by Kip Hanson
In response to lightweighting initiatives, cutting tool suppliers are tackling new materials, forming new partnerships
Automobile drivers might be getting a break at the pump right now, but none should doubt that fuel prices will rebound at some point, quite likely with a vengeance. At the same time, concerns over greenhouse gases and air quality continue unabated, driving governments across the globe to mandate improved fuel economy with fewer emissions. Together with competitive market pressures, these policies force automakers to increase engine efficiency while lowering vehicle weight, both of which test the mettle of manufacturers, machine tool builders, and cutting tool suppliers alike.
Aluminum vs. iron
Tom Chevalier is the business development manager for automotive at www.sandvik.coromant.com and responsible for the North American market. He sees tremendous activity across the region, but particularly in areas such as Ontario and Detroit, much of it centered around aluminum machining. “Perhaps 75 per cent or more of the engine blocks used in passenger vehicles today are made of high silicon aluminum, material that is very tough on carbide. Because of this, we’ve spent several years developing new types of cutting tools able to achieve the wear resistance and quality requirements called for by automakers.”
One example of this is Sandvik Coromant’s CoroMill M5B90, an indexable polycrystalline diamond (PCD) face mill designed for high volume finishing of engine blocks, cylinder heads, and other aluminum components. Chevalier says the cutter achieves surface finish and waviness to R4 and W4 respectively, two attributes important to automakers, at cutting speeds of 3,800 m/min (12,467 sfm) and feedrates of 9,000 mm/min (354 ipm), resulting in part cost reduction of up to 30 per cent.
Another attribute is ease of use. “Automakers are very focused on setup time,” he says. “These cutters are very simple to use, needing only a single screw to clamp the insert. There’s none of the time-consuming adjustments seen with traditional types of aluminum finishing cutters that require each insert to be set individually.”
Aluminum is unable to withstand the extreme friction of a reciprocating piston, so many automakers line engine block cylinder bores with cast iron. Since weight must be kept as low as possible, however, these liners are generally quite thin, often just a few millimeters (0.12 in.) or less. The result is a tendency towards chatter during boring operations. To counter this effect, Chevalier recommends a tooling solution popular in another transportation industry: aerospace. “We find that the silent tools (anti-vibration bars) used to machine the long, thin-walled bores found in aircraft landing gear to be quite effective in cylinder boring applications as well.”
Ice is nice
Sandvik Coromant isn’t the only tooling supplier involved in the automotive market , nor is iron the only material used to line cylinder bores. Oleg Eliezer, global automotive industry manager for Iscar LTD., Tefenm Israel, says the company is heavily engaged with the automotive industry’s Internal Combustion Engine (ICE) Optimization program, and that a number of automakers have replaced cast iron liners with thermal spray processes such as PTWA (Plasma Transferred Wire Arc Spraying) and APS (Atmospheric Plasma Spray), which use a plasma jet to atomize metal feedstock and deposit it on cylinder walls. This metal is quite hard—around 52 to 62 Rc—and thin, measuring roughly 0.5 mm or less (0.019 in.), requiring but a small amount of material removal to clean up the slightly irregular surface.
Until recently, automakers were finishing this layer by taking numerous passes with a hone or grinding wheel. Iscar has since introduced what it says is a more effective alternative, a polycrystalline cubic boron nitride (PCBN) boring head that uses a drawbar to retract the inserts at the bottom of the stroke, or a programmable actuator to adjust their position during the boring process. Either way, what was once a lengthy series of honing passes has been reduced to a single boring step performed at cutting speeds up to 700 m/min (2,300 sfm) and feedrates to 1.2 mm/rev (0.047 in.).
Eliezer points to another recent automotive development: replacement of forged or cast aluminum pistons with friction welded ones made of steel. “They’re thin all over, like sheet metal, but much stronger than aluminum,” he says.
Whatever the material, throughput is always a top priority in automotive machining. Iscar has responded in this case with a specially engineered tool to machine the ring grooves near the top of the piston—rather than profiling each one individually, the tool plunges four grooves at once, using 80 bar (1160 psi) high pressure coolant to break chips and extend tool life.
Sometimes the best way to lightweight a vehicle is to eliminate metal altogether. Jay Ball, product manager solid carbide endmills NAFTA at Seco Tools, Troy, MI, says that’s exactly what some automakers are doing. “Car companies are no longer restricting CFRPs to body panel use. They’re now looking at steering knuckles, drive shafts, seat pans, and other structural components.”
CFRP, or carbon fiber reinforced plastic, enjoys a strength to weight ratio greater than steel or aluminum, making it a logical choice for many lightweighting applications. As manufacturing costs have come down, CFRP has become a darling of aerospace manufacturers, with automotive not far behind. Unfortunately, it’s not much fun to machine.
Ball says CFRP can be machined quite successfully using brazed PCD tooling, but that a rigid setup is required. “You can run very high speeds and feeds while still achieving excellent tool life, but it comes at a price. PCD is quite expensive. And if there’s any part movement at all, the tool will almost surely fail.”
The most common alternative, he says, is CVD diamond coating. “Probably 80 per cent of CFRP machining today is done with diamond-coated carbide—in our case, the Jabro and Niagara line of cutters. They come in a wide variety of geometries to accommodate the different types of ‘sandwich’ layers found in CFRP, and have a relatively thick coating—up to 12 microns (0.0005 in.) is not uncommon, nearly twice that of other CVD coatings. This substantially prolongs tool life in this extremely abrasive material.”
Not off the shelf
Ball also sees an increased call for specialty tooling in automotive. “It’s a challenging industry, with long reach requirements, difficult quality and material requirements, and a focus on minimizing cost per part,” he says. “Because of this, specials are about 25 per cent of our business.”
William Radtke agrees. Engineering coordinator Americas for Walter USA, Radtke says there’s an increased emphasis on overall solution development in the industry, and that automakers are partnering with cutting tool and machine tool suppliers alike.
“Take turbochargers as an example,” he says. “Newer engines put out more power per cubic inch. This means automakers have had to switch to more heat-resistant materials. Working in partnership with them and the machine builder, we’ve developed a complete process for the manifold and turbocharger components. This includes both standard and special tooling, grades designed to effectively cut these tougher materials, and
proper application of ‘interpolation turning’ technology.”
The irony of using relatively heavy steel as a way to build lighter vehicles extends to the crankshaft as well—since bearing journals are smaller and counterweights narrower on today’s more fuel-efficient engines, traditional cast iron cranks are being replaced with stronger steel versions. This creates problems with rigidity and increased tool wear, something that Radtke says toolmakers are addressing through tool design, insert geometry, or new carbide grades.
“When I first started working in automotive 16 years ago, machine builders would mandate what the tooling should look like,” Radtke says. “Today you see more of a collaboration. The end users producing these parts work directly with the cutting tool companies, asking for advice on what we believe the tool should be, and then that information is fed back to the machine builder. Part of this is due to decreased emphasis on dedicated automotive transfer lines and more on standard machining platforms, so it’s easier to deliver off the shelf or nearly off the shelf solutions. Still, it remains a fiercely competitive business—saving even a fraction of a penny per part is a big win. That’s something we’re continually working on.” SMT