Turning the hard way

Author Holly B. Martin
Published
February 01,2021 - 12:15pm

When turning metal, the tool material must be four times harder than the workpiece. This becomes a problem when cutting metals with a hardness greater than 45 HRC, such as alloy steel, high-speed steel and chilled cast iron.

The hardest material is diamond, and polycrystalline diamond cutting tools work well for turning nonferrous materials like ceramics and aluminum. But the iron in steel reacts to diamond at the high temperatures generated by hard turning, causing excessive tool wear.

“Certain aluminum oxide ceramic-grade inserts can be used for hard turning, but not all ceramics can hard-turn,” said Brian Sawicki, business development manager for the Northeast region at Tungaloy America Inc. in Arlington Heights, Illinois. “Sometimes you can get away with using ceramic inserts, but you can’t use them in interrupted machining, such as face turning gear teeth, because they will fracture.”

An alternative to ceramics and diamond, cubic boron nitride is the second-hardest material, and CBN tools of many grades have been developed for hard turning applications like transmission shafts, constant-velocity joints and toolholders. These inserts are used to finish-turn hard materials and produce fine finishes and tight tolerances, replacing grinding, which takes longer and costs more.


Chips entangling on workpieces or piling up at the bottom of machine tools can be prevented with the HP-style chipbreaker on the BXA10 range of coated CBN-grade inserts.  Image courtesy of Tungaloy America

“Depending on the specifics of the job, you could source a $500,000 grinding machine, or you could buy a $100,000 lathe and do the same thing with hard turning,” said Travis Coomer, national key account manager for GWS Tool Group in Tavares, Florida. “As long as you can attain all the tolerances given to the part, then it’s absolutely a no-brainer to use hard-turn inserts as opposed to grinding.”

Choosing Inserts

Hard turning is commonly considered to start with materials of 45 HRC, with a transition zone up to 50 HRC. In that zone, some harder carbide and cermet inserts still work. Coated and uncoated ceramic grades work for a hardness range of 45 HRC to about 60 HRC while CBN inserts are designed for about 50 HRC to 68 HRC.

Within the overlap of these ranges, choosing between ceramics and CBN depends on the application.

“Most ceramic grades don’t have a high breaking strength,” Sawicki said. “Whisker-reinforced ceramics generally have the highest breaking strength. But through an interrupted cut, it’s way more predictable to run CBN than ceramic because once a ceramic insert starts to show flank wear, many times it just chips out due to excessive tool pressure.”

But he advised against making the mistake of using a CBN insert on material with a hardness of less than 50 HRC.

“When you put CBN in a soft material,” Sawicki said, “it begins to feel drag, which quickly erodes the insert, so your in-cut time is much lower in softer materials.”

In contrast with ceramics, CBN signals it’s starting to wear.

“You may start to see a change in surface finish that tells you this edge has reached its (end of) life,” said David Essex, turning product manager at Tungaloy America. “Typically, operators will have worked out a part count and know, for example, this grade on this part will give you 50 pieces easily. So they change the insert at that point because it is
predictable.”

Many CBN grades are available and often are designed for continuous-cut steel, heavily interrupted steel or moderately interrupted steel. Once a grade is selected, Coomer said choosing the right edge preparation and hone for the insert can minimize tool wear, lighten tool pressure, improve crater wear resistance and help a tool last longer.

“To protect the cutting edge,” he said, “we can vary the angle coming off the top of the insert anywhere from 10 degrees to about 35 degrees down from the horizontal to direct the cutting forces into the tool and away from the edge.”


GWS Tool Group’s custom high-performance ISO inserts with various tip options, including single tip, double tip, full top and special, can accommodate any hard turning application.  Image courtesy ofGWS Tool Group

It’s important to match the combination of the CBN grade and the edge preparation to the application, said Brian Wilshire, technical center manager for Kyocera Precision Tools Inc. in Hendersonville, North Carolina.

“There’s a fine balance you walk to pick a large enough edge preparation so that you get standard wear and no chipping,” he said. “But if you pick too large an edge preparation, it’s not going to last as long. You can consider it almost pre-worn because you’re grinding a portion of the cutting edge away to strengthen it.”

The Question of Coolant

Whether to use coolant is a substantial concern when hard turning and depends on the tool and application.

“Our ceramics definitely get better tool life running dry,” Wilshire said. “So in most cases, when we recommend ceramics for hard turning, we prefer no coolant.”

For CBN, he said, coolant can be used in a continuous cut with little difference in tool life.

Cooling does produce a part that’s more in tolerance.

“If there’s gauging immediately after machining, it’s easier to get a true measurement if the part stays cooler,” Wilshire said. “But if you run without cooling, the part heats up and tends to expand. So the part may be in tolerance when it’s gauged, but as it cools off, it may shrink enough to go out of tolerance.”

Interrupted cuts don’t fare well with coolant, however, due to rapid thermal expansion and retraction as the tool moves in and out of the workpiece material.

“Using coolant for an interrupted (cut) will create thermal shock in the CBN,” Wilshire said, “which will cause small fractures and quickly destroy your edge. We always tell customers, if they are running coolant, to make sure the stream is not interrupted, which could happen if the pump cavitates or if the geometry of the part shields the coolant line from the insert itself.”

Where running coolant won’t work, he said, an air blast could be used to keep a part cool, though that’s less effective.

Chip Control

In general, chip control is less of an issue with hard turning due to high temperatures that cause ribbons of bright red, molten steel to flow off workpieces. Coomer said chips from hard materials are so brittle that they tend to crumble in one’s hand, so CBN inserts for hard turning don’t often come with chipbreakers.

However, even during hard turning, there are cases when chips may wad up and create issues with fixturing, workholding or surface finish — and harm tool life as well.


The KBN05M indexable insert works for high-speed finishing, as well as interrupted machining of hardened steel. Image courtesy Kyocera Precision Tools

“For example,” Coomer said, “when you’re hard turning internal bores, even though the chips may be free flowing, you might get into some bird nesting issues where chipbreakers might help.”

Wilshire said another application for a chipbreaker would be when turning casehardened materials.

“The outer surface is hard,” he said. “But as you get deeper into the part, it gets softer, and a chipbreaker can help curl the chip to keep it from scratching the softer surface underneath.”

“Nowadays,” Sawicki said, “a lot of high-production cells use robots to take parts out of one chuck and put them into another chuck or onto an inspection table, so you cannot afford to have chips hanging up on the workpiece. The ultimate chip control is using a chipbreaker plus high-pressure coolant at the tip with a coolant-through toolholder, so you’re pretty much guaranteed the chip is going to go into the bed of the machine.”

More Considerations

For parts with ultratight tolerances, Sawicki said CBN holds size much longer than ceramics. The lubricating and cooling effect of coolant helps with tolerances while providing a better finish, he said, and wiper flats on the inserts may improve the finish and reduce cycle times as well.

“Because the material is much harder, the cutting forces in hard turning are much higher, and any play in the ballscrew or in the ways of the machine is going to be magnified,” Wilshire said. “So select the most rigid machine available and go up to the largest shank on the toolholder, with the shortest overhang, the strongest insert shape and the biggest corner radius possible for the part.”

Coatings are available for ceramic and CBN inserts to resist wear, prolong tool life and increase speed, which is all-important for production machining. Coated inserts are more expensive, but Wilshire said the increased cost for a CBN insert is not that much.

“I would say 90% of the time coated is going to be better,” he said. “But I have seen in some cases where uncoated might give a slightly better surface finish than coated, especially with the ceramics.”

Another tip is to pre-chamfer the edges of the part and any drilled holes before it is heat-treated.

“This lets the hard-turn insert ease into the cut from a lighter depth of cut to the full depth of cut without hitting that hard corner,” Wilshire said. “That minimizes the shock as it goes from no load to cutting loads, which can help prolong tool life — and it’s less machining they have to do in the hard turn.” 


Best Practices for Hard Turning

Brian Sawicki, business development manager for the Northeast region at Tungaloy America Inc. in Arlington Heights, Illinois, shared best practices for hard turning.

“If the size of the workpiece you’re turning is 1.25" (31.75 mm) or greater, I prefer using an 80-degree rhombic insert with a 32nd nose radius, which is the most common type of CBN insert,” he said. “As the diameter gets smaller, you want to transition into a screw-down positive insert with a front relief angle because it’s more important to be on centerline on smaller components than it is larger components. You’ll be able to hold size and finish better.”

Sawicki said good starting parameters for hard turning alloy and tool steels include:

  • 122 m/min. (400 sfm)
  • 0.1016 mm (0.004 ipr) feed rate
  • 0.1016 mm to 0.254 mm (0.01") per side depth of cut

“You’re safe 99.5% of the time running those operating parameters in any hard turning application for most materials above 50 Rockwell C hardness,” he said. “If you’re closer to 50 Rockwell C, start with 500 sfm (152 m/min.). And if you’re hard turning D2 tool steel, start at 300 sfm (91 m/min.) or 350 sfm (107 m/min.).”

The goal is not to maximize speed.

“If you double the speed from 400 to 800 sfm (244 m/min.), you’ll get far fewer parts per corner and perhaps less than five minutes of in-cut time,” Sawicki said. “You may get done much faster, but you’ll need to offset your tool more frequently in order to compensate for the tool wear that higher speeds induce.”

He recommends programming a CNC for a constant sfm rather than a constant rpm.

“I prefer coding G96 for constant sfm,” Sawicki said. “Then, as the diameter changes and you get closer to the centerline of the spindle, it’s programmed to speed up the rpm to keep that sfm constant and to give you more consistent and predictable wear on your insert. If it were programmed in G97, which is a fixed rpm, it’s going to feel drag as the part gets smaller. And CBN is sensitive to drag.”    — Holly B. Martin

 

   

 

Related Glossary Terms

  • aluminum oxide

    aluminum oxide

    Aluminum oxide, also known as corundum, is used in grinding wheels. The chemical formula is Al2O3. Aluminum oxide is the base for ceramics, which are used in cutting tools for high-speed machining with light chip removal. Aluminum oxide is widely used as coating material applied to carbide substrates by chemical vapor deposition. Coated carbide inserts with Al2O3 layers withstand high cutting speeds, as well as abrasive and crater wear.

  • ceramics

    ceramics

    Cutting tool materials based on aluminum oxide and silicon nitride. Ceramic tools can withstand higher cutting speeds than cemented carbide tools when machining hardened steels, cast irons and high-temperature alloys.

  • chipbreaker

    chipbreaker

    Groove or other tool geometry that breaks chips into small fragments as they come off the workpiece. Designed to prevent chips from becoming so long that they are difficult to control, catch in turning parts and cause safety problems.

  • chuck

    chuck

    Workholding device that affixes to a mill, lathe or drill-press spindle. It holds a tool or workpiece by one end, allowing it to be rotated. May also be fitted to the machine table to hold a workpiece. Two or more adjustable jaws actually hold the tool or part. May be actuated manually, pneumatically, hydraulically or electrically. See collet.

  • computer numerical control ( CNC)

    computer numerical control ( CNC)

    Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.

  • coolant

    coolant

    Fluid that reduces temperature buildup at the tool/workpiece interface during machining. Normally takes the form of a liquid such as soluble or chemical mixtures (semisynthetic, synthetic) but can be pressurized air or other gas. Because of water’s ability to absorb great quantities of heat, it is widely used as a coolant and vehicle for various cutting compounds, with the water-to-compound ratio varying with the machining task. See cutting fluid; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.

  • cubic boron nitride ( CBN)

    cubic boron nitride ( CBN)

    Crystal manufactured from boron nitride under high pressure and temperature. Used to cut hard-to-machine ferrous and nickel-base materials up to 70 HRC. Second hardest material after diamond. See superabrasive tools.

  • cubic boron nitride ( CBN)2

    cubic boron nitride ( CBN)

    Crystal manufactured from boron nitride under high pressure and temperature. Used to cut hard-to-machine ferrous and nickel-base materials up to 70 HRC. Second hardest material after diamond. See superabrasive tools.

  • depth of cut

    depth of cut

    Distance between the bottom of the cut and the uncut surface of the workpiece, measured in a direction at right angles to the machined surface of the workpiece.

  • edge preparation

    edge preparation

    Conditioning of the cutting edge, such as a honing or chamfering, to make it stronger and less susceptible to chipping. A chamfer is a bevel on the tool’s cutting edge; the angle is measured from the cutting face downward and generally varies from 25° to 45°. Honing is the process of rounding or blunting the cutting edge with abrasives, either manually or mechanically.

  • feed

    feed

    Rate of change of position of the tool as a whole, relative to the workpiece while cutting.

  • flank wear

    flank wear

    Reduction in clearance on the tool’s flank caused by contact with the workpiece. Ultimately causes tool failure.

  • grinding

    grinding

    Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.

  • grinding machine

    grinding machine

    Powers a grinding wheel or other abrasive tool for the purpose of removing metal and finishing workpieces to close tolerances. Provides smooth, square, parallel and accurate workpiece surfaces. When ultrasmooth surfaces and finishes on the order of microns are required, lapping and honing machines (precision grinders that run abrasives with extremely fine, uniform grits) are used. In its “finishing” role, the grinder is perhaps the most widely used machine tool. Various styles are available: bench and pedestal grinders for sharpening lathe bits and drills; surface grinders for producing square, parallel, smooth and accurate parts; cylindrical and centerless grinders; center-hole grinders; form grinders; facemill and endmill grinders; gear-cutting grinders; jig grinders; abrasive belt (backstand, swing-frame, belt-roll) grinders; tool and cutter grinders for sharpening and resharpening cutting tools; carbide grinders; hand-held die grinders; and abrasive cutoff saws.

  • hard turning

    hard turning

    Single-point cutting of a workpiece that has a hardness value higher than 45 HRC.

  • hardness

    hardness

    Hardness is a measure of the resistance of a material to surface indentation or abrasion. There is no absolute scale for hardness. In order to express hardness quantitatively, each type of test has its own scale, which defines hardness. Indentation hardness obtained through static methods is measured by Brinell, Rockwell, Vickers and Knoop tests. Hardness without indentation is measured by a dynamic method, known as the Scleroscope test.

  • indexable insert

    indexable insert

    Replaceable tool that clamps into a tool body, drill, mill or other cutter body designed to accommodate inserts. Most inserts are made of cemented carbide. Often they are coated with a hard material. Other insert materials are ceramic, cermet, polycrystalline cubic boron nitride and polycrystalline diamond. The insert is used until dull, then indexed, or turned, to expose a fresh cutting edge. When the entire insert is dull, it is usually discarded. Some inserts can be resharpened.

  • interrupted cut

    interrupted cut

    Cutting tool repeatedly enters and exits the work. Subjects tool to shock loading, making tool toughness, impact strength and flexibility vital. Closely associated with milling operations. See shock loading.

  • lathe

    lathe

    Turning machine capable of sawing, milling, grinding, gear-cutting, drilling, reaming, boring, threading, facing, chamfering, grooving, knurling, spinning, parting, necking, taper-cutting, and cam- and eccentric-cutting, as well as step- and straight-turning. Comes in a variety of forms, ranging from manual to semiautomatic to fully automatic, with major types being engine lathes, turning and contouring lathes, turret lathes and numerical-control lathes. The engine lathe consists of a headstock and spindle, tailstock, bed, carriage (complete with apron) and cross slides. Features include gear- (speed) and feed-selector levers, toolpost, compound rest, lead screw and reversing lead screw, threading dial and rapid-traverse lever. Special lathe types include through-the-spindle, camshaft and crankshaft, brake drum and rotor, spinning and gun-barrel machines. Toolroom and bench lathes are used for precision work; the former for tool-and-die work and similar tasks, the latter for small workpieces (instruments, watches), normally without a power feed. Models are typically designated according to their “swing,” or the largest-diameter workpiece that can be rotated; bed length, or the distance between centers; and horsepower generated. See turning machine.

  • polycrystalline diamond ( PCD)

    polycrystalline diamond ( PCD)

    Cutting tool material consisting of natural or synthetic diamond crystals bonded together under high pressure at elevated temperatures. PCD is available as a tip brazed to a carbide insert carrier. Used for machining nonferrous alloys and nonmetallic materials at high cutting speeds.

  • relief

    relief

    Space provided behind the cutting edges to prevent rubbing. Sometimes called primary relief. Secondary relief provides additional space behind primary relief. Relief on end teeth is axial relief; relief on side teeth is peripheral relief.

  • shank

    shank

    Main body of a tool; the portion of a drill or similar end-held tool that fits into a collet, chuck or similar mounting device.

  • tolerance

    tolerance

    Minimum and maximum amount a workpiece dimension is allowed to vary from a set standard and still be acceptable.

  • tool steels

    tool steels

    Group of alloy steels which, after proper heat treatment, provide the combination of properties required for cutting tool and die applications. The American Iron and Steel Institute divides tool steels into six major categories: water hardening, shock resisting, cold work, hot work, special purpose and high speed.

  • toolholder

    toolholder

    Secures a cutting tool during a machining operation. Basic types include block, cartridge, chuck, collet, fixed, modular, quick-change and rotating.

  • turning

    turning

    Workpiece is held in a chuck, mounted on a face plate or secured between centers and rotated while a cutting tool, normally a single-point tool, is fed into it along its periphery or across its end or face. Takes the form of straight turning (cutting along the periphery of the workpiece); taper turning (creating a taper); step turning (turning different-size diameters on the same work); chamfering (beveling an edge or shoulder); facing (cutting on an end); turning threads (usually external but can be internal); roughing (high-volume metal removal); and finishing (final light cuts). Performed on lathes, turning centers, chucking machines, automatic screw machines and similar machines.

  • wear resistance

    wear resistance

    Ability of the tool to withstand stresses that cause it to wear during cutting; an attribute linked to alloy composition, base material, thermal conditions, type of tooling and operation and other variables.

  • wiper

    wiper

    Metal-removing edge on the face of a cutter that travels in a plane perpendicular to the axis. It is the edge that sweeps the machined surface. The flat should be as wide as the feed per revolution of the cutter. This allows any given insert to wipe the entire workpiece surface and impart a fine surface finish at a high feed rate.

Contributors 

GWS Tool Group
877-497-8665
www.gwstoolgroup.com

Kyocera Precision Tools Inc.
800-823-7284
www.kyoceraprecisiontools.com

Tungaloy America Inc.
888-554-8394
www.tungaloy.com/us