Workholding's Clever New Concepts
Don't overlook this path to productivity
This is the first part of our workholder series. It chiefly covers workholders that use magnetism, hydraulics, vacuum, and adhesive, as well as multiple methods, as the means of gripping the part. Part two will feature those manufacturers that emphasize mechanical clamping or who offer multiple workholding styles.
By Robert Aronson
Many manufacturers of workholding equipment report a significant sales increase or a backlog of orders. Several factors contribute to this rosy picture in addition to the generally positive mood for manufacturing in the US.
Companies are looking at the total manufacturing operation, from materials in the door to product shipping, instead of basing buying decisions on issues such as initial cost. And that approach certainly includes workholding. The overall goal is increased productivity. It's seen as one of the few factors that US manufacturers can use to beat foreign competition.
Aside from improved mechanical elements and better software, there is a strong effort to get the workman out of the loop. Initially, this was an emphasis because of the labor cost and, in some processes, the fact that manual processes introduce error.
More recently, the emphasis has been on increased productivity. Robots and hard automation are replacing manual functions because of the increased production speed advantage.
But automation isn't the answer in every case. Because of low volume, some companies don't find it cost effective to install special handling equipment. Another negative is that, despite its versatility, there are some motions and reaches automation can't achieve. Only humans can handle that work. Yet in many cases even the speed of manual operations can be increased by more efficient workholding systems.
Here's a sampling of what is currently being offered to meet these needs.
"Everyone wants to automate," says John Powell, president, Wen Technology Inc. (Raleigh, NC). "This has caused us to become integrators or turnkey system providers because we must now interface our workholding units with robots. This work includes loading and unloading pallets and feeding the parts to the machine. Plus, there must be quick change between setups."
These jobs also include interfacing with the control software.
One trend that has worked to favor magnetic chucks is the move to high-speed cutters operating at small depths of cut and lighter chip loads. Force of the tool on the workpiece is therefore reduced. Magnetic chucks, which have had limited use in some applications because of their fixed holding power, can now be used. "A problem we now have is convincing potential customers of this advantage," concludes Powell.
Wen Magnetics has also introduced an improved series of chucks. A common problem with existing designs is the resin between the magnetic poles—chips tend to erode it and become embedded in it. The new chucks use a stainless steel grid in its place for improved durability and accuracy.
During 2006 Bunting Magnetics Co. (Newton, KS) saw a lot of new interest in their magnetic assemblies for use in workholding. Magnetic assemblies, especially those made from new high-strength rare earth (neodymium) magnet material, hold ferrous parts together much like clamps, but are much faster to set up.
Simple magnetic assemblies, magnetic channels, and even electromagnets are easily mounted to jigs and fixtures. Adjustment is rapid with a permanent magnet workholder. And because the magnetic field is constant, there are no power sources or wires to get in the way. If on/off fields are needed, the company offers electromagnets with higher holding power.
Magnetic assemblies come in a variety of shapes, sizes, mounting styles, and magnet materials. Ceramic assemblies are great for general-purpose use in temperatures to about 180°F (82° C). Rare earth magnets are 5–10 times stronger than ceramic, and are useful in temperatures to approximately 300°F (149°C). For temperatures up to 840°F (449°C), Alnico magnets offer the widest range of temperature stability.
LAAG (Light Activated Adhesive Gripping) is a unique workholding system from Master Work-Holding Inc. (Morganton, NC) that relies on UV-cured adhesive to hold a part. The workpiece is positioned on a fixture, adhesive is applied and then cured by UV light. After machining, the part is removed from the fixture and the process repeated.
According to the company's Mike Rice, they are targeting the aerospace and defense industries, but at the same time are working with the automotive, heavy equipment, fluid power, and many other industries. LAAG has been successfully demonstrated with hard-turning applications with exotic as well as more traditional materials. Aerospace/defense seems to be the hungriest segment for a new workholding solution, but the bearing, automotive, and heavy equipment industries are also pursuing LAAG solutions.
LAAG offers 6000 psi (22,690 N) of holding power and new adhesives are being developed, including one for bonding to a workpiece with a powder coat surface.
While some manufacturers diversify their production, Fuji Machine America (Vernon Hill, IL) believes in single-source responsibility, so they engineer their own custom workholding to optimize the process for each application. The design employs built-in robotic automation, inprocess autogaging for quality control, and custom workholding, all from one source. Fuji has developed numerous optimal workholding solutions for a wide variety of automotive, aerospace, and defense-related components. This single-source responsibility allows Fuji to guarantee takt time, cycle time, and Cpk with turnkey and runoff solutions that are built for lean manufacturing.
"Our vacuum chucks are moving well," says Bill Popoli, president of Ibag North America (North Haven, CT). "In the past, companies often made their own. Now they don't have the people or time to do the work themselves. But they still want a fast solution to their work problems."
Most of the company's applications are for nonferrous parts, chiefly those made of aluminum or plastic. Also, the customer frequently wants to eliminate external clamps, so it's possible to machine as much of the surface as possible in one setup.
For accurate applications, it's necessary to use a grid-type chuck with a seal, usually as a gasket around the edge. When the manufacturer of the part requires holes, a different segmented design must be used. It has a rubber pad over a slotted chuck. The part is placed on the pad and held down by the vacuum, creating a number of compartments. Although machining may puncture several compartments, there is still enough holding power in the remaining compartments to hold the part in place. Another product, called "Vac Mat," uses a plastic pad containing a series of molded suction cups. Each suction cup provides holding force on the part surface. This is widely used in the aircraft industry to hold large sheets of aluminum and composites.
The massive order is now rare. "At our company, certain part runs are smaller than before," says Paul Yeko, Pascal Engineering (Elk Grove Village, IL). "In the past, machine tools were dedicated for long-running piece parts. If a new job came in, an additional machine might have been purchased if all others were at capacity. Now, shops are looking to be more flexible with their existing machine tools due to smaller part runs and contracts with a wide variety of piece parts. The changeover time to switch out fixtures needs to be as short as possible to maintain machine uptime."
Pascal has developed quick-change pallet systems to facilitate this changeover on both vertical and horizontal machining centers.
Another key trend they see is greater use of robotic piece part loading on machining centers. These fixture applications often require clamping components with built-in position stroke sensors. The machine cell controller receives confirmation that the clamping components are open or closed in relation to the piece part positioning.
Pascal has also introduced a work support with partcontact confirmation for use in automated applications. Work supports are a separate element used on many fixtures. Along with clamping devices such as swing clamps and link clamps, work supports provide added rigidity against deflection from cutting-tool or clamping forces. Hydraulic advance work supports have been in use for many years, but Pascal added the part confirmation feature.
For work supports to be effective, the plunger needs to be in full contact with the piece part after actuation. Any debris or casting imperfection where the work support needs to make contact can affect the piece-part quality after machining. The air sensor built into the plunger tip confirms to a controller that the work support is in proper contact.
Many of these robot cells are also using Pascal's pneumatically operated link clamps to hold the piece parts onto the robot end effector. The link clamps offer greater flexibility for holding certain castings compared to conventional multijaw grippers. Robots typically have an interface device attached to the end of the arm that supplies multiple air feeds to actuate standard or custom-made gripper elements. Depending on the complexity, size, and weight of the piece part, complete custom tooling for the end of the arm is required. Grippers with linear movements are sufficient for many smaller piece parts. The Pascal pneumatic link clamps offer a toggle-style movement that can better clamp on internal or external casting flanges. Pneumatic swing clamps are another option depending on the part's configuration. Both style clamps are unique in that they can be manifold-mounted to keep the air-feed lines further away from the piece part for improved safety. Sensor feedback is also an option on these clamps to confirm an open or closed position.
When piece parts with multiple heights and a common footprint are to be machined on the same fixture, a quick-change arm system can be of help. The swing clamp arms go back in the same repeatable position.
Pasal has placed a spindle-driven, pallet-mounted pump unit so there is no need for an external pump source. The compact pump remains mounted onto the machining fixture, and is connected to a single-acting hydraulic clamping circuit. The machine-tool carousel will have a specific tool to engage into the driving mechanism on the pump. The spindle rotates, providing power for the pump that supplies oil to actuate the fixture. The driving tool is exchanged for a conventional cutting tool to start the machining process. After machining is complete, the driving tool is again exchanged into the spindle. The center of the driving tool is used to actuate a release valve that unclamps the fixture.
"Integration of electronic controls and feedback is now more common," says Tom Eggert, workholding product manager, Enerpac (Milwaukee). "Customers are generally very comfortable with hydraulic workholding, and now they want to take those devices to the next level of performance. That includes real-time pressure-monitoring, PLC control of clamping functions and sequences, and even data collection for use in maintenance schedules."
The company's latest product is the ZW-series electric-power unit. It can be custom-configured at the factory, and there are also several prepackaged control packages available. Each pump includes the option of an integrated microprocessor control box which, when coupled with pressure transducers and level and temperature switches, provides additional automation and feedback to the hydraulic workholding system.
The control box can automate the motor and valve controls to maintain clamping pressure throughout the clamping cycle. It can also provide some programmable pressure settings and timers to further increase the automation. Finally, the control box provides data collection in terms of number of hours and cycles run by the motor and the valve solenoids; it provides information on low supply-voltage problems, high oil temperature, and low oil levels.
To take full advantage of a complex machine tool, the workholder must be compatible. For example, to use a five-axis machine requires more versatility from the workholder. One answer is a five-axis clamping system from Kurt Manufacturing (Minneapolis). It allows continuous five-axis cutting of complex pockets, sculptured and contoured surfaces, and intricate 3-D features--with repeatable high accuracy.
Designated the VB 5AX100, this clamping system provides a clamping force up to 8992 lb (40 kN) for obstruction-free, high-speed machining in all five axes. The system's two clamping jaws, one stationary and one moveable, can be positioned at any desired distance from each other.
Jaw deflection and/or part misalignment is eliminated during machining operations. Locating the tension spindle directly under the workpiece prevents the jaws from flaring out under tension, and the machine table does not distort. Specifications of the VB 5AX100 include a clamping width from 0.0787 to 9.29" (2–236 mm) that can be extended with optional spindle screws, a clamping depth of 0.315" (8 mm), and height above the machine table of 6.889" (175 mm), with extended height options to 7.874 and 8.858" (200 and 225 mm).
This article was first published in the April 2007 edition of Manufacturing Engineering magazine.