Rough Turning of Difficult Superalloy

Instead of a knee-jerk reaction to lighten the cut and increase the passes to ‘take a load off the insert’ when facing an insert rupture when running Inconel, this shop went to a stronger insert with a gentle lead angle to make fewer, deeper passes.

Many machine shops have a love-hate relationship with the high temperature alloys. They’re proud to claim the ability to machine the Inconels and titaniums efficiently, and grateful for the resulting competitive advantage in a growth niche of the metal fabrication business.

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But first, they need to perfect their practices on such difficult materials. These exotic alloys are abrasive to tools and impose high cutting forces due to their very strength. They also generate hot, stringy chips that tend to stick to the cutting edge and overheat the tool as a whole. As a result, machining superalloys too often creates bottlenecks, ties up machines and drives up tooling costs.

However, one company that “got it right” in rough turning Inconel is Scot Forge, a diversified forging company with plants in Clinton, WI and Franklin Park and Spring Grove, IL. Management of the employee-owned company, which has been in business more than 100 years, noticed an uptick in orders for Inconel parts, which they took as a trend.

SAVING $2,400 PER PART
Recently on a huge Inconel NO 7718 hub that goes into nuclear turbomachinery, a simple retooling saved $1,500 in cycle time and $900 in tooling costs – per piece. The switch was from square flat-topped zero-lead inserts to Hex-Turn inserts from Ingersoll Cutting Tools (Rockford, IL) with a 45 deg lead angle and aggressive chipbreakers on the top face.

“We get about six reorders a year for the part, and rising order levels for other Inconel and titanium forgings, so we decided it was time to develop a best-practice for machining them,” says Howard Dempsey, supervisor of the machine and saw shop at the Clinton plant that houses a 200-man operation, 50 of them machinists, and runs 24/5. By customer preference, most of the company’s output is delivered in the forged and rough-machined condition.

The hub requires rough turning and facing to remove about 0.800 in all around, mainly for truing up. As forged, it measures 98 in diameter by 7 in long, and has a step down to a 93 in diameter on one end. Previously, machining took 105 hours and 40 passes, required different inserts for the turning and facing and ate through 69 inserts per part. Parameters were 0.020 ipr feed, 0.075 in depth of cut at 38 sfm.

“Tool failure was so bad, we typically had to index every half inch and run it 100 percent attended,” notes Dempsey. “Moreover, the main mode of failure was edge rupture which renders the insert useless, even though it may still have unused edges.”  All the while, the operation tied up a big G&L vertical CNC lathe that Scot needed for other work.

WIDE NET CAST
Dempsey asked all his tooling vendors for their best ideas and tested them all with an eye toward a better standard practice for turning this class of alloy. Most tools tested produced minor improvements at best. However, Ingersoll’s Jim Whitley suggested taking a deeper cut and doing it with their new Hex-Turn insert, which features a 45 deg lead angle presentation and top-face chipbreakers.

The improvement was dramatic. A Hex-Turn insert lasted through the whole piece, despite more than doubling the cutting rate. “We would have been happy with just the tool life improvement” smiles Dempsey. “The throughput gain was an unexpected bonus.”

After optimizing the operation, parameters with the new tool were standardized at 0.015 ipr feed (slightly lower than before) and 0.150 in depth of cut (double the previous setting) and 60 sfm (60 percent higher).

“By cutting deeper, you reduce the number of passes and therefore the exposure of the cutting edge to the hot, abrasive environment at the cutting edge,” explains Whitley. But such a deeper cut requires a much stronger insert, which the hex shape provides. The only insert shape stronger than a hexagon is a round.

The other big difference was the 45 deg lead angle, made intrinsically possible by the insert’s hex shape, and the chipbreaker top face. “The lead angle provides a gentler entry, which reduces cutting forces and protects the insert,” adds Ingersoll product manager Raymon Avery. “The contoured top face breaks up those characteristic long, hot, gummy chips and deflects them away from the insert so the operation runs cooler.”

Judging from the results, these theories certainly worked. The drop-in retooling literally produced a $2,400 saving per part.

Based on success on that one hub at Clinton, Scot Forge has standardized on Hex-Turn and those same parameters for roughing a growing variety of Inconel parts at all three plants. Cost savings aside, having a secure process for such a difficult material also enables the company to bid more confidently on Inconel jobs and compete more effectively in a high-tech niche.

“This improvement allows us to maintain competitive costs for machining Inconel,” states Dempsey. “Had we not found this improvement together, we would have been forced to raise prices to cover the excessive cost of tooling and the resulting sub-par productivity. For now, our standard settings are identical with those in Jim Whitley’s first test.

UNMASKING THE ROOT CAUSE
The Scot Forge success illustrates an often overlooked point, according to Whitley. “When faced with insert rupture running Inconel, the knee-jerk reaction is to lighten the cut and take more passes to ‘take a load off the insert’. This can make matters worse. They forget that rupture often begins with an edge going dull, which in turn overloads the insert to the point of breaking down.”

All of this can happen is a few seconds, “so it’s easy to miss wear as the root cause,” adds Whitley. “Assuming that adequate machine horsepower is available, the better answer is to go to a stronger insert with a gentle lead angle, and make fewer, deeper passes.”

Ingersoll Cutting Tools, 845 S. Lyford Road, Rockford, IL 61108-2749. Phone 815 387 6600, fax 815 387 6968, info@ingersoll-imc.com, www.ingersoll-imc.com.