Hard turning? Hardly

Advanced cutting tools have made what was once a highly improbable machining operation into a highly productive reality

By Kip Hanson

Steel shafts and bearing races, gears, splines, pistons, and mold components. These are just a few of the hardened metal parts—many of them used by the automotive industry—whose finish machining has long depended on cylindrical or centerless grinding. This status quo has changed over the past few decades, however, as an increasing number of manufacturers adopted a faster, more efficient alternative to grinding. It’s called hard turning, a process that presents a few challenges as well as many significant benefits. 

Ironically, hard turning and grinding share many similarities. Both depend on rigid, highly accurate CNC machine tools. Both deliver fine surface finishes and precision best measured in microns. And both work best with advanced cutting tools—a cubic boron nitride (CBN) wheel for grinding operations, and a single-point tool of the same material for turning.  

How hard is hard?

Yet the industry’s definition of hard turning varies. Some experts consider it the cutting of hardened steel parts in the range of 58 HRC to 70 HRC, while others expand this to include materials starting at 45 HRC, covering hardened irons and heat-resistant superalloys (HRSAs). Despite the difficulties it presents, though, hard turning is quite manageable—and often preferred over grinding—given the right equipment, tooling, and techniques.

“Hard turning is a mature science, but we and other cutting tool manufacturers continue to introduce new grades and insert geometries that can make it more productive and predictable,” says Doug Evans, a grade development and turning specialist at Sandvik Coromant. “For us, at least,
much of this activity centers around wiper technologies, which allow our customers to feed much faster than in the past. That’s a big plus.”

The grades he’s referring to vary in CBN content, with those on the low end recommended for light finishing cuts at higher cutting speeds, and high CBN inserts that use a metallic binder the first choice for heavy interruptions like those encountered with cross-holes and keyways. All are available with various edge preparations, and as Evans mentions, many boast specialized wipers for increased feedrates. 

How much of an increase are we talking about? According to Evans, twice as fast in some cases, with no negative impact on the part’s surface finish. Conversely, shops might choose to maintain their existing feedrates and leverage wiper technology to improve surface quality. “We’ve also introduced a proprietary insert—the Xcel—that has a 10-degree lead angle on the tool nose that allows us
to thin the chip out while also spreading the wear over a larger surface,” he says. “So now we can take inch-per-rev feedrates into the teens and still maintain excellent RA values.”

Skipping the phonograph

Unfortunately, even the widest of wipers does not eliminate what some in the industry call the phonograph record effect, a miniature spiral groove that comes as a natural consequence of any single-point turning operation. This groove can allow small amounts of oil to seep under and around rubber seals and contaminate whatever’s on the other side, making it one of the main reasons why OEMs will call for a ground surface rather than one that’s turned. 

Here again, modern technology is turning the tide in hard turning’s favor. “For shops with a Y-axis lathe, there are inserts and toolholders available that allow you to sweep the outside diameter in a manner similar to a skiving operation, eliminating this groove,” says Evans. “It’s called lead-free machining.”

Another checkmark on the hard-turning side of the ledger is its greater sustainability. Raymon Avery, product manager for turning products at Ingersoll Cutting Tools, a member of the IMC Group, notes that the environmental cost of grinding is higher due to the slurry it generates, tiny bits of metal that must be filtered from the cutting fluid before disposal. 

“Grinding is a bit less expensive in terms of tooling cost, as you can just re-dress the wheel as needed and keep machining,” says Avery. “But hard turning provides several benefits that easily offset this. As noted, it’s a much greener process, but it’s also much faster and more flexible.”

Break it up

Like the other cutting tool providers in this article, Ingersoll offers a wide range of CBN grades, wipers, and edge preparations, including a new bi-directional, hi-feed insert, reportedly capable of feeding three times faster than wiper inserts. These cover the lion’s share of applications, from light profiling cuts to those with heavy interruptions. They also have a large ceramic offering, although generally speaking, these are limited to roughing and semi-finishing operations. “Ceramics can take relatively heavy cuts of a millimeter or more per side, but the tool life doesn’t compare to CBN,” says Avery. 

He points out another consideration, one that’s often overlooked in the hard turning arena: chip control. “Hardened materials are typically brittle, so there’s little need for chipbreakers. Yet some parts have varying levels of hardness, and as you cut deeper you’ll encounter soft areas. Here, the chip tends to wrap around the part, leading to a recut situation. We’ve addressed this by grinding a small flat on the top face of some of our CBN inserts, which does a great job of breaking those chips into little pieces.”

Whatever the hard turning operation, Avery and the others agree that a high-quality CNC lathe is required. The cutting forces are high when removing hardened steel, iron, and HRSA, and neither ceramic nor CBN cutting tools respond as well to vibration and deflection as their carbide equivalents. In addition, the diametral and surface finish tolerances are typically quite tight in this environment, making an extremely rigid and accurate machine tool a must. 

Fast and flexible

“Hard turning has certainly become more prevalent over recent years, especially in automotive and other high-production environments,” says Darryl Latoski, milling product manager at Iscar Tools Inc. “That’s because it presents some definite advantages. For one thing, grinding is extremely expensive, both for the machine tools and the production costs. With the right lathe, you can achieve comparable surface finishes in most cases, and while grinding is a bit more precise, hard turning can maintain tolerances of half a thousandth or so (.0127 mm), which is plenty good for most jobs.”

Flexibility is another advantage. Unless you’ve invested in a dedicated form wheel—an expensive proposition except in very high production volumes—grinding is limited to a single diameter per operation. But as Latoski points out, CNC turning can turn, bore, and face in a single part handling, greatly reducing work-in-process, lead-time, and part cost. 

For shops willing to give hard turning a try, he recommends that they first attain a good general knowledge of the process. Evaluate the available tooling options (like its competitors, Iscar has plenty). Semi-finish the part as close to the final dimensions as possible, leaving perhaps 0.2 mm or so (0.008 in.) per side for finishing, depending on the workpiece size and geometry. And seeing that CBN is much more brittle than carbide, minimizing shock to the insert is critical. 

“You need to use the right programming approach,” he says. “Arc into the cut or use a shallow taper, and avoid burying the tool against the shoulder. And pay special attention to your feeds and speeds. That’s good advice no matter what you’re cutting, but particularly when using CBN on the very hard alloys and HRSA materials—here, you might be running cutting speeds that are three to four times higher than carbide, and it’s crucial to get everything dialed
in right.” SMT