Trending to turning

Courtesy of Seco Tools

Turning hardened steel with a CBN060K insert from Seco Tools.

The ability to hard turn is a good reason to trade in your old grinder for a lathe.

Holding a few tenths tolerance on a piece of 60-HRC steel was once the domain of expensive grinding equipment. Today, it’s no big deal.

A growing number of machine shops are shoving their OD grinders into the corner and replacing them with relatively inexpensive lathes. The ability to inexpensively, accurately and quickly cut hardened steel on commodity turning centers is opening doors for part manufacturers and job shops looking to reduce costs and increase throughput.

A Hard Turn

Can a Plain Jane CNC lathe really eliminate a high-precision grinder and hold a few tenths tolerance on a piece of 60-HRC steel? Blake Bailey thinks so. He’s the senior manufacturing engineer at Bronson (Mich.) Precision Products, a division of Royal Oak Industries Inc. Bailey explained how his shop turns 60- to 62-HRC 8620 steel driveline components for Harley-Davidson, as well as 4130 casehardened parts with similar hardnesses for Caterpillar. “We hold 0.0003 ” to 0.0004 ” and 16 µin. Ra all day long on 15-year-old Wasinos, no problem,” he said.

Back up the truck! Using gang-style lathes bought during the Clinton administration to turn hardened-steel motorcycle components? “When you’re only removing 0.010 ” [of material], you don’t need a super rigid machine,” Bailey said. “We use an old linear-way machine that might have cost $50,000 back in the day, compared to a grinder that cost 10 times as much.”

Better yet, it’s faster to hard turn. A typical grinding operation might remove 0.007 ” to 0.010 ” and take 45 seconds. “We can hard turn the same thing in 25 to 30 seconds and get a minimum of 100 parts between tool changes. It works pretty dog-gone good,” Bailey said.

According to Rich Parenteau, director of applications development at Methods Machine Tools Inc., Sudbury, Mass., Bronson Precision could improve tool life 20 to 30 percent with a box-way machine. “In my mind, it’s almost mandatory that you use a box-way machine for hard turning,” he said. “From a machining aspect, hard turning separates the men from the boys, and everything from tooling performance to size control and surface finish becomes a factor when you’re comparing box way to linear way. Linear rollers are pretty sturdy, but when you’re buying inserts that cost $50 and up, tool life is important.”

Courtesy of Greenleaf

The WG-300 whisker-reinforced ceramic insert from Greenleaf is effective for turning hardened steel.

Parenteau pointed out one more consideration when shopping for a machine to perform hard turning. “I’m a firm believer in not using an integral spindle for hard turning,” he said. “There’s a fine line between the heat generated by the spindle and the preload on the bearings.” This is because higher spindle heat can cause bearings and their housing to expand.

The additional clearance induced by bearing expansion can make it more difficult to make a round part. “You’re always better off with a spindle that has the motor to the side of the spindle,” Parenteau said. “This gives you a rigid platform across six bearings, and, when it comes to roundness, that means you’re as close to zero as you can get.”

Hard vs. Hard

Another company successfully hard turning is Reliance Tool & Manufacturing Co., Elgin, Ill., which has more than 15 years of experience with the process. Jeff Staes, director of technical services, said: “We do both short-run and production turning, and 99 percent of it is heat treated—A-2 and D-2 tool steels and 400-series stainless steels. We try to stay away from grinding as much as possible because it’s more expensive.

“Sometimes you have to grind, such as small internal diameters, where you can’t get in with a boring bar, or journals, where you can’t chuck the part and turn it at the same time. But if we can reach the part with the right cutting tool, we’re going to hard turn every time.”

A range of cutting tool materials can be applied when hard turning steel and other materials. According to Staes, even carbide tools are effective. “Carbide works as well as some of the original ceramic and CBN grades, which have traditionally been the standard for hard turning,” Staes said. “For example, Seco has new carbide grades (TH 1500 and PH 1000) that are especially good for interrupted cuts, which have always been a problem for hard turning. We’ve had great success with them.”

Chad Miller, advanced materials product manager for Seco Tools Inc., Troy, Mich., agreed that carbide can be an effective tool material choice. “It depends on the workpiece material, but as a rule we can use carbide to cut 62-HRC materials at starting speeds of 350 sfm and achieve reasonable performance.”

Miller noted carbide tools aren’t for everyone. “Smaller job shops or other operations doing limited batch sizes can get away with using carbide, but if you’re looking at a production situation—automotive components, for example, where you’re making thousands of parts a day—you’ll definitely want to go with CBN [because of its longer tool life].”

Miller said much of Seco’s PCBN, or CBN, tool sales come from auto parts makers, which typically turn 1018, 1024, 4340 and 8620 casehardened steel. “Look at a typical differential,” he said. “You’ll have a hardened axle shaft, pinion, ring and spur gears—all of those are being hard turned with CBN.”

However, chip control is a common problem when turning casehardened parts. Miller said: “On many of these components, a majority of the turning is in the 55-plus HRC range, but then you’ll hit an area of the workpiece that’s soft, say, 30 HRC. This happens a lot with induction-hardened parts or when you cut a groove through the case hardening. That’s where you can get into problems with chip control. To counter this, we’ve developed a chipbreaker for our CBN inserts. We use a laser to cut a groove with some bumps, for lack of a better word, on the tip of the insert.”

Rich Maton, engineering manager at toolmaker Sumitomo Electric Carbide Inc., Mt. Prospect, Ill., agreed that chip control is a common problem for casehardened parts. For example, the middle sections of a transmission shaft are kept soft so they remain tough, while outer sections are hardened for wear purposes. After the part is casehardened, the hardened area is removed via hard turning from some areas.

This presents challenges not only with chip control, but also with grade selection. “Let’s say you’re setting up a job for 50-HRC casehardened material,” Maton said. “You have to pick the correct CBN grade and dial in the cutting parameters accordingly. But that setup might not work on the softer parts of the shaft. So you have to adjust [feeds, speeds and DOCs] on the fly and try to generate more heat in the cut. If not, the softer material tends to adhere to the insert, and you can tear off big chunks of the CBN. Sometimes the entire tip will come off.”

Courtesy of Seco Tools

OD turning, with interrupted cuts, and facing operations for a 54-HRC 420 stainless injection mold component using a WNMG 432 insert from Seco Tools with a MF 1 chipbreaker and made from TH 1000 carbide.

Despite this dire possibility, Maton cited a couple reasons why part manufacturers are moving from grinding to hard turning. “A CNC grinder can easily cost five times as much as a decent CNC lathe, and the operating costs are higher. Because of the high torque requirements and long cycle times, grinders consume at least 50 percent more power.”

The versatility of a lathe is another reason to consider hard turning, Maton noted. “Now you can rough and finish on the same machine, you can do ID or OD work—it doesn’t matter,” he said. “With the grinder, you have to set it up specifically for the application, preparing the grinding wheel to match the part profile, whereas with turning you can pretty much do whatever you need in one shot. This is especially important for job shops, which need a lot of flexibility. And, of course, you can hard turn most parts in a third or even a fifth of the time it would take to grind them.”

Sticker Shock