Until now, turbine blades have been manufactured in the usual creep feed grinding process using grinding tools with a diameter of 400 mm or greater. Preferred grinding strategies were either grinding with the grinding wheel in constant contact with the roller dresser – otherwise known as CD grinding – or without CD grinding. In these processes, the cooling lubricant is injected directly into the grinding zone at 3 to 10 bar, usually at a speed ratio of 0.8 to 1 for the peripheral speed of the grinding wheel.
Tyrolit, a leading manufacturer of bonded abrasives, has developed a new product line, Viper Ultra specifically for use in the turbine industry. This advanced open structured grinding wheel, when used in conjunction with the Viper grinding process, is able to deliver higher productivity and lower production costs whilst achieving the ever more stringent quality demands of the aerospace industry.
Small wheel, big bite
The Viper grinding process, developed by Rolls Royce, Tyrolit and Raysun, uses a completely new approach. Firstly, grinding wheels with a diameter of 300 mm are generally used without CD grinding. Secondly, special nozzles are used to inject the cooling lubricant at 50 to 70 bar at right angles into the grinding wheel and away from the grinding zone.
The grinding wheel turns at a peripheral speed of up to 50 m/s and creates a layer of air on the wheel surface that the cooling lubricant has to break through. The high coolant pressures of 50 to 70 bar are important to ensure the coolant breaks through this layer of air and into the wheel. The specially developed open-pore grinding wheel is able to absorb the coolant and transport it into the grinding zone.
This special method of delivering cooling lubricant into the grinding zone enables particularly high metal removal rates to be achieved. In comparison to the usual creep feed grinding processes, with stock removal rates Qw of 5 to 25 mm3/smm, this patented system enables values of 50 to 100 mm3/smm to be reached. By further combining this Viper technology with the HSCD (High Speed Continuous Dressing) grinding process, it is possible to reach stock removal rates of up to 300 mm3/smm.
Grinding the hard stuff
The VIPER System is, at present, exclusively employed on Bridgeport and Makino machining centres. In addition to achieving a step change in the processing of difficult-to-machine nickel-based materials, the process also enables relatively small grinding tools to be used (up to 300 mm in diameter). These wheels can be switched in seconds with the aid of a tool changer, allowing the wheels to be optimised for either rough or finish grinding. As a result, machining times can be reduced and the high demands for profile accuracy and surface quality can be met.
Tyrolit has developed special high-porous grinding tools for the Viper grinding system, which exhibit exceptional features. Firstly, the coolant and grinding chips can be absorbed and transported extremely efficiently by means of the open-porestructure, and secondly, profile retention remains high.
With the new Viper Ultra product range, a special bond matrix has also been developed, which has double the strength of previous bond systems and thus improves grain retention. This enables high dynamic loads to be absorbed without damaging or breaking the bond bridges, resulting in optimal use of the abrasive grain. This bond system is capable of achieving the same high performance associated with sintered aluminium oxides with cheaper aluminium oxides without compromising on performance in any way. High priced sintered aluminium oxides also have a very aggressive effect on the dressing tool and consequently, promote increased diamond wear and higher dressing costs.
Achieving high precision & accuracy
The Viper machining centres enable several dressing tools to be stored on one or more spindles at the same time and for these to be deployed individually as required. The function of the roller dresser is to transfer the profile to the grinding wheel and to generate optimum topography on the grinding wheel surface. Special natural diamonds are used for this, which are galvanically embedded in a layer of nickel. The roller dresser is manufactured using a reverse process to enable the particularly high requirements of profile accuracy to be fulfilled. This production method allows the largest possible diamonds to be used and guarantees maximum profile accuracy and dimensional precision with minimal wear.
The benefits of the Viper Ultra wheel when combined with the Viper grinding system and an optimised high-pressure coolant delivery are high stock removal with an exceptionally cool grind. The lower heat development, therefore, reduces the risk of burning the component and of any unwanted change to the material structure. The grinding forces are additionally lower with the viper process, which results in reduced stress on the component and less force on the work holding system. The cool grind is especially welcome when machining heat-sensitive materials, such as, the nickel-based alloys used within the turbine industry. The following two examples highlight the efficiency of the Viper Ultra system.
Case 1: A fir-tree profile of a nickel-based turbine blade of aircraft turbine
For this application, a 245x25x32 mm Viper Ultra wheel was used, which is applied at an operating speed of 40 m/s. With the Viper Ultra grinding wheel, itwas possible to grind 30% more components. The lifetime, thereby, increased from 200 to 260 parts per wheel. The Viper Ultra grinding wheel optimised the process and dressing parameters, such that it was possible to achieve infeed values of 0.1 to 2 mm, depending on the process stage, at a traverse speed of 2000 mm/min. The required dressing amount was also reduced to 0.4 mm. The result was a productivity increase of 35%, achieving an annual cost saving of EUR 42,000 based on a total production volume of 35,000 components per year.
Case 2: A fir-tree profile of a nickel-based turbine blade of industrial gas turbine
A 250x25x32 mm Viper Ultra wheel was used in this case, and was also applied at an operating speed of 40 m/s. The infeed was increased up to 1.5 mm during pre-grinding, at a traverse speed of 1500 mm/min. This grinding wheel achieved an increase in service life from 150 to 250 components, which equates to a 67% rise. The optimisation of the process parameters resulted in a productivity increase of 30%, achieving an annual cost saving of EUR 20,000 based on a total production volume of 20,000 components per year.
The Viper grinding process facilitates higher productivity and lower production costs. By using modern machining centres, it is also possible to produce components in a single clamping operation, thus guaranteeing that the highest quality requirements are achieved.
A close collaboration between all partners, users, machine manufacturers and Tyrolit have combined to develop an outstanding process and will continue to make it possible to overcome any future challenges.