Flying High on a new technology

Centrax makes a number of components for the aerospace sector, including turbine blades used in the Rolls-Royce Tay engine, a two-shaft, high-bypass-ratio engine.

A grinding wheel mounted on the FGC 1000 spindle grinds a form into a turbine blade.

A Nikken 2-axis fixture holds blades so that the grinding wheel can grind profiles at the correct angle.

Two turbine blades before and after after grinding on the FGC 1000. The blade on the left shows the finished root structure, while the one on the right shows that the forged component has a great deal of material to be removed by the grinding process.

It pays to be on the lookout for new technologies, reports Tony Wiltshire, general manager of manufacturing and quality at Centrax Ltd., a supplier to the aerospace industry. While visiting a major customer, Rolls-Royce, in October of 1999, Wiltshire and his group spotted an intriguing machine that looked like a standard vertical machining center. Closer inspection, though, revealed that the FGC 1000, built by Bridgeport Machines, A Goldman Industrial Group Company, Leicester, England, also did grinding. The Centrax group quickly recognized the machine's potential, and within a year, had several of its own.

In the case of the FGC 1000, Centrax spotted a machine that represented a radical move away from conventional methods previously used for the production of components such as small gas-turbine blades. Wiltshire says that it was obvious that the FGC 1000 was ideal for machining turbine blades and would reduce setups and lead times.

"The biggest advantage is our doing several operations on one machine," comments Wiltshire. "We gained the same sort of benefits we got when we converted our labor-intensive, manual operations from milling machines to machining centers," he explains. The machines have also given Centrax the flexibility to tackle new jobs.

The company currently has seven FGC 1000s in its Turbine Components Division, five in turbine-rotor-blades production and two in process development.

The machine
Basically, the FGC 1000 is a 5-axis Bridgeport VMC 1000 configured to do creepfeed grinding as well as to integrate milling, drilling, tapping, probing, and deburring operations. Besides milling cutters, the tool magazine holds up to 10 small, aluminum-oxide grinding wheels mounted on standard BT 40 taper shanks. The wheels, each of which has its own profile, are changed automatically in the machine.

The FGC machines also incorporate a Rolls-Royce-developed technology called Viper (Very Impressive Performance Extreme Removal) that fits them with a special coolant-delivery system. A 90° indexing nozzle assembly accurately directs coolant, pumped at 70 bar pump pressure, into the grinding wheel. This configuration lets the machine grind in both the X and Y axes and remove stock many times faster than conventional creepfeed grinders.

The ability to change wheels is a significant way that the FGC differs from a conventional creepfeed grinders. Generally creepfeed grinders use either a single, large wheel to do all the work, or the job is done in several setups. On creepfeed grinders, the wheel is dressed with the forms of all the component features to be ground. This means that only part of the grinding-wheel periphery is in-cut at any time. Re-dressing then takes place as a post-operation. On the FGCs, conversely, the wheel is dressed in-cycle.

Centrax's FGCs contain up to nine 220-mm-diameter, 35-mm-wide wheels in the toolchange carousel. Diamond-roll units affixed to the machine beds dress these wheels. In some applications on the FGCs, Centrax uses cutting tools in addition to the grinding wheels.

The machines have a high-speed spindle that delivers 6,000 rpm. This speed is governed by the use of aluminum-oxide wheels, which can explode at higher speeds.

Making parts
Centrax saw the FGCs working most effectively for turbine rotor blades in reasonably close-to-shape cast forms. "Oversize forgings with lots of stock don't benefit fully from the Viper process because of the extra operations needed," Wiltshire comments. "Also, with a forging, we have to split the operations to give ourselves some rigidity in holding the part to have sensible roughing times."

The company also found the machine lends itself well to smaller components. This was proven on the first part Centrax made on the machine, a 6-in. blade previously done on four creepfeed grinders. The FGC made the part in a quarter of the time the grinders did.

Although Centrax is not currently making this test part on the FGCs, it could. Instead, the company opted to use the grinding centers exclusively in the production of new parts never machined through conventional means. While some might consider this a risky step, Centrax thinks the new business the machines have helped it bring in indicates the company made the right decision.

If the company had opted to make new parts through conventional means, then bring the FGCs on-line at a more leisurely pace, says Wiltshire, the tooling cost would have been astronomical. "It wasn't a viable proposition, so we bit the bullet," he explains.

Its next step was to address production engineering issues so it could get more from the process. "We got a reasonable amount from the Viper process," Wiltshire comments, "but there's plenty of scope for further development."

Method Development Engineer Martin Fisher agrees, stating that Centrax is still evaluating the optimum parameters to use on its components. "We've had a lot of process development to find the best way to machine, including what sort of stock removal is possible, the most efficient way to hold parts, and how to increase wheel life," he explains. "It's been a steep learning curve, but we're getting there. We do know the Viper concept is the way forward as far as making turbine rotor blades."

The machines have proven their worth quickly, say Patrick Moore and Matt Far-rant, two Centrax production engineers who have worked on the FGC 1000 project since its inception. The machines let them complete turbine rotor blades in one or two operations, rather than the seven or eight needed when using conventional creepfeed grinding. This cuts down on machine loading and increases part quality.

"With the Viper process," comments Far-rant, "you load the fixture once, and the machine grinds the root end. Then you manually turn over the component and grind the tip end. You only have one suite of tooling to concern yourself with instead of seven or eight different types of fixtures and seven or eight different operations."

Using the conventional process, each time you load a component, you are dealing with different fixtures and operations — all chances where you might scrap the component," points out Moore.

The Viper process also eliminates the need to encapsulate parts, a costly, time-consuming method of fixturing for conventional machining. Encapsulation involves injecting a liquid metal around parts to hold them at a predetermined angle for machining.

With the FGCs, encapsulation is not needed. Operators simply load the parts into the machine's special fixturing, and the machine automatically probes the part before grinding commences.

Wiltshire says the tooling associated with doing the job without encapsulation has been a challenge. However, he believes the benefits have far outweighed the difficulties. "Encapsulation becomes an industry in itself with employees having to encap and decap. And it's non-value-added."

Another advantage of the FGCs is that they free up operators to do other work while a job is in cycle. "In a conventional grinding shop," says Wiltshire, "several creepfeed grinders would be placed in a line, and each machine would handle a separate operation." These operations could take anywhere from 45 sec to 3 min, and each would require an operator to stand at the machine. Because the FGCs perform several operations during a 10 to 15-min cycle time, Centrax engineers can operate several machines at once or inspect parts while they are waiting for a job to finish.

Combining several operations on one machine has also improved part quality at Centrax. "You get the benefit of having one set of datums, which you machine off of," says Wiltshire. This means there's no stackup of tolerances.

Yet another benefit of the FGCs, says Moore, is that they give Centrax a degree of flexibility that conventional creepfeed grinders can't. "Where this machine gains is its ability to change the grinding wheel in-cycle, which can't be done on standard grinding machines. With the VMC concept and toolchanging, we do different features on one machine."

Moore says shops should consider these machines any time they have a particular component that demands a re-think in cycle time and costing. "We, as a company, have to continually push for cost reductions and method improvements. The only way you can do that is by considering new technology. This, obviously, is a big part of that," he explains.

The evolution of a machine tool
Since Centrax brought the FGC 1000s into its facility, it has recommended minor changes to the machine's guarding, coolant nozzle, and software.

As Fisher explains, "When you get into the early stages of a process like Viper grinding, and you use it in a production environment, you're going to come across problems that the manufacturer has never before seen." Bridgeport took heed of many of the recommendations Centrax made and modified its new machines. It also refurbished the original two machines installed at Centrax to bring them up to spec.

Bridgeport personnel, such as David Martin, manager of new machining processes, have spent time with Centrax, finding such problem areas and fixing them. The machine guarding, for instance, has seen several revisions. "One of the biggest issues we had initially," says Bill McCaskill, Bridgeport general manager --aerospace, is how to contain 70-bar coolant pressure. We had to re-engineer the guards and interfaces."

Centrax also found that the original aluminum coolant nozzle was vulnerable to wear and grit ingress. Therefore, Bridgeport and its partners designed a more robust unit made of steel.

The two companies have also collaborated in developing software specific to the Viper process. Operators have a series of canned cycles specific to grinding, which are driven by macros within the Heidenhain 426 control that allow simplified parameter entry fields.