How smart fixturing affects throughput
April 17, 2013
A shop is only as productive as its constraint process—that is, its bottleneck. All that adaptability in upstream processes may not make a part cost less if it takes days or weeks to build a new weld fixture. A modular approach to weld fixturing can help.
Welders must fit workpieces securely and precisely before laying down a bead to ensure dimensional accuracy and prevent distortion. Most manufacturing operations make jigs and fixtures as unique, one-off creations that must be designed by an engineer and milled by a machinist before being used by the welder. Creating custom jigs is time-consuming in the short term and creates space issues in the long term, as jigs accumulate on walls and shelves around the work area.
In today’s manufacturing environment, many owners and managers are moving toward just-in-time concepts and away from the traditional batch-and-queue approach. Instead of dozens or hundreds of components being pushed downstream, a JIT operation pulls kits of disparate workpieces to downstream processes. This helps reduce or eliminate those all-too-familiar piles of work-in-process between operations and shortens overall manufacturing time. The result: A part spends more time flowing downstream and less time sitting in the queue before the next process.
At least that’s the ideal. But small batch sizes inevitably require more changeovers, and this can wreak havoc in a high-product-mix shop that doesn’t have its changeover times under control. This includes changeover at the welding cells.
In many situations, creating dedicated welding fixtures can take three or four weeks, or even more, especially if the fixture calls for custom components that the shop can’t machine in-house. All that engineering, machining, and fabrication adds to overall lead-time. And if a customer changes a part design, the weld fixture must be changed with it.
Contract fabrication shops sell their ability to adapt. Say a customer changes a part design so that it now requires a new blank shape and size, and perhaps even bending geometries that call for an entirely different press brake setup. Within minutes, a programmer can insert new parts into a nest for laser cutting. Offline bend simulation and modern CAD/CAM capabilities have shortened press brake programming time considerably. But a shop is only as productive as its constraint process—that is, its bottleneck. All that adaptability in upstream proc-esses may not make a part cost less if it takes days or weeks to build a new weld fixture. Here is where a modular approach to weld fixturing can help.
Modular fixturing entails reusable, off-the-shelf components, and the manufacturer does not have to fabricate individual fixture elements for every new piece. It lets a shop take on a variety of work without having to create a specialized jig for every job—especially useful for short runs and prototyping. Many components are adjustable and universal.
Modular fixturing tables feature precision-machined mounting holes for the setup of clamps, supports, and other components at any point along the tabletop. The mounting holes are set in a dimensionally accurate grid pattern for measurement and visual alignment during setup. Hole accuracy is extremely important for receiving the modular components on the table.
The mounting holes serve an indexing and locating function, offering accuracy and repeatability. If a fixture clamps to a specific hole for one job (for instance, indexed to the hole “three from the top and two from the left” on the grid), a welder can unclamp the component after the job, then re-clamp it to the same location the next time the job comes up, be it several hours or weeks later (see Figure 1).
Modular fixture components comprise various bars, right-angle brackets, and large riser blocks. All components contain holes and slots for mating with each other and the table using special bolts. The table and modular fixtures use the same hole-grid patterns and hole diameters. The bolts may be simple machined screws, or they may be ball locking bolts that can be tightened by hand. Ball locking bolts eliminate the need for reaching under the table to tighten components. Specialized components include V-notched holders for round tubing (see Figure 2), high-strength magnetic plugs for holding sheet metal, and special plates for holding pipe flanges.
The fact that fixtures are modular doesn’t change fixturing fundamentals. Jigs should stop and locate the workpieces while allowing space for the welder to operate. They should be designed with possible heat distortion problems in mind and work to minimize those effects. Specific fixture components also should account for variability in upstream proc-essing—such as a spring-stop that can conform to the part while still holding components well within tolerance limits.
Clamping options include specialized clamps, traditional C or sliding F clamps, toggle clamps, or a combination thereof. Which to use depends on the part geometry and volume. Traditional clamps integrate well along the edges of the tabletop frame, or when building a large, three-dimensional structure. They can be more difficult to work with toward the center of the table, because the welder must reach in to tighten them manually.
Toggle clamps affix the workpiece with one quick motion, making clamping even faster. They can be adapted to modular fixturing tables with adapter plates, which come either predrilled or as blanks. The downside: Toggle clamps are not as flexible as manual clamps, and certain toggle designs may be required for specific jobs. Traditional clamps take longer to tighten, but they can be used for a wide range of products.
Say a welder works with a dozen short runs throughout a shift. If he works with fixtures designed mainly with quick-action toggle clamps, he may be able to swap fixtures on a modular table within minutes. But because these toggles work for only certain jobs, the shop may need to purchase more toggle clamps to handle the entire product mix. Moreover, a welder may need a specific toggle that’s being used by another welder in the adjacent cell. In this case, he may need to move on to another job or wait until the adjacent welder is finished with his run. Either option disrupts part flow.
Even if specific toggles are available, he still must spend time swapping them out between jobs. This may involve walking back and forth from a central fixture storage area. All this walking takes time and, ultimately, costs money. In many cases, a mix of toggles and traditional clamps may work best. The traditional clamps take longer to affix, but they can handle a greater portion of the product mix.
Modular fixturing tables can hold custom fixtures, too, creating a hybrid—that is, a fixture entailing both custom and modular components (see Figures 1 and 2). This can make setup even faster for the occasional repeat job.
Here, storage is another consideration. Imagine a welding department that runs several dozen short-run jobs over two shifts. This may call for several dozen fixtures, each with components located precisely onto its own board or base plate. This can require significant storage space. On a conventional fixture, a 4- by 3-foot weldment may require 5 by 4 ft. of space on a storage rack.
A modular fixturing approach changes the storage situation. Because the holes in a modular fixture provide an indexing function, so that a welder can locate fixture components to the same spot every time the specific job comes up, components need not be mounted permanently on a board. In a modular setup, the welder disconnects components from the table after he finishes the job. He then may stage the modular components for use with another job, and place the remaining job-specific, custom components in a 2- by 1-ft. bin.
How quickly welders set up fixtures for the next job is critical, to be sure, but it isn’t the only factor. Any time a manufacturer spends working with a fixture is worth scrutinizing, and this includes fixture design time. Manufacturing time may comprise only a small portion of the entire order-to-cash cycle: that is, the time between receiving an order and finally shipping and getting paid for it. A shop’s engineer may work for days or weeks designing or tweaking a fixture, and that time costs money.
Many operations may not use CAD for low-volume or one-off components. Still, CAD takes the guesswork out of the setup for fitters and welders, and standard libraries and design procedures in CAD can speed the process (see Figure 3). Most modular fixture manufacturers provide downloadable libraries of tooling component models. Saving a setup in a CAD library also serves as a reference for future setups, and is more complete than impromptu methods such as a welder taking a photo snapshot of a fixture.
Decreasing costs of welding automation and increasing capabilities of systems and software mean they are no longer confined to high-volume products. And more businesses are incorporating the use of welding robots to address the shortage of skilled welders. This is why fabricators, even those with high product mixes, continue to adopt robotic welding.
In the past many shops may have purchased a robotic system for a specific high-volume job. But what if the contract ends, or the demand level changes? A high-volume, repeatedly ordered job may not stay that way forever. The result: A robot cell eventually sits idle for much of the time, simply because it takes too long to develop fixtures and welding programs for other low-volume work.
Modular fixturing systems can help change this situation. Such fixtures can be integrated into robotic welding cells on turntables, skyhooks, and headstock/tailstock positioners. The precision and repeatability of modular fixtures mesh well with those of robots. Such fixturing also provides flexibility for welding operations that incorporate robots but nonetheless experience frequent job changes.
Consider a job that’s robotically welded with a modular fixture setup entailing two groups of clamps on the same base plate, so that the assembly may be clamped in one orientation first, welded, then reoriented in a second group of clamps so that the robot can access the remaining joints. Finally, the part is sent to a manual welding station for the remaining joints.
This breaks with conventional thinking—that “done-in-one” setup ideal. If the robot can’t access every weld joint, why isn’t this assembly clamped once and welded manually? In reality, it depends on the job. A part’s design tolerance may demand that the part be fixtured as few times as possible. But for other parts, a robot may be able to weld joints faster and more accurately than a person could, even if the assembly must be refixtured several times.
The benefits of a well-designed modular fixturing system include savings through greater productivity and reduced lead-times. Engineers experience reduced time and expense required for jig design and fabrication. Welders experience reduced setup time and increase welding throughput with more precise and more comfortable positioning. Organizations respond to design changes and varying volume requirements with greater agility.
Perhaps most significant, a modular approach makes the fixturing itself less of a barrier to welding efficiency. Developing fixtures is costly. That’s the traditional thinking, at least. But for the right application, modular fixturing can change the story, because it takes less time to design, change, and set up a fixture for a workpiece.