a Holroyd Edgetek SAT super abrasive grinding machine as part of a three year programme to develop a deeper understanding of the mechanisms of metal removal using HEDG (High Efficiency Deep Grinding technology). The investigation culminated in a series of tests on a Cinetic Landis LT2 used to cylindrically plunge grind automotive crankshafts with metal removal rates up 1000mm3/mm.s. No thermal damage to sidewalls was recorded.

Achieving these increased removal rates has unlocked the potential of rough grinding to make the HEDG process competitive against more traditional methods.

The study at Cranfield was a collaborative project sponsored by EPSRC between the University, Holroyd, Cinetic Landis Grinding, St Gobain, Liverpool JMU, Castrol, Stesstech, and Element 6.

It was organised and undertaken under the direction of Professor David Stephenson, a world authority in grinding technology.

'The objective of our research was to establish a model which explains the thermal characteristics of metal removal under HEDG conditions and then apply this to machining standard automotive crankshafts,' said Prof Stephenson.

'Many production engineers are aware of the potential that exists to optimise existing industrial process chains using today's high performance grinding machines and the latest super-abrasive technology'.

'HEDG is one such example; its high material removal rate offers the potential to improve cycle times whilst maintaining surface integrity, form and finish requirements.' One of the key requirements on automotive crankshafts is that the pins and mains both require journal diameters and web sidewalls to be ground.

The pins require wide short path grinds, typically using high spindle power levels, while the mains need narrow and longer path grinds, which are not normally limited by the available spindle power, but by issues of thermal damage.

The major ramification of this is that the webs or sidewalls of the crankshaft are particularly suitable for the application of HEDG technology.

HEDG delivers improved 'Burn Threshold' qualities that result in lower grind temperatures of the workpiece, and overall improvement in the quality of the finish itself.

In addition, HEDG offers great potential to improve process times and, as a consequence, helps manufacturers to gain a competitive advantage.

The Cranfield research comprised grinding trials, which were carried out in two stages: initially utilising test-pieces in a laboratory environment, followed by automotive crankshafts on production machinery.

The test pieces themselves were selected steel and cast iron materials.

One of the major issues faced by the research team was that, in order to increase specific material removal rates (Q'w) to levels far higher then currently used commercially, it was necessary to understand how the heat energy is partitioned to the workpiece.

This was achieved using thermal models specifically developed for high removal grinding; and by employing a novel temperature measurement technique adapted for cylindrical grinding.

During the initial work, tests were carried out over a range of Q'w from 100 to 2000mm3/mm.s on steel samples.

Power data were recorded and then used to predict surface temperatures using the thermal model.

A second set of trials was then carried out on production machinery with both steel and cast iron crankshaft components.

A similar range of Q'w rates was investigated.

In this case the simple shaped grinding wheels were replaced with two fully profiled left and right hand wheels, to enable the production of the undercut form, blending sidewall and diametrical features, and any differing width requirement.

The initial set of trials was conducted using a Holroyd Edgetek SAT super abrasive turning machine, while the second set, which were validation trials, took place on an LT2 machine at the Cinetic Landis factory, under actual production conditions.

The CNC controlled Edgetek system provides cutting speeds of up to 12,200m/min, courtesy of a high power, variable speed 37kW spindle motor operating at 9000 rev/min max, and CBN electroplated wheels, which provide long wheel life.

Coolant nozzle design had to be improved too.

Delivery of the correct amount of workpiece coolant and lubricant to the work zone is important to reduce the heat being transferred into a workpiece.

The Edgetek machine is fitted with a very stiff grinding spindle, which uses hybrid ceramic bearings that facilitate the very large cuts possible with HEDG.

In addition, the granite polymer composite base offers excellent damping properties, virtually neutralising resonant frequencies within the machine that could impair its accuracy.

The result of this synergy is a machine that allows heavy stock removal on straight outside diameters (ODs) or special form diameters to grinding machine tolerances on virtually any difficult material or Rockwell hardness.

* Results - the results for both the initial and production stages of the trials correlated well.

The industrial application of the process initially considered a small number of steel crankshafts ground over a range of Q'w, and then a batch of 1000 with a constant stock removal rate.

The batch run demonstrated process stability with grinding power levels remaining consistent throughout.

Crankshafts were checked for form and surface finish requirements and remained well within specification; typically 50 micron for concentricity and 2.3 micron Ra for surface finish.

The application of the increased removal rates - 1000mm3/mm.s for sidewall and 200mm3/mm.s for diameters on the crankshafts, offers the potential to save cycle time in full production for both steel crankshafts and cast iron crankshafts.

In addition, even with the increased removal rates, the specific grinding energy (SGE) versus Q'w relationship showed a similar trend to previous surface HEDG investigations, with the SGE reducing at higher Q'w values.

This means that a greater proportion of the energy is removed by the grinding chips, resulting in a reduced energy input to the workpiece within the HEDG regime.

Summing-up the results of the trials, Prof Stephenson said: 'Our study has shown that the performance level of standard production machinery currently in use within the automotive industry can be extended to provide a number of major advantages'.

'These are, firstly, more efficient grinding regimes that compete with conventional cutting processes in terms of stock removal rate; secondly, reduced manufacturing costs as a result of higher production rates and lower capital investment; and thirdly, improved surface integrity and, therefore, better component performance and reliability due to relatively low workpiece temperatures.