Secure, accurate workholding sets the stage for consistent machining productivity. Depending on the parts and processes involved, workholding can be as simple and temporary as a plain vise or clamp or as complex and permanent as a machined and fabricated fixture that is custom-designed to hold a unique part. Between the two extremes is modular workholding. The trend to lower manufacturing production volumes and the increased capability of CAD technology have focused new attention on this multifaceted concept.
Modular workholding typically employs standard locating and gripping components that are assembled as needed for short part runs. When machining is completed, the fixture is taken apart and the components reassembled to hold different parts. The goal is to speed setups and teardowns while saving the time and expense of building and storing permanent fixtures.
Modular workholding is ideal for machining prototypes and in today’s high-mix, low-volume production environments. Other candidates for modular fixturing include the early stages of high-volume production runs, where engineering updates may require fixturing changes, and the smaller part runs that occur after a mass production job wanes.
Modular workholding equipment varies, but most systems use drilled base plates, blocks, clamps and risers. Although modular fixtures are often labeled with toyshop terms like Tinkertoys or LEGOs, they provide serious advantages beyond setup cost savings. A shop can design and assemble a modular fixture much faster than a permanent one. Standard components promote assembly speed and repeatability while minimizing chances for mistakes. A crashed modular fixture often can be repaired quickly by replacing only the damaged components.
Setting a Foundation
As noted, modular workholding fills the gap between permanent fixtures and temporary, general-purpose workholding. In essence, modular and permanent fixturing are a lot alike. Both achieve the same basic purpose and the components are often similar. However, all modular components can be assembled for a certain part or project and be reused after disassembly. Because modular components are standardized and adjustable, they’re not as compact as components for permanent fixtures.
Perhaps what distinguishes modular workholding most are the special tooling plates and blocks to which the rest of the components are bolted.
“Any building is only as good as its foundation,” said Aaron Hull, senior manufacturing engineer for standardized workholding products pioneer Carr Lane Manufacturing Co. (St. Louis) “Fixturing is the same way; without a solid foundation, everything else will be suspect.” Base plates feature precision holes arranged in standard grid patterns. Some suppliers’ base plates alternate threaded holes for fastening components with plain holes for alignment. To maximize flexibility, the holes in Carr Lane base plates feature two-part construction. The top of each hole has a precision bushing for accurate locating, while a threaded insert below permits fastening of other components. Each hole can provide alignment, a mounting site, or both.
While modular fixtures are suited for short part runs, lot size isn’t the only reason to use them. When a job frequently repeats, the time required to reassemble the modular fixture for each repetition adds up. Thus, it may be more economical to create a permanent fixture that can be reused.
When there’s a need to reassemble a previously-used fixture, access to setup information is critical. Large companies may design a fixture and store the work instructions in job tracking systems. Many smaller shops simply photograph setups to show the components and their locations.
However, Hull said, the best documentation includes detailed assembly drawings showing the location of every component with a bill of materials, including modular component part numbers. “That way there is no question which components go where, and if a replacement component is needed you have the part numbers,” he said. “Photos are good, but you often can’t see details or tell which components are supposed to be used.”
Careful storage and organization of components expedites both the initial setup and the reassembly of a modular fixture. “They should be removed from the fixture when not in use, cleaned, protected from rust, and stored in individual locations by part numbers,” Hull said. “That way, you know where the parts are, how many of each you have, and if you should need to replace anything you find out before it is an emergency. You will also have the replacement part number if needed.”
Another factor driving the growth of modular fixturing is the wide availability and utility of CAD technology. According to Brian Dodsworth, director of sales for Bluco Corp. (Aurora, IL), modular workholding setups previously were designed and built around tangible parts. Now potential users can match CAD files of parts with CAD images of fixturing components and quickly explore workholding options.
Bluco, a modular fixturing supplier, offers new customers a free design service. “We encourage people to send us their CAD files. We can show them how to handle a specific project, or let them know how the tooling can hold their type of parts,” Dodsworth said. “We size a tooling package around their needs that they can use for hundreds of projects.”
Off-machine fixture design and construction is fast. When components are available and operators familiar with the design process, a fixture can be designed, built, and ready to run in less than an hour, said Dodsworth. Compared to building a fixture from scratch, “You’re not having to figure out as much geometry because the geometry is built into the tooling,” he said, “so it is really just a configuration process that will go fast for an experienced user.”
Large OEMs often use modular workholding in prototype machining and through first-article production. “Then they apply what they’ve learned to build or contract out their hard tool,” Dodsworth said. When demand for a part slows down, use of modular fixturing may be revived to meet episodic needs. “Modular makes sense again because a shop doesn’t have to hold and store fixtures it may only need once every three years,” Dodsworth said. Not keeping old fixtures, he added, requires “a champion inside the company” who can own the modular concept and motivate action to get rid of old fixtures when a part moves into an aftermarket life cycle.
Fixturing Expense or Capital Investment?
Many companies, especially contract manufacturers, include workholding equipment costs when bidding a job. However, modular workholding equipment has features that add flexibility and usually make it more expensive than a one-time, single-project fixture. The modular system cost should not be charged to one project, according to Dodsworth: “Modular workholding can do hundreds of projects. It’s not fair to justify the cost of it on one part or one project.”
For many shops, viewing modular fixturing as capital equipment instead of an expense requires a new mindset. Management should look at modular workholding in the same light as a press brake or a laser cutter. “You don’t justify that equipment over one job. You want a capability that will let you win more work,” Dodsworth said, adding that eliminating a majority of fixturing expenses can enable shops to bid lower for some jobs.
While CAD technology has increased acceptance of modular workholding, Dodsworth also noted that the combination of the two may help address changes in the manufacturing workforce. Manufacturers are losing skilled tradesmen to retirement and their replacements do not have the same level of experience. However, younger workers have grown up with computers and CAD, and that familiarity, along with the standardization inherent in modular workholding, can help offset the loss of older workers’ experience and skill.
The essence of modular workholding is its adjustability and flexibility. Tom Stimac, sales director, distribution North America for global workholding and automation equipment supplier DESTACO (Auburn Hills, MI), pointed out that clamping adjustability complements modular concepts. Some cam-action clamps from DESTACO, for example, provide variability up to about 5/16″ (7.94 mm), facilitating clamping of different thicknesses of rough castings in the same setup.
Another style of clamps offers both vertical and angular adjustability. DESTACO’s Carver line of buttress clamps is used on thin or low-profile workpieces, replacing ordinary strap clamps. The buttress clamps feature an adjustable, self-aligning swiveling pivot that lets the tightening bolt stay vertical, eliminating side forces. The clamps provide up to 18,000 lb (8165 kg) of force.
Swing clamps can also hold parts of varying thicknesses. “They swing 90˚ over the workpiece and then go straight down,” Stimac said. Clamping position can be varied without making any adjustments. “One of our lines of swing clamps can adjust the swing to the right or left as well as straight up and down. They are flexible in that you can repurpose that clamp to go in different directions based on the existing setup. You don’t have to get another clamp and set it up,” Stimac said.
Some machining vises feature elements that make them modular. For instance, Kurt Manufacturing (Minneapolis) markets quick-change jaws consisting of a set of master jaws and machinable jaw sets. The company’s DoveLock quick-change jaw system permits jaw changeover in less than one minute, with repeatability of ±0.001″ (0.0254 mm). The jaws are machinable to custom-match certain part profiles, and each jaw can be turned 180˚ to handle two different setups without leaving the machine tool. Kurt produces the vises in 4–10″ (101.6–254 mm) lengths in 6061 aluminum or 4140 prehardened steel.
Kurt also has developed modular-style workholding for palletized machining. Its SeraLock Towers and SeraLock Pallets feature interlocking V-shaped serrated surfaces that facilitate precise and repeatable alignment when equipped with the WedgeLock and MoveLock clamping modules. The combination of repeatable mounting location and standard clamping modules lets users quickly configure custom high-density workholding systems. The modular components can be disassembled and reused. Kurt provides SeraLock Towers and SeraLock pallets in configurations engineered to match most HMC and VMC applications.
Fixturing for Five Axes
As in three- and four-axis operations, modular workholding can be an important part of five-axis machining, particularly when shops need to move quickly from job to job. It wasn’t always this way. Early five-axis setups often weren’t as productive as they could be. According to Tim Easton, national sales manager at tooling and workholding maker Jergens Inc. (Cleveland), at first it was difficult to find people who could program the machines efficiently and to find shops willing to invest in a technology with an uncertain return on investment. “Oftentimes an expensive machine ran only 10 hours out of a 40-hour workweek,” Easton said. “The machine would cut a few parts and then sit idle for a couple of days. It was difficult to go from job to job in a reasonable amount of time.”
Today, quick-change and modular workholding systems help users maximize five-axis machining. Accurate, repeatable machine table mounting systems, combined with a variety of modular pallets, risers, adapters and part-gripping top tooling, provide adaptability, flexibility and speed. A shop can set up a part off the machine on a pallet while another is being processed, then quickly move the new part in. “For the last 10 years, it has been about getting that part in and out of the machine as fast as possible, while repeating and holding it as accurately as possible,” Easton said.
Jergens’ Fixture Pro multiaxis quick-change system follows a “bottom-up” approach, Easton said. At the lowest level of a fixture, a sub-plate with a grid pattern of precise holes bolts to the machine table. A series of risers and adapters mounted on the sub-plate elevates vises, collets and other top tooling to the desired level.
Rapid exchange of adapters and top tooling permit a shop to move smoothly from machining a five-axis-style part to a job that requires only simple milling or drilling. The accuracy and repeatability of the system can enable less-skilled and experienced operators to perform near the level of seasoned machinists, according to Easton.
Further advantages of quick-change modular fixturing include the ability to take a part locked in top tooling directly to a CMM whose table has the same grid pattern as the machine tool. After critical dimensions are checked, the part can be put back in the machine tool without losing position.
“The system provides flexibility and adaptability that can be applied anywhere parts need to be taken in and out of machines and relocated accurately, including three-axis machines, CMMs, EDMs and operations such as assembly and welding,” Easton said. Quick-change modular fixturing can facilitate a shop’s use of automation, he added, when modular systems enable robotic loading of multiple jobs.
When pallets are moved on and off machines, they must be accurately remounted without the need to relocate them in relation to the machine spindle. Zero-point clamping systems—which establish a consistent alignment location for the clamping system—make pallet location fully repeatable.
In the Power-Grip zero point clamping system from Röhm Products of America (Suwanee, GA), zero-point clamping pods are mounted on base carriers installed on a machine table or tombstone. The clamping pods engage centering/locking bushings mounted on removable pallets that hold fixtures or workpieces. Each time the pallet is loaded into the machine tool, its position repeats to within 2 µm over a 300 mm length, while overall system accuracy is within 5 µm.
Key engineered details enable the Power-Grip system to maintain that consistency. Röhm sets the zero point of the system in the center of the pod carrier. As a result, when operating temperatures change and thermally alter the dimensions of the pallet, the central position of the zero point assures that the relationship of the four pods to the machine remains constant, according to CEO Matthew Mayer.
When a shop moves unwieldy pallets in and out of a machine, collisions and misloads can occur, whether loading is via automation or by an operator using a crane. Any damage affects system accuracy. Röhm designed its system to protect the clamping components during pallet exchange.
In the event of a misload, critical mating and reference surfaces are shielded from impact. A pre-centering ring around the pod’s ball cage guides the pod into correct position in the centering/locking bushing. The connection itself features flexible axial cushioning and centering “lips” that flex and dampen the mating of the pod and bushing. When the clamping is deactivated, the lips provide a 2-mm lifting action that promotes clean release of the bushing and pallet. Mayer said the design makes it impossible to damage the pod during a misload.
Overcoming Spindle Interference
Overcoming interference problems in five-axis machining is a central focus of a zero-point modular workholding system from BIG Kaiser Precision Tooling Inc. (Hoffman Estates, IL). The problems often occur when shops transition from three- to five-axis machining and the machine spindle and workpiece move in relation to one another. “The distance from the centerline of the spindle to the outside of its housing becomes a new limitation,” said John Zaya, product manager, workholding. “It limits the size of the workpiece that can be machined, what length the tools have to be, and obviously how the fixturing is handled.” Clearance is generally not a problem when a part nearly fills the machine table. But when a smaller part occupies only a portion of the table, multiaxis movement of the spindle and/or table can cause interference between the spindle and the machine table or part.
For smaller parts, the solution is to raise the part high enough above the table to eliminate interference. BIG Kaiser’s modular system consists of base components of varying heights along with different sizes of extensions, reductions and shims that can be stacked to eliminate clearance problems and let the cutting tool reach and machine complex part features. The base support components are clamped to the table or to a baseplate that can be locked to the machine table via BIG Kaiser’s Unilock system, permitting quick turnovers and setup outside the machine.
The Manufacturing Continuum
Analyzing the status of modular workholding, DESTACO’s Stimac said the systems represent an interim solution, “and at some point people need to graduate from there and go to more repeatable, quicker acting, more ergonomic, less user-intensive workholding products.” However, he said, there is a continuum in the machining process. It proceeds from prototypes through high-volume production and automation, where modular workholding loses its advantage. The advantage then returns as product volumes decrease. “For low volumes or when you are perfecting a process or a product, modular workholding makes total sense,” Stimac concluded.