Grinders go with the flow

Grinders go with the flow

Creepfeed grinders provide the thrust for Pratt & Whitney's flowline production strategy.

Creepfeed grinders provide the thrust for Pratt & Whitney's flowline production strategy.

The low, fixed table of the Blohm Profimat MC creepfeed grinder makes manual part transfers effortless within Pratt & Whitney's flowline production cells.

Because they are narrow and all maintenance is done from the front and back of the machines, Profimat MC grinders sit side-by-side, metal-to-metal, in flowline cells at Pratt & Whitney's Turbine Module Center.

Flowline production at Pratt & Whitney's Turbine Module Center in East Hartford, Conn., depends on grinders — 38 to be exact. That's how many make up the company's blade and vane flowline cells. All the machines are Blohm Profimat MC creepfeed grinders that fit, literally, into the cell environment, accommodate shop's special workholding demands, improve part quality.

The Turbine Center arranged its flowline cells according to all the operations to complete a part. Machines involved are grouped together as close as possible, so one operator can oversee a whole This same operator is also responsible for manually moving parts from one machine to the next.

To make his route obstacle-free and more ergonomic, each Profimat measures only 79-in. wide and has a control console that hangs above and out of the way, a joint-engineering design between Pratt & Whitney and Blohm. "These consoles drop down when needed, but that isn't often," says Thomas Horn, equipment engineer at Pratt & Whitney. "In our flowline cells, there is minimal operator interaction. In fact, the only reachable switches on the machines are the cycle-start-and-stop buttons." machine maintenance access is from the and back, which lets them sit side-by-side, metal-to-metal. A traveling-column machine design frees up space for a low-profile, fixed that not only provides easy operator access but also the flexibility for Pratt & Whitney to grind various blade and vane sizes.

Profimat MC series machines, which handle both conventional and surperabrasive grinding wheels, offer X-axis column travels of 28 in. and Y-axis wheelhead vertical travels of 22 in. Column cross positioning, or Z-axis travels, are either 12.8 or 20.8 in., depending on machine model. And the Profimat's heavily ribbed, welded-plate base construction features a three-point leveling system that needs no special foundation, making the machines even more adaptable to flowline production cells.

All the Blohm grinders at the Turbine Center are the same and from United Grinding Technologies in Miamisburg, Ohio. The only difference is the part setups, so switching over for different operations and parts sizes involves simply exchanging fixtures. And the Profimat's multiple grinding wheels and dressing rolls make changing machines over from one operation to another or from one part size to another even easier.

Pratt & Whitney has four flowlines in which it uses 21 of the Blohm machines to grind all the Inconel turbine blade surfaces, including the fir trees, or dovetails. Workholding is critical for these parts because all surface tolerances must be ±0.0006 in. in relation to each other.

Normally, the blades are held by an expensive method called encapsulation, in which a low-melt zinc alloy is cast over the part. This casting contains the datum points worked off of during the grinding process, and once that's complete, the encapsulation material is melted off. However, the alloy contains heavy metals, and any trace amounts left on the part can mar it and cause a weak spot. So strong chemicals are used to thoroughly clean parts.

Other common setups include holding and locating from the blade section itself, which is difficult and involves complex fixturing. Another is to grind one end of the blade and use it for holding and locating. "This works for short blades, but when they are longer, there is more chance for part vibration," says Horn.

For current blade production, the Center holds and locates parts using material added to the blade's end and root during its casting stage. Blades rest securely between centers as in a cylindrical-grinding setup. Pratt & Whitney developed this less-costly workholding, and Blohm integrated it into the grinders.

Blade flowlines include EDMs, EDM hole machines, CMMs, and the Profimat grinders. These machines have table-mounted dressing units that accommodate numerous widths of diamond dressing wheels by exchanging arbors in minutes.

In the first steps of the blade flowline, raw castings get some of their holes polyfilled to reduce vibration and keep them clean. The next two steps involve EDMing and grinding blade locating surfaces and the proprietary pattern in the extra material at the blade's root and end for workholding — Profimat grinders handle this.

In the final steps, a CMM verifies that blade-locating surfaces and datum are in the right places. Then the blades move to other Profimat grinders that complete the many other surfaces.

Turbine vanes are stationary parts that manage gas flow in a jet engine. Tolerances on these parts aren't as tight as with blades. However, they involve hard-to-hold complex part shapes such as angles, radial cuts, and shoulders — all of which are ground.

As with blades, the Center holds and locates individual vanes by extra material cast on the parts. Four of the five flowline vane cells incorporate a total of 17 Profimat grinders. The machines first grind locating surfaces and then inner and outer buttresses. These include surfaces and slots that must mate precisely to other vanes for creating a complete circular assembly.

What makes this possible is that the Profimats provide true 5-axis grinding capabilities. Vanes mount on a rotary table that tilts. The table's rotation and tilt, together with three machine axes, let the Center create a perfect arc so vanes match.

Prior to the flowlines, the Center had to use a machine with a table big enough to accommodate the radius of a complete vane assembly's diameter — all the vanes put together. Grinding meant situating numerous vanes on the machine's table in a circular pattern, similar to how they would be in a jet engine.

Overall, these grinders were big and required platforms for accessing work areas. Operators had to step up and down off these platforms, so movement from one machine to the next was difficult and tiring.

Accurate and efficient
"The most important benefit," says Kip Wyman, director of manufacturing engineering programs at the Turbine Module Center, "is the improvement in quality and efficiency. As a result of the new grinding cells, CpK values have risen an average of 65%, exceeding a 2.7 Cpk. In addition, blade and vane castings are worth $2,000 to $5,000 each, and running them through the cells one at a time lets us correct problems before a skid-full of parts is scrapped."

The basic Blohm cellular machines replace 12-axis CNC grinding centers that were complicated to run and tough to maintain. These old machines also ran a pallet-full of parts, so if something went wrong, it meant a lot of scrap.

For increased efficiency, flowline production involves a takt (a German word for pace) time. It is the required rate a part needs to come out of the flowline. The goal, says Horn, is to get all the machines within the cell to complete their particular operations within a determined takt time.

At Pratt & Whitney, blade and vane takt time for the flowline grinding cells is less than a few minutes. Even part changeovers within the cell stay within this time. Such takt times are possible because operations are spread out among smaller and less-complicated machines, like the Profimat grinders.

"And since flowlines depend on minimal operator intervention, machines in the cells must be adjustment free, so operators are not having to make any required offset modifications or the like," says Horn. "The Profimats are accurate, but more importantly repeatable," he adds. "Once we get them set up, the operators simply keep them loaded."

As a result of flowline production, lead times at the Center dropped from 6 to 10 days down to 2 hr, and part-size changeovers plummeted from 8 hours to within takt time. In addition, required floorspace for blade and vane production went from 16,000 ft 2 to 4,000 ft 2 with the new cells.

"Flowline cells are perfect for families of parts, such as blades," comments Horn. "The old way, which was basically batch production, was producing, but it wasn't flexible enough. Equipment involved was too dedicated and physically spread out within the plant. Queue times, along with part changeovers, were just too long."

The flowline cells are also more environmentally friendly. The Center filters and reuses coolant from the Profimats and recycles the grinding swarf. And because of its workholding innovation that eliminates the use of encapsulation alloys, the Center has no need for melting equipment or harsh chemicals for parts cleaning.

For machine maintenance, Pratt & Whitney staffs a United Grinding Technology service technician full time at the Center. Both companies have a partnership, so to speak, that keeps the flowlines up and running. "Because of this joint effort concerning not only maintenance but also the complete application development," says Wyman, "the entire flowline project has exceeded our expectations."

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