Traditional manufacturing methods based on CDCF achieve very high production rates in both the aeroengine and land-based gas turbine markets. Extremely complex multispindle machines by OEMs such as Blohm, Elb, and Excello [n. d.] were designed in the late 1970s and 1980s that could grind all the surfaces of a blade in cycle time of the order of a minute. Novel machine design concepts such as opposing twin wheel spindles could grind two opposing faces such as both dovetail forms at the same time to balance the forces at the higher metal removal rates [Salmon 1984]. The output of these machines is still unrivalled but the process was found to have several drawbacks. First, the machines were very complex, some with up to five spindles and over 1,000 control feedback inputs, making them difficult to maintain and requiring a very skilled workforce to maintain them. Second, the processes were geared to high production where change over times could be lengthy. Consequently, large quantities of expensive components would be produced generating a high “work in progress” inventory and total manufacturing times in terms of months. In the 1990s, manufacturing moved toward a JIT strategy in conjunction with a policy for 6 (Six sigma) quality control. This, in turn, led to cell approach for manufacturing where all the equipment to make a part, including CMM inspection and other manufacturing processes such as electrodischarge machining (EDM), were contained within a single cell with a minimal number of operators. Grinders were now much simpler, lower priced, and with tooling designed for quick change over times where necessary. An operator under these conditions could only efficiently function with cycle times of the order of 4 min. The need for CDCF under these conditions was eliminated.
16.5.4.3.2 Stock Level
Older casting technology produced casings with very heavy levels of stock up to 10 mm deep making them particularly suited to CDCF. Modern near net shape casting methods have now reduced these stock levels to as little as 1 to 2 mm on many aeroengine components, eliminating the need for a high Q operation and making processing much more suited to a CBN solution.