Cells Keep Spindles Turning
Pooling works for single or multiple machines
By Jim Lorincz
Cellular manufacturing is widely touted as a way to overcome the limitations of legacy shop-floor layout that historically separated processes by machine type. In that vintage scenario, shop floors were organized by process with milling machines, turning machines, and grinding machines—all located in their own respective departments.
The manufacturing cell transforms the factory from a process layout to that of a product layout. The product layout is proven to reduce WIP, material handling, and delays caused by scheduling sequential operations.
Today's manufacturing cells are configured to position machines with different process capability in close proximity to one another. Milling, turning, grinding, and even parts cleaning and deburring, as well as measuring functions can all be accommodated within the cell if so desired.
The basic building blocks of manufacturing cells include:
- Single-function machines, such as VMCs and HMCs, multiple-function multitasking mill-turns, and four and five-axis machining centers, among others.
- Pallet pool systems, rotary and linear, that are easily integrated onto machines and between machines.
- Tooling systems sufficient to support the redundancy required of untended machining and integral probing systems to monitor tools and keep cells up and running.
- CNC control and software that manages scheduling and captures data for quality and other management reporting purposes.
One obvious benefit of the manufacturing cell is the better use of labor. Cellular manufacturing frees the shop from the inefficient one-operator-one-machine mentality and offers a cost-effective opportunity for untended operation.
The benefits of manufacturing cells are especially attractive to manufacturers focused on lean production:
- Schedule Variety. Production run quantities can range from one at a time to continuous manufacturing of high-volume parts. The cell can build to daily demand, monthly demand, or work order quantities.
- Flexible Operation. Properly cross-trained operators can handle more than one machine/process, reducing labor requirement. The number of operators required can be reduced to a minimum for normal operation or eliminated for untended operation.
- Process Simplification by reduction in the number of process steps or operations. Multi-tasking or four and five-axis machines can be combined with single-process machines so that palletized parts can be shuttled among machines for single setup, done-in-one machining.
- Setup Time Reduction. Single-machine cells or multiple-machine cells can be chosen according to which delivers the most efficient way to process parts for just-in-time delivery. Twin-pallet changers, commonly associated with HMCs, and multiple pallet pools are increasingly being combined with VMCs.
Setup time reduction was realized by Cox Machine Inc. (Wichita, KS) by acquiring three Bridgeport XR 1000 APC VMCs with integrated front-load automatic pallet changers and 48-tool ATCs as a focal point for its lean and cellular manufacturing. Each of the XR 1000 APCs replaced two standalone machining centers.
"With the integrated front-load pallet changer, we can create a one-piece flow in a cell with one machine," explains chief technical officer Jason Cox. Cox Machine produces structural and control-system components and assemblies for aerospace companies including Cessna, Spirit AerosSystems, Hawker Beechcraft, Eclipse, and Boeing, among others.
"We try to load them with stable and repeatable work, such as brackets and sockets. Because of this arrangement, we haven't entered new markets, but we have gone after work that's a bit of a specific breed—that is, small envelope, high-quantity, long-term contract parts," says Cox.
"It's stable work, repeatable and high quality. When we operate this way, we're able to invest more in the front end of the cell, into standardization of job and process, into our staging areas. We end up with reduced lead times for the customer and a lot quicker response time to forecast changes or engineering changes, for example," says Cox.
"The idea for a pallet changer is that you want to keep the spindle running at all times," explains Andy McNamara, product manager for milling machines, Hardinge Inc. (Elmira, NY). "When you aren't cutting, not making parts, you're not being productive. When the machining center stops cutting and the operator's loading parts, you're losing productivity."
At Core Manufacturing (Conneaut Lake, PA), both VMCs and HMCs with pallet pools are used to machine parts for the transportation, injection molding, medical, and material handling industries, among others. The company has competencies in fabrication, welding, machining, injection mold building, and assembly. "We are a totally integrated company. We buy our own plate, plasma-cut it to shape, weld fabrications, and machine the completed units," explains Kevin Keisel, director of engineering and R&D.
Applications are found in the transportation industry, including locomotives, and power generation, such as coal burning plants. Smaller fabrications and production machining support material handling products such as fork lift trucks and manlifts, injection molding, and medical components. Materials machined include structural steel, tool steel, and aluminum. "We are a huge user of steel and have our own steel supply center. We consume over one million lb (454 t) of steel per month," says Keisel.
At the heart of Core Mfg.'s selection of single-machine VMCs and HMCs from Haas Automation Inc. (Oxnard, CA) is process simplification and scheduling variety. Core's Haas machining centers feature pallet-changing systems that enable them to function as single-machine cells. Core's Haas HMCs feature two or six-pallet changer systems. The VF3 has a four-pallet system; the VF6 has a four-pallet system. An EC400 HMC has a six-pallet changer, and the HS1 has a two-pallet changer.
"One of our principal customers wanted just-in-time delivery, which the single-machine cells make possible. We don't ship to them once a week or every two weeks. We ship to them every day. We'll ship ten completed parts today, eleven tomorrow, nine the next day...from over 500 different part numbers," says Keisel.
"Pallet changers save us by enabling us to do so many parts and fixture them. While machines are running one part, we can be changing fixtures and setups on the other pallets. Basically, changeover time is zero. And part changeout time from chip to chip is quick because we don't have to take the part out and put another part in and start over again."
"The probing system on the Haas machines is especially effective for locating centerlines on weldments so that machine programs can be adjusted according to variations in weldments," Keisel explains. Machining weldments represents about half of Core's volume.
Combinations of VMCs and HMCs are being joined by four and five-axis models and multitasking machines to provide end users with the ability to replace multiple legacy machine tools with individual machines that can be set up to machine a variety of parts around the clock if needed.
"One of the most exciting developments in cellular manufacturing is the development of mill-turn machines based on machining center platforms," says Greg Hyatt, VP-engineering and chief technical officer, Mori Seiki USA Inc. (Rolling Meadows, IL). "Our NMV machines offer an option for high-speed rotation of the fifth axis, allowing turning or lathe operations to be integrated with five-axis milling and are supported by the pallet pools."
Hyatt explains that five-axis VMCs are able to access more facets of the workpiece and can be combined with four-axis HMCs to consolidate the number of operations for certain classes of workpieces. "We can put four-axis HMCs and five-axis NMVs in the same pallet pool. The customer can rough the part on the four-axis HMC, leave it on the same fixture, and shuttle it to the five-axis machine for finishing. That will allow him to get a lot of five axis throughput without buying a lot of five-axis machines. Applications for these cells include blocks and heads for lower-volume internal combustion engines for racing, industrial, and marine engines and other prismatic parts and components that require precision milling and boring."
To support its lean manufacturing strategy, Roberts Tool Company Inc. (Chatsworth, CA) has dedicated several manufacturing cells to eliminate production bottlenecks and reduce cost of parts production. This setup enables multiple machining processes to occur simultaneously, seamlessly moving the part from one stage of production to the next. It gives the operator complete control over the process and creates a start-to-finish manufacturing cycle that improves both quality and efficiency.
Roberts Tool has adopted both single-machine and dual-machine flexible manufacturing systems. The FMS from Toyoda Machinery (Arlington Heights, IL) comprises two HMCs that are paired with eight buffer stations, 12 palletized work settings in linear configuration, and a rail-guided vehicle (RGV). Machines feature 30-hp (22-kW), 14,000-rpm spindles.
The FMS allows the company to deliver high-quality products at a faction of former production costs and run time and has eliminated the need for manual setup and removal of fixtures, reducing changeover time. Fixtures are stored at the buffer stations and delivered to the work area by a single RGV. Each machine has a 120-tool magazine. Replacement tools are readily available and automatically sourced and installed in the case of tool breakage. Before the installation of the system, staging and setup of each fixture was a manual process.
The single-machine cell at Roberts Tool features an HX 400i HMC with 25-hp (18-kW), 13,000-rpm geared spindle from Kitamura Machinery of USA Inc. (Wheeling, IL). The cell features 21 palletized work settings with 400-mm pallets. Tool storage is 150 tools as back-up and redundant tooling. Programmable probing for tool, part, and fixture measurement is included.
"We have integrated rotary pallet changers into our products for a number of years, particularly in the high-speed milling [HSM] products," says Gisbert Ledvon, GF AgieCharmilles (Lincolnshire, IL). "Our machines, specifically the five-axis HSM machines, are designed to accommodate pallet changers such as those from System 3R. Machines are built from the ground up to be automation-ready. The worktable is designed in such a way that the pallet receiver of the chuck is integrated into the fifthaxis table design. In other words, you don't mount on a standard T-slotted table receiver or chuck as you would do on other machines."
Ledvon explains: "We designed the lower table so that the chuck is directly mounted into the casting so you don't lose height and you don't have to deal with any air hoses opening and closing chucks on the table because that could interfere with tool changers and loading and unloading the pallet. The other benefit is that you don't lose accuracy because you are really keeping the pallet as close as possible to the pivot point of your AB axis on the five-axis machine." The Sigma FMC allows the shop to identify pallets with a chip reader that is integrated into the pallet changer. When the carrousel door is opened, the reader reads the new pallet information and feeds the information to the control.
"There is a tendency for customers to look at pallet technology as good only for production applications. That's not true. A normal job shop, or a mold shop, or an aerospace shop can benefit from a single-machine cell rather than investing immediately in a multi-machine cell, which would require a robot or some other material-handling device to service the multiple machines," says Ledvon.
Contract manufacturer, Eldorado Tool & Mfg Co. (Waterloo, ON, Canada) selected a Palletech flexible manufacturing system and Nexus HMCs from Mazak Corp. (Florence, KY) to manufacture prototypes and deliver high-volume production almost seamlessly. The Palletech systems consists of three HCN 6000 HMCs with 160-tool magazines, 40 pallet locations, two load/unload stations, and one stacker/transporter robot. There is also a stand-alone Mazak HCN 6000.
"We make one hundred production parts for off-road vehicles," explains Peter J. Harry, managing director. "That's one hundred setups. You can't make that many setups without something like Mazak's Palletech. If we didn't go for these flexible production systems, we would still be in business, but our growth would be extremely slow," he says.
The Mazak Palletech system enables Eldorado to provide just-in-time, low-to-medium lot manufacturing with minimum lead time. For repeat orders, the system eliminates setups and first-off inspection and minimizes inventory.
Pallet systems are available for both HMCs and VMCs offered by Okuma America Corp. (Charlotte, NC). The MA-H series of HMCS feature a twin-pallet rotary-shuttle APC with pallet pool control, with the option of a multi-pallet APC FMS application. An off-the-shelf pallet changer is readily accepted by Okuma's MU-V series five-axis VMCs.
"Between the Okuma MU-400 and the Midaco flexible pallet changer, we have wiring to tell it to change the pallet and let it know when it's done changing the pallet," explains Brian Sides, director of engineering. "At this time, the method in which we use to exchange pallets is generic. It's called spare M-codes. An M-code is a discrete signal that goes out and says 'change pallet' and a reply is received back that says 'I changed the pallet.' So then we can start the program and cut metal."
"A pallet changer is a good thing," says Sides. "In this case, it's an off-the-shelf pallet changer, so Okuma doesn't have to design a pallet changer for a specific machine. That's not unique to Okuma—Midaco does it for everyone, but it saves Okuma a lot of cost in engineering time that we can use building better CNC machines."
With more than 1700 multi-machine systems installed worldwide, Enshu Ltd. (Hamamatsu, Japan) has adopted a building-block approach to systems design and implementation. Customers can choose one or more standalone machines with option to add pallets and tools through expansion of the ATC as the workload increases.
"Customers can no longer afford to buy a machine that doesn't have a migration path to a flexible manufacturing system," explains Mike Germann of Enshu USA Corp. (Schaumburg, IL). "Since 2001, all of our S and G series HMCs are available from stock in a system-ready configuration. Many companies are asked to support their customer's prototype, PPAP requirements, or initial production, then have to wait for production quantities. Being able to acquire a machine quickly that has an open architecture to systems migration is essential to a self-financing spend as you grow strategy."
"Multi-pallet systems are best suited to high-value families of parts and low to mid-volume parts that have lengthy setups and are ordered on an 'as required' basis," Germann explains. "Once the initial introduction into the system takes place, no setup is required on repeat production requirements. FM pallet systems have been successfully implemented down to an EOQ economical order quantity) of one," Germann concludes.
This article was first published in the June 2008 edition of Manufacturing Engineering magazine.