Major reductions in capital investment, set-up times, production costs and lead times have resulted at Rolls-Royce group factories as a result of viper grinding on Makino M-Cs.

Major reductions in capital investment, set-up times, productioncosts and lead times have resulted at Rolls-Royce group factoriesin Glasgow, Derby and Bristol as a result of viper grinding onMakino machining centres. Required process capability oftypically Cpk 1.3 is being achieved easily on components rangingfrom turbine blades to compressor casings. During a recent openhouse at NCMT's Coventry technical centre, productionengineers from Rolls-Royce as well as other OEMs andsubcontractors were shown demonstrations of so-called'viper' grinding of inconel on a Makino A99-CDmachining centre.

The technique was developed in collaborationwith grinding wheel manufacturer, Tyrolit, for producing aircraftengine parts.

The first operation on an Inconel test piece duringthe open house showed a 15mm wide aluminium oxide wheel roughgrinding an 8mm deep slot in six seconds, using continuousdressing of the wheel.

The same operation would take 3 minutesusing a carbide end mill on the same machine.

Operation 2involved using a similar wheel, this time 30mm wide, to produce a25mm wide flat on the flange into which the previously mentionedslot had been ground.

Stock removal depth was 8mm, as evidencedby the disappearance of the slot, and the process took just 12seconds.

It would have taken a face mill much longer to do thesame job.

If the expense of a diamond continuous dressing wheeland arbour cannot be justified for a particular application,Makino can equip the machine with an hydraulic, 3000 rpm,table-mounted diamond dresser, as was demonstrated at the openhouse.

One factor behind the success of viper grinding is the useof very high pressure, high flow rate, refrigerated coolantdirected at the machining area - 70 bar through an NC nozzledirected continuously just above the point of grinding and 100bar through the spindle.

To withstand these pressures as well asthe copious generation of grinding dust, the Makino A99 isconstructed with a double skin and has its slideways protected byspecial guards.

The application takes advantage of themachine's rigid structure, efficient swarf and coolantcontainment and management, and advanced spindle technology witha wide speed range of 50 to 12,000 rpm, all consistent with highmetal removal rates.

Both internal and external grinding may beundertaken.

Owing to the difficulty associated with milling ofInconel, conventional creep feed grinding has traditionally beenused wherever practicable.

The rationale behind finding analternative stems from the significantly higher capital cost ofgrinders compared with that of a machining centre.

Otherfinancial considerations are the high price of standard,large-diameter profile grinding wheels and the greater levels ofoperator attendance associated with conventional grinding ascompared with a machining centre.

In particular, a grinder maytake up to a day to set up for a new component whereas a Makinomachining centre can be reconfigured in one to two hours, andoften considerably faster.

Another relevant factor is that asingle, large wheel has to incorporate the forms of all thefeatures to be ground on a component, so whilst only a small partof the periphery may be grinding the component at any particulartime, continuous dressing of the whole wheel peripheryunnecessarily wastes this expensive consumable.

Herein lies thekey benefit of translating the process onto a machining centre.Small grinding wheels, each with the profile of a specificfeature on the component, are held in the tool magazine.

They arechanged automatically in the same way as, for example, a millingcutter, and dressing is restricted to the profile that is in use.For complete machining of components, milling cutters, drills andother tools may be stored in the magazine as well and called upautomatically within the cycle.

It is also possible to carry outstatic tool work such as broaching.

In the viper grindingprocess, vitrified, aluminium oxide wheels are employed in creepfeed grinding mode.

They have been found to machinelong-chipping, sticky, nickel-based alloys faster than cubicboron nitride.

Additionally, the porosity of aluminium oxide isbeneficial in retaining coolant and assisting in its delivery tothe point of cutting.

The open grain carries the coolant throughthe point of cutting, providing both lubrication and cooling andavoiding the problem of push-off caused by the wheel floatingover the component if the jet is aimed directly at the point ofmachining.

Accurate control over the coolant is achieved bymounting a 2-axis programmable coolant nozzle around the spindlenose, one axis (U) providing rotation, which allows bothhorizontal and vertical grinding, and the other (V) allowingradial positioning of the nozzles to take account of grindingwheel wear.

Automatic nozzle changing is effected from a cabinetmounted on the rotary table (B) axis, behind the rotaryhorizontal A-axis on which the workpiece is fixtured.