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.