Continuous dress (CD), high-speed machining of a 700mm long, industrial turbine blade made from inconel 718 was simulated on a Makino A99 horizontal machining centre at Makino UK agent NCMT's technical centre recently. An air-cut cycle showed the strategy for roughing, semi-finishing and finish-grinding the root block using three separate wheels. A single dresser was used continuously to maintain the profiles on all three wheels, quadrupling stock removal rate to 200cm3/min compared with non-CD VIPER grinding.

Crucial when machining a turbine blade root form is to maintain close tolerance on the seating faces that oppose the centrifugal force when the engine starts.

Allied to this, symmetry of form also needs to be very accurate, as otherwise the blade tries to twist out of alignment.

It is difficult to maintain the required accuracy if the machining centre's rotary axes are interpolated during grinding.

Consequently, a narrow (30mm), 300mm diameter roughing wheel was used to grind most of the material from each fir tree serration in turn while simultaneously interpolating the rotary axes, as accuracy was not an issue during this operation.

The component was then swivelled through 180 deg and the same wheel was similarly used to rough the serrations on the other side.

For the finishing passes, however, both rotary axes were clamped and only the linear X-axis of the table was employed, while an 80mm wide wheel carrying the entire root form profile plunged vertically in Y.

To minimise 'stagger' between the two sides of the root form, the component was maintained in the same orientation and the wheel was swung below the component to finish grind the other side.

In a conventional production route, each side of the root form has to be machined on different grinders, compromising accuracy, unless the manufacturer invests in an extremely expensive, double-sided grinder.

Between 4 and 6 micron total tolerance for the stagger between the two sides of the root profile is maintained, even on large blades, which is three to four times better than traditional, multiple set-up manufacturing processes involving, say, eight operations on six different machine tools.

The application again highlighted the benefit of one-hit grinding, (plus milling, drilling, etc, as appropriate) using inexpensive, aluminium oxide wheels on a more or less standard machining centre with a large rotary axis.

On 5-axis grinders, the rotary axis tends to be much smaller.

In the demonstration, the turbine blade, which was originally fixtured off the aerofoil alone, was inverted and reclamped off the machined root block and the aerofoil for machining of the shroud end, Z notch and seal fins.