Turning and drilling Inconel, Waspaloy and other hard metals used in jet engines generally requires far longer cycle times per part than machining common alloys. However, when engine maker Pratt and Whitney (P&W) looked to hike component repair productivity, it found dramatic savings in a partnership with a knowledgeable tool supplier. 

Working with toolmaker Sandvik Coromant, the Pratt and Whitney PSD in Springdale, Ariz., cut 12 hours machining time on a troublesome engine case in half. It also uncovered potential annual savings greater than $380,000 in a single machining cell. Coordinated cutting tool and process changes enhanced throughput on individual parts as much as 30 percent.

Pratt and Whitney PSD is a repair center for cases, gearboxes, and bearing housings in many turbo fan aircraft engines. Its repair programs are intended to optimize component performance and lower total engine ownership costs. However, machining operations on some Heat Resistant Super Alloy (HRSA) parts were exceptionally slow and costly. Depending on their condition, Inconel 718 and Waspaloy HRSAs are rated up to 410 Brinell hardness. Slow turning, milling and drilling speeds extend tool life but increase cycle times. Multiple operations on an Inconel 718 engine housing, for example, required 12 to 14 hours cumulative machining time.

Slow grooves
In one particularly time-consuming operation on the housing, cutting 0.073-inch-wide grooves on a vertical lathe using dogbone-style carbide inserts took 9.5 minutes. The 300 Brinell alloy required three passes to reach 0.195 inch cutting depth and took a heavy toll in the negative-geometry cutting inserts. 

“Our turn times were killing us,” recalls machine operator Dan Broxson.  In addition, rapid insert wear necessitated time-consuming rework. “Inserts would break or develop flat spots. That would leave you with a nasty finish, and you’d have to re-run the part a time or two.”  

To make matters worse, a broken insert typically ruined the cutting tool.

Through Pratt and Whitney’s tool integrator, Van Asche Industrial Supply, Sandvik Coromant field representative Jeff Clemence addressed the engine maker’s difficulties. 

“I knew they wanted to reduce cycle times, increase efficiency and achieve general cost savings,” he recalls. “I first made recommendations to help them do that in the grooving operation.”  

Clemence suggested Sandvik Tailor Made inserts with wear-resistant H13A-grade carbide. The uncoated carbide is formulated to resist abrasion and stand up to the high temperatures encountered when machining stainless steels and HRSAs. Tailor Made tools supplied in the correct width also eliminated lengthy in-house grinding.

Instead of three pauses, the replacement carbide grade and positive rake chip breakers grooved the part to finish depth in a single pass without rework. Actual cutting time fell from 7.5 minutes to 2.5 minutes. Projected savings in machining time and tooling costs totaled over $12,000 over a run of 600 parts a year.

Productivity improvement program
The initial success led Pratt and Whitney and Sandvik to partner on a productivity improvement program on the five machines in the PSD military case shop.

Sandvik Coromant productivity teams conduct several hundred such programs a year without cost to manufacturers. Experience shows the partnerships boost plant-wide machining throughput 20 percent on average and generate significant savings in machining time and maintenance, repair and operating costs. Like all such productivity programs, the Pratt and Whitney effort began with a careful audit of part cycle times and machine data. 

“We brought in our specialists and did a lot of legwork,” explains Clemence. Sandvik parting, grooving, and threading specialist Mike Campbell and others joined the team at Pratt and Whitney.

Analysis of typical machining cycles in the split case cell uncovered more opportunities for improvement. A face, turn and plunge operation on an Omega computer numerically-controlled lathe, for example, prepared an engine case for flame spraying. The engine case is about 23 in. in diameter by 16 in. long and is made of Titanium 24 Rockwell. Actual length of cut is 2 in., but rapid wear on uncoated carbide inserts routinely forced machine operators to make two passes on each case. 

“Interrupted cuts due to the cooling holes were breaking inserts down so fast we couldn’t get through a full cut,” says lead machinist Dave Framstad. 

Cutting time per engine case was 4.2 hours.

The productivity team suggested a change to a CoroCut insert with wear resistant 3115-grade coated carbide optimized for hardened materials. The carbide insert has a layered chemical vapor deposited coating of titanium carbon nitride, aluminum oxide, and titanium nitride to resist wear and reduce friction. Sandvik representatives recommended only slight changes in machine data, making the new insert nearly a drop-in replacement. 

“Basically, we just changed out the insert,” says Framstad. “We increased our feed rates and rpm just a tad, but not much.”  

Feed was notched up from 0.010 to 0.011 in. per revolution. Spindle speed was increased from 19 rpm to 23 rpm and surface cutting speed from 139 sfm to 169 sfm.

With only modest increases in speed and feed, the new insert provided dramatic cycle time reductions by completing the cut in a single pass. 

"We used to go back and touch-off each time we changed inserts, and we changed inserts a couple of times a part," says Framstad. “Now, it’s one insert, one cut. We saved a lot of cycle time.”

Cutting time per part fell from 4.2 hours to 1.58 hours. Even with 5.5 hours of non-cutting time unchanged, overall cycle time per engine case fell 28 percent, saving $281 per component. Total machining costs per component fell from $1,015 to $734, and projected savings over a full year were nearly $16,900.

In another turning operation, a difficult undercut on an Inconel casing also required multiple passes with conventional carbide inserts. Substituting 3115-grade carbide inserts made it possible to boost spindle speed slightly from 19 rpm to 22 rpm and increase feed from 0.010 to 0.011 in. per revolution. Overall, cycle time fell 6 percent, and projected savings over 600 parts a year topped $26,000.

Drilling solution
Drilling 72 holes 0.218 of an inch in diameter and 0.1200 of an inch deep in the rear turbine case posed its own challenges. Making accurate holes in the Waspaloy part took between two and three hours work on an Ikegi horizontal milling machine using two different tools.

“We had to first center-drill each hole, then drill each hole to size with another carbide drill,” says  Pratt and Whitney machinist Kelsey Moran.

Rapid edge wear commonly required up to five drill changes to finish each part. Worse, as the conventional carbide drills wore, they would sometimes oversize the holes and cause rework and scrap.

The productivity team suggested the Sandvik CoroDrill Delta C to improve the productivity and profitability of the operation. Like its predecessor, the Delta C drill fed coolant through the spindle. However, the self-centering Delta C drill eliminated the need for a pre-drilling step and centering drill. 

“We were using two to four drills to make the 72 holes before. This one drill now does the whole part,” explains Moran.

The GC 1020 grade carbide eliminated the premature tool wear that frequently stopped the operation and enabled Pratt and Whitney machinists to increase machine settings. Spindle speed was increased from 900 rpm to 12,500 rpm and feed from 0.003 of an inch to 0.005 of an inch per revolution. Drilling time fell from 65 minutes to just 20 minutes, and longer tool life eliminated up to four 15-minute tool changes per part.

Ongoing savings
Despite the higher initial cost of the Delta C drill, greater tool life reduced tool cost per part from $89 to just $12. The cumulative time savings improved productivity 34.77 percent and saved $295 per part. Total machining cost per component fell from $937 to $642. Over the course of a year, potential savings could total more than $28,320.

Ongoing tool and process changes continue to give Pratt and Whitney PSD dramatic savings. Projected annual savings from the changes recommended in the productivity program and thereafter exceeded $380,000. 

“Jet engine manufacturers face some of the most difficult machining tasks around,” explains Clemence. “I think we showed how the right cutting tool technology and technical support can make manufacturers more competitive and more profitable.”

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