Crankshaft Plunge Grinding Trials Use HEDG

Manufacturing Engineering, Sep 2006

At Cranfield University (Cranfield, Bedfordshire, UK), the School Of Industrial and Manufacturing Science has employed an Edgetek SAT superabrasive grinding machine made by Holroyd (Milnrow, Rochdale, UK) as part of a three year program to improve understanding of the mechanisms of metal removal using High Efficiency Deep Grinding (HEDG) technology. The work culminated in a series of tests on a Cinetic Landis LT2 (Cinetic Landis, Keighley, West Yorkshire, UK) used to cylindrically plunge-grind automotive crankshafts with metal removal rates up to 1000 mmVmm/sec. No thermal damage to sidewalls was recorded.

Researchers believe that 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 the UK government's Engineering and Physical Sciences Research Council (EPSRC) involving the University, Holroyd, Cinetic Landis Grinding Ltd., St. Gobain, Liverpool JMU, Castrol, Stesstech, and Element 6. It was organized and undertaken under the direction of Professor David Stephenson, an expert on 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," says Stephenson. "Many production engineers are aware of the potential that exists to optimize existing industrial process chains using today's high-performance grinding machines and the latest superabrasive technology. HEDG is one such example; its high material-removal rate offers the potential to improve cycle times while maintaining surface integrity, form, and finish requirements."

One of the key requirements on automotive crankshafts is that the pins and mains both require journal diams 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. Consequently, 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 workpiece grind temperatures, and overall improvement in the quality of the finish. In addition, HEDG offers the potential to shorten process times and, as a consequence, can help manufacturers gain a competitive advantage. The Cranfield research comprised grinding trials, which were carried out in two stages: initially utilizing test pieces in a laboratory environment, followed by grinding automotive crankshafts on production machinery.

The test pieces themselves were selected steel and castiron materials. One of the major issues faced by the research team was that, in order to increase specific material removal rates to levels far higher than currently used commercially, it was necessary to understand how heat energy is partitioned to the workpiece. This was achieved using thermal models specifically developed for highremoval grinding, and by employing a temperature-measurement technique adapted for cylindrical grinding.

During the initial work, tests were carried out over a range of specific metal removal rates from 100 to 2000 mmVmm/sec 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 castiron crankshaft components. A similar range of specific metal removal rates was investigated. In this case, the simple shaped grinding wheels were replaced with two fully profiled left and right-hand wheels, to enable 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 superabrasive turning machine, while the second set, which were validation trials, took place on an LT2 machine at the Cinetic Landis factory, under production conditions.

The CNC-controlled Edgetek system provides cutting speeds of up to 12,200 m/min, courtesy of a high power, variable speed 37-kW spindle motor operating at 9000 rpm max, and CBN electroplated wheels, which provide long wheel life. Coolant-nozzle design also had to be improved. Delivery of the correct amount of workpiece coolant and lubricant to the work zone is important to reduce the heat being transferred intoia workpiece.

The Edgetek machine is fitted with a very stiff grinding spindle, whicfh uses hybrid ceramic bearings that facilitate the very large cuts possible with HEDG. In addition, the granitepolymer-composite base offers excellent damping properties, virtually neutralizing resonant frequencies withlin the machine that could impair its accuracy. The result of this synergy; is a machine that allows heavy stock removal on straight ODs or special form diameters to grinding machine tolerances on virtually any difficult material or Rockwell hardness.