Grinding technology streamlines manufacture of jet engine compressor blades

Citation

(2005), "Grinding technology streamlines manufacture of jet engine compressor blades", Aircraft Engineering and Aerospace Technology, Vol. 77 No. 5. https://doi.org/10.1108/aeat.2005.12777eab.001

Grinding technology streamlines manufacture of jet engine compressor blades

Keywords: Aircraft engines, Grinding

A major step forward in the grinding process employed to manufacture compressor blades for jet engines has improved consistency and halved product cycle times. The improvements are so pronounced that one of the worlds top aero engine manufacturers has recently placed an order for three of Holroyd's new Edgetek grinding machines, all of which are being supplied as full turnkey systems complete with programming, fixturing and grinding wheels to produce engine parts (Plate 1).

Plate 1 A major step forward in grinding technology streamlines the manufacture of jet engine compressor blades

The basic crystalline structure of metals is sidestepped when turbine blades are created, the metal, typically a derivative of inconel or waspalloy is effectively grown at a precisely controlled rate and temperature. Each ingot that is to be machined into a turbine blade is formed as one crystal. As most engineers know, metal fatigue occurs along crystalline boundaries, which explains why fatigue breaks create a ragged line delineated by the crystal boundaries within the material.

Since a turbine blade has no crystalline boundaries, there are fewer weak points in the material due to its structure. The result is that bending forces and temperature fluctuations, provided they are kept within a certain safety boundary, cannot cause metal fatigue. The blade's fatigue strength will not therefore deteriorate through usage and throughout its life.

Another useful property of a single crystal structure is that heat expansion and physical behaviour under stress are consistent and therefore, predictable. This factor allows internal compressor stages in the engine to be constructed using many blades that are simply slotted together and key into each other to make a complete fan. As the engine is brought up to working temperature, heat expansion locks the blades together and the fan becomes a single rigid structure, also retaining the uniform and predictable thermal and stress characteristics.

As with most “ideal” scenarios there is hitch: the alloy is extremely hard and difficult to machine. The hardness means that each blade has to be ground rather than milled, grinding creates heat and if the work piece is heated beyond a certain temperature boundary the crystalline structure changes. The single crystal can revert to a lattice structure that can include multiple crystal boundaries and is immediately susceptible to fatigue breakage.

The temperature boundary is critical and hence the grinding of turbine blades is normally a lengthy and involved process using vitrified type grinding wheels, which have a tendency to produce both mechanical and heat damage to the metal surface unless they are very tightly controlled. A damaged part is often not immediately detectible, so exhaustive safety checks are carried out using elaborate testing procedures such as nital etching and X-ray stress analysis.

Holroyd's solution to the problems of safely grinding turbine blades and consistently retaining a single crystal structure is to employ a grinding technique that does not raise the temperature of the work piece significantly, but is aggressive enough to quickly machine the hard material.

The process also imparts desirable surface qualities such as high compressive stress, which improves the creep life of the material.

The process, termed “High Efficiency Deep Grinding” (HEDG), is a relatively new machining technology that uses recently developed cubic boron nitride (CBN) super abrasive grinding wheels at very high-speeds; the result is that the heat generated by the grinding action is transferred to the metal being removed and is carried away from the work piece so quickly that it does not have time to conduct into the material substrate. Thermal damage is therefore minimised if not eliminated and although testing is still carried out, failure rates are effectively zeroed (Plate 2).

Plate 2 HEDG, is a relatively new machining technology that uses recently developed CBN super abrasive grinding wheels at very high-speeds

Compared to more conventional creep feed grinding, HEDG has much higher specific removal rates, typically 50-2,000 mm³/mm/s compared to 0.1-10 mm³/mm/s. In addition, the lower finished surface temperature of the work piece also adds an overall improvement in the quality of the finish itself.

The superior qualities of HEDG were determined in extensive machining trials conducted by Professor David Stephenson at Cranfield University using a Holroyd Edgetek 5 axis super abrasive grinding machine. The Edgetek machine is fitted with a very stiff grinding spindle, which uses hybrid ceramic bearings that facilitate the very large cuts possible with HEDG. In addition, the specially designed and formulated granite polymer composite base offers excellent damping properties, virtually neutralising resonant frequencies within the machine that could impair its accuracy.

Details available from: Paul Hannah, Machine Tools, Holroyd. Tel: +44 (0) 1706 526590; Fax: +44 (0) 1706 353350; E-mail: phannah@holroyd.renold.com; Web site: www.holroyd.com