July 2011 / Volume 63 / Issue 7|
Powertrain: The Hole Story
By Jim Destefani, Senior Editor
| Improved holemaking efficiency drives powertrain machining throughput.
Courtesy of Chrysler Group
“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
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
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
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
Process parameters for the Kennametal drill (with the competitor’s in parentheses) were: spindle speed of 4,897 rpm (2,448), cutting speed of 200 m/min. (100), feed rate of 0.35 mm/rev. (0.25) and surface speed of 1,714 mm/min. (612). “Of course, it does require machine tools that are able to run at the speeds the tool needs without excessive vibration or other issues,” Merlin said.
Kennametal is producing standard YPC tools for drilling 3, 5 and 8 diameters deep.More on Specials
With all the various types of holes—very deep through-holes, blind-holes, cross-holes and so on—required for automotive powertrain components, special tools are often required to achieve the necessary quality and productivity.
“Very often we can take a standard technology but adapt it into a special tool,” Nehls said. “It’s common to provide dozens of special tools on, for example, an engine block. For one project, we had 25 different special drills on an engine block producing something like 7 million holes per year.”
For Seco Tools Inc., Troy, Mich., specials also include increasingly popular combination tools, according to Mike Smith, product manager, reaming and toolholding. “At both the OEMs and in the Tiers, they are using a lot of combo tools to produce multiple features simultaneously, improve quality and reduce cycle time,” Smith said. “That holds true in both reaming and drilling applications.”
He added that the toolmaker focuses a lot on specific parts. “We try to optimize holemaking for the component the shop is working with, whether it’s a cylinder head or another drivetrain component,” Smith said.
Smith cited valve-seat reaming as an example of an application where a combination tool is particularly applicable. “If you can hit multiple angles on the valve seat with a single tool, that saves a lot of time,” he said.
A combination tool for valve seat reaming might use a traditional carbide reamer to produce the valve guide and a PCD insert to produce the angle on the seat, which must be precise to ensure maximum fuel efficiency. “Seco has two different setups to do that kind of work, depending on whether the shop is running a transfer line or a CNC machine,” Smith said. “On a transfer line, the reamer is actually fed out from the head. For flexible cells that use CNC machines, there’s another style. Both are very effective.”
Regardless of whether the shop is using dedicated or flexible equipment, the numbers of components required for automotive powertrain applications are so massive that a short time reduction is significant. “Those fractions of a second add up to minutes, hours and days over the course of a year,” Smith said. “And, of course, time is money.” Just one component—the cylinder head—accounts for more than 50 percent of the tool cost for engine machining, he added.
Seco also does a lot of work in transmitting power from the engine through the transmission and out to the wheels. Here again, combination tooling can reduce cycle time and improve quality. According to Thomas Sandrud, drilling product manager for the toolmaker, a Seco PerfoMax combination drill is being applied to produce a 0.866 "-deep hole, a chamfer and a back chamfer in gray cast iron wheel hubs in a single operation. The through-coolant insertable tool, which circular interpolates a back chamfer on the exit hole and eliminates deburring, runs at 525 sfm with a feed rate of 0.006 ipr. “Tool life is approximately 100 ' per cutting edge,” Sandrud said.
Low-carbon steel half shafts require shallow holes, and Seco’s Crownloc system comes in stub lengths as short as 1.5 times diameter. “The shorter the drill you can use, the more accurate the hole geometry you can hold, the better positioning you get and the more reliable the process will be,” Sandrud explained. “In one case, we switched a customer to the Crownloc drill at 1.5 times diameter and increased tool life due to increased stability.”
Seco’s Crownloc tools are available with geometries optimized for ISO P, M and K materials, Sandrud noted, and both the crown and the drill body are standard products.
Perhaps nowhere is productive holemaking more important than in automotive powertrain applications. The sheer numbers of holes and throughput rates required make it a challenge, but it’s one that machine and cutting tool suppliers are up to. CTEAbout the Author: Jim Destefani, senior editor for CTE, has written extensively about various manufacturing technologies. Contact him at (734) 528-9717 or by e-mail at firstname.lastname@example.org.
Courtesy of Mazak
Courtesy of Mazak
Holemaking at Penske Racing
Seco Tools Inc.
University of Michigan
Manufacturing Research Center
Walter USA LLC
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