Powertrain: The Hole Story

Improved holemaking efficiency drives powertrain machining throughput.

Courtesy of Chrysler Group

A typical automotive engine—in this case Chrysler’s Hemi V-8—requires hundreds of holes of all types, produced using drilling, reaming, boring and tapping.

Learn more about deep-hole machining

For more information on deep-hole machining, view a video presentation here.

“In automotive powertrain machining, the OEMs and their suppliers are striving for balanced processing—cycle times that are more or less the same for each process. That’s why holemaking can become a bottleneck or require more than one machine to meet throughput requirements.”

That’s how Jun Ni, professor of mechanical engineering, University of Michigan, Ann Arbor, sums up the potential problems facing manufacturers of engine and transmission components.

The good news, according to Ni, is throughput when machining engine blocks and other major powertrain components can be improved by 20 to 25 percent simply by optimizing holemaking tools and processes. Ni is also the director of the university’s S.M. Wu Center for Manufacturing Research, which conducts basic and applied research in machine tools and machining, among other topics. He has more than 20 years of experience working with the Big Three and other automotive OEMs on machining optimization.

On one project, Ni worked with a Japanese automaker to improve throughput when single-tube gundrilling oil holes in crankshafts. “They were using four stations to make those holes, and if any of those stations went down they had a backlog,” he said.

By changing cutting parameters and drill geometry, the company was able to maintain the same production level with three machines, enabling the fourth to be used if another machine was down.

Courtesy of Kennametal

Optimized for drilling of cast irons, Kennametal’s YPC drills use a three-margin design and other features to reportedly improve roundness, straightness and cylindricity.

He cited another project from a few years ago that aimed to drill a 9mm-dia., 1 “-thick A-319 cast aluminum alloy in 0.15 seconds. “This was work for a consortium of the Big Three and Caterpillar,” Ni recalled. “Spindle speed was 20,000 rpm, and, at that speed, the operation was more like punching than drilling.”

Ni noted that the project required comprehensive analysis and redesign of the holemaking process, including tool material, flute and point design, tool coating and coolant delivery. “We had to look at the tool cross section,” he said. “Torsional strength was important, but so were chip evacuation and point design. The point geometry had to produce very small chips that could be easily and quickly flushed out.” A couple cutting tool suppliers eventually adopted elements of the twist drill design, he added.

Research Drivers

Driving the emphasis on holemaking productivity in powertrain components are the sheer numbers of holes involved, the switch at OEMs from dedicated transfer lines to more flexible CNC equipment and evolving powertrain materials, according to Ni.

“Automotive powertrain components require hundreds of holes, so holemaking accounts for a disproportionate percentage of tooling costs, machining time and total cycle time consumed, and scrap/rework costs,” he said.

Courtesy of Walter

Walter reports that its Titex XD drills can run up to 10 times faster than gundrills and eliminate pecking when drilling holes up to 70 diameters deep.

Swapping a dedicated transfer machine that can drill multiple holes simultaneously for a single CNC spindle also creates a throughput challenge that can be solved only by adding capital equipment or by faster hole production, Ni added. “You may need to drill 15 or 20 times faster with a CNC machine to get the same hole production as a transfer line drilling station.”

In terms of materials, twin emphases on weight reduction and improved engine efficiency are resulting in use of stronger materials that are more difficult to machine, according to Patrick Nehls, product manager for Walter USA LLC, Waukesha, Wis.

“An example is compacted graphite cast iron, which is so tough that an engine with a CGI block can actually be lighter than an engine with an aluminum block of the same displacement,” Nehls said. “CGI engine blocks can have much thinner cross sections than aluminum blocks.”

Compared to gray iron, CGI is 75 percent stronger and up to 75 percent stiffer. Audi, BMW, Jaguar and Hyundai were early users of CGI in automotive engines, and nearly all NASCAR teams have CGI engine blocks or blocks with CGI liners.

According to Nehls, Walter does significant internal and external testing to determine what types of holemaking tools are needed to help users continue productivity improvements. He described results of one test at the University of Darmstadt, Germany, that involved drilling 7mm-dia., 10 “-deep oil line holes. “We ran at about 200 sfm, 21.5 ipm in CGI, and tool life was 175 holes, or roughly 138 ‘ of drilled holes.”

Courtesy of Seco Tools

An example of a special tool from Seco is this long-length reamer for finishing of camshaft bores.

Deep-hole drilling also requires a piloting strategy. “In our case, the pilot drill is sized just a few microns larger in diameter than the long drill, so you’re essentially going into a clearance hole for about the first 1.5 to 2 diameters of depth,” Nehls said. “That’s done so the long drill isn’t cutting at first, which will produce excessive wear and chatter.”

For holes up to 70 diameters deep, Walter reports that its Titex XD drills can run up to 10 times faster than gundrills and eliminate pecking. “Very often at that kind of aspect ratio you’re looking at gundrilling, but a gundrill has a relatively low penetration rate,” Nehls said. “The XD drills can run faster because they have two effective cutting lips, which enable us to basically go twice as fast.”

The through-coolant tools don’t require high-pressure coolant, he added. They are available in diameters from 0.197 ” to 0.472 “, and can be specified with a tip coating for either soft steels or cast irons.

Optimized for Iron

For holemaking in cast iron, solid-carbide YPC drills from Kennametal Inc., Latrobe, Pa., use a three-margin design to minimize drill “walking” when entering the work material and improve hole straightness. Automotive powertrain applications for the cutting tools include drilling differential and planetary gear carriers and housings, engine blocks and manifolds.

The tools feature the company’s “Y-Technology,” which uses three margins and creates predictable, unbalanced motions offset by a counterbalance effect. The result is good hole quality in terms of roundness, straightness and cylindricity, according to Chris Merlin, global team leader, holemaking for the toolmaker. “The YPC drill gives greater cylindricity, especially in deeper holes, which have a tendency to ‘run’ because of the grain structure of cast iron. That not only improves quality of as-drilled holes, it has the potential to improve tool performance in secondary tapping operations,” he said.

“In one test—using a 13mm-dia. YPC drill and a competitor’s drill to produce 45mm-deep holes in GG25 gray cast iron—our drill ran at basically twice the speed and achieved identical tool life,” Merlin added.

Courtesy of Seco Tools

Efficient valve seat machining requires a combination tool that can finish the valve guide and the seat angle quickly and precisely. Such tools often combine a carbide reamer for the guide and PCD tool for the seat.