Materials Force Equipment Development

Manufacturing Engineering, Oct 2007 by Morey, Bruce

The substrate for the company's tools uses ATI Stellram's patented X-Grade technology, available in three grades, that employs a ruthenium and cobalt alloy rather than standard carbide. The combination results in superior thermal cracking and propagation resistance, according to ATI Stellram, achieving higher metal removal rates. Their X500, and X700 grades were specifically designed for titanium and nickel-based alloys, while the X400 was designed for hardened steel.

When cutting titanium, Hoffman suggests reducing the arc of engagement between the tool and the work surface. This reduces heat while enabling higher spindle speeds. "For proper tool life, reducing the arc of engagement anywhere from 2 to 15%, compared to 50-100% for common steels, actually produces higher material-removal rates without a loss of tool life," he says.

Parag Hegde, manager of the Global Machining Technology group at Kennametal, agrees that a titanium cutting tool needs a positive rake angle. Edge preparation is another matter. "For titanium alloys, the conventional wisdom is they should be machined with a sharp [or a smallhone] edge," says Hegde. "We did a lot of work to test this theory. We find that the edge preparation is very application-dependent. For example, we have found that a honed T-Land edge preparation works very well in some specific titanium applications."

Kennametal recommends their Grade KC5010 in the -UP geometry for finishing to medium-machining turning operations in heat-resistant materials and titanium. Using a PVD AlTiN coating with a microfinished edge over a fine-grain tungsten carbide substrate helps the substrate resist the high heat generated, while providing deformation resistance.

They are also trying out coolant delivery through nozzles built into the tool pocket, rather than flood coolant, to provide a jet of coolant where the edge is hottest. The coolant is directed through the tool and exits a nozzle directly in front of the insert. "We have just introduced this coolant technology into our new Z-Axis cutter," explains Hegde.

Composites present their own tool design challenges; whether they are the traditional epoxy-based carbon-reinforced variety, or the more advanced metal-matrix composites. Drilling is probably one of the more common-and demanding-machining operations on traditional composite materials, according to several sources. Drilling issues include delamination, fiber pullout, and burr formation. Kennametal, in partnership with Novator AB (Spanga, Sweden), offers an orbital drilling approach that Hegde says is particularly suited for composites, especially stacks of differing material often found in aerospace fuselages. He describes it as a helical interpolation process that allows making multiple features on the hole in the same operation, such as holes plus countersinks. They have also introduced a new diamond-coated solid-carbide drill that is specifically designed for epoxy-based composites, and report significantly reduced delamination and pullout in tests.