Right now, the tooling development unit of Kelsey-Hayes Co. (Livonia, MI) is working with Daewoo Machinery Corp.'s systems division (Plymouth, MI) on special fixturing for their horizontal-spindle machining centers. The work is being done on behalf of a new joint-venture plant in China, where Volkswagen components for Beijing taxicabs will be manufactured.
quot;It's typical of the kind of jobs we're getting involved in now," says Daewoo engineer Len Elliott. "We're putting four steering knuckles on a two-sided upright fixture that can be rotated in 0.001-deg increments. That allows machining on any feature of the knuckle." Other aspects of the job include employing 180-psi coolant nozzles on each side of the machine's work area, plus through-the-spindle coolant, to control chips; using step drills and backfacing cutters that reduce out-of-cut time; and perfecting a specialized straddle mill that cuts both sides of the steering arms of both knuckles on each face of the fixture.
An interesting aspect of the project: While tooling and fixtures are being developed on a demonstration machine in Michigan, the six production machines are being delivered from their builder in Seoul, South Korea, and installed in China for preproduction operator training. The machining center (MC), Daewoo's ACE-H50S, has several features specifically aimed at the Tier-One automotive parts market, including four controlled axes using the Fanuc 16, twin automatic pallet changer with pallet rotation at the load station, a screw conveyor for chips, and an oil jacket cooler for the spindle that helps deliver guaranteed 0.002-mm repeatability. The design items fit today's demand, says Jim Janis, VP of sales at the builder's U.S. headquarters in Carlstadt, NJ. "The call is for quick setup and changeover, easy chip removal and part access, and simple maintenance, as opposed to the previous tendency toward single-purpose transfer-type concepts."
If faster throughput in order to compete with dedicated metalcutting lines is the aim, many MC designers seem to be getting the hang of it. Cincinnati Milacron, Inc. (Cincinnati, OH) conceived a proof-of-concept exercise not long ago that shows how gains can be made. Mark Adkins, director of market development for the Mill's machine-tool group, recalls that the three-scenario project involved machining typical features on an aluminum exhaust manifold for a vintage, 1989-design Chrysler four-cylinder engine.
The baseline first scenario uses an old-generation Milacron T-10 horizontal-spindle machining center with a changer for its 500-mm pallet. Operating two shifts per day, output is calculated at 14,000 parts per year.
The second-generation situation involves upgrading to the builder's present-day 500-mm-pallet horizontal, the Maxim 500, and using features like axis speeds to 100 ipm, fast pallet shuttle, and tool-storage capacity to 180 tools. Hypothetical two-shift production of the same exhaust manifold grows to 35,000 parts per year.
The third scenario entails moving to what Adkins terms the "coming generation" of machining centers. It includes features like an indexing pallet that allows eight parts on a cubic tombstone fixture (two on each side), toolchange speeds that are triple those of the old T-10, an improved spindle carrier that enables acceleration to cutting rpm in one-third the time, and triple-speed slides. Production of that Chrysler exhaust manifold now zooms to 70,000 parts per year.
Over the past 10 years or so there has emerged a new class of machining centers designed for high production while retaining the flexibility that endeared MCs to generations of job-shop owners. The newest ones in that class—whether they are called "production machining centers" as in the PMC-V750 from Monarch Machine Tool Co. (Cortland, NY), "production centers" as in the SPN50 from Niigata Machinery (USA) Co., Inc. (Rolling Meadows, IL), or simply have no special class designation as in the FS-510 horizontal that Mazak Corp. (Florence, KY) introduced at IMTS-96 or the new A77 horizontal from Makino Inc. (Mason, OH)—all tend to challenge the conventional idea of where machining centers fit in the overall scheme of things.
You remember the old conceptual scatter graph from all those 1980s articles and promotional materials for flexible manufacturing systems. As you ascend the graph's vertical axis, production volumes increase. Move out along the horizontal axis, and part variety grows. So in the upper-left portion of the graph (high volume, low variety), transfer lines rule. Out at the bottom-right (low volume, lots of variety), stand-alone machining centers dominate. The middle cluster (mid volumes, moderate variety) was to be the natural dwelling place for FMSs, those multi-machine installations linked with rail-guided cars and mind-melded with a central supervisory computer the size of a Jeep Grand Cherokee.
Costly, full-blown flexible manufacturing systems may not have caught on as predicted, but many of their elements were incorporated into smaller-scale manufacturing automation in the form of flexible cells of two or three linked machines. What we're seeing today are the elements of those cells being integrated into single machining centers. These, then, can be operated singly or ganged for increased output and/or redundancy.
"We're talking about a single machine that's aimed at `medium-volume production,'" says Bruce Kiwala, national sales manager for Monarch Cortland. "Although it's designed to accommodate a family of parts, it runs the same part for long periods of time."
For this class of machine tool, it turns out, there's a list of attributes that have to be re-thought, or at least thought out further. For example, the 70,000-manifolds-per-year "next-generation" machining center that Cincinnati Milacron postulates will employ a magnetically levitated super-high-speed spindle. While such spindles are not yet widely used, there are other ways of increasing tool rotational speed, and the trick for many will be putting together the right package of productivity-enhancing features.
So what are those features that volume producers look for? Several areas of machine design stand out.
Pallet shuttles are most often used in conjunction with horizontal spindle orientation, such as on the newest automotive-oriented MC from Mazak. In fact, Mazak formalized its designs two years ago in its so-called Palletech approach for larger MCs that modularizes pallet-system components and makes expansion easier. Then, there can be the non-pallet approach.
Automated loading is often the norm at first-tier manufacturers like New Process Gear Div. of New Venture Gear (East Syracuse, NY). But a line of five newly installed Monarch PMC-V750s there are clustered into "man-loaded cells." The MCs make transmission components on a fully dedicated basis, but specialty load and unload automation was bypassed, reportedly because of the uncertain life cycles for parts in the fast-changing four-wheel drive transmissions the plant makes. Multiple parts are hand-loaded into fixtures on both faces of a trunnion worktable by an operator who moves up and down the line and "bellies up" to the fast-swap fixtures on the worktable. An open design at the front of the vertical-spindle MC lets him do so.
Mag-lev spindles are virtually frictionless and thus capable of enormous rotational speeds, and they have been just over the horizon for more than a decade. They're only now starting to become commercially available—Zurich-based IBAG A.G. through its U.S. subsidiary (Milford, CT) demonstrated one at the last IMTS, for example, and Milacron may soon bring its model out from behind closed university-lab doors. But "high-speed" is a relative term, and for many production jobs the 50,000-plus-rpm speeds of a magnetic spindle may be overkill.
Still, no one will argue that fast spindles aren't productive. In a demonstration, Makino completely machines a 4.0-liter aluminum V-8 block with cast-iron cylinder liners in one setup using its 50-taper-size Model A77. The horizontal-spindle MC has top spindle speeds of 18,000 rpm with conventional spindle bearings, and more than half of the features in the demo are produced at speeds exceeding 10,000 rpm. Similarly, the FS-510 from Mazak, aimed specifically at automotive customers, offers spindle speeds to 15,000 rpm, and Niigata points to its SPN 50 with a 12,000-rpm top speed for its 50-taper, 40-hp spindle.
In another new machining center aimed at the auto market, the Mach 1 machine from Lamb Technicon Machining Systems (Warren, MI), there are two 28-kW, 24,00-rpm spindles. One can be cutting while the other is undergoing a tool exchange.
Perhaps as important as top spindle speed is the time it takes to get to operating speed. An integral motor in the Makino A77 MC provides acceleration from 0 to 10,000 rpm in 2.5 sec and up to 18,000 rpm in 6.5 additional sec. Sheer horsepower is necessary to make those kinds of jumps, but another major factor is the toolholding system, which for the A77 conforms to the HSK standard now becoming popular in Europe.
More automotive customers are demanding HSK toolholders, say several machining-center builders. The reduced mass of the HSK holder helps hasten tool changes, another factor in reducing nonproductive time. The HSK 63-A system is the one used, in the new machining center that Fiat affiliate Comau S.p.A. (Turin, Italy) introduced at the last EMO in Milan. The machine—designated the "1g," with "g" as in acceleration—offers a tool-to-tool exchange time of 2.9 sec. Add to the lowered mass the fact that HSK designs are inherently self-centering—a boon in very-high-rpm work—and it's easy to see why the pattern is quickly becoming asked for in machining centers as well as in the transfer lines where the design got its start.
Plug-in machines.In a series of MCs set in a fixed line, if it's going to take more than 45 minutes to get a stopped machine back to running, it may be easier to replace one MC and get the line back to running. Ease of snuggling into a lineup has been one of the hallmarks for horizontal machining centers from Cellular Concepts (Detroit, MI). At the last IMTS, four of its Model H15W were integrated into a flexible dial machining system. Several builders now offer machines with base configurations that allow them to be used in a stand-alone mode or integrated into a line. The 1g from Comau offers a choice of bases.
Builders and users have similar observations on a number of related issues. On the footprint of machining centers, for instance, smaller is all of a sudden much better. Some experienced builders are amused; they recall when automotive customers would cavalierly add a series of new operation stations—and hence plant acreage—to a transfer line, the better to alleviate potential bottlenecks at one machining station. This change in attitude also has a profound affect on tool management. More users want to machine more of the component in a small area, and the new MCs need to concentrate a variety of cutters (and probes and tool-type sensors) into that focal point. Not to mention the desire to have back-up tools available, now that breakage detection is trustworthy. A related issue, CNC reliability had been a problem in the past, especially compared with the extremely rugged switches and relays that Detroit had developed over decades. But as electronics have gained command, it's no longer an issue. In fact, modern-day controls open whole new areas for interactivity—and cost savings.
Wish List Persists
Nearly two years ago a group of six U.S.-owned machine tool builders, including machining-center producers Cincinnati Milacron, Kingsbury, and Monarch, got together with representatives from GM, Ford, and Chrysler under the auspices of the National Center for Manufacturing Sciences (Ann Arbor, MI), the 200-company-member not-for-profit consortium for collaborative research. Their goal was to launch a project on requirements for machining centers used in the automotive industry. A formal project wasn't started because anticipated funding under the federal Advanced Technology Program dried up. But a working "hit list" was developed. It included development of
•Temperature control for accuracy-enhancing thermal stability across the board in production machine tools. Many builders offer some kind of system, usually refrigerated coolant removing heat from areas surrounding the spindle (Boston Digital Corp. in Milford, MA takes the opposite tack and heats the spindle to a known, stable temperature.)
•Feedback devices to enable the CNC to perform condition monitoring of parameters like torque, horsepower consumed, etc., on a continuous basis.
•Improved Mean Time Between Failure rates and, perhaps more important, Mean Time Between Repair. The latter reportedly was a new concept to some machine-tool builders that did not have extensive experience with automakers.
•Designs for standardized machine components. Common vertical- or horizontal-spindle modules could be interchanged for similar modules from other manufacturers, but each builder could customize its own product.
Although the specific research project never got underway formally, it started a dialogue that continues. It's possible that a follow-on consortium project will renew the program, and in the meantime, many of the concepts are likely to see further development in future MC offerings to the huge automotive market.
Some Things to Know About Machining Centers
•Definition of a machining center: a multifunction machine, typically combining boring-drilling-milling tasks. Machining centers have three or more axes of motion, have a contouring-type numerical control and an automatic toolchanger (according to the builders' trade group, AMT-The Association for Manufacturing Technology).
•First commercial machine to fit that definition: the 1958 Milwaukee-Matic Model II from Kearney & Trecker Corp., a horizontal-spindle machine with random-access 30-tool changer and four-position rotary table. (Tool-change time: 8.5 seconds, reported in American Machinist 100th Anniversary Issue.)
•Machining centers purchased in U.S. in 1995: 11,638 machines, of which 5,046 were imported (Census Bureau reports EM545 and IM145).
•Value of U.S.-produced machining centers that year (`95): $699 million; value of station-type machines: $477 million.
•Purchasing intentions for machining centers, buyers in automotive standard industrial classifications 3711-3714: Increasing from 1995 to 1996 (Capital Spending Survey, Gardner Publications, Inc.)