Grinding is 30 times faster than milling

Citation

(2002), "Grinding is 30 times faster than milling", Aircraft Engineering and Aerospace Technology, Vol. 74 No. 1. https://doi.org/10.1108/aeat.2002.12774aab.007

Grinding is 30 times faster than milling

Keywords: Grinding, Machinery

During a recent open house at NCMT's Coventry technical centre, production engineers from Rolls-Royce as well as other OEMs and subcontractors were shown demonstrations of so-called "viper" grinding of Inconel on a Makino A99-CD machining centre. The technique was developed in collaboration with grinding wheel manufacturer, Tyrolit, producing aircraft engine parts.

Major reductions in capital investment, set- up times, production costs and lead times are said to have resulted at Rolls-Royce group factories in Glasgow, Derby and Bristol as a result of viper grinding on Makino machining centres. Required process capability of typically Cpk 1.3 is reportedly being achieved easily on components ranging from turbine blades to compressor casings (Plate 2).

Plate 2 The Makino FMS at Flight Refuelling, Wimborne, is capable of all operations needed to machine critical Eurofighter Typhoon missile launch system components

The first operation on an Inconel test piece during the open house showed a 15mm wide

aluminium oxide wheel rough grinding an 8mm deep slot in six seconds, using continuous dressing of the wheel. According to NCMT the same operation would take 3 minutes using a carbide end mill on the same machine.

Operation 2 involved using a similar wheel, this time 30mm wide, to produce a 25mm wide flat on the flange into which the previously mentioned slot had been ground. Stock removal depth was 8mm, as evidenced by the disappearance of the slot, and the process took just 12 seconds. NCMT is also of the opinion that this would have taken a face mill much longer to do the same job.

If the expense of a diamond continuous dressing wheel and 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 open house.

One factor behind the success of viper grinding is thought to be the use of very high pressure, high flow rate, refrigerated coolant directed at the machining area – 70 bar through an NC nozzle directed continuously just above the point of grinding and 100 bar through the spindle. To withstand these pressures as well as the copious generation of grinding dust, the Makino A99 is constructed with a double skin and has its slideways protected by special guards.

The application takes 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 12,000 rpm, all consistent with high metal removal rates. Both internal and external grinding may be undertaken.

Owing to the difficulty associated with milling of Inconel, conventional creep feed grinding has traditionally been used wherever practicable. The rationale behind finding an alternative is said to stem from the significantly higher capital cost of grinders compared with that of a machining centre. Other financial considerations are considered 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.

NCMT also asserts that another relevant factor is 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.

The company believes that 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. They are changed automatically in the same way as, for example, a milling cutter, and dressing is restricted to the profile that is in use. For complete machining of components, milling cutters, drills and other tools may be stored in the magazine as well and called up automatically within the cycle. It is also said to be possible to carry out static tool work such as broaching.

In the viper grinding process, vitrified, aluminium oxide wheels are employed in creep feed grinding mode. They claimed to have been found to machine long-chipping, sticky, nickel-based alloys faster than cubic boron nitride. Additionally, the porosity of aluminium oxide is beneficial in retaining coolant and assisting in its delivery to the point of cutting. The open grain carries the coolant through the point of cutting, reportedly providing both lubrication and cooling and avoiding the problem of push-off caused by the wheel floating over the component if the jet is aimed directly at the point of machining.

Accurate control over the coolant is achieved by mounting a 2-axis programmable coolant nozzle around the spindle nose, one axis (U) providing rotation, which allows both horizontal and vertical grinding," and the other (V) allowing radial positioning of the nozzles to take account of grinding wheel wear. Automatic nozzle changing is effected from a cabinet mounted on the rotary table (B) axis, behind the rotary horizontal A-axis on which the workpiece is fixtured.

Details available from: NCMT Limited, Tel: +44 (0)20 8398 4277; Fax: +44 (0) 20 8398 3631; E-mail: richardtrimbee@ncmt.co.uk