Shop roughs it in Texas

Rough turning forgings made of nickel-base superalloys isn’t something that the uninitiated are likely to get into, but Heartland Enterprises Ltd. has extensive experience — and expertise — with the process.

“It’s something we do day in and day out,” said Chris Easterwood, plant manager of the production job shop in Fredericksburg, Texas. “I wouldn’t call it a challenge.”

Being in the heart of Texas enables the company to capture a lot of business from the oil and gas industry, such as machining natural gas compression parts, including piston rods and power cylinder liners, as well as components for powering fluid ends and frack pumps, said President Jay Mallinckrodt. He began that leadership role in summer after Dave Campbell, longtime president, majority owner and his father-in-law, semiretired and became CEO.

The shop also serves the aerospace industry, for which the heat-resistant alloys come into play, including 615 and 725 Inconel, Waspaloy and Hastelloy. The parts involved are primarily large discs and shafts that go into the hot section of commercial jet engines, ranging from 152.4 to 1,066.8 mm (6″ to 42″) in diameter and up to 1,270 mm (50″) long.

“People see the Inconel forgings,” Easterwood said, “and they have never seen Inconel pieces as large as what we’re working on.”

Machinist Jorge Diaz loads a workpiece into a turning center. Image courtesy of Heartland Enterprises

“Our objective is to work with customers long term,” Mallinckrodt said. “We love orders that are 100, 200 pieces, but a lot of work is 10-, 15-, 25-piece jobs. But we want to do those once a month or once a quarter or two or three times a year so we are not doing a lot of reprogramming, setup and design.”

As a result, he said Heartland Enterprises doesn’t take on one-off, short-run jobs.

“You have to do some production work so you can see how to make these parts right,” Mallinckrodt said. “It’s hard to get up to speed making two pieces out of Inconel.”

In the Rough

The company only rough-turns the age-hardened superalloy forgings before sending them to jet engine original equipment manufacturers for finishing in-house or by a subcontractor, Easterwood said. Nonetheless, tolerances can be fairly tight at ±0.0254 mm (±0.001″).

He said the process begins by removing the scale on the forgings, an interrupted turning process that breaks down inserts. Round ceramic inserts are the go-to tools because round is the strongest insert shape, but square and 55-degree inserts with four or six nose radiuses also are used, depending on the required part features. The inserts are uncoated.

“We will get the scale off using a slow surface footage, around 180 to 200 sfm (54.9 to 61 m/min.), a 0.1″ (2.54 mm) DOC and a 0.01 to 0.012 ipr (0.254 to 0.305 mmpr),” Easterwood said.

He said removing the scale typically requires several passes. In addition, each pass usually doesn’t cover the entire length of a workpiece. For example, the first pass might travel half the length of the part before an insert is indexed and cutting continues.

“It depends on what the forging looks like, how clean it is and everything like that,” Easterwood said. “Once the scale is off, it’s a lot easier cutting, with no interrupted cuts.”

With the scale removed, he said Heartland Enterprises turns superalloys at a vigorous but slow cut to prevent the cutting tool from pushing off the material and to correctly produce the part dimensionally.

“We try to be pretty aggressive with the time,” Easterwood said.

Even with the assertive approach, Mallinckrodt said the company rarely scraps the pricey workpiece material.

“We’ve been machining these parts for 20 years now,” he said. “It’s almost like second nature to us how we do it. Someone coming into this cold would be like, ‘I’m not touching that stuff.’ It’s hard, and coming out of the forge, it’s a little bit nasty.”

Cycle time varies widely, depending on part size and the number of required operations.

“There are some smaller tubes that we can get 50 parts a shift,” Mallinckrodt said. “There are also some big shafts and discs where an operator may only get one or two parts a day. We might do one part with one or two operations, and another may take six operations.”

Overcoming Issues

Because nickel-base superalloys tend to workharden, turning parameters must be controlled carefully to prevent too much heat from entering a workpiece and damaging the tool, Easterwood said.

In addition, workhardening causes the tool to be pushed off the workpiece. For example, push-off might cause a 152.4-mm-dia. workpiece to measure 152.91 mm (6.02″). In that scenario, he said the tool should not be offset 0.51 mm (0.02″), because the part will be turned to its specified dimension when the tool is run at the correct speed and feed.

Heartland Enterprises also applies a semisynthetic coolant to help remove heat.