Believed to be a 'world first', a horizontal machining centre was demonstrated the complete VIPER grinding of root slots on a turbine disc at rates up to four times faster than milling.
A Makino A99 VIPER horizontal machining centre recently demonstrated the complete grinding of root slots on a turbine disc - a world first - at Makino UK agent NCMT's technical centre in Coventry. To simulate this in a demonstration, an inconel billet was inserted into one side of a cast iron fixture. Using continuous dressing (CD), roughing was achieved in two passes using a 300mm diameter by 25mm wide Tyrolit wheel.

One half of the dresser imparted the square form to the first wheel for roughing, while the other half was used to dress the second wheel, which finishes the profile of the slot floor, in this case using dab rather than continuous dressing.

A third grinding wheel was automatically exchanged into the machining area, the operation also requiring automatic PCN change.

A CD cut was again employed to semi-finish grind the fir tree root form, first on one side and, following 180 deg rotation, on the second side as well.

A fourth and final grinding wheel used the same dresser in 'dab' mode to finish grind the root form in two passes to an accuracy of 10 micron.

A steel billet clamped on the other side of the cast iron fixture was used to illustrate how much more efficient VIPER grinding is than milling.

First, a 50mm diameter face mill with three 12mm diameter ceramic button inserts made a 34mm wide by 4mm deep pass at 380mm/min, resulting in a metal removal rate of 52cm3/min.

Then, with a 300mm diameter, aluminium oxide wheel cutting at 1.5m/min feed rate, 60m/sec peripheral speed, the same width and depth of cut was completed at a metal removal rate of 200cm3/min - four times faster than milling.

It was noteworthy that the effects of heat in the metal plus severe burring could be seen on the side that had been milled, whereas with the relatively cool grinding operation, there was no discernible discoloration and minimal edge burring.

Various other machining operations were demonstrated on this machine in addition to grinding of the root slots.

They included drilling and spiral milling of air cooling holes in the root slot, precision OD grinding of a bearing journal, grinding of a high-precision, curved-tooth coupling, and turning of a hook groove using a static tool in the spindle.