Machining centres developed for grinding
Machining centres developed for grinding
Keywords: Rolls-Royce, Tyrolit, NCMT, Grinding, Machining
Rolls-Royce has developed a new technique, called viper grinding, for producing Inconel aircraft engine parts on a machining centre instead of on a creep-feed grinder, resulting, it said, in major reductions in capital investment, setup times, production costs and lead times. The project, believed to be a world first, has been carried out in collaboration with grinding wheel manufacturer, Tyrolit. Machine tool supplier NCMT was selected to supply the machining centres under its sole UK agency agreement with the Japanese manufacturer, Makino.
So far, 13 Makino machining centres have been installed in Rolls-Royce group factories in Glasgow, Derby and Bristol for grinding compressor blades, turbine blades and engine casings respectively. The latter involves milling as well as grinding on the same machine, whilst the Glasgow application includes deburring in the cycle. A further three Makinos are in Rolls-Royce's Manufacturing-Technology Process Demonstration Cell at Bristol. The majority of the machines were installed during the course of 1999, although the first went into the Process Demonstration Cell in 1997.
According to NCMT, the rationale behind finding an alternative to conventional grinding of these aircraft parts stems from the significantly higher capital cost of grinders compared with that of a machining centre. Other financial considerations are thought to be the high price of standard,large-diameter profile grinding wheels and the greater levels of operator attendance associated with conventional grinding as compared with a machining centre. In particular, a grinder may take up to a day to set up for a new component whereas a Makino machining centre can reportedly be reconfigured in one to two hours, and often considerably faster.
Another relevant factor is believed to be that a single, large wheel has to incorporate the forms of all the features to be ground on a component, so whilst only a small part of the periphery may be grinding the component at any particular time, continuous dressing of the whole wheel periphery unnecessarily wastes this expensive consumable.
In NCMT's opinion, herein lies the key benefit of translating the process onto a machining centre. Small grinding wheels, each with the profile of a specific feature on the component, are held in the tool magazine instead of, or as well as, prismatic metal cutting and other tools. They are changed automatically in the same way as, for example, a milling cutter, and dressing is restricted to the profile which is in use.
In the viper grinding process, vitrified, aluminium oxide wheels of typically 220mm diameter by 40mm wide are employed in creep feed grinding mode. These inexpensive wheels have, it is reported, been found to machine long-chipping,sticky, nickel-based alloys faster than cubic boron nitride. Additionally, the porosity of aluminium oxide is said to be beneficial in retaining coolant and assisting in its delivery to the point of cutting, as described below.
The Makino horizontal machining centre, a standard model but with the slideways protected by special guards to prevent ingress of grinding dust, is configured with seven CNC axes and is capable of both internal and external grinding. The application is said to take advantage of the machine's rigid structure, efficient swarf and coolant containment and management, and advanced spindle technology with a wide speed range of 50 to 20,000rpm, all consistent with high metal removal rates.
The success of the application is thought to hinge on Makino's patented,programmable coolant nozzles which direct high pressure, refrigerated coolant tangentially on to the wheel a short distance in front of the component. The open-grained aluminium oxide carries the coolant to the point of cutting,avoiding the problem of the wheel floating over the component if the jet is aimed directly at the point where grinding takes place.
The accurate control claimed over the coolant, which is delivered at 70 bar/110 litres per min (higher than for normal grinding), is achieved by mounting a two-axis CNC unit around the spindle, one axis (U) providing rotation, which allows both horizontal and vertical grinding, and the other (V)allowing radial positioning of the nozzles relative to the workpiece. The radial movement is needed to compensate for wear of the grinding wheel, which can halve in diameter during a machining cycle due to abrasive sacrifice during the machining process.
The workpiece is clamped on a rotary horizontal A-axis on a special fixture which itself is mounted on a rotary B-axis on the table, these accounting for the sixth and seventh computer controlled axes. Full form, hydraulic rolls are mounted on the table for rotary dressing of the grinding wheels.
The first production machines were installed at Glasgow in March 1999. Here,two manually-loaded Makino A55-5X.R machining centres are used for grinding compressor blades. In March this year (2000), two similar automated machines were installed and the first models have had automation equipment retrofitted.
Radial dovetail root forms are ground individually, a feature which was historically turned (with difficulty owing to intermittent cutting) by assembling the blades in a ring. Up to three grinding wheels are needed to complete the cycle, and there is also provision in the toolchanger for milling cutters and for a nylon abrasive brush for in-cycle deburring. Previously,radial cut-outs in the components were machined by putting them onto a separate milling machine – an expensive, double handling procedure. The grinding part of the cycle is now said to be completed in a floor-to-floor time of 4.8 minutes, approximately half the previous grinding time.
Process capability of Cpk 1.3 had to be met and this is reportedly being achieved easily. Consumables cost per part is said to be reduced by 86 per cent. A further advantage claimed is that the time to reset the machines to produce a new blade is just 30 minutes, far shorter than for the former process.
The largest Makino Viper grinding installation is at Rolls-Royce, Derby,where ten machines similar to those at Glasgow are deployed for grinding the fir tree root form and shroud end features of turbine blades. Traditionally, a tandem wheel grinding machine is used for this complicated process, but it has been replaced by single wheel grinding and automatically changing up to 15 wheels into the spindle from the Makino tool magazine.
Included in each of the five cells are two machining centres, two co-ordinate measuring machines and a cleaning booth, all served by a Fanuc robot in conjunction with an conveyor system built by NCMT Automation.
Based on the successful implementation of Viper grinding at these two sites,Rolls-Royce decided to extend its use to machining larger components. At the end of last year, a Makino MC1516-5XA machining centre was installed at Bristol for producing complex Inconel compressor casings up to 1.2m diameter. Around two-thirds of the cycle is grinding, the remainder being various prismatic machining operations. These are necessarily thin wall components, to keep the weight down. Turning would be slow and costly on inserts so the traditional method of machining the casings is electro-chemically, which in NCMT's opinion is also expensive. The flexibility of viper grinding together with the high stock removal rates, which are possible without exerting undue force on the component, are reported to work well.
Three further Makinos are in the Rolls-Royce Manufacturing-Technology Process Demonstration Cell at Bristol – two A55-5XRs and a larger A99-5XR. Here, in collaboration with Tyrolit and NCMT, the company is continuing its research into additional applications for this versatile new production method. Rolls-Royce holds the patent; and the viper grinding process is available through Tyrofit. NCMT is also developing further applications of the technique.
Details available from NCMT Limited. Tel: +44 (0) 208 398 4277; Fax: +44 (0)208 398 3631.
