Precision workholding leads to greater production efficiency, increasing profits.
By Michael Hurley
Vacuum technology achieves effective, reliable hold-down of materials for printing, cutting, machining, measuring, application of coatings and adhesives, and other types of processing without using clamps or chucks. Inherent advantages include: faster, more accurate setups; access to entire surface of the workpiece; and lower potential to damage material or machine
From large industrial facilities cutting and machining steel to cleanrooms recording precise measurements of aerospace grade composites, vacuum table systems provide logistical solutions and operational advantages.
Although the principles of operation are the same in all vacuum table systems, design and construction vary widely depending on the application. Small tabletop systems may have a vacuum surface area of 1 in² while modular systems for industrial use may be expanded to hundreds of square feet. Stainless steel and aluminum are the most common surface materials – and generally preferred for heavy-duty use – while common variants include anodized aluminum, and plastic.
The precision of computer-controlled industrial processes such as CNC mills, lathes, routers, plasma cutters, and EDMs has led to an increased use of vacuum table systems during the last two decades. At the same time, high-tech composites and synthetic materials have grown more popular, creating a demand for clamp-free workholding solutions.
“I don’t even know all of the potential customers, because I regularly hear from people in new industries with new applications,” says Michael Green, president of vacuum table maker Graphic Parts International (GPI), a division of the Chicago-based A.W.T. World Trade Group. “Meeting the specific needs of new customers has led to some of our most important design innovations.”
GPI’s customers demonstrate the versatility and variety of vacuum workholding solutions. Multi-national aerospace and high technology manufacturing corporations such as Boeing, GE, and Agfa use vacuum tables in their close-tolerance and highly regulated manufacturing operations. Vacuum tables are also used for precision measurement of flat and three-dimensional objects. In these instances, the systems can be optimized to accommodate delicate materials.
Peak performance under pressure
A vacuum table system typically consists of a penetrable flat, rigid work surface, usually perforated or grid-style depending on the application; a plenum or vacuum chamber; and a vacuum pump large enough to create a sufficient pressure differential between the chamber and the ambient or atmospheric pressure at the surface to hold a workpiece in place. It is not pure vacuum that holds the part in place but the force (in this case suction) created by this pressure differential. Hoses and clamps are used to connect the vacuum pump to a port or series of ports on the vacuum table, typically on the sides or bottom of it.
In a properly configured and calibrated vacuum workholding system, a high level of hold-down force may be generated with a relatively small pressure differential; balancing suction and system airflow achieves the best results. Working in only one zone of the vacuum table surface requires effectively blocking the unused zones with gaskets or air dams to eliminate air leakage.
If the process generates excessive heat it may require the application of coolant during processing; if it generates excessive scrap material it may require cleaning during processing with blasts of compressed air.
In such instances it is advisable to use a grid-style vacuum surface with larger air holes, vacuum ports, and integral drainage channels. Because it is vital coolant doesn’t flow into the vacuum pump, systems will use a coolant collection system.
Look out for leaks
Air leaks and airflow inefficiencies account for most of the problems encountered when calibrating vacuum workholding systems. These problems aren’t solved by simply adding a larger vacuum pump.
A vacuum table system draws air through the spoilboard (usually medium-density fiberboard if one is used); the table surface; the plenum; a sequence of ports, flanges, hoses, valves; and fittings to the vacuum pump. There must be no air leaks at any juncture in the configuration, no kinks or obstructions, and minimal use of elbows and sharp changes of direction.
Again, efficient hold-down of the workpiece comes from balancing suction and system airflow. It’s critical the part is flat against the vacuum table surface. It is also important that the plenum be deep enough, and fittings be large enough to prevent reduction of the necessary pressure differential to maintain hold-down of the part as it is processed. An improperly configured or installed system may not achieve adequate suction until such errors are corrected. Vacuum table systems should be regularly tested to ensure system integrity. Vacuum gages and ultrasonic leak detection devices may be used for this purpose.
An airtight case
With an appropriate system properly configured and installed, business owners and production managers can reap the rewards of vacuum workholding technology. Clamps, chucks, fasteners, and adhesives can be eliminated along with the time-consuming manual processes they require. This results in faster setups and changeovers with reduced downtime and waste.