A research project using superabrasive machines has achieved a significant breakthrough in machining superalloys - in terms of reduced cycle times and better surface finish and accuracy.

A research project undertaken by a team at CranfieldUniversity's School Of Industrial and Manufacturing Sciencehas employed Holroyd's superabrasive Edgetek machines toachieve significant breakthroughs when machining Nickel basedsuperalloys; materials which are notoriously difficult to work.The breakthroughs, in terms of reduced cycle times, bettersurface finish and improved accuracy, have been achieved usingthe latest high efficiency deep grinding (HEDG) techniques. Theyare likely to have a profound influence on the way thatsuperalloys are machined in the future within the aerospaceindustry. The Cranfield team, led by Professor David Stephenson,carried out lengthy 'Burn Threshold Studies' on Inconel718 - a superalloy commonly employed for gas turbine components,using Holroyd's 'Edgetek' Superabrasive Machiningprocess.

Manufactured at Holroyd's UK factory in Rochdale,the multi-axis, CNC controlled Edgetek machines deploy provenhigh efficiency deep grinding (HEDG) techniques, using CubicBoron Nitride (CBN) wheels at surface speeds of up to 200m/s.This enables them to achieve high depths of cut and optimisedmetal removal rates far exceeding those of more conventionalmachines, such as CNC lathes, milling and machining centres andgrinders.

An Edgetek machine (a five-axis unit) was specified forthe Cranfield research project due to its ability to machineNickel-based superalloys such as inconel 718, which are extremelydifficult to work.

The ability of these alloys to retain much oftheir strength at elevated temperatures means that wear rate onconventional tooling is rapid, even when cutting at low speeds.In addition, problems such as workpiece burn and poor surfaceintegrity are common, due to the alloys low thermal conductivity.Over the last 30 years developments in creep feed grindingtechniques have addressed some of these problems, notably inachieving higher rates of metal removal, but economic pressuresto reduce manufacturing costs further, and to improveproductivity, still remain.

As a direct result of thesepressures, a growing interest in HEDG (high efficiency deepgrinding) has developed, which can be considered as a combinationof creep feed grinding and high speed grinding, using high wheelspeeds often in excess of 200m/s.

A main area of research hasbeen to attempt to prove a commonly held HEDG theory that veryhigh stock removal rates are associated with low work-piecetemperatures.

Key to this, were the grinding trials carried outat Cranfield University.

The trials were undertaken on blocks ofInconel 718 measuring 40 x 40 x 40mm, using a 'downgrinding' mode and grinding parameters were varied over thefollowing range: depth of cut: 0,05 - 1,5mm, work speed: 0,05 -55mm/s and wheel speed: 50-150m/s.

For each test a series of 15mmwide cuts was taken and resultant forces were measured using athree-axis dynamometer.

Although the results of the trials arenaturally very detailed, the general trend shows a reduction inspecific grinding energy as specific stock removal ratesincrease.

Commenting upon this, Professor Stephenson said:'The thermal modelling results correlate well withexperimental observations of grinding burn.

Modelling indicatesthe potential benefits associated with HEDG, using high workspeeds and large depth of cut to minimise the finished surfacetemperature.

Thermal modelling also highlighted the significantcontribution that CBN abrasives can make to heat removal.'These results, highlighting the large depths of cut andaccompanying lower temperature work-piece characteristic of HEDG,have significant practical relevance in terms of aerospacemanufacturing processes, in that the two promote dramaticallyimproved productivity, less surface deformation, improvedaccuracy and better surface finish.

'We are delighted, butnot surprised by these results,' said Holroyd SalesDirector, Paul Hannah.

'The massive productivity gains andcost savings experienced by those who have made the switch toEdgetek superabrasive machining speak for themselves and provethat the technological developments pioneered by our machinesrepresent a highly significant breakthrough for the future ofmachining.' In addition to its key role in the BurnThreshold Studies, Holroyd's Edgetek machine is also centralto another Cranfield project, 'Ultraflex', which isrunning simultaneously.

'Ultraflex' is a European /automotive industry-funded project, for the development ofinnovative manufacturing technologies for reducing processchains.

Its specific objectives are to develop and to demonstratethe following two processes: * Cost-effective and highproductivity machining process of high-alloy steel beforehardening operation and hardening process.

* Ultra-precision andenvironmentally friendly machining process of high-alloy steelafter hardening operation.

The Edgetek machine is playing a keyrole in developing these processes and in achieving the twoperformance targets for the project: (1) working towards opticalquality, machined surface finish levels of Ra 0.0micron and (2)the ability to rough finish a high alloy steel CVT gearshaft inless than 2 minutes.

The groundbreaking research carried out byCranfield University on both Burn Threshold Studies and projectUltraflex and the positive results obtained, proves conclusivelythe effectiveness of Superabrasive machining on difficult to workmaterials, not only within its 'traditional' sector ofhigh volume production associated with milling machineaccuracies, but also on very close tolerance work with highlevels of surface finish.

However, despite these results, neitherCranfield University nor Holroyd are resting upon their laurels.Holroyd is conducting ongoing technical developments at itsfactory in Rochdale which complement the innovative researchcarried out by Cranfield University, developing further thepotential that exists in this rapidly growing area of machinetool technology.

Stressing the latter point, Holroyd's PaulHannah said: 'Feedback from customers and the results ofdetailed research into superabrasive machining both support theview that this exciting technological breakthrough has massiveimplications for the future of a wide range of machiningoperations.

The dramatic reduction in cycle times and toolingcosts which result from a switch to Holroyd's Edgetek'superabrasive machine tools contribute to significantimprovements in overall productivity for every company usingthem.'.