Grinding nickel-alloy blades

The Middle East is home to one of the fastest growing power requirements in the world. With the Arabian Peninsula's industrialisation and commercial development continues at a rampant pace, meeting electricity demand is critical to achieving the goals the Gulf nations have set for themselves.
Nickel alloys are commonly used for power-generating turbine engines because heat transfer through these materials is very slow. However, the properties that enable nickel-alloy blades to perform effectively in the hot section of turbine engines also make the blades challenging to machine. A grinding technique akin to high-speed machining offers an alternate method for grinding nickel-alloy blades used in turbine engines.

Speed-stroke grinding

A grinding concept, called speed-stroke grinding, is analogous to high-speed machining. The technique takes light, fast passes with the grinding wheel so that any heat generated during the operation will be expelled with the chips rather than into the surface of the blade.

The Blohm Jung Prokos machine grinds lightly and allows extremely fast workpiece movement. It uses linear motor drives to move its aluminium worktable along the X and Z axes. The linear drives allow the table to move at 4,700 ipm in X and 2,000 ipm in Z. The table has an integral 200-rpm rotary torque motor that enables B-axis workpiece rotation. Simultaneous rapid movement of the worktable and rotary table allows the grinding wheel, which is brought to proper position in the A axis, to follow the curved profile of a blade or vane.

In terms of process control, speed-stroke grinding tends to be more forgiving than a traditional creep-feed grinding operation. Effective creep-feed grinding requires tight control of speeds, feeds, coolant pressure and application, as well as coolant nozzle shape. If any of these elements fall out of that tight control band (say a coolant nozzle gets bumped or coolant concentration levels are too low), problems can quickly develop as heat is generated at the workpiece surface. Because heat is delivered away from the workpiece with the chips, the process control band for speed-stroke grinding is more open. Speed-stroke grinding is also said to use a fraction of the amount of coolant that would typically be required for creep-feed grinding.

In addition to providing heat removal with the chips, speed-stroke grinding's light passes lower grinding wheel pressure on the blade. This reduces the chance that the blade's airfoil will deflect or material compression will occur at a microscopic level during grinding.

Although blades have contoured surfaces, programming such grinding operations is not as complicated as creating five-axis machining tool paths, for example. Grinding operations generally call for motion that's either straight, to produce grooves in a blade's root, or convex or concave, to match an airfoil's profile.

If the huge power needs of the Gulf region are to be met, engines plus turbine solution must be reached. Meeting power and district cooling needs is vital to the success of the region's master plans. Lights-out in the Middle East is not an option. A practical application of the best-fit solution is critical to continuing economic growth. So care must be taken when grinding the blades used in turbine engines to minimise the amount of heat that is delivered into the workpiece. Because grinding heat won't dissipate throughout the entire nickel-alloy blade, it will remain localised at the blade's surface. This can cause the material to melt and re-solidify as a recast layer that may measure 0.001 to 0.002-inch thick. The recasting of the material can change the alloy's physical properties, causing the blade to become more brittle, softer or harder. And because of this, blade performanceEntrainement Nike