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CUTTING TOOLS |
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Dry Steel Turning Pays Off With Lower Part Costs, Longer Tool Life |
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Wet turning is an old habit that costs money and can compromise productivity, especially with advances in carbide turning insert technology that now make dry turning more attractive than ever before. Modern carbide grades provide equal metal removal rates, part finishes, and tool life, wet or dry. With cutting performance equal, shutting off the cutting fluid can reduce the cost per part 15% and eliminate persistent fluid problems. In fact, the experience of one high-volume automotive drivetrain supplier shows how you can find greater productivity in dry turning, when your tools can take the heat. In select turning operations at a busy drivetrain component plant in the northeast, turning off cutting fluid ended persistent chip jamming, and prevented needless scrap. Dry turning promises the plant operations manager significant savings in fluid cost and treatment. With advanced Grade 4015 carbide cutting inserts and technical assistance from Sandvik Coromant, the auto parts maker also showed dry turning could double tool life. Costly Cooling Cutting fluids are meant to lubricate cutting tools and reduce cutting zone temperatures to protect both tools and workpieces. They reduce cutting edge pressure. Fluids also flush away metal chips and suppress troublesome dust. Cutting fluids nevertheless pose workplace health hazards, and they impose a constant economic burden on machine shops. In addition to the cost of fluids and dispensers, coolants require expensive cleanup and treatment to prevent the growth of disease bacteria and satisfy environmental regulations. By one automotive industry estimate, cutting fluids account for up to 15% of the total cost of machining a part. Despite the premium paid, the benefits of fluids are questionable. At the high speeds encountered in computer numerically controlled lathes, just 15% of the coolant directed at the workpiece evaporates on the cutting edge to dissipate heat. About 84% goes down the drain, and 1% remains on the part after machining, sometimes interfering with quality control measurements. Worse, the temperature variations created by liquid coolants promote tool wear. Cemented carbide cutting inserts ordinarily fail due to flank wear. However, wide temperature fluctuations due to irregular coolant flow and heat dissipation accentuate thermal shock and accelerate cracking and insert failure. With interrupted cuts, inserts hot from the workpiece plunge into cold coolant again and again to crack even faster. Alternatively, continuous turning cuts create sustained high temperatures that soon cause plastic deformation in conventional carbide inserts. While dry turning eliminates the expensive nuisance of cutting fluids, it takes special inserts to survive increased heat and tool pressure. To achieve equal performance wet or dry, Sandvik Coromant combined two powerful advances in substrate and coating technology. Sandvik Coromant’s 4015 carbide grade has a gradient-sintered fine-grain substrate. The low-cobalt tungsten carbide microstructure at the core of the insert is protected by a cobalt-enriched microskin. Unlike conventional carbides that grow more brittle as they grow harder, the gradient sintered composition creates a substrate that resists both plastic deformation and impact. To protect insert edges in dry turning, the extra-hard substrate is covered with a thick coating of titanium carbon nitride oriented in columnar crystals. A proprietary medium temperature chemical vapor deposition process uses temperatures lower than previous processes to enhance adhesion without embrittling the zone between substrate and coating. Together, the advanced substrate and coating create a cutting tool that withstands higher pressures and temperatures. Dry turning creates more heat and more edge pressure. Without cutting fluids, ordinary carbide inserts show rapid edge flaking. By comparison, the columnar microstructure Series 4000 insert coating undergoes more progressive wear. While the lay-flat microstructure of ordinary insert coatings loses large random portions in dry cutting, uniform columnar crystals slow tool wear. The radius cutting edges and MT-CVD process provide constant lamellar growth and a uniform protective coating all around the insert surface. The result is enhanced edge security in the face of heat and pressure when compared with ordinary turning inserts. Hot Chips, Small Chips Axles, steering linkages, and suspension and steering components are big business for a busy drivetrain plant supplying Big Three automakers. However, a new line turning forged steel axle tubes, axle shafts, and ring gears came to a halt as chip augers clogged with long stringers. With two CNC lathes idled by each clogged auger, chip cleaning typically stopped production for 30 minutes every two hours. The shop manager asked for help from a knowledgeable tool supplier. Sandvik Coromant field representative Dave Lewandowski had already introduced GC4015 turning inserts into the plant to maximize throughput. “The Ultraspeed GC4015 inserts are designed to withstand the extreme heat and high loads encountered in faster machining. They were the best technology to minimize cycle times and increase productivity,” Lewandowski explains. To remedy the chip problems at the drivetrain plant, Lewandowski first suggested changes in chipbreaker geometries and cutting data. When chip stoppages continued, he got the shop manager to break with tradition and turn off the coolant. Dry turning puts most of the heat of machining into the chips, and hotter chips are more brittle and readily broken. With the axle tubes turning at 1,394 sfm and feed set at 0.0091 ipr, dry turning reduced troublesome two-foot long stringers into manageable two-inch long “sixes and nines.” Significantly, tool life of the Grade 4015 inserts in dry turning equaled that with coolant. Dry Turn, Long Life The success on the axle tubes was repeated in multiple dry turning operations on ring gears. Grade 4015 inserts withstood cutting speeds from 800 to 1,000 sfm with feeds from 0.018 to 0.022 ipr. Again, turning off the coolant reduced chips to manageable lengths. Based on the success with the ring gears, the plant intends to use dry turning on more machines producing a growing range of gears. Dry turning axle shafts provided a triple payoff in chip control, process accuracy, and tool life. Like the wet turning operations on the gears and axle tubes, chip control for the finishing cuts on the axle shaft flange face posed a problem. Moreover, the wet-turned shafts were frequently marked for scrap by a new in-line gauging system, but manual measurements found the same parts within specified 0.001" tolerances. Analysis showed contaminated coolant residue on the parts was misleading the gauging system and could lead to unnecessary scrap. To address the production problems, the Sandvik Coromant representative again suggested dry turning with Grade 4015 carbide inserts. However, this time the standard chipbreaker inserts would be replaced with a wiper-geometry insert. Wiper geometries on turning inserts follow the cutting nose radius with additional radii. Like conventional turning inserts, wipers use their leading edge to remove metal and leave a surface roughened with peaks and valleys. Unlike conventional inserts, the trailing wiper radii remain in contact with the work to wipe down or burnish away the peaks and leave a smoother finish. Wiper geometries typically permit double the feed to achieve the same finish or double the finish quality with the same speed. To preserve or improve finish in dry turning, feed per revolution was increased just 25%, from 0.0160 to 0.0200 ipr. Surface cutting speed was reduced from 1,300 to 1,050 sfm. Without coolant, the inserts produced manageable chips that keep production running. Dry turned axle shafts passed automated inspection. In addition, insert life doubled from 300 to 600 parts per edge. Projected savings in inserts and tool changing time, and a 1% increase in productivity, were estimated to be worth $6,700 a year. Dry It Out Dry turning is not appropriate everywhere. Using carbide inserts, thin-walled parts or those with tolerances tighter than 0.001" can be subject to excessive thermal distortion if turned without coolant. (Cubic boron nitride [CBN] inserts may hold 0.0005" tolerances dry.) Stainless steels full of abrasive alloying elements create higher cutting forces and more heat, and generally require coolant. Cast iron requires exceptional dust collection capability if turned dry. However, in many steel turning applications, advanced insert technology lets you turn off the coolant and a variety of coolant-related problems. Sandvik Coromant - July 2002 |