A five-axis CNC grinder from Blohm Jung GmbH is helping with the production of nickel-alloy turbine components at the Lincoln factory of Siemens.
This machine was purchased — via Blohm’s UK agent JRA Bennett Ltd (
www.jrabennett.co.uk) — in response to a significant increase in the factory’s output of industrial gas turbines (IGTs) in the past two years.
An increase in engine rebuilds, following the opening of Siemens’ new service facility nearby, has also contributed to the higher workload.
To raise grinding efficiency, the machine — a Profimat MC 607 five-axis CNC grinder — is one of the first in the world to be fitted with Blohm’s new RazorTec wheel-cleaning technology, which reduces cutting forces and allows the use of higher feed rates.
Whereas on other creep-feed grinding machines on site, table speed is restricted to between 600 and 800mm/min to prevent burning of the nickel-alloy compressor parts, the Profimat MC 607 achieves 1,000mm/min — and productivity improvements ranging from 25 to 67%.
A wheel speed of 21m per sec and an in-feed of 1µm/rev are the same on all Siemens’ blade machines, as this cutting data has been established over many years as optimal for grinding and minimising wheel wear.
The Lincoln factory specialises in producing Siemens’ smaller IGTs for power generation up to 15MW. The components ground on the new Blohm machine are for the hot part of the engine, notably low- and high-pressure nozzle guide vanes for the rotor and vane segments for the stator. Production is round the clock during weekdays and extends into the weekend.
Blade shop investment
Phil Howard, senior manufacturing engineer at Siemens in Lincoln, says: “We needed investment in our blade shop and wanted a new machine that could grind both nozzles and segments. We investigated VIPER grinding on a machining-centre platform and looked at other makes of five-axis creep-feed grinders, but we finally selected the Blohm machine for a number of reasons.
“First was the high productivity and accuracy achieved during trials. Despite the faster cycles, we immediately matched the tolerances that we were getting on grinders of a different make on which we had been tweaking the process for the past 12 years.
"Vane output is significantly higher, yet the tolerances are now superior to those we were achieving before. The Blohm grinder also had the smallest footprint — just 3 x 5m — of the machines we considered.
"Another reason for choosing the Blohm was our successful use here of two 22-year-old Profimat MTs, which are still holding tolerance grinding the teeth of Hirth couplings.”
An engineer from Blohm was at Siemens for three weeks to ensure that the installation, acceptance and training went smoothly. Mr Howard wrote the programs at an off-line station running Blohm software, which provides the user interface to the machine’s Siemens 840D control.
The Profimat MC 607 is configured for large production runs up to 70 parts, rather than one-offs and small batches. Between two and five wheels of 400mm diameter are exchanged automatically from the tool changer to complete a grinding cycle.
To maximise production output, the wheels are continuously dressed during grinding using a rigid, twin-station, overhead dressing unit (this swivels under program control and normally accommodates two diamond dressing rolls).
Wheel-cleaning technology
To maximise productivity, the Profimat MC 607 has a wheel-cleaning system — RazorTec — that continuously projects filtered coolant at 40 bar towards the grinding wheel. Consistency is achieved by automatically maintaining the same angle and distance between the nozzle and the wheel as the latter wears.
The high-pressure coolant forces metallic debris out of the porous structure of the aluminium oxide wheel, keeping it sharp and consequently raising stock removal rates while lowering heat transmission to the workpiece. The risk of burning and micro-cracking of the nickel alloy in a heat-affected zone near the surface of the material is consequently much diminished.
Further advantages are reduced wear on the grinding wheel and dresser, better profile integrity, lower grinding forces and a decrease in power consumption. The wheel-cleaning technology was developed in collaboration with IWT Bremen, a research institute located on the University of Bremen campus.
The nozzle positioning unit is servomotor-driven and programmable, and the cleaning jets are mounted just above the coolant nozzle, which is angled directly at the point of grinding.
The carriage carrying the two nozzles is also adjustable automatically or manually in the Z axis. It is this level of precision that allows such efficient wheel cleaning and coolant delivery.
Weighing in at 8.5 tonnes, this travelling-column machine has a 45kW spindle drive, a cartridge-type HSK-A 80 grinding spindle, and a magazine for up to 24 tools that can include milling and drilling cutters in addition to grinding wheels. Axis travels are 700, 650 and 520mm in X, Y and Z respectively.
Furthermore, the machine is equipped with linear guideways for high dynamic performance, while glass linear scales provide high-precision feedback of axis positions. The machine at Siemens is equipped with an indexing table that provides the two rotary NC axes; also featured is a System 3R pallet system to clamp the IGT components for grinding.