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Linear Pallet Systems: Straight Road to Profitability?

Ed Sinkora
By Ed Sinkora Contributing Editor, SME Media

Improved software gives linear pallet systems a major boost

Adam Hansel, chief systems and sales officer, DMG Mori (Hoffman Estates, IL) sums it up perfectly: “Go into any shop. Ask them if they want to automate. The answer is yes. 100%.” But what kind of automation? And which machines and processes? For a wide variety of companies, the logical answer is a linear pallet system that integrates multiple metalcutting machines, and possibly additional operations. On the other hand, “logical” doesn’t always mean “easy” or “worry free.”

Don’t Confuse FMS with Pallet Pool Systems

A basic linear pallet system, also known as a flexible manufacturing system (FMS), automates the loading and unloading of one or more machining centers with workpieces from a central storage location. The workpieces travel on a straight rail, carried by a vehicle with additional axes of motion that enable it to move the work into a machine and onto its table and out again when finished. Operators set up jobs at a central station, not at the individual machines, and a central computer controls the distribution and execution of the work.

In the most common configuration, workpieces are bolted to fixtures, which are in turn bolted to uniform pallets. This maximizes the flexibility of the system since any job can go to any machine at any time as long as the machine is configured for that standard pallet. Another key feature of linear pallet systems is the ability to expand them after their initial installation to include additional machine tools, more storage, and even nonmachining operations like inspection. Some systems have 10 or more machining centers and dozens of pallet shelves stacked in three levels.

Thus a linear pallet system differs from a pallet pool system, the latter of which can be thought of as an “unintelligent extension of the machine,” as Bob Baldizzi, specialist for systems and large machines, Doosan Machine Tools America, (Pine Brook, NJ) explained. “In a pallet pool system, additional software on the machine itself runs the system. You are typically limited to telling the software what program is associated with what pallet in the pool and the sequence that you want those pallets to go through the machine. That’s it. There’s no production scheduling. No data management. No reporting. It’s just a larger pallet system extension to the machine.” Pallet pool systems are usually circular arrangements, though they can be linear, adding to possible confusion.

“The pallet pool systems I’ve seen typically have the same part on all five or six pallets,” said David Suica, president, Fastems LLC (West Chester, OH). “If you want to change parts, you generally have to change tooling inside the machine, the part program, and any offsets needed for each part. Pallet pools are fine for a series of the same part. But if you want to make just what you need in the quantities you can sell, a linear pallet system is generally a better way of going.”

Spindle Utilization Over 90%

Doosan’s linear pallet system features a straightforward user interface (control shown to the left) and an overhead buss to supply the high voltage three-phase power to the pallet vehicle without cables, which are subject to wear and breakage.

Situations differ, but it seems no machine shop reaches over 50–60% spindle utilization without an FMS, while anyone using one properly will see rates of 90% or more. And in some cases the disparity is even greater. Hansel said “spindle utilization is probably 30% or less in most cases if they have a mix of machines that are being manually fed. It’s just impossible to do workpiece changeovers, fixture changeovers, and scheduling a lot of resources efficiently on a clipboard.” As Suica explained, “Most of the downtime on a machine is caused by humans…waiting on a tool…figuring out the program…manually punching in tool offsets. Once that hour of setup is spent, you never get it back for production.”

This points to the biggest and most significant improvement in linear pallet systems over the last few years: software. To one degree or another, all FMS providers offer capacity planning software that makes it possible to maximize system throughput. Hansel described a typical scenario: “Let’s say you have several machines, 180 tools in each magazine, 24 fixtures, and certain jobs that have to get out to meet your contracts. You set parameters like the priority of the jobs and the numbers you need by certain times. Or say you’re building a car. You wouldn’t produce two axles and three wheels. You’d need four wheels. So you can tell the software to produce lot sizes. In either case, the software automatically calculates how to use your resources properly to get the production you need. If you plan properly and prepare fixtures and tooling and then let the software do the work for you, a run rate of 90–95% is completely achievable.”

Good Fit for Job Shops… and More

Who benefits from an FMS? Anyone from a small job shop to an operation making 100,000 or more of a part, in some situations. John Einberger, product line manager, Makino Inc. (Mason, OH) said, “The sweet spot for this technology is someone who has a variety of products to run, in a somewhat random order, with relatively short production runs. By doing the work-setting off the machining center, you increase spindle utilization.

“But there are also applications for higher production volumes among people who have seasonal production,” he continued. “For example, let’s say they make 80,000 of something a year, but they make it only for three months, and then they run a variant or something different for the next four months. The recreational vehicle powertrain industry fits this kind of production scheme. During the off-season they’re making what they need during the season, like boat motors when snowmobiles are selling, and snowmobile motors when boats are selling. Our FMS has been a very useful tool for customers in these high-volume situations.”

This flexible manufacturing system from Makino (which they call MMC2) illustrates the basic components of any FMS.

Hansel added that “it’s pretty common for people who run just five or six different part numbers to put them on a linear pallet system. That’s because you have a lot of redundancy in such a system. Horizontals typically have large tool magazines that allow you to put redundant tools in the magazine to run longer shifts. If the tool breaks or wears out, you can just go to the next one.” On the other hand, the folks at Liebherr (Saline, MI) pointed out that pallet-handling cells are most cost effective when the cycle time for all the parts on a given pallet totals over five minutes. Otherwise you’d waste time waiting for the vehicle to switch out the pallet.

To give you some sense of how popular these systems have become, Einberger said Makino has installed 575 of its linear pallet systems in the last 20 years in North America alone.

Not Just for Horizontals

Traditionally, linear pallet systems generally connected horizontal machining centers, because “a horizontal machine almost always comes with a pallet changer,” explains Baldizzi of Doosan. “You always have a second pallet out in a load station area that—in a stand-alone setting—the operator uses to load and unload parts while the machine is running. That’s a benefit in an FMS because the crane can load and unload off of the load station area without interrupting the machining cycle. A typical vertical machining center doesn’t have that, nor do most five-axis machines, though manufacturers like Doosan are now equipping five-axis machines with pallet changers that can be hooked up to an FMS.”

Besides mixing types of machining centers, it’s also possible to integrate other functions into an FMS. You can also integrate machines from different builders, though in that case you might be better off working with an “agnostic” automation company like Fastems: “We can have a Brand X machine for a roughing operation, a Brand Y machine for a tight tolerance application, a Zoller with automatic tool offsets writing to a chip, a CMM, a wash station, and a deburr system, all fully integrated,” said Suica. “Our software would even automatically manage tool offsets and the inspection data.”

Some of these options require a robot in one or more stations, or replacing the traditional FMS vehicle with a robot on rails. Einberger said Makino offers the latter hybrid “as an answer to some requirements, primarily in the aerospace structural arena. Some of these customers want to autoload parts into a machine using a robot on a rail because they want the advantage of being able to start by loading the part onto a flat surface, and then take the subfixture and load it into a horizontal machining center.

“This approach is also very advantageous on multi-operation parts where the first operation may be a four-axis operation and the second operation is a five-axis operation within the same machining center,” he continued. “So pallet one on the machining center would be a four-axis tombstone that accepts the part on a subplate, while the second pallet on the same machining center would be on a fifth-axis table that accepts the same part on the same subplate for a subsequent five-axis operation. The robot on rail systems bridge the gap between a pallet-handling FMS and a discrete part-handling robot.”

On the other hand, Baldizzi cautioned that getting one kind of flexibility causes you to lose another. “We’re starting to see the demand to adapt to different pallets, like performing some functions on a horizontal and others on a five axis machine with turning capabilities, requiring a different pallet. The plus is you can put all these machines you need to perform the operation on the FMS. The minus is you lose some of the flexibility of the FMS because the software has to make sure that this pallet only goes on this particular machine at this particular time. Whereas if you have multiple machines on an FMS that are all the same, the work that’s bolted onto a pallet can go anywhere, anytime, anyhow.”

Costs and Challenges

According to Einberger, the cost of an initial FMS including the rail system, vehicle, work-set station, and pallet stocker stands is roughly two-thirds the cost of a given machining center, excluding fixtures. So given the promise of doubling your throughput, that’s a good pay-off. But Hansel said that “while people really like the idea of having a lot of pallets, they sometimes underestimate the cost of fixturing. So when they’re making a decision about what machines to buy, the real cost of fixturing is sometimes not included—which can prevent them from using the system fully. For example, you get the benefit of lights-out operation, or eliminating a third shift, only if you can load up enough work to keep the machine running for eight hours or more.”

A Makino FMS with several horizontal machining centers. A key advantage of any FMS is the ability to set up both workholding and programs for all jobs at a central station (foreground) and then let the software determine how to route them among the machines to maximize throughput.

However, everyone agrees the biggest challenge with an FMS is operational. As Suica puts it: “It’s the same as your PC: garbage in equals garbage out. If you have good programs, tool offsets are known; if you have your schedule worked out, it works very well. The biggest thing for most companies is the discipline required to make sure they’re ready. It’s no different from the discipline that every company puts into their accounting department. You match invoices, make sure they’re correct, make sure you pay the tax. It’s not easy, but it’s lined up exactly and completed. You can do the same thing on the manufacturing floor and have it integrated, and you can remove the human element that causes the error.”

Suica also cautioned that “most of the linear pallet and robotic systems out there tend to be islands of automation. They know what they know and they’re not connected to anything else inside your operation. You may have an ERP [enterprise resource planning] package that tracks your orders, the intake of raw material, and so forth. You’ve spent a lot of money on tools, determining the tool offsets, and measurement systems. But the information flow between all these systems still ends up being manual.” This relative lack of integration prevents companies from achieving major savings in manpower.

Suica said it’s not uncommon to have eight or nine machines running with just a few people if you fully integrate these systems. “We’re working on a system right now that has eight machines that will be manned by three people, one of whom is just doing a check because they didn’t want to automate that. It will run two shifts, so there will be six people on that line, whereas now it’s 14 people. But you need a high level of systems integration to do that, otherwise you still need a bunch of people to tell the automation what to do. Integration and the software is key.” So is training and support, which is why DMG Mori, Makino, and others have dedicated automation teams and facilities.

Software doesn’t solve everything, though. Baldizzi pointed out that mechanical reliability is absolutely paramount: “If you hook a machine—or more importantly multiple machines—to an FMS and it goes down, the spindle utilization of those machines goes down to zero. Remember, the place where the operator used to stand to load the horizontal machining center is now enclosed in the FMS. The pushbuttons are gone. There is very little chance of you being able to efficiently load and unload parts in that machine manually. So look under the hood to make sure that the FMS is mechanically capable of delivering that 90% up time.”

For high-quality builders, that includes running the three-phase, high-voltage power through a buss bar above the vehicle, like the third rail of a subway, with no moving parts other than the contact point. “Less robust systems tether the vehicle to a series of cables in a cable track that follows the vehicle as it goes back and forth. This causes fatigue points as the cables are continually bending,” said Baldizzi. “Secondly, we use optical eyes enclosed in a tunnel for the data communications that have to occur between the servodrives, motors, control, and so forth, taking the need for those cables out of the mix.

“I would also look at whether the FMS has coolant channels, ducts and piping to make sure that any coolant that remains on the pallets once they go into storage is recovered and returned to either the machine coolant tanks or a central repository, rather than making a mess on your floor,” Baldizzi concluded.

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