Doing it the Hard Way

Requirements for milling hardened steels.

Thinking about taking the leap into milling hardened steels? Why would you? After all, if your shop has been successfully grinding, jig boring and EDMing hardened materials, why change?

For starters, hard milling might be more profitable. Michael Minton, national application engineering manager for Methods Machine Tool Inc., Sudbury, Mass., explained that hard milling not only eliminates costly finishing operations, but produces a higher quality part than does traditional machining methods.

Courtesy of Greenleaf

Roughing hardened tool steel with a facemill tooled with ceramic inserts.

“By machining after heat treatment, you not only avoid secondary operations, but also eliminate problems with workpieces being twisted, bent or otherwise out of shape due to heat treating,” Minton said. “This allows you to make a part with higher accuracy and better dimensional characteristics compared to parts heat-treated after machining.”

In general, hard milling involves cutting primarily tool steel or precipitation hardening stainless steel, such as 15-5 or 17-4, that has been hardened to at least 50 HRC. After a workpiece is roughed in the soft state, it is sent to the furnace for hardening and then finish machined with coated carbide, ceramic or PCBN tools. The amount of metal removal in the hardened state is minimal—perhaps just 0.010 ” to 0.020 ” per surface—making this process feasible for most hardened parts.

Depending on the workpiece configuration, production volume and amount of stock removal, however, it may be feasible to machine the workpiece entirely from a hardened state. Modern machine tools, advanced cutting tool materials and sophisticated CAM programs make what was once a highly improbable machining operation into one within the reach of many shops. According to Minton, the advantage of machining from a hardened state is having to cut it only once.

Good Machines

However, you’re not going to accomplish this on an old knee mill. “As always, machine tool construction plays a large part in the level of success you’ll achieve with hard milling,” Minton said. “You can be successful with linear way machines, provided they are made well.”

Another key is spindle design and construction. “That’s the only thing holding the tool and keeping it stable, so very solid, HSK-capable spindles typically are required,” he added.

Minton noted that volumetric accuracy is also important. What does accuracy of the machine tool have to do with cutter life? “Because of the need for consistent chip loads and predictable stock removal, the more square the spindle is to your workpiece, the better your tool life.”

Danny Haight, national milling product manager for Mitsubishi machine supplier MC Machinery Systems Inc., Woodale, Ill., agreed. “You need a solid machine tool, one with bridge construction and a rigid spindle. You also need hand-scraped machine surfaces because they are more accurate and better at dampening vibration than conventionally ground surfaces.”

Of course, the level of machine rigidity depends on what type of hard milling you’re doing. “There’s a big difference between the cutting forces generated when finish machining a mold for a hearing aid vs. roughing a mold cavity for a telephone or an automobile turn signal lens,” Haight said. “Bigger DOCs mean higher cutting forces, which in turn require a more rigid machine tool.”

Haight explained that because of the demanding toolpaths required for typical hard milling jobs—3-D contouring consisting of many short, high-speed movements designed to deliver consistent cutter engagement—the machine controller is also very important.

He said: “The control should have good look-ahead capabilities and fast calculation times. The machine should be very responsive when changing directions or going in and out of corners. You need to maintain a constant chip load if you want your cutters to last.”

Tough Tools

When milling materials from 50 to 60 HRC, not just any cutter will do. Not only is the workpiece very hard, but to reduce thermal fluctuations on the cutting tool, machining is typically done dry. For this, you need tough tools.

One company offering such tools is SGS Tool Co., Munroe Falls, Ohio. Product Manager Jason Wells said SGS’ Z-Carb MD solid-carbide endmills are designed for hard milling, including a negative rake to strengthen and support the cutting edge, an AlTiN coating for heat resistance and an eccentric relief with a radial grind to also strengthen the cutting edge.

Courtesy of Makino Inc.

Hard milling miniature mold cavities.

“Due to the high level of energy needed to create a chip in hardened steels and the abrasive action of the workpiece, you need a tool with a lower-volume cobalt and fine-grain substrate to endure the high loads and temperatures seen in dry machining,” he added.

Coated carbides offer a good trade-off between heat and wear resistance and between strength and toughness, according to Wells. “It’s all about compromise. Ceramics and PCBN definitely have good heat and wear properties, but are more fragile when it comes to shock and imperfect cutting conditions.”

Applications Engineer Dale Hill of Greenleaf Corp., Saegertown, Pa., concurred. “Ceramics don’t do well in situations where you have vibration, excessive tool overhang and less-than-rigid spindles or fixtures. Failures in ceramics are generally mechanical in nature.” Even under normal milling conditions, the ceramic tool flexes as it enters and exits the cut. “It’s unavoidable,” he said. “This flex causes chipping of the cutting edge at a microscopic level. What appears as flank wear is actually microchipping caused by deflection and forces on the tool; as the microchips propagate, the tool eventually fails.”

Despite this, however, ceramics are widely applied for milling hardened steels, irons and superalloys. That’s because carbide’s cobalt binder begins to soften at around 1,600º F, while ceramics can operate effectively at temperatures up to about 4,000º F. “Ceramic comes in where carbide leaves off,” Hill said, explaining that the higher the hardness, the more heat generated during machining.

Courtesy of SGS Tool

Three-dimensional contouring of hardened steel using a ballnose endmill.

Because of this, ceramics can successfully machine into HRCs in the mid-60s. “We’ve even pushed into the high 60s,” Hill said. “Because ceramic is indifferent to heat, cutting speeds can be much higher. In many cases, carbide’s toughness allows a higher chip load per tooth, but the significant speed increase offered by ceramic offsets that higher feed rate. In most cases, ceramic tools will produce much higher metal-removal rates.”

It comes down to economics because a high-quality carbide insert might cost $7 to $8 compared to $20 for a ceramic one. And when the insert costs nearly three times as much, you have to ask if it can remove at least three times as much metal. “The answer is generally yes, but you’ve got to weigh all the machining factors,” Hill said. These include insert life, the cost of the inserts, time to change out a worn set of inserts, required part accuracy and machine capabilities. “It’s all about total metal removal.”

Geometries at Work

As with carbide, toolmakers change the geometry of a ceramic tool for cutting hardened materials. Hill is a strong believer in positive geometry tools for softer materials, where built-up edge can be an issue. “Chip flow is critical when you’re under 45 HRC. But as the hardness goes up, we go to negative tools. We don’t have BUE problems with hard machining.”

Courtesy of Surfware

Constant cutter engagement is critical in hard milling operations.

When hard milling, it’s important to avoid shocking the tool, especially a ceramic one. This can be accomplished by reducing the feed rate on entry and exit, taking a circular toolpath into the workpiece and ramping into pockets and cavities. CAM systems do this automatically, according to Hill.