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Case Study - DT40 - Delta Turn

The Deltaturn Super Precision Lathe was designed and built to meet the specifications necessary to turn hard disc drive hubs for the computer industry. Cranfield Precision were contracted to manufacture and demonstrate capability on a range of customer components before delivering two machines to the US production facility in California.

The original workpieces, shown at the bottom of the collection of hard and soft turned samples, were made from either soft steel or brass. The outer surfaces and the bore were turned from the semi-finished blanks. A hydrodynamic, sinusoidal, bearing scroll had to be broached or grooved into the finished bore without leaving either swarf or a burr behind.

Hard Turned Parts

The required accuracy, repeatability and surface finish of the component exceeded specifications normally seen on conventional chucking lathes. A new concept in machine design was needed to ensure that the parts could be manufactured in volume production and within tolerance.

Key precision machine design techniques were invoked, in particular, hydrostatic bearing systems, high machining loop stiffness and direct drive control. The most notable aspect of the design is the use of a two-axis plate grating scale to provide position control for the principle orthogonal linear axes and to provide intrinsic correction for straightness, squareness and cutting force errors.

The machine was built and successfully achieved the necessary productivity targets and accuracy criteria. The DT40 machine platform was later redesigned to allow the development of a high precision hard turning processes. The original aerostatic workhead was replaced with a higher stiffness hydrostatic bearing design capable of reaching cutting speeds of 220 m/min on small and medium sized hardened bearing parts. Trials using CBN tools have demonstrated semi-finishing material removal rates at cut depths up to 0.25mm in 60Rc bearing steels. High quality finish turned surfaces as fine as 20nm Ra have also been produced in similar hardened bearing steels.

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