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Few people have Albert Ferrara's tough, five-axis contouring problem. The machine operator at Boeing Defense & Space Group (Kent, WA) (see lead photo) must mill a titanium panel for the Air Force's F-22 aft fuselage. When completed, the part is about 2' (0.6 m) wide x 3' (0.9 m) long and has an average thickness of 0.1" (2.5-mm). The work material is difficult to machine; the part geometry complex; the required CNC program long. Many, however, share his processing goals -- to improve CNC milling productivity, and that isn't as easy as it sounds. The most common machining process can also be the most demanding. The following are characteristic of many CNC milling applications: *Complex Programs. Cutting a mold for an automotive quarter panel with a 25.4-mm-diameter flat-end milling cutter can require almost 28,000 tool moves. During their research into three and five-axis machining of sculptured surfaces, Robert Jerard and Susan Li at the University of New Hampshire programmed the quarter panel test part, described graphically by 37 surface patches, in 34 minutes on a Silicon Graphics Indigo R4000 workstation. Most conventional computer-aided manufacturing (CAM) systems aren't so fast. *High forces acting on the machine frame. During high-speed machining of a pocketed aircraft part, a tool must pass through a corner about once every 100 mm of travel. According to Professor J. Tlusty at the Machine Tool Research Center, University of Florida (Gainesville, FL), accelerations on typical milling machines are about 2 m/sec sup 2 . Passing through a corner with a feed rate of 0.17 misec takes about 0.5-1.0 sec; traversing between corners 0.6 sec. Therefore, Tlusty estimates cornering accounts for 38-54% of cutting time. With this in mind, machine designers should minimize mass of the machine's moving structures and draw the highest torque possible from the servomotors, especially to handle tight-tolerance machining. Tlusty is currently designing a new three-axis horizontal milling machine with lightweight X and Y-axis motion components, features machine tool builders often don't pay enough attention to. To optimize CNC milling, you must work with its basic building blocks: the machine tool (often a CNC machining center), programming, and tool management. Your machining application will only be as efficient as the weakest of the three components. SMART MACHINE MOVES Machine tool builders now offer buyers a somewhat bewildering array of CNC machining center configurations. Consider the options for both one-setup five-sided machining and five-axis complex contouring. One way is to use a table-on-a-table construction, where a rotary-tilt table adds a fourth and fifth axis of motion to a conventional three-axis vertical machining center (VMC). Several large and small-VMC builders use the technique. "You give up some rigidity for added flexibility," says Yusuf Venjara, general manager, engineering, Hitachi Seiki USA Inc. (Congers, NY), "but the tradeoff is usually minimal for smaller parts. Moreover, users can reduce cycle times by machining five sides of a part in a setup, as well as doing some limited contouring." Hitachi Seiki's VK Series VMC has several design features tailored for high-speed milling. A long-nose spindle provides good clearance at the part. There also is 5-sec tool-to-tool swing-type automatic tool change and 20-tool storage expandable to 120 tools. The no-overhang tables edges never move beyond the bed to improve rigidity. In addition, the spindle never moves outside the table dimensions. Positioning accuracy is +/- 0.0003" (0.008 mm), and repeatability is +/- 0.00008" (0.0020 mm). "Thermal distortion also can be a problem during high-speed milling," says Venjara. "The VK design, however, relies on pretensioned ballscrews anchored at both ends and centered between the guideways to eliminate thermal effects on X, Y and Z axes movements. High-accuracy linear motion guideways have zero-clearance bearings with automatic lubrication to maintain accuracy over a wide temperature range." Another example of a table-on-a-table configuration is the VF-4 VMC from Haas Automation (Chatsworth, CA), which has a standard fourth axis and provides a fifth-axis option using a rotary table. Unlike other VMCs in its price range, this machine provides expanded travels of 50 x 20 x 25" (1270 x 508 x 635 mm) to accommodate large parts, fixtures, and tools. Options include a chip conveyor and a Fanuc TM compatible 32-bit control with up to 1800' of expanded memory and user-definable macros. Maho Machine Tool Corp. (Naugatuck, CT) provides five-axis simultaneous processing on its dual-spindle universal machines without stacking a table on a table, says s company vice president Mal Sudhakar. Its C-series machine, for example, uses built-in rotary tables -- either an NC rotary table, two-axis NC table, or NC rotary/tilt table. Some models also provide fifth-axis simultaneous processing using a built-in rotary table and a C-axis vertical head with programmable contouring of +/- 60 deg left to right. A built-in encoder provides angular measurement resolution of 0.001 deg. Cincinnati Milacron (Cincinnati), which makes VMCs with up to 200 hp (150 kW), provides complex contouring capability using machines with X and Y-axis table motion, Z-axis spindle movement (up and down), and A and B-axis spindle tilt (side to side and front to back). Its smallest five-axis VMC is the 20-hp (15-kW) 20V, a which can have X-Y-Z travel of 120 x F 30 x 24" (3048 x 762 x 610 mm). WHEN IS A MILL A MACHINING CENTER? "We rarely use the term milling machine anymore," says Cincinnati Milacron manager Chip Storie, "instead calling machines either five-axis profilers or machining centers and using the terms interchangeably. Most mold-related contour milling is still done with VMCs, although we also offer horizontal five-axis machines that can handle some heavy-duty cuts." The horizontal machining center's advantage is typically in repetitive processing of smaller parts. Fixturing for complex contouring applications can be more costly than with the five-axis VMC, but palletizing can reduce some of the impact. "For large lot runs," says Storie, "you can build dedicated fixtures on pallets and then store those for later use." The VMC, in contrast, usually has an advantage machining long, skinny parts the horizontal can't tackle cost effectively. Palletizing isn't as common as on horizontals, and changeover typically involves setup at the machine tool. To improve machine uptime, some VMCs allows operators to set up parts on one end of the worktable while the spindle runs. For safety, you can program the machine so the spindle never encroaches into the setup zone. "Chip control problems are still major when machining vertically if you don't pay attention to removing chips from the part and the table," says Storie. "Although the 20V doesn't have a chip conveyor, larger machines do. Operators must remove chips either using air blowing or flood coolant." One benefit of five-axis vertical machining is the ability to simplify fixturing and reduce its costs. Often, on five-axis machines, users can standardize fixturing and reduce its subplates or use a vacuum table to hold long parts. THE HORIZONTAL EDGE E.H. Wachs Co. (Wheeling, IL), a manufacturer of pipeline industry equipment, improved its short-run milling productivity by switching from VMCs to three-axis HMCs. The shop processes some 2500 assembly components in batches of 1-75 parts, averaging three setups and two shifts per day. Innovative fixturing allows four-sided machining in one setup on most parts, five-sided processing on others. "Before we purchased our first Mach 3 HMC from Saeilo (USA) Inc. [Blauvelt, NY]," says Shop Supervisor Bill Adams, "we used mostly VMCs and CNC lathes, and the milling department couldn't keep up with our assembly schedules. Switching from vertical to horizontal machining required rethinking everything from workholding to tooling, but the productivity gains made the change worth the effort. "Our goal was to change our entire way of machining. Parts processed were often small, which compounded fixturing requirements on the VMCs. We switched to horizontal processing to reduce part handling and add a pallet changer. Now, where a VMC required fixturing a part four times to access its four sides, the Mach 3's indexing pallets can present four sides to the spindle in one setup Using an arbor and different tooling, operators can sometimes machine five sides at a time." Tombstone fixturing is key to efficient changeover for Wachs. The dowel-pinned tombstones allow bolting fixtures to exact locations. The Z-axis location is constant; X,Y,Z coordinates are predetermined. Operators mount the fixture, load the parts, set tool lengths, update offsets, and usually run a good part the first time. "As part of the learning curve, we had to determine how to optimize the Mach 3's power and spindle rpm," says Adams. "We were used to 50-taper VMC spindles, but the Mach 3s have 40-taper spindles and less power so we had to rethink tooling. We now use tooling designed for high spindle speeds and can program tools for several passes at smaller depths of cut and faster speeds." Wachs's first Mach 3 application was a large cast-iron spindle housing, 100 of which are made per year. The complex part required milling, drilling, boring, and tapping on four sides When machine operators ran the job on verticals, setup took four hours; run time 45 min. With the Mach 3, setup dropped to one hour and machining to 25 min. The scrap rate fell significantly, from 10 out of every 100 parts to one or less per year. Another problem application for VMCs was a 4140 steel centering mandrill, which Wachs makes in seven sizes. Each mandrel has two keyways the entire part length, 180 deg apart. Location accuracy is critical. "We tried machining these parts on a vertical with a rotary indexing head, but the indexer was difficult to dial in and we couldn't hold accuracy. Now, we hold the part in a three-jaw chuck mounted to the HMC table, machine the first keyway from top to bottom, and then index the part 180 deg to cut the second one. This n improves location accuracy." A MIXED BAG More often than not, a company will have a variety of machining centers on the shop floor, as is the case at K2 Corp. (Vashon, WA), which recently bought a Cincinnati Milacron five-axis V20 VMC. The 700-person company also uses a three-axis VMC from Mazak Corp. (Florence, KY) and a 2-1/2-axis CNC Bridgeport from Bridgeport Machines (Bridgeport, CT). According to Tooling Manager Nancy Beall, each machine tool offers unique benefits. The five-axis machine has its strengths in complex contouring; the three-axis machine in short-run mold production; and the Bridgeport in small part machining. "During ski manufacturing," says Beall, "we used to place the layup in a hot-wrap mold with a continuous 3 deg side angle and resin-cure it with applied heat. The blank produced was then removed from the mold and a ski top applied. "Five-axis machining capability allowed redesigning molds so we can reduce processing steps by embossing the cap (which replaces the top in the new design) directly into the redesigned body mold. The reason is complex contouring allows producing a finished top with the appearance we wanted. The cap wraps over the top and sides of the finished ski body, instead of just covering the top surface as in the old design. "In another style of mold, five-axis machining allowed cutting the ski rails from one block of material instead of two, which improved structural integrity. In both cases, the end user benefited from enhanced performance characteristics of the new ski designs." Around the time Beall introduced the five-axis to the shop floor, she also switched to a PC-based SurfCAM system from Surfware Inc. (San Fernando, CA) that handles programming for all three machines. "The move to five-axis machining went smoothly," she adds, "although it was intimidating at first, mostly because everybody warned us about how difficult five-axis machining would be. Within four months, in though, three machinists were programming and cutting parts on the V20. The CAM system blended the is critical small fillet radiuses on the in molds with no problems. As another benefit, programs were sometimes shorter because the V20 uses simpler NC code language to interpret contours." K2 also simplified fixturing by using a full vacuum table to hold all long parts. This allowed fixturing three caps at a time without angle in fixtures (common on the three-axis system). Combining less fixturing with increased machining speeds and feeds also improved manufacturing productivity on a shaping cam, where cutting time dropped from 6.5 hr on the three-axis machine to 45 min on the Cincinnati Milacron VMC. "The five-axis machine, with its speed advantage and user-friendly PC-based programming, also aids our R&D efforts," says Beall. "Designers can program and machine a new ski design on it, try out the product, and make modifications without disrupting production." CAD/CAM also is a key feature of Burlington Technologies's (Burlington, Ontario, Canada) machining operations. The Tier I supplier to Ford produces aluminum die castings for applications such as automotive engine and power train components. It moved to a Cimatron CAD/CAM system from Cimatron Ltd. (Southfield, MI) about five years ago to address all aspects of daily business, such as die design, layout, detailing, advanced surface design, and user-friendly NC programming. "The automotive industry requires milling complex castings with many drafted and radiused surfaces blending together," says Ivan Zuccolin, Burlington engineering supervisor. "Doing that without an advanced CAD/CAM tool would be too time-consuming." For example, before using Cimatron, Burlington manually programmed its CNC machines, using a simple software program to determine points of intersection. Designing a tool took about three weeks. It now takes about three to four days. "Before CAD/CAM, we produced it plastic models from manually developed CNC programs and then used a tracer to create a steel version. Now all jobs are done on CAD/CAM, and productivity has doubled in the past two years." "Whether you are doing three or five-sided machining or complex contouring," says Nadav Katz, vice president of engineering, Cimatron Ltd., "a lack of CAM system functionality can reduce productivity. Programming takes longer if the system isn't automated enough or is too sensitive to geometric problems such as gaps, break points, and singular points. Without toolpath optimization, machining time increases. Moreover, for complex molds, the amount of manual hand work required after machining depends on the quality of toolpaths. System reliability and ability to verify, simulate, and control the NC operations are crucial in making the first cut both high quality and functional. "When necessary," says Katz, "you can do gouge control in five-axis machining locally, the same way it is done for three-axis. Approximate the surface from the current Z-direction and look for the nearest facet (in three axis, the highest facet). Also pay attention to controlling cusp height between cutter passes. Most CAD/CAM systems use an X-Y plane projection calculation, which can give a substantial error in steep areas. Another approximation used by some software systems is distance as measured along the surface. This gives a better result, but only for ball cutters. For a square tool, a constant distance will not give a constant cusp height when cutting a steep slope. Katz recommends the following CAM system features for complex milling applications: *Ability to construct tool trajectories based on motions such as zigzag, spiral, radial, and parallel while taking into account user-defined contours and surface flow lines *Ability to control the penetration technique during rough machining, including ramp-down technology so there is no need to drill before roughing *Ability to skip gaps or overlapping surfaces within the NC functions without correcting geometry (important if receiving part files from other companies) *A real cusp calculation to obtain the minimum passes to produce the required surface quality *Ability to limit the area to be machined with a large number of contours, islands, and check surfaces in the CAD file. AT THE MACHINE CONTROL George Yamane at Mazak recommends examining the nuances of the various machine tool controls. For complex CNC milling applications, the control needs power to handle lengthy programs and processing flexibility. For example, Mazak's Versatech V-40 five-face double-column machining center uses an automatic head changer to switch between a horizontal and a vertical machining spindle, a process taking just 17.9 sec. Without the V-40's Mazatrol M-32 control's automatic plane conversion function, adjusting the X-Y-Z reference for each spindle would be difficult. With the automatic function, operators can machine with the horizontal head using the same X-Y-Z coordinate system as they did for the main spindle. The Mazatrol control also optimizes tool length compensation selection by determining the axis to be compensated based on horizontal-head orientation. Another key feature is a same-tool-priority function. In an application machining both a part's left side and front with a 160-mm-diameter face mill, a 9.6-mm-diameter drill, and a M12 tap, the system can automatically select the face mill first, machine the left face, index the spindle 90 deg, and mill the front face, then repeat the procedure for drilling and tapping to reduce required tool changes. To reduce programming time, operators input machining data into the Mazatrol control either conversationally or in EIA/ISO format. The control automatically determines the optimum toolpath and cutting conditions based on the tool, workpiece material, and machining shape. The system's toolpath registration function also helps eliminate unnecessary air cutting. When a tool breaks, for example, the operator simply pushes a button to store five points on the toolpath (including the point of breakage). After the tool change, the system automatically moves the new tool at the rapid traverse rate to two points before the break point, and then moves it at the cutting feed rate to the break point where machining begins. TAMING TOOLING After specifying the CNC machine tool and CAM software for your five-axis machining or contouring application, you're still not finished. Rethinking how you tool the turret can reduce problems you thought you would have to live with. For example, manufacturing engineers tending two automated cells at the Center for Advanced Technologies at Focus: Hope (Detroit) solved a processing problem by adding Valenite's (Troy, MI) 8" (203-mm)-diameter slotting mill cutter to the 32-tool toolchangers on eight Cincinnati Milacron Maxim 630 horizontal machining centers. Instead of programming the 1/4" (6.4-mm)-thick tool to mill a feature, they used it to saw off four casting lugs on the mounting face of an aluminum intake manifoid. "We broke the rules on some operations in the cells to improve efficiency," says Valenite Engineer Bob McAnally. "The slotting cutter, for example, acts as a circular saw to cut four lugs, each extending 3/4" [19 mm] above the manifold surface. We take off each lug with relatively thin slices instead of using a large insert on a conventional face mill to convert the entire lug to chips. The process is more efficient, there are less chips, and tooling costs are lower." Derek Giles, manager, Carboloy Inc. (Troy, MI) recommends tooling the turret to maximize machine uptime. This requires tools to meet four basic criteria. *Machining efficiency. Tools should provide high metal removal rates with low power consumption, good surface finish, and acceptable tool life. Some cutters, for instance, can machine parts at fast finishing speeds and produce 15-micron surface finishes. This often allows eliminating a grinding step. *Functionality. Multipurpose cutters reduce setup and allow maximizing tool turret utilization. Typical characteristics include ability to combine operations such as facing and chamfering or end milling and drilling, ability to process a range of parts and work materials simply by changing inserts, and ability to work in a simultaneous three or five-axis cutting mode. Carboloy's Minimaster TM modular end milling system, for example, allows combining any of four reusable steel shanks with a variety of disposable uncoated or physical-vapor-deposition-coated carbide inserts for spot drilling, slotting, chamfering, or ball-nose end milling in ferrous and nonferrous work materials. In one application, an aerospace manufacturer machines a vibration isolation mount from 15-5PH stainless steel hardened to 40-45 R sub c on a 10-hp (7.5-kW) horizontal machining center. Previously, machinists used a combination of HSS center drills, drills, and roughing mills to produce a 3" (76-mm)-long, 1.5" (38-mm)-deep, 0.53" (13-mm)-wide slot. Machining required three passes at 0.5" (13 mm) DOC and took 74 minutes. Using a Minimaster slotting mill with a 12-mm PVD-coated insert, operators increased cutting speed and feed rate and reduced the DOC to 0.15" (3.8 mm). While 10 passes were necessary to complete the slot, machining required only 46 minutes -- a 38% time savings. *Reliability. Tool design improves processing reliability in many ways. A combination of positive axial and negative radial rake geometries can help reduce power consumption and vibration while promoting good chip flow. For example, Carboloy's Seco Plus TM -76 chip-breaker geometry can reduce machining edge temperature by as much as 400 deg F (204 deg C). A +34 deg rake angle can reduce power requirements up to 20%. DON'T FORGET TRAINING In process R&D at Delco Remy (Anderson, IN), George Carter uses a Maho 800C universal machine with a horizontal and a vertical spindle and a rotary-tilt table for a variety of five-axis machining projects. Delco uses the machine, which has a work envelope of 31 x 20 x 23" (787 x 508 x 584 mm), for mold R&D. Carter supervises processing and provides another equally important role -- training. As a C4 training coordinator, he must train personnel with a range of machining and programming skills in the universal's use. "We program off-line with Unigraphics workstation-based software," says Carter. "Controlling the five-axis universal machine isn't more difficult than running another CNC machine. The hardest part is understanding that you are cutting one job with a vertical spindle, the next with a horizontal one. "Everyone downplays programming, but I must train moldmakers, as well as engineers right out of school. No two training requirements are exactly alike. The advantage to training a moldmaker is that you know the person is well grounded in toolroom knowledge. The advantage to training a young engineer is that the person is probably eager to learn. "When machining in five axes, you push a lot of metal past the cutter and must put a lot of thought into how chips are moved out of the way," continues Carter. "You also must rethink the way you address each axis. On the universal we use the Z axis to represent the horizontal spindle. Then the vertical head represents the Y axis. The G17 code is the startup plane for the horizontal spindle. G18 is the startup plane for the vertical one. Normally on a true three-axis machine, G17 is the table, and the tool comes down vertically. On a universal machine such as Maho's, G17 references the part length plus the spindle. G18 references the top of the part, but you also use that reference plane when working with the vertical spindle. "In one case, an operator I supervise couldn't produce the required machine tool movements dictated by the NC program, and I couldn't see any immediate problems with the program. It took a little time to figure out that he had moved the vertical spindle into position, referenced the tools on the top of the part, and left the plane G code as G17, instead of switching to G18." To develop a successful training program, Carter recommends tailoring training to the needs of the individual when possible. Moreover, management should define just exactly what level of training they want that person to receive. Will he or she just be tending the machine or also doing off-line programming? The answer makes a difference in the length of training required to successfully produce parts. Die/Mold Engineer Tim Jones at LeBlond Makino (Mason, OH) reports horizontal machining centers are beginning to provide some competition to VMCs in complex moldmaking applications. Automatic toolchangers, tool monitors, advanced cutter technologies, and adaptive control also are enhancing processing. The true test of either a VMC or HMC's ability to make the cut, however, lies with the spindle and whether it can meet the needs of high-speed milling. "Traditionally," says Jones, "finish machining is done on a massive machine with limited spindle speed (2500 rpm) and poor servo response. Thermal effects from high cutting speeds cause spindle growth and compromise finish accuracy. Not only are cycle times long, hand work accounts for almost 255 of the total manufacturing process." LeBlond Makino's GN spindle design uses three-point bearing support (front, center, rear) to provide high spindle stiffness even when running at the full 8000 rpm. The spindle is preloaded at the front bearings only (one tapered contact roller bearing and two angular contact ball bearings), but also proves a center tapered roller bearing and a real roller bearing. A built-in conical disk spring mechanically clamps each tool into the 50-taper spindle with 4400 lb (enough force to hold a Cadillac up to the spindle). Jones reports Z-axis growth of the GN spindle measures only 0.001" (0.3 mm) at the full 8000 rpm, with less than 0.0002" (0.005-mm) variance during continuous running. The reason is automatic lubrication of all spindle bearings, input shaft bearings, and intermediate shaft bearings -- the hot spots in the rotating spindle. Lubricant temperature is determined by a thermocouple mounted in the machine bed. "Depending on the bed temperature," says Jones, "the oil chiller maintains a temperature differential of +/-2 deg C or less. The casting typically lags room temperature change by six to eight hours. Therefore the spindle oil and spindle variation remain consistent." http://findarticles.com/p/articles/mi_qa3618/is_199403/ai_n8726432/pg_1 |
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