Fixturing for Flexibility
High-volume, low-mix and mass customization are all the rage these days. But in the world of automated assembly, the need to accommodate a variety of products in a single system has long been a fact of life. Assembly machines aren't cheap, and companies have been manufacturing diverse products since the beginning of time. It only makes sense that a system be able to produce more than one widget in an effort to improve the bottom line.
"We've been using flexible fixturing for 40 years," says Richard P. Bodine Jr., CEO of Bodine Assembly and Test Systems (Bridgeport, CT). "It has been a part of our business for as long as we've been in business."
Rick McInnes, vice president of sales at Central Machines Inc. (Wheeling, IL) agrees. "It's not a new problem. Flexible fixturing has been around as long as people have been assembling things," he says.
When designing an assembly system, there is inevitably the question of part holding and creating fixturing nests to accommodate more than one product. Obviously, engineers have the option of using different fixturing for each line of products being assembled. However, this solution can be both cumbersome and expensive: cumbersome in that changing over tooling sets can be time- and space-consuming; expensive because tooling costs money.
Then there is the simple challenge of tracking all this loose equipment. Whether it's a disgruntled employee dropping a component in his pocket or an overburdened inventory system, changeout fixturing can be vulnerable to getting lost. The very thought of machine operators digging through parts boxes midway through an already costly changeover is enough to send a shop manager over the edge.
For this reason, automation systems manufacturers have devised a number of strategies for creating flexibility in fixturing, including everything from conventionally machined dedicated nests to more complicated variable systems that automatically adapt to whatever part is being assembled at any given moment.
Ultimately, parts fixturing performs a role that is simple, yet crucial to the success of any assembly system. On the one hand, fixturing does nothing more than hold an assembly in one place while the next component is added. However, it must do so securely and precisely. Any movement at all, and the assembly is doomed for the reject bin from the outset.
Nests, or fixturing, are therefore very carefully manufactured parts, often made from hardened tool steel. They are then precisely mounted on an indexing dial or belt in a synchronous system, on a tabletop in a manual or semiautomatic workcell, or on pallets in a nonsynchronous system. Other material options include stainless steel, anodized aluminum, ceramic or plastics, depending on the application and the materials from which the assembly's parts are fabricated.
Fixturing can also be made by molding materials, such as two-part polyurethane, around a part. This option works especially well when working with delicate plastic components like cell phone housings or other finish-sensitive assemblies. In cases like these, even the smallest scratch on the parts would cause them to be rejected, so a forgiving fixturing material is crucial.
No matter what the materials, the resulting fixturing is largely static. Once you've created a piece of steel tooling, it can be pretty challenging getting it to change shape from one product run to another. Still, there are a number of ways to incorporate flexibility into a system. Specifically, flexibility can be accomplished by using interchangeable inserts, machining multiple nesting options into a single piece of equipment, or designing the products being assembled with common location features to facilitate a variety of parts being assembled using the same nest.
With regard to interchangeable inserts, or loose tooling, the advantages are that a single fixture can accommodate a variety of parts geometries. Granted, there are limits. As Bodine says, "You can't make a spark plug on Tuesday and a faucet on Wednesday." Nonetheless, when assembling a family of products with similar geometries, say, a series of different doorknob assemblies-to cite an example from Bodine's experience-inserts can accommodate a variety of models.
Unfortunately, this kind of flexibility has its downside. For one thing, there are changeover times to take into account. The inserts can be attached using setscrews, plunger push pins or a simple snap-fit configuration, but it still takes time to swap them out. Then there is the fact that you are still faced with the possibility of lost parts, as was the case with conventional fixturing.
To remedy this situation, engineers can provide multiple nests on a single indexable tooling. One way this can be done is by simply creating two separate nests on a single face of the tooling. The system can then be adjusted, depending on the assembly being produced, so it performs its work in the correct location.
Another option is having different nesting features on more than one face of the tooling, so that flipping the tooling accommodates a different product. With this system in place, changing from one product to another requires manipulating the tooling. But there is no need to dig up an entirely different set of fixtures as part of the process.
Similarly, ATS Automation Tooling Systems Inc. (Cambridge, ON) has developed a system for fixturing windshield wiper motors on a pallet conveyor system in which the locating fixture on the pallet has pneumatically powered, revolving stops that automatically adjust to the correct position for the part being run. Because the stops are adjustable, small changes to the dimensions of a part can be accommodated. As an added benefit, the fixtures were designed with extra stops for added flexibility when new models of parts are added in the future.
Finally, engineers can create a single nest configuration that can accommodate more than one type of assembly. Again, the different parts need to be similar. But they don't need to be identical. The fact that assembly X doesn't fill a small cavity in the fixture necessary to accommodate assembly Y doesn't mean the same nest can't hold them both securely.
Al Stone, mechanical engineering manager at Cox Automation Systems (Bloomingdale, IL), describes candidates for this kind of fixturing as parts that, although varied, retain the same basic fixturing registration datums. "In this case, we try to use the common features between the parts," Stone says.
Francisco Carillo, a senior applications engineer at ATS Automation Tooling Systems Inc. agrees, adding that forethought in product design can help facilitate the process. "ATS will work with customers to modify their product designs to achieve more common elements within a product family," he says of his company's efforts to develop flexibility.
McInnes notes that this is one of those areas where planning ahead, designing for assembly, can mean gains in productivity and return on investment.
"When I have a family of parts, I'm looking at commonality between parts," he says. "Can you put a feature in all products that will allow them to use a common piece of tooling?"
Before putting multiple nests on all your fixturing, though, it's important to stop and think about how the variation between those different parts will affect the rest of the system.
Steve Kinderman, sales engineering manager for Isthmus Engineering and Manufacturing (Madison, WI), warns that conventional changeout fixturing for multiple product lines is often still the best solution to ensure system reliability and quality product.
Matt Osler, director of engineering for Flow Automation (Burlington, ON), adds that flexible fixturing needs to be seen within the context of the overall assembly system to avoid problems further down the line. "Sometimes [different] assemblies will all fit in the same fixturing, but various parts of interest end up in different points in space," he warns. "It's important to look at it as a whole and come up with a solution...That's where flexible tooling, especially robotics, can play a role."
Then there is the question of expense. According to Stone, flexibility generally adds cost to a system, which means systems engineers need to weigh the pros and cons of a methodology before going ahead just to be "flexible." Even in those cases where flexibility exists, engineers need to know where to draw the line.
"We assess...requests for flexibility, and not all components are good candidates," Stone says. "For example, if one of the styles of parts runs 2 percent of the year and has a different profile, maybe it should not be integrated into this flexible system where it could compromise the rest of the machine."
Nonetheless, in the right application, flexibility can make all the difference in the world. As James Cranford, design engineer for Automation Tool Co. (Cookeville, TN), puts it, "While flexible fixturing takes additional upfront effort in concept and design, this additional cost can create savings in both machine flexibility and longevity."
All well and good: But what about manufacturers assembling products with markedly different geometries? Thanks to today's motion control technology, they now have options for flexible fixturing as well.
For example, Southern Engineering and Automation (Tarpon Springs, FL) has built a system that constructs more than a half-dozen different fence panel assemblies. This system uses servomotors to control a kind of adjustable jig, which holds the different parts in place while they are joined using threaded fasteners.
According to Southern Engineering and Automation engineering sales manager Beryl Lawrence, the tubing for the fencing arrives in carts carrying a bar code ticket that instructs the assembly machine as to the kind of fence panel being assembled. Lawrence says it's not unusual for the company to build two different types of fence per shift. Some days three or more different styles can be assembled on the line.
Another area in which this kind of dynamic flexibility is enabling greater productivity and efficiency is in the auto industry. The trend toward mixed-model production on a single line requires flexibility in everything from aftermarket parts production to vehicle final assembly lines.
For example, Bosch Rexroth Corp. (Hoffman Estates, IL) has developed what it calls its programmable lift platform (PLP), which can be used to accommodate multiple car bodies in a single production line.
According to Bosch Rexroth director of product management George Martin, each PLP acts as a "multiaxis lifting device," basically a Cartesian robot with Z axis ability. In an automotive assembly application, six or eight PLPs will be installed on a single pallet to support the different chassis models coming down the line. Each PLP then adjusts in three dimensions to accommodate the pin holes and geometries of the chassis so they will be securely fixtured.
Currently, Bosch Rexroth is using this technology in a car body welding application. But Martin sees tremendous growth potential for this kind of fixturing in the coming years. "For any company to be doing mixed-model production, you've got to have something like this," he says.
Similarly, FANUC Robotics (Rochester Hills, MI) has developed what it calls its subcompact robot platform, a system that provides both flexible fixturing and positioning in a single package. In a current pickup truck box welding application, a pair of these platforms working in concert is able to accommodate two different styles of front panels being attached to three different styles of boxes.
To perform the assembly, an articulated robot loads a front panel onto the paired platforms. These panels each have their own locating datums, so the platforms automatically adjust their end-of-arm tooling for whatever panel is being welding.
The platforms then position the panel against the pickup box where it has already been secured in the welding station. Because of a phenomenon known as "matchboxing" the pickup boxes are sometimes slightly twisted about their longitudinal axes. This means the platforms must adjust the front panel accordingly. In essence, the fixturing is adjusting both for the type of component it is holding and the orientation of the part to which that component is being attached.
"By simply making a few key strokes on the positioner's teach pendant, an electrician or toolmaker can adjust the location of the front panel tooling, such that the top edge of the front panel is flush with both the top edge of the right- and left-hand box sides," says Allen Grzebyk, program manager of FANUC's body structures group. "In conventional tooling, the shimming would have been accomplished mechanically through the time-consuming manual addition, removal or grinding of metal spacers."
Equally flexible, is a technique used by Motoman Inc. (West Carrollton, OH) and its parent company Yaskawa Electric Corp. (Fukuoka, Japan) to weld exhaust pipes and assemble truck axles.
In the first application, three different robots are employed in a single workcell-two that position the tubes being joined and a third that performs spot welds that hold the assembly in place for final welding. In a sense, the first two robots provide a kind of infinitely flexible fixturing system.
According to Motoman market segment manager for welding Chris Anderson, the key to the system is his company's multiple-robot controller, which can coordinate the actions of up to four separate robots. "You get maximum flexibility with this cell," he says, noting that the three robots can handle dozens of different configurations, making them ideal for aftermarket, low-volume, high flexibility applications.
Similarly, in the truck axle application, one robot positions the axle while two other robots perform a variety of welding and capping operations before the assembly moves on to the next station. According to Anderson, the company using the system manufacturers a variety of air-ride and standard suspension axles that can all be processed by this single workcell. As is the case with the exhaust pipe application, lot sizes tend to be very small. According to Anderson, each axle carries a bar code, which tells the workcell the operation that it needs to perform.
Finally, it's important to keep in mind that flexibility is possible in the clamping that is used to secure different assemblies in a single piece of tooling. De-Sta-Co Industries (Madison Heights, MI), for example, manufactures a number of clamps, including its straight-line action clamp line, that incorporate a variety of different manual and automatic adjustment features, enabling them to hold pieces or laminates of varying thicknesses.
"Part of what we do is try to encourage flexibility in fixturing," says De-Sta-Co Industries' industrial products manager Tom Stimac. "We'll do walk-throughs [at customers' facilities] to show ways that this can help save money."
As an example of this kind of flexibility, Stimac cites the use of his company's clamps to fixture electric motor housings on a line. Because they can be adjusted, a single clamp can hold down the bottom flange on a range of motors, even if those flanges are of a variety of thicknesses.
Then there are modular fixturing systems, like those marketed by companies like Flexible Fixturing Systems Inc. (East Granby, CT) and Yuasa International (Elk Grove Village, IL). Built around various types of base plates, with dozens of holes for attaching dozens of different brackets, blocks, clamps, spacers and locating bolts, these systems are primarily used for machining and prototyping. However, in some cases they have assembly applications as well.