May 2010 / Volume 62 / Issue 5|
Not your father's VMC
By Bill Kennedy
Courtesy of B. Kennedy Advanced vertical machining centers provide performance and value.
Although vertical machining centers have traditionally trailed horizontal machines somewhat in terms of flexibility, spindle utilization and sophistication, many shops rely on VMCs as cost-effective workhorses. Machine tool makers are overturning tradition, however, by building VMCs that operate faster and more accurately than earlier models while also offering productivity-boosting features such as 5-axis machining, multiple spindles, pallet pools and turning and grinding capabilities.
Key among the historic advantages of VMCs is their generally lower cost, compared to horizontals. Bill Howard, VMC product manager for Makino Inc., Mason, Ohio, said HMCs typically have pallet changers, large toolchangers and additional axes that add to their cost. In regard to performance, Howard noted that VMCs are often preferred for heavier cutting.
Verticals provide a stronger platform because they typically have fewer moving parts than horizontals, and gravity adds stability to workpiece fixturing. “Your part is laying flat on a table, typically with the stiffness and rigidity of the table behind it, so you are not getting part deflection and distortion,” Howard said. The work zone of a vertical is relatively smaller than a horizontal, he added, making it “much easier to maintain alignment, straightness and parallelism.”VMC Drawbacks
On the other hand, the VMC’s basic configuration can make it difficult to maximize spindle utilization. The pallet changers on most horizontals permit parts to be preloaded into fixtures and swapped in and out of the machine in a few seconds, minimizing spindle downtime. On a traditional vertical, however, the spindle sits idle while the completed part is removed from the fixture and a new part is mounted. “The traditional vertical utilization time was way less than half, 30 percent, something like that. The good news is that the machine is less expensive, but the bad news is you are only getting a 30 to 50 percent bang for the buck out of it,” Howard said.
Courtesy of Chiron America
As a result, machine tool builders are adding capabilities to VMCs aimed at increasing productivity. The improvements start with elements as basic as rapid traverse rates and spindle speeds. “It used to be a 400-ipm rapid traverse on a vertical was great. Now, you are looking at most verticals having a 1,600-ipm rapid traverse. That reduces nonproductive positioning time, which reduces cycle time and cost,” Howard said. “Today, we are even seeing people using linear motors on high-end verticals, for even higher speeds.”
Higher spindle speeds represent another upgrade. While 12,000 rpm is a typical maximum spindle speed for VMCs, builders are now offering spindles that turn at up to 45,000 rpm. “That can improve productivity, specifically in applications in tool and die or medical, where they are doing very small features with small tools,” Howard said.
Tool change speeds are rising too. When VMC toolchangers held 10 or 15 tools and the machines were milling simple parts, a 30-second tool change was acceptable. Now, according to Mark Abell, sales and marketing coordinator for VMC builder Chiron America Inc., Charlotte, N.C., faster tool changers are a key factor in making verticals more competitive. A few seconds can yield a major net increase in production over time, he said. Chiron accomplishes fast changes with a “basket” toolchanger system, which stores the tools in a magazine around the spindle and produces chip-to-chip times as fast as 1.3 seconds. Because the basket changer doesn’t have to move from the spindle during the changing process, the system offers both high speed and positioning accuracy, according to Chiron. Abell added that even the company’s larger-capacity, chain-style toolchangers are fast, at 2.4 seconds chip-to-chip. He said the push for speed includes “not only tool changing, but also higher feed rates and the ability to use the latest cutting tool grades and cut material faster than conventional tooling.”New Magazines
The use of larger tool magazines is another VMC technology trend. Makino’s Howard said verticals of the past stored one or two dozen tools. “But as you put more and more work on the table, particularly when five-face or 5-axis machining, you need more tools.” VMC tool magazines now commonly hold as many as 80 tools, and machines such as Makino’s D500 5-axis, tilt-trunnion vertical can be configured to hold 300 tools or more.
A large selection of tools permits a machine to produce complex part features and a variety of parts without the need to change tools in and out of the magazine. “Previously, VMCs required added time to set up a new job,” Howard said. “You only had a 15-tool capacity and you had seven or eight tools for the job, so you’d have to pull some tools off and put some tools on for a new job. Now you have all the tools on the magazine already.” Makino’s D500 has a tool load/unload station at the back of the magazine that permits loading, unloading and maintaining tooling while the machine is cutting, because the magazine is isolated from the machine’s work envelope.
VMCs are capable of higher accuracy than past models. Today, accuracy of ±0.0001 " to ±0.00015 " is average, but builders are increasing machine precision in a variety of ways, including tightening tolerances on machine components and alignment, and maximizing machine stiffness through engineering techniques such as finite element analysis. Machine movement is controlled more precisely via feedback mechanisms such as glass scales. Makino fits most of its VMCs with glass scales, providing 0.05µm accuracy. The technology permits users to “program to submicron levels,” he said.
Makino plans to introduce a HSK 32-taper, 45,000 rpm-spindle vertical called the iQ 300 at IMTS 2010. It will have scales that provide feedback of 0.005µm. Also in the interest of accuracy, the machine’s axes will have linear motors, not so much for ultimate speed but to impart fine surface finishes.
Courtesy of Mori Seiki Maxing Axes
One way to increase VMC spindle utilization is to add axes of motion beyond the standard X, Y and Z. “If you put a trunnion on a vertical, load/unload time is eliminated,” said Tom Jonas, president of DSM Machinery, Warrendale, Pa., a Gosiger company and a Haas Factory Outlet. Distinct from a rotary table, in which a part can be chucked and machined on four sides, a trunnion adds two axes of movement and, Jonas said, permits a shop to “not only work around four sides; you can tip the part straight up and work on the fifth side, and any angle thereof.”
With five-sided machining capability provided by a trunnion, part features can be produced in one clamping that would otherwise require the spindle to be stopped and the part to be refixtured (see sidebar on below).
Jonas pointed out that the 5-axis positioning capability of a trunnion can be employed without high-level, continuous, 5-axis CAM software. “Just about any CAM package can handle five-sided machining,” he said.
Scott Rathburn, marketing product manager for Haas Automation Inc., Oxnard, Calif., said Haas offers several 5-axis machines, including VMCs with trunnion tables and VMCs with dual-axis spindle heads, dual-axis tilting rotary tables and dual-axis trunnion rotary tables. “Our latest trunnion rotary table, the TR160Y, mounts in the Y direction on a medium-sized mill, freeing up the rest of the table for additional fixtures,” he said.
Howard noted that the addition of automation can multiply the benefits of a VMC equipped with a tilt-trunnion table. Pallets have been developed that will clamp into a tilt-trunnion table. “You can add a robot and a material handling system to provide a flow of various workpieces already mounted on pallets that the robot can load into and out of the machine using the pallet clamped directly to the tilt-trunnion table,” he said.
Courtesy of Makino
This eliminates the major objection to VMCs—lower spindle utilization and productivity, Howard said. “By automating, you have not only 5-axis machining capability, now you’ve got high spindle utilization because your only spindle downtime is the time it takes for the robot to unload one pallet from the tilt-trunnion table and load the next pallet onto the table.”
Howard cited several applications where shops have a Makino VMC with a full contouring, 5-axis tilt-trunnion table integrated into a cell with robot loading capability. “These folks have reduced their costs by increasing machine and spindle utilization and are able to compete globally,” he said. The VMC with robot setup enables these shops to queue workpieces and feed them to the machine in an untended operation. The shops see the approach as a way to reduce costs and labor content. “It’s not super-duper, high-technology stuff, but it keeps the spindle busy, the costs low—and the work onshore,” he said.Crossover Machines
At the leading edge of VMC evolution are major changes to machine structure and capabilities that begin to blur the definition of a VMC. For example, multiple spindles enable one VMC to do the work of two or more, in nearly the same footprint. For example, each spindle of Chiron’s twin-spindle VMCs machines its own part at the same time. Both spindles slave off the same X, Y and Z axes.
Abell reported a situation where a manufacturer of small appliances had been machining product housings and stands on dedicated equipment. “We’ve been replacing a lot of their dedicated equipment that requires multiple operators and we put them on twin-spindle machines,” he said. “The manufacturer has already replaced nine machines and they also have [reduced the number of] operators.”
Abell also described Chiron’s offering of what he called “very different vertical machining centers, where we actually tilt the spindle to be horizontal.” In the company’s Mill series, orienting the spindle horizontally enables it to approach a bar-fed part from the end and permits turning and milling in the same machine.
In some cases, VMCs are providing capabilities similar to multitasking machines, according to Greg Hyatt, vice president and chief technical officer for DMG/Mori Seiki U.S.A. Inc.’s Machining Technology Laboratory in Hoffman Estates, Ill. “We are integrating turning and grinding operations with 5-axis verticals,” he said. For example, the company’s NMV series is a 5-axis VMC with a pallet pool and turning as options. By giving one of the machine’s two rotary axes the capability to spin at higher speeds, it can be used to turn the part for lathe-style machining. Grinding wheels are mounted on arbors and loaded into the tool magazine. “Then we can perform a rough-milling operation, perform a tool change to put a grinding wheel in the spindle, and then finish the feature,” Hyatt said.
He pointed out that integrating processes on one machine helps maintain part accuracy. When a part is milled and turned on separate machines, the refixturing makes it difficult to hold tolerances between the turned features and the milled features. With separate operations, fixturing errors stack up—even if they are on the micron level. “You get the best possible concentricity and position between the turned features and the milled features if the operations are integrated into a single fixturing,” said Hyatt. “Combining processes on one machine is especially attractive if the cycle times are not well matched. If the cycle times are imbalanced, a cell will suffer from poor utilization of the quicker machine.”
Makino’s Howard said advances in VMC technology are aimed at today’s global manufacturing arena. “The easy stuff is finding its way into the very price-competitive global market, and what’s left are the jobs that involve challenging materials, demanding tolerances and difficult features, or products and designs that are proprietary in nature.” CTE
About the Author: Bill Kennedy, based in Latrobe, Pa., is contributing editor for Cutting Tool Engineering. He has an extensive background as a technical writer. Contact him at (724) 537-6182 or by e-mail at email@example.com.
Courtesy of B. Kennedy
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