On a horizontal machining centre, using the VIPER grinding technique, 5-axis machining of a nickel alloy compressor blade involved grinding a radial dovetail root form, including a milling operation.
On a Makino A55 machining centre, three separate demonstrations at UK Makino agent's technology centre in Coventry, underlined the flexibility of the machine as a platform for producing components by grinding and other metal cutting operations in very few set-ups, or even one hit. Five-axis machining of a nickel alloy compressor blade involved grinding a radial dovetail root form followed by a combination of grinding and finish milling of a small-radius scallop, which the grinding wheel was unable to access. Total cycle time was 5.5 min.

Conventionally, blades are first ground and then set up for a second operation on a machining centre.

Savings in handling, fixturing and cycle time are considerable with the Makino solution.

The second component demonstration involved grinding the fir tree root form for a high-pressure turbine blade from solid nickel alloy in a cycle time of 3.5 min.

Normally, such components are ground from a closer-to-form forging on a dedicated grinder.

The most usual production process for this popular aerospace component is in a 'Nagare' cell comprising, say, 10 separate machines tended by five operators to clamp every part once on each machine.

On a Makino VIPER machining centre, parts generally come off complete in two operations, drastically reducing production costs.

Grinding of a radial seal slot, 2.6mm wide by 7.3mm deep over a 200mm chordal length, formed the third demonstration.

Instead of being produced in Inconel, the part was of a less expensive steel, which in this application is more challenging to grind.

Two small, aluminium oxide wheels were used, rather than traditional cubic boron nitride, one for the outside radius and another for the inside radius to achieve perpendicular groove sides.

Dressing was by NC profiling, avoiding the expense and long lead time of full-form dressing (ideal for fast-make situations).

Two-axis control of the grinding wheel plus B-axis positioning of the component achieved the required contouring of the slot.

Another aspect of this demonstration was the manual-assist loader at the front of the machine.

Components were fixtured outside the machining area and transferred with minimal effort to a zero point fixture on the table ready for machining.

Believed to be a 'world first', a horizontal machining centre was demonstrated the complete VIPER grinding of root slots on a turbine disc at rates up to four times faster than milling.
A Makino A99 VIPER horizontal machining centre recently demonstrated the complete grinding of root slots on a turbine disc - a world first - at Makino UK agent NCMT's technical centre in Coventry. To simulate this in a demonstration, an inconel billet was inserted into one side of a cast iron fixture. Using continuous dressing (CD), roughing was achieved in two passes using a 300mm diameter by 25mm wide Tyrolit wheel.

One half of the dresser imparted the square form to the first wheel for roughing, while the other half was used to dress the second wheel, which finishes the profile of the slot floor, in this case using dab rather than continuous dressing.

A third grinding wheel was automatically exchanged into the machining area, the operation also requiring automatic PCN change.

A CD cut was again employed to semi-finish grind the fir tree root form, first on one side and, following 180 deg rotation, on the second side as well.

A fourth and final grinding wheel used the same dresser in 'dab' mode to finish grind the root form in two passes to an accuracy of 10 micron.

A steel billet clamped on the other side of the cast iron fixture was used to illustrate how much more efficient VIPER grinding is than milling.

First, a 50mm diameter face mill with three 12mm diameter ceramic button inserts made a 34mm wide by 4mm deep pass at 380mm/min, resulting in a metal removal rate of 52cm3/min.

Then, with a 300mm diameter, aluminium oxide wheel cutting at 1.5m/min feed rate, 60m/sec peripheral speed, the same width and depth of cut was completed at a metal removal rate of 200cm3/min - four times faster than milling.

It was noteworthy that the effects of heat in the metal plus severe burring could be seen on the side that had been milled, whereas with the relatively cool grinding operation, there was no discernible discoloration and minimal edge burring.

Various other machining operations were demonstrated on this machine in addition to grinding of the root slots.

They included drilling and spiral milling of air cooling holes in the root slot, precision OD grinding of a bearing journal, grinding of a high-precision, curved-tooth coupling, and turning of a hook groove using a static tool in the spindle.

Incorporating latest CNC and servo technology to provide dynamic performance and smooth running characteristics a grinding machine produces accurate long lead/high helix components quickly.
Matrix Machine Tools is demonstrating the new 6950 High Helix Thread grinding machine at IMTS 2006. The machine has been designed specifically for the fast and accurate grinding of long lead/high helix components. The machine incorporates the latest advances in CNC and servo technology to provide the dynamic performance and smooth running characteristics required for precision grinding applications.

The radical Matrix 6950 is the first machine which controls the grinding of the high helix via software rather than by mechanical means, the only technology available until the introduction of the innovative Matrix technology.

Controlling the grinding of the high helix by software is a far better production method because it is more flexible and faster than any mechanical system.

In grinding a high helix, the geometry changes constantly because the wheel is always getting smaller.

The Matrix system dresses out the interference, which would be caused by the change in grinding wheel shape without operator intervention or the need to change the diamond roll.

The Matrix-created software programme 'NutOpt' predicts the grindability of the component.

The innovative technique developed by Matrix uses a corrected compound angle approach of the spindle, in conjunction with the two axis CNC dresser, which continually corrects the grinding wheel profile and allows components with a higher than usual helix angle to be successfully ground.

The NutOpt software allows the component designer to predict the grindability of part and make any necessary changes at the design stage.

The interaction of design and production engineers creates a better component which is easier and less expensive to manufacture.

The software has been created by Matrix research and development engineers in response to the recognised production problems being caused by the need to grind a high helix.

The development of the linear motion systems to produce faster axis movement on any product controlled by ballscrew and nut assemblies has meant that pitch/lead has increased and that thread helix angles have become larger.

On these larger angles it is difficult to insert a grinding wheel within the bore of a nut at the correct helix.

To overcome this problem the grinding wheel has to be introduced at a smaller helix and the profile corrected.

The Matrix system for high helix grinding gives substantial cost savings in this traditionally difficult area of grinding because a reduced number of tools are required, set up times are reduced, and human error is removed from the process.

This is because the Matrix process is entirely programmable, eliminating the need for specialised mechanical tooling.

Only one diameter disc is needed to profile the wheel, completely removing the need for specialised tooling specific to individual combinations.

This diminishes substantially set up times and operator involvement in the process, improves accuracy and minimises scrap.

David Giles, vice president of Matrix Machine Tools, said: 'This is a new standard and sets the benchmark against which all other machines operating in this manufacturing sector can be judged.

The NutOpt software allows the component designer to predict the grindability of part and make any necessary changes at the design stage.

The interaction of design and production engineers creates a better component which is easier and less expensive to manufacture.

'Very impressive performance extreme removal' - or VIPER - grinding is 10 times faster at removing metal than milling when machining aerospace and industrial turbine blades.
Broaching is a notoriously expensive and time-consuming manufacturing process, with machines costing up to GBP 2 million and suppliers quoting lead times as long as two years. Consumable costs are also high. The technique dates back to the industrial revolution and if you asked most production engineers which machine tool they would most like to eliminate, it would be the broach.

Its main benefit is that it has been used for decades, so the metallurgy of broached components is known precisely.

This is important when manufacturing safety-critical parts such as turbine and compressor discs and blades, and broaching has long been approved by companies building aero engines and turbines for power generation.

Makino-NCMT Grinding Division has been making significant strides towards replacing broaching with grinding for critical part production.

The British firm is a recognised world leader in so-called VIPER (very impressive performance extreme removal) grinding, introduced in 2001 as a replacement for creep feed grinding of inconel and other nickel-based alloys using CBN wheels.

While grinding is traditionally regarded as a low-stock-removal, finishing process, the VIPER process has also been shown to be 10 times faster at removing metal than milling.

Rolls-Royce in collaboration with Japanese machine tool builder, Makino, its UK agent NCMT, and Austrian grinding wheel producer, Tyrolit, developed the patented VIPER technique.

VIPER uses inexpensive, vitrified aluminium-oxide wheels of small diameter, typically 220mm diameter by 40mm wide, mounted in the spindle of a horizontal machining centre (HMC).

They are up to eight times faster than CBN when machining long-chipping, sticky, nickel-based alloys and set-up times are faster, lowering economic batch size.

High-pressure coolant is employed, both through the spindle and via a programmable coolant nozzle (PCN) to direct it accurately to the point of cutting.

The process is gradually being adopted by the major aero-engine companies and their subcontractors to replace some, if not all, broaching and milling operations.

Land-based turbine producers have been quicker to change, as their components are not so safety-critical.

Rolls Royce has for many years employed VIPER grinding on Makino HMCs for the manufacture of compressor blades, turbine blades and engine casings.

More recently, a land-based power generation company has invested in the technology for rough grinding instead of broaching the root form of nickel alloy turbine blades for power generation, and is close to replacing the finish broaching operation with grinding.

It is noteworthy that metallurgical analysis by Makino-NCMT shows lower residual stress in a ground component compared with its broached equivalent.

A big advantage of VIPER grinding is that it is carried out on an HMC using small grinding wheels that may be interchanged from the tool magazine.

A milling cutter, drill or other driven or static tool for turning can also be exchanged automatically into the spindle during a cycle, allowing second and subsequent machining operations to be carried out in the same clamping.

Multi-functional machining including VIPER grinding as one of the operations was pioneered by the Japanese machine manufacturer, whose UK agent, NCMT, was appointed earlier this year to market the technology not only in the UK, but also across Europe.

It follows the completion of many successful VIPER turnkey installations by NCMT in the UK over the past five years.

Versatility is almost unlimited using the Makino-NCMT solution owing to the ability to mix and match machining processes.

Maximum advantage is taken of the VIPER process for roughing and semi-finishing.

If finishing is not appropriate using the technique, perhaps because a grinding wheel cannot access some features, or a manufacturer's 'lifing' engineers do not accept grinding for the final operation, then milling or broaching can complete the machining cycle.

Related areas that have been researched by Makino-NCMT to achieve required process capability and throughput include gear grinding software, flexible fixturing, and manual or in-cycle part measurement, verification and truing.

In addition to aerospace and land turbine components, VIPER grinding is also suitable for cost-effective manufacture of medical components, especially as their increasing complexity is making manufacture more difficult on conventional or modified tool and cutter grinders.

At a European seminar and exhibition of VIPER grinding held at NCMT's Middlemarch technical centre near Coventry, UK, on 14th and 15th June, six 7-axis Makino machining centres were on show.

One was static with guards removed to give visitors an opportunity to inspect the build quality of these Japanese machines.

Metal removal rate on this size of machine - an A55 - when machining nickel alloys is of the order of 70 cm3/min.

As with all VIPER-specification machining centres, it had three linear axes, two rotary axes, two further NC axes for precise positioning of the coolant nozzles, reinforced guarding and pressurised rather than grease lubrication of the slideways.

Coolant through the programmable coolant nozzles is 110 litres/min at 70 bar, while the coolant through the spindle is 30 litres/min at the same pressure.

Overall coolant flow is 200 litres/min including the wash for the vertical covers and table.

Air in the electrical cabinets is at positive pressure to prevent ingress of dirt and contaminants.

The five other Makino HMCs at the seminar and exhibition demonstrated different applications of VIPER grinding, both in isolation and combined with other machining operations.