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Turnover in vertical turning
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Vertical turning for chucked parts 8-20" (200-500 mm) diam has been literally turned on its head in the '90s particularly for high-volume production of automotive wheels, rotors, drums, and hubs. High volume calls for automated workhandling, and the cost of that automation has been reduced 30 to 40% by the rise of CNC vertical chuckers with mobile integral spindle motors. This machine-tool approach became practical once the integral motor spindle technology was perfected. The motorized spindle performs workpiece gripping and transport, moves the workpiece to the tools, and rotates the workpiece for turning, drilling, boring, and possibly some milling. The spindle grips the workpiece from above and the clamping force must resist not only centrifugal and cutting forces, but also gravity. "Many of the high-volume parts are castings or forgings and have a short cutting cycle time," says Gary Sihler, president, Index Corp (Shelton, CT), "making it important that the workhandling times be reduced to the absolute minimum." This new class of mobile-spindle verticals are quite compact for short traverse distances, and offer short workpiece change times as well as quick turret indexing. These vertical lathes may be employed singly, but are best suited to a cell of two or more units to complete parts on both sides. Since the spindle does most of the work of part transfer, much of the gantry-style automation associated with automated workhandling in vertical turning can be eliminated. The traditional vertical lathe has the rotating chuck, spindle, and motor mounted in a fixed position at the machine base. The turret or toolblock moves up and down and laterally on slides above the table. The traditional design has the benefit that gravity helps keep the work in the chuck. Single-spindle and twin-spindle vertical CNC chuckers in a variety of sizes and with driven tools, part gaging, and automated workpiece handling systems, have been available since the early '80s. Gantry-type loading devices have, until recently, represented the main form of automation for these verticals. The gantry loader picks up the workpiece from outside the machine and transports it to the first chuck, after which it turns the part over and transports it to the second chuck, and after that removes the part from the second chuck to pass it out of the work area. Such loaders represent a large investment in automation. For workpieces above 19.7" (500 mm) diam, or for particularly heavy workpieces, this traditional-design vertical may still be best. For workpieces in the mid-size range, however, the vertical chuckers now come in two distinct configurations--the traditional design and newer machines with two-axis linear travel integral spindle motor above and tools below, in a turret or toolbocks. Even for chucked parts smaller than 8" (200 mm) diam, says Robert Cramer, president, Emag (Farmington Hills, MI), "inverted verticals may be the machine of choice, depending on part volumes. For low-volume job shop work, it is hard to beat the low cost of basic horizontal chuckers. For high-volume production, pairs of the new verticals are faster than traditional horizontal chuckers and less complex and costly than the dial-index machines used on some high-volume parts today." The majority of builders of vertical chuckers noted here, including Daewoo, Gomiya, Hardinge, Index, and Motch, also sell horizontal CNC chuckers. That some are now offering verticals for the first time, indicates that a significant shift in chucking is afoot. Advantages of Inverted Verticals The key advantage of the inverted verticals is that the motorized mobile spindle can pick up and drop off parts. This eliminates a loader to move the workpiece from the first to the second spindle. Parts may move by conveyor to the point where the spindle picks them up, and likewise move by conveyor from the point where they are completed on the second side. Pan turnover methods vary from one design to the next. Noncutting time is short. Work transfer speed from the pickup point to the starting of machining is typically not more than five seconds, which is much faster than gantry work-transfer operations of typically 15 to 20 sec. Another advantage is that chips fall away from the work and down into a chip conveyor rather than on to the rotating table and workpiece of the traditional VTL. Many of these machine tools are equipped with cooled spindles, advanced axis drives, linear measuring systems, optional integrated part gaging, fast turret indexing, and a chip removal system to boost speed, reliability, and precision. Automakers and some key suppliers have already adopted this new technology. Emag's Cramer has been surprised at its rapid acceptance in the US, and notes that "since 60 of turned parts are chucked parts, there is a potentially large market for this technology." GM, for example, has been purchasing them by the dozen. Five Inverted Verticals T here are five builder of inverted vertical CNG lathes for fast production of mid-sized chucked parts in the US market, with additional builders likely to enter soon. Emag Bohle L.L.C. introduced its VSC vertical turning machines in 1991 and now offers several sizes. The spindle travels front-to-back on the slide, picking up parts from the conveyor system at the rear of the machine, and bringing them to the 12-station turret at the front of the machine. Following machining, the part is returned to the conveyor. Workpiece change time may be as low as 3 sec. An optional probe may be added to check the diameters and lengths of selected parts and compensate for tool wear. The polymer-aggregate base provides excellent vibration dampening. Coolant circulates through the machine and spindle to remove heat. Equipped with a linear measuring scale in the X-axis and a rotary measuring system in the Z-axis, positioning accuracy is less than one micron. Many systems of linked multiple machines have been installed. The Hessapp line of inverted verticals from Thyssen Production Systems (Auburn Hills, MI) offers an unusual way to address both sides of the workpiece without turning the workpiece over, thus eliminating an additional piece of automation equipment. In a patented configuration called the DV Transfer, the first operation is performed using an inverted spindle. Following the first operation, that spindle moves the workpiece over to the second spindle, which is fixed in place and oriented in the "old" way, facing upward toward the tools above it. The inverted spindle positions the part directly in the second spindle without a turnover station, and the part is machined in the traditional position in the second spindle. Following the second operation, a gripper mounted on the vertical slide removes the completed part and places it on the discharge conveyor. Hessapp machines take chucks from 6.5 to 16" (160-400 mm), with maximum spindle speeds from 6500 rpm for the smallest to 3200 rpm for the largest size. Spindle power is from 19 kW for the smallest to 35 kW for the largest. Machines may be equipped with 12-station turrets, including live tool capability, or block toolholders. Hessapp has built vertical turning machines for 40 years and was acquired by Thyssen within the last year, reports Randy Stohler, Thyssen Production Systems vice president. Hessapp's focus in the last 15 years has been in automated systems and the DVH/DVT models are a result. "One Hessapp configuration, says Stohler, "uses a DV transfer machine with two spindles to machine both sides of the part, and then a DVH single-spindle inverted machine finish-turns the most critical dimensions for maximum process consistency. We are selling new verticals, in part, because the old Bullard VTLs are not holding tolerances close enough for today's requirements, even when they are reworked." Thyssen Machine Group also includes Diedesheim, which produces medium-to-large size Vertifront vertical CNC single- and twin-spindle lathes in the traditional configuration of fixed spindle below and moving tools above. The Index V200 reduces the workpiece load/unload time to a quick four seconds with a rapid traverse of 1180 ipm (30 m/min). It may be optionally equipped with a gang-style tool arrangement, a 14-position turret, or both. Gang tools allow for larger tools or special devices that do not fit in a turret. There are several options for transporting parts to and from the machines: The belt conveyor carries any part--small or large diameter, short or long--provided it can stand upright on the transporting conveyor. It permits extensive part storage in a compact space. The circulating pallet system consists of two rows of 15 pallets each, for parts up to 200 mm. The V200T (Tandem), introduced in late 1995, integrates two vertical workspindles in one machine, with a unidirectional pallet system. If necessary, a turnover station is incorporated in the line The Tandem is cost efficient because its two machining stations use one control, hydraulic system, chip conveyor, and frame. Index's Sihler says that a larger-sized machine of similar design, the V300 model, will be available shortly. The Motch (Cleveland) MV SL 250/315 vertical motor spindle, self-loading chucker picks up parts from a variety of feeder systems and moves them to the working position. It is a single or twin-spindle model. The spindle is 32 hp (24 kW) with top speed of 5400 rpm. The chuck supplied is 10 or 12.4" (250 or 315 mm). The preloads on the twin nuts of the ballscrew are programmable. The preload on the twin nut of the ballscrew moving the spindle laterally is relieved during the 1575 ipm (40 m/min) rapid traverse moves for part transport but restored for machining. This minimizes backlash and wear on the ballscrew, while permitting high accuracy and rigidity. Motch continues to build an extensive line of medium-to-large single and twin-spindle CNC vertical lathes with the spindle, motor, and chuck in the base. Many of these go into multimachine production cells. A six-machine group of Motch VTC vertical turning centers, with traditional spindle orientation, is being built for turning differential cases for American Axle. The six turning machines will; be followed by a Turmat rotary transfer machine (from Turmatic Systems, St. Louis) for the drilling and milling. Rich Legan says that Motch now offers a choice of traditional or hanging-spindle verticals, as well as a new line of self-loading single or twin horizontal-spindle chuckers, to cover many turning applications. Limits to the Hanging Spindle "Workholding is the major limiting technology in traveling-spindle vertical turning," says Legan. "We offer a spindle that goes over 5000 rpm, but the 12" chucks can only go about half that, before they start to open up." Only smaller chucks can go beyond 5000 rpm today. This limitation impacts high-volume applications for all the builders in this market. "At Motch," says Legan, "we prefer to offer the 5000-plus rpm now and hope that the chuck builders catch up in the next couple of years, rather than limit the machine to what today's chucks can withstand." The other limit he sees: is weight: he does not recommend heavy parts be targeted for the hanging-spindle design today. "Gravity must be given its due," be warns. Kingsbury (Keene, NH) designed its new VTC-4 traveling vertical spindle machine on the base of a horizontal slant-bed CNC chucker, except that the integrated pickup spindle motor, rather than the turret, is placed on linear guideways on the slantbed: The turret is mounted where a tailstock is usually found on a typical slant-bed chucker. Maximum turning diameter is 10" (250 mm). Rapid traverse in X-axis moves is up to 1000 ipm (25.4 m/min), and in the Z it is up to 500 ipm (12.7 m/min). Like other machine builders who serve automotive, Kingsbury builds single and twin-spindle vertical chuckers of the traditional design. The traditional design for vertical lathes is standard in the larger VTLs, such as those from Diedesheim (part of Thyssen Production Systems) and Motch. It is also found in a number of the small verticals, especially single-spindle and twin-spindle models designed for manual operation. One advantage of this orientation is that some of the machining forces are directed down into the table. This is particularly true of the thrust in drilling. The Daewoo (Carlstadt, N) Puma V15-2SP twin-spindle vertical with gantry automation was shown at IMTS 94. Specs on the machine include 25.6" (650 mm) maximum turning diameter, 60 hp.(45 kW) spindle drves, 630 ipm (16 m/min) rapid traverse rates in the Z-axis and 480 ipm (12 m/min) in X, and dual 10-station turrets. An automated gantry-loaded, twin-spindle Daewoo system for turning Class 8 truck hubs has been installed in the US. "The parts involved are too heavy for the gravity-defying inverted spindle type machines," says Daewoo's Don Wood, machine tool systems general manager. The gantry loader can handle workpieces to 90 lb (40.8 kg). In one application, Wood says, "97 different truck hubs are machined from seven different castings. A fiber-optics system identifies the casting to ensure that the right casting is entering the machine for the part program running at that time. Automated tool management is incorporated in the cell. After turning, a conveyor system carries the hubs to one of two multispindle drills, where dual bolt-hole patterns are drilled. All the workhandling automation is between the two turning spindles, so access to the work areas is good." Hardinge Brothers (Elmira, NY) introduced its V200 and VT100 single-spindle CNC vertical lathes with traditional configuration to serve automotive customers. The VT 100 has a 10 or 12" chuck and 20-hp C15-kW) spindle drive; the VT 200 has a 17" (430 mm) chuck and 30-hp (22-kW) spindle drive. The 12-station turret may have up to six live tools. Other options include high-low chucking force, a programmable tailstock, part probe, high-pressure coolant, and a chip removal system. Rapid turret indexing, and rapid traverse rates of 472 ipm (12 m/min) in the X axis and 630 ipm (16 m/min) in Z, make for fast cycle times. Gomiya (USA) Inc. (Elk Grove Village, IL) offers twin-spindle, twin-12-station-turret CNC verticals in two sizes. The Twin-Merit 12 can have a 10 or 12" chuck and 30 hp (22 kW), the Twin Merit 15 can have a 12 or 15" chuck and 40 hp (30 kW). The two chucks are located relatively close to each other and to the center of the machine, minimizing operator fatigue. The two sides of the machine may be operated independently or may run the first operation on one side and the second operation on the second side. The turret indexes in a fast 0.6 or 0.8 sec. "The machines are built with very heavy construction and are super-rigid," reports Ed Sanders of White Hydraulics (Hopkinsville, KY. "I'm a firm believer in verticals, and we use a range of vertical and horizontal CNC chuckers. We take heavy interrupted cuts on castings, about 0.150" without vibration. The rigidity allows us to push the speeds and feeds about 25% over what we could do on a horizontal chucker and still hold the tolerances." The shop runs both low and high-volume parts across the verticals. About 70% of the work runs first operation on one chuck and second operation on the other, while 30% of the work is managed with two low-volume unrelated jobs running at the same time in the two chucks. "Bore tolerances are regularly held to 0.0003" 0.01 mm , says Sanders. "Sometimes we run 10" 1254 mm shafts on these verticals because the part doesn't whip around as it does on the horizontals, and the turning tool pressure is down, into the chuck, which is good." Okuma & Howa (KGK International, Buffalo Grove, IL) has a line of traditional twin-spindle CNC verticals. The largest machine in the line is the ACT 2SP-V5 with twin independent 40 hp (0 kW) spindle motors, and 21.65" C550 mm) maximum cutting diameter. The smallest, the 2SP-V2, offers a 9.84" (250 mm) maximum cutting diameter. The machines have separate controls, spindle drives, and coolant pumps, so they are actually two machines in one. The two work areas are separated so a part can be running on one side while the machinist is working on the setup on the other side. The machine may run two different parts or two sides of the same part. http://findarticles.com/p/articles/ |
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