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Consider a shop that lands a contract for large-diameter work. The work represents a new direction for the job shop and, perhaps, calls for a hurried capital-equipment purchase. Why? Because the lathe on the shop floor can't handle such large-diameter bar work.
Pneumatic chucks can provide an alternative. True, reliability concerns have surrounded pneumatic chucks for years. A hydraulic leak can be clearly visible while a slow air leak can be tough to detect. But thanks to new sensor technology, this concern may eventually go by the wayside.
Pneumatic chucks simplify componentry, expand machine capacity and, as the technology gains greater market acceptance, could greatly simplify workholding setups in machine shops across metal manufacturing.
THE PNEUMATIC ALTERNATIVE
Most lathes use a hydraulic-mechanical chuck. Actuating the front chuck is a draw tube that is pulled and pushed via the hydraulic cylinder in the back of the lathe spindle. Roughly 95 percent of the lathes in the U.S. use this kind of technology.
But to fit a larger part on a smaller machine—without having to invest in larger equipment—requires more through hole clearance on the chuck. This becomes possible by taking the draw tube and hydraulic components off of the spindle and mounting a front-actuated air chuck. The spindle bore is then clear, with no hydraulics and no draw tube, thereby increasing the chuck's holding capacity.
The front-actuated air chuck maximizes the through-hole capacity of the turning center, giving the same through-hole capacity as the machine's spindle. On a large turning center this may increase capacity by about 1.5 to 2 inches.
Most applications for this technology have found a home in turning down-hole pipe in the oil industry, but such workholding has applicability for various applications. The price of a pneumatic chuck—though higher than traditional chucks—can be a lot less than the cost of a new machine tool. And although most applications today, like the down-hole pipe described, do not require relatively high clamping forces, pneumatic chucks are designed with as much clamping capability of equivalent hydraulic chucks.
These chucks offer a two-piece design, with an air-distribution ring surrounding the main chuck. That ring has so-called "profile seals" that resemble a large-diameter O-ring—not touching, mounted just off the chuck body. When the chuck is in the "stop" position and ready to be closed, air is introduced to the profile seal, which then collapses on the body of the chuck and charges it with air. A timer on the air inlet times how long it takes for the chuck to fully charge. Once charged, the air stops flowing into the profile seal, and the distribution ring and profile seal no longer touch the chuck body. A check valve in the chuck body retains the pressure. With checked air inside the chuck, and profile seals no longer touching the chuck body, the chuck can spin with the profile seals and distribution mounting rings remaining stationary. This all happens in reverse when the pneumatic chucks open.
CHALLENGES WITH PNEUMATIC TECHNOLOGY
One concern arises with this technology: safety. Consider if a small leak starts in the chuck's valve. What if the machine is set up with a part and stays dormant for the weekend? The operator comes in on Monday and doesn't re-chuck the part, but simply starts the machine, which thinks the chuck is closed. This, obviously, leads to a very dangerous situation.
In recent years, sophisticated sensors have come into the industry. Thanks in part to development of pneumatic pressure sensors that show drivers of high-end vehicles their current tire pressure, these sensors can show exactly how much pressure lies within the chuck. A transmitter and air gauge are mounted inside the chuck body, which transmits a radio frequency to the machine control, continuously monitoring the pressure inside that air chuck. If the chuck loses pressure, the sensor immediately signals the machine control to shut down.
Other systems use stroke monitoring with a switch on a part that engages just before the chuck lets go. Still others involve poppet-valve-style pressure switches that kick out below a certain pressure.
The bottom line: Thanks to these innovations, safety issues traditionally associated with these air chucks have been largely resolved.
THE AIR DEBATE
Shops that use hydraulic clamping vises require a hydraulic pump, lines and other auxiliary equipment that may not be immediately available. Pneumatic chucks, on the other hand, just use an air line, and virtually every manufacturer has pneumatic pressure running throughout the shop floor.
For anyone in pneumatics, "non-clean air" usually means "water," or condensation accumulating inside the pneumatic lines. Over the years, this issue has improved immensely with better compressor technology. And besides, many machine tools, in fact, need that dry, clean air to run indexers, automatic tool changers and other equipment. So, why not use it for workholding?
True, hydraulic systems are sure to remain the industry standard for years to come—and for good reason. They represent a proven, cost-effective technology with a price point that makes sense for most machining applications.
But pneumatic systems could potentially simplify workholding systems within the coming decades. Consider non-rotating, stationary chucks on a tombstone or other device. Although not yet commercially available, in these tombstone setups the check valve would not necessarily have to be within the chuck body; instead, it could be integrated into the inlet tube in the manifold, regulating pressure for all the chucks in the system at once. Since the manifold and all other chucks would be tied together, only one chuck would integrate the pressure sensor, to ensure constant pressure within the system.
For hydraulic chucking complications can arise with quick-changeout pallets, for example. The hydraulic clamping force has to be checked. And hydraulic oil is non-compressible, so when a little oil is lost, the pressure goes as well. Even a slight leak can mean the end to total clamping force. So any time the hydraulic system is disconnected from the hydraulic pressure source, the tombstone has a hydraulic accumulator with extra oil to make up for any lost oil during the changeover.
Pneumatic workholding doesn't have these issues. Why then has the technology not become more widespread? The reason comes back to monitoring pressure. How long has technology been available to monitor pressure in a car's tires? Not too long. But these sensors are now just now being available in the workholding sector. In the coming decades, sensors could help pneumatics gradually gain greater acceptance in workholding throughout metal manufacturing.
Editor's Note: Ron Wright is sales manager, tooling and workholding, SCHUNK Inc., Morrisville, N.C., www.schunk.com. Artwork courtesy of SCHUNK.
author: By Ron Wright