Turbine parts ground precisely, no burning

Subcontractor is using open-grained, aluminium oxide wheels in 'Viper' grinding - maintaining a sharp, accurate profile in Inconel 939 with minimal heat generation.

Three varieties of land turbine component are currently produced by Viper grinding on a Makino A55 horizontal machining centre at subcontractor, Teleflex Aerospace Manufacturing UK.

Two are stage-one nozzle guide vanes for Siemens Industrial Turbomachinery AB in Finspong, Sweden, while the third is a shrouded turbine blade for Rolls-Royce in Derby.

To achieve the required tolerances and avoid burning, all have their finish grinding operation completed by wheels supplied by Saint-Gobain.

The nozzle guide vanes are produced in two set-ups from nickel-based Inconel 939 alloy.

The surface of these heat-sensitive components used to crack during the all-important final grinding operation after the previous wheel became clogged with sticky swarf.

This not only led to surface cracking and porosity, but also made it difficult to hold fine edge features.

Said Phil Chadwick, Customer Service/Quality manager at the Teleflex works in Dunnockshaw, near Burnley, UK, which is one of the first subcontractors in Europe up to use Viper grinding, "The solution was to replace the wheel for the final grinding operation with a Vortex wheel from Saint-Gobain".

"It is one of the most open-grained, aluminium oxide wheel on the market and is able to maintain a sharp, accurate profile with minimal heat generation, even when machining difficult metals." The porosity is naturally and evenly spaced, without using artificial pore inducers, in a patented manufacturing process at Saint-Gobain's Stafford factory, allowing coolant to be readily absorbed and diffused to the point of cutting.

The temperature of the long alloy chips is therefore kept low, discouraging clogging of the cutting edge.

"You could physically see the cleaner cut after using the Vortex wheel," commented John Armitage, quality compliance leader at Teleflex.

"The finer features of the guide vane did not drop off as previously and the cooler cut improved the surface finish." The shrouded turbine blade for Rolls-Royce's high pressure facility in Derby is made from a nickel-based, high temperature alloy casting and has its fir tree root form and shroud end ground in two successive, single-hit operations.

For this component too, Saint-Gobain provides the wheel for the final grinding operation.

Called Sirius, it is another very porous, aluminium oxide wheel in a vitreous matrix.

It is used by Rolls-Royce themselves for the last grinding pass on the turbine blade, which is also manufactured at Derby using identical Makino cells.

A spokesman for Rolls-Royce said that the fir tree root requires radii as tight as 0.3 micron.

The previous wheel tended to break down, struggled to hold tolerance and required frequent dressing, shortening wheel life.

It also caused blueing on the surface of the component.

So the same type of Sirius wheel was specified for the process at Teleflex.

It contacts the component at a surface speed of 45m/s for only 10s out of a total cycle time of 12 minutes, during which time it imparts the required fine profile to the root and shroud features, with negligible burning.

Four interchangeable Erowa fixtures are employed to speed component changeover during production.

Continuous dressing of the grinding wheels during production is available as an option, but Chadwick feels that high volume production is needed to justify the extra cost.

At Dunnockshaw, the wheels are preset off the machine to minimise the amount of subsequent form dressing required when the wheel periodically visits the diamond rolls mounted on the rotary table beneath the workpiece.

Continued Chadwick: "The Viper project is helping us to grow our business".

"In addition to increasing our aerospace work, we aim also to manufacture in-house our medical division's components that are currently subcontracted out".

"We have an option on a second Makino A55 Viper machine, which we hope will be installed in 2006." The company's vision is to set up a two-machine, conveyor-fed cell with robot loading, 5-axis deburring, washing and shop floor inspection by a co-ordinate measuring machine (CMM).

If things go well, several such cells will be created and older production plant sold off.

Saint-Gobain products from the group's 'Universal' grinding wheel range are used by Teleflex for general machining duties on its Elb grinders.

Creep-feed and reciprocating surface grinding, which both involve a large contact area between wheel and component, account for 70% of non-Viper throughput and the subcontractor is currently evaluating Sirius wheels for these applications as well.

* About 'Viper' grinding process - Viper is an acronym for 'Very Impressive Performance Extreme Removal'.

Developed by Rolls-Royce, it is capable of stock removal rates up to eight times those achievable when grinding nickel alloys conventionally using a plated CBN wheel.

High pressure coolant at 70 bar has to be directed at the point of grinding to keep the component and chips cool.

For example, traditional grinding of nozzle guide vanes at Teleflex previously involved a cell containing 10 machines and employing five operators to clamp every part once on each machine.

Viper grinding requires only two set-ups on the Makino and one operator.

Repeatability has improved with Viper grinding owing to fewer clampings.

As there are no longer repeated set-ups on different machines, cumulative error does not use up the drawing tolerance, so it is rare for a part to be out of tolerance and scrap is therefore minimal.