August 2011 / Volume 63 / Issue 8|
By Susan Woods, Contributing Editor
Courtesy of Sumitomo Electric Carbide
Grooving hardened steel parts with PCBN cutting tools.
Over the past decade, finish grooving hardened steel parts using polycrystalline cubic boron nitride inserts has increasingly replaced grinding. “Typically, grinding is a more stable process that provides more accurate tolerances than grooving,” said Tyler Economan, proposal engineering manager for INDEX Corp., Noblesville, Ind. “However, people are trying to do more complete machining on workpieces if the turning machine is capable of completing the necessary processes.”
Various work materials that are through-hardened include HSS, die steels, bearing steels and alloy steels. Only ferrous metals are casehardened and that process is typically reserved for low-carbon steel. Casehardening produces a hard, wear-resistant exterior over a tough core. Parts made from hardened steels include arbors, axles, link components, driving pinions, camshafts, gears, bushings, transmission shafts and bearings.
“Hard,” however, is a moving target. Some consider materials from 40 to 55 HRC to be hard, while, for others, hard is 58 to 60 HRC and harder, which is where PCBN comes in.
For induction-hardened, or casehardened, parts, the outer, hardened surface, can be as much as 1.5mm thick, and up to 58 to 60 HRC, while the undersurface is typically much softer. In situations like this, it is important to ensure the majority of the cut happens below the hardened, outer surface.The Right Stuff
Rigid machine tools with adequate power are a must for hard grooving. “The more rigid and powerful machine you have, the more aggressively you can cut hardened material,” Economan said. “For applications involving machining materials harder than 50 HRC, many light-duty machine tools won’t hold up to the demanding cutting conditions. This holds true when the machining requirements exceed what the machine is capable of, as far as horsepower, torque and especially rigidity.”
Rigidity is important for setups and workholders. Because grooving involves significant edge contact, it produces significant tool pressure. When fixturing hardened workpieces, wide clamps can distribute the clamping surface. “You definitely need to support the area being machined,” said Paul Ratzki, marketing manager for Sumitomo Electric Carbide Inc., Mt. Prospect, Ill. “With hard material, you have much greater vibration and tool pressure, which can cause the part to fly out of the machine, or chip or even break the CBN insert.”
Courtesy of Sumitomo Electric Carbide
Toolholders for grooving inserts should be as short as possible to minimize overhang, which increases rigidity. And while solid, or integral, holders are typically better suited for hard grooving, according to Matthew Schmitz, national product manager—GRIP Products for Iscar Metals Inc., Arlington, Texas, Iscar offers modular systems as well.
“Modular holders are going to be used where you see a lot of catastrophic failure,” Schmitz said. “Instead of replacing the entire holder, you just replace a less-expensive component.”
Modular holders also provide many options. “Iscar’s Modular-Grip system mounts various products. You can use one shank with seven different blades to run seven product lines, or any number of blades for the same product line but with different widths,” Schmitz said.
Courtesy of Mitsubishi Materials
Sumitomo’s holders for CGA-style inserts have a top clamp that pulls an insert back into the pocket, and the holders also use a side screw to help enhance stability and tool life. “We only offer it for hard grooving,” said Rich Maton, assistant manager, engineering department for Sumitomo. “If you have an insert that is moving in the pocket, over time the insert will wear and tool life will vary, which is especially significant for the high-productivity requirements of the automotive industry, where going from 50 to 100 or 150 pieces per edge makes a huge impact.”
Mitsubishi Materials USA Corp.’s GY series Tri-Lock modular system reportedly offers rigidity equal to that of an integral insert holder. The system secures the grooving blade to the holder in three directions: the periphery and front and top sides. Two features decrease insert movement during grooving. The convex “V” formation eliminates side-to-side movement, and the safety keys eliminate forward movement caused by cutting forces when plunging.Insert Information
Some common groove features for hardened steel parts are simple square, profile and plunge grooves. Typically, grooved parts require a fine surface finish because there is a mating part or an O-ring or snap-ring groove. “The applications are split between internal and external operations, but the majority of applications require finishing passes,” said Mark Menconi, product specialist for Mitsubishi Materials, Schaumburg, Ill. “The finishing passes range from a slight touch-up with a low DOC near 0.010 " to a full finishing pass near a 0.020 " DOC.”
Grooving hardened steel requires harder and more wear-resistant cutting tools with appropriate tool geometry. The trick is to know whether to apply carbide, ceramic or PCBN inserts. “For an application less than 50 HRC, I almost always choose carbide,” Schmitz said. “Ceramics are a very economical choice for applications from 50 to 58 HRC. CBN should not typically be considered until the hardness reaches 58 HRC. This is where it performs best.”
Courtesy of Mitsubishi Materials
CBN works well in such hard materials because instead of cutting the material, CBN molts the material at the tool/workpiece interface.
Chip control is not an issue for 58-HRC and harder applications. Dry grooving is the norm, and the chips are more like dust or very small particles, which can be removed with a manual air blast. “If a chip touches anything, it usually just falls apart,” said Sumitomo’s Maton, adding that chips won’t mar parts upon contact. “If you grab a pile of chips, they would just crumble in your hand.”
Courtesy of Iscar Metals
Part of the reason CBN is run dry is because, while it can take the heat, it does not operate as well with temperature fluctuations. “When a CBN insert actually interfaces with the material, heat is created at the tip of the tool,” Economan said. “But because the CBN doesn’t like shifts in temperatures, it is difficult to adequately cool it to where you can maintain a more constant temperature.The hard CBN is very brittle and could crack due to temperature changes.”
When cutting softer steel, say 45 to 50 HRC, with carbide, ceramic or PCBN, chips should be as short as possible. This effectively removes heat from the material during the cutting process because the heat stays with the chip.
Iscar’s Schmitz also suggests running the cutting tool “upside down.” He explained: “Mounting a tool in a machine so the insert is facing upward during the cut is the machine tool builder’s preferred way to mount tools because the rotation of the part exerts downward pressure on the machine ways or rails, stabilizing the machine tool. But as the insert goes across that material, a chip forms and may remain on top of the insert and the workpiece. If I take that tool and flip it upside down so I no longer see the insert, the chips are flowing away from the working zone and I’m using gravity to my advantage.”
Courtesy of INDEX A Simple Case
Casehardening is a simple method of hardening low-carbon steel whereby carbon is added to the outer surface of the steel to a specific depth. Problems can arise when the groove depth exceeds the thickness of the casehardening because the grooving insert transitions from a harder material to a softer one. Therefore, toolmakers have developed several grades for various types of materials.
Courtesy of HORN
“Customers don’t always want to change the insert when going from harder to softer, so we have to find the next best tool for the operation,” said Duane Drape, national sales manager for HORN USA Inc., Franklin, Tenn. “If they use carbide, they have problems with the insert wearing too quickly on the harder outer casing. If they are cutting the softer area with CBN that was made for cutting very hard material, it would be destroyed. So we work on a compromise. We may work with a very hard carbide with a super lubricated coating or a soft CBN grade with an edge prep that is suitable for cutting rather than for hard machining.
“You can cut 45- to 50-HRC material effectively with CBN, but you have to adjust the geometry,” Drape continued. “Typical CBN is manufactured with a negative land on the cutting edge. On softer materials with that negative land, the CBN is going to get pulled out of the material and then you lose tool life. If I use a softer CBN and adjust the geometry, I can have very good success cutting 45- to 50-HRC material.”
Another option is changing the cutting parameters. “Once you cut through that casehardening, you can be more aggressive,” said Index’s Economan. “If the case depth only goes down to 0.005" or 0.0010", then after that you should increase the cutting parameters to appropriate levels with either the same tool or a different one.”
PCBN grades are evolving to cover a wider application range. Harder grades allow for elevated speeds while tougher grades can run in more unstable environments. Different grades of PCBN are also available for continuous or interrupted cutting. Because PCBN is brittle, a sharp cutting edge would start to chip when machining hardened steel, according to Sumitomo’s Maton. “We have to protect the edge, especially when interrupted cutting where the edge prep is much larger and at a greater angle than when continuous cutting.”
Iscar Metals has advanced its Groove Turn PCBN product line with the newly created IB10H and IB20H grades. IB10H is a fine-grain PCBN intended for continuous cutting, while IB20H, composed of fine- and medium-grain particles, offers a good balance between wear and impact resistance to withstand more severe cutting conditions. The failure mode with any PCBN should be gradual edge wear—not sudden cracking or rupturing.
Sumitomo has introduced BNC30G, a coated PCBN grade for parts with interruptions. For continuous grooving, the toolmaker recommends its general-purpose BN250 grade. “With the continuous cut, you’re in the cut for a long time, generating a great amount of heat, so you need a high wear-resistance grade,” Maton said. “With interrupted cutting, you are coming in and out of the cut, banging the corner of the insert. Therefore, you need a very tough grade to withstand the interruption, and a coating helps the tool last longer.”
No matter what kind of groove they are making, shops that previously relied on grinding for finishing hardened steel parts can improve productivity by transitioning to grooving with PCBN tools. They achieve comparable tolerances while dramatically reducing machining time. CTEAbout the Author: Susan Woods is a contributing editor for CTE. Contact her by phone at (847) 973-2271, or by e-mail at firstname.lastname@example.org.
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