May 2013 / Volume 65 / Issue 5|
By Alan Richter, Editor
| The quality of the sealing, braking and bearing systems must be considered when specifying a rotary table.
Job shops may believe that all CNC rotary and index tables are essentially alike. As a result, they often base their selection on price and hope they can use their machining creativity to maximize a rotary or index table.
However, to expand the capabilities of their machine tools when purchasing new ones or retrofitting existing equipment, considering the quality of a table’s sealing, braking and bearing systems and how those systems impact a shop’s current and projected applications can pay big dividends down the road. Shops are also wise to consider the effectiveness of a cable management system—which ensures a rotary table is always connected to the machine without operator intervention and extends connector life—when palletizing tables.
Courtesy of Koma Precision
“The initial quality of the rotary table makes a big difference,” said Lee Flick, national sales manager for HPI-Pioneer, “especially at job shops.” The Elk Grove Village, Ill., company supplies CNC rotary and index tables from Yukiwa Seiko Inc., Japan.
Predictably, many end users—especially shops on limited budgets—push the limits of a rotary table’s weight capacity and capability. “Some of our customers will put a car on a little table and try to turn it,” Flick quipped. But by specifying the correct rotary table for the job, shops can extend equipment life while reducing maintenance issues and continuing to machine high-quality parts.
A table axis’ lack of durability and inadequate clamping power can cause out-of-tolerance features and rough surface finishes. One solution is a dual-bearing system that supports the table on the front and rear with large bearings, according to Flick. Even with an overhanging weight load, this design prevents the table from leaning, he added.Sealing and Purging
One of the key factors that determine the life span of a rotary table is how well it is sealed against potential contaminants, such as metalworking fluids. Flick noted the most common rotary table service is repairing electronics damage caused by coolant infiltration. “Within 20 years I believe the market will be dry cutting,” he said. “But as long as coolants are involved, sealing is extremely critical.”
Flick indicated that there are three basic sealing arrangements: sealing just the motor cavity, sealing the entire table and sealing the table and incorporating an automatic-purge system similar to check-valve pressure on an airplane. “You pressurize the cabin and if you were to get a leak, it blows outwards,” he said, adding that a rotary table with that type of seal typically lasts twice as long as one that doesn’t, whether it’s an entry-level or higher quality table.
HPI-Pioneer President Nobu Kiriyama explained that the Yukiwa air-purge system uses the 70-psi shop air that also supplies a table’s braking system and enables a table to effectively function when submerged for EDMing. For those applications, an optional nickel-plating is appropriate when the table is submerged in water-based dielectric fluid.
Of course, chips, swarf and other debris produced during cutting can abrade any seal material and provide a passageway for contaminants into a table’s inner mechanisms. Therefore, John Arnestad, tooling product manager for Koma Precision Inc., recommends against aiming a high-pressure coolant line at a seal or an area where a seal is located to reduce that risk. The East Windsor, Conn., company supplies rotary tables from Tsudakoma Corp., Japan, which have seals directly behind the faceplate. The tables also have a metal cover that shields the main seals to decrease the chance of debris abrasion.
He added that coolant infiltration primarily dilutes and washes away bearing lubricants, causing premature failure. Depending on the type of coolant, Arnestad noted a coolant mixture that is too rich, or has an improperly high ratio of coolant to water, can soften seals and cause failure.
Courtesy of CNC Indexing & Feeding Technologies
For high-pressure coolant applications, Rotec Tools Ltd., Millwood, N.Y., recommends a labyrinth seal on the rotary table. Rotec distributes rotary tables from Peter Lehmann AG, Switzerland. Ivo Straessle, Rotec president, noted the air-supported labyrinth seal attaches to the spindle nose of the table, extending the standard spindle seal, and creates an air cushion underneath the seal to keep coolant out. “We prefer oil-based over water-based coolants because they’re not so aggressive on the rotary tables,” Straessle said. “Many water-based coolants attack seals, cables and electrical components.”
In addition, Lehmann rotary tables have an air-purge seal on the motor housing, and the gear housing has a positive oil pressure at all times. “That makes it very difficult for debris or coolant to penetrate the seal system from the outside,” Straessle said.
To ensure longevity of the rotary table and its sealing system, pressure is reduced to the internal components of the pressurization system during rotation on most of the table models offered by L.C.M. Srl, Italy, said Mike Bickham, president of distributor ITI Tooling Co. Inc., Ramsey, N.J. Then, the pressure increases when the table reaches the preprogrammed working positions.
“The big enemy is coolant contamination,” Bickham said, noting that more flood coolant at a higher pressure is often required as machining speeds continue to increase.
Courtesy of Rotec Tools
Unless it’s an older table, coolant infiltration behind the rotary table seals isn’t an issue in 99 percent of machining applications, according to Jamie Schwarz, national sales manager for CNC Indexing & Feeding Technologies, Mason, Ohio, which offers Taiwanese-made Ganro rotary tables. A combination of oil seals, O-ring seals and silicone caulking keeps contaminants out of the tables’ inner workings, and the company offers an air-purge system as an option.
Schwarz explained that oil seals and O-ring seals are used in all areas where there are key components that require lubrication. Silicone is used around any connecting edge with internal electrical connections inside, such as motors and solenoids. Other examples include areas where one piece of sheet metal meets another and where sheet metal meets a nonmachined casting. In addition to silicone caulking, the sheet metal has a rubber/silicone adhesive seal. Areas where it’s critical to keep lubricating oil from escaping are also sealed, such as behind the faceplate and around the worm gear and worm shaft.
When coolant penetrates a rotary table, it can short out a servomotor, and fine chips can wreak wear-related havoc on the brake, worm wheel, worm shaft and other parts. “That can get expensive when you start replacing internal components that are important to the table’s accuracy and repeatability,” Schwarz said.
Pressure also comes into play when selecting the appropriate braking system for a rotary table. According to HPI- Pioneer’s Flick, two basic variations exist: a disc brake system and a hydraulic version that provides higher braking pressure than a disc brake. Although hydraulic systems are prevalent, he pointed out that the company offers models with the less-typical air-over-hydraulic booster inside the table’s motor cover, eliminating the need for a hydraulic pump or other external hydraulic supply. Instead, 70-psi shop air does the trick. This is because the table clamp mounted inside the air-hydro structure boosts the hydraulic pressure and achieves higher clamping power simply by providing air pressure. (See image below.)
“The higher the braking pressure, the more aggressive machining you can perform on the rotary table,” Flick said. “Hydraulic boosters are becoming more common because people want to machine harder and harder on the table.”
He added that there’s no angular pressure on any braking system when using a rotary table to cut on-center, but the type of system makes a significant difference when machining off-center because the pressure created on the cutting location increases based on its distance from the center. Hydraulic systems permit more aggressive cuts, even when machining farther out on the table.
The heavy-duty, zero-backlash braking systems on the rotary tables from ITI Tooling can use hydraulic pressure up to 870 psi, according to Bickham, but 580 psi is more typical. In addition, the tables can use an air supply together with an air-over-oil intensifier, which provides benefits because shop air is generally more readily available and is restricted to a safer pressure than a hydraulic system. Bickham added that whether it’s an air- or a hydraulic-actuated brake, the brake operates in the same manner by expanding a 360° pressurized sleeve that wraps around the table’s center axle, holding it rigidly in place.
Cost considerations can play a prominent role when selecting a rotary table’s braking system. The disc brake is the most popular type because repairs are fairly inexpensive, according to CNC Indexing’s Schwarz. The repair is usually performed on tables 10 years and older used for heavy machining, he said. “We can get the repair done within a day for a few hundred dollars.”
Arnestad indicated that Koma offers several braking system options. Called the RNE series table, the pneumatic, dual-disc system is the most economical, while offering good accuracy and a good torque drive, he noted. The mid-range RNA series table has a dual-taper clamping system that uses a pressure intensifier, where a ring of ball bearings are captured between the clamping ring and the piston.
“The piston has an angular feature on it, as well as an angled member inside the braking system so, as the piston moves forward, it displaces that ring of ball bearings and triples the clamping force,” Arnestad said. This enables taking heavier off-center cuts without the fear of the rotary faceplate turning under force.
Arnestad added that the hydraulic, dual-disc clamping system, called the RBA series table, is the company’s most robust. “That’s targeted for high-volume production,” he said, “where they need to hold very close tolerances and want to machine at the fastest rate possible.”
According to Rotec’s Straessle, Leh-mann rotary tables have the same braking system for any application, using a built-in air-over-oil intensifier. This booster coverts 90-psi shop air into more than 4,000 psi of hydraulic pressure, he added, and a clamping ring equally applies 360° clamping force to the rotary axis. “A high clamping force allows high feed rates and high workpiece accuracy.”
End users can also increase machining parameters when a rotary table’s bearing configuration enhances table rigidity. For example, preloaded axial and radial needle bearings in front of the table next to the spindle nose and preloaded axial needle bearings in the rear of the spindle (all inside the gear housing) provide a high level of rigidity, Straessle explained. Preloading eliminates the undesirable “give” for the bearings in the axial and radial directions. “This design allows the customer to work with heavy workpieces and, at the same time, reduces overall vibration,” he said.
According to Arnestad, a cross-roller bearing construction provides the best rotary table support. “It gives better stiffness and less runout than, say, tapered roller bearings, which have a chance of losing their preload over time,” he said.
Courtesy of HPI-Pioneer
In terms of runout, HPI-Pioneer’s Kiriyama noted the bearings in Yukiwa’s rotary tables provide a 0.0001" runout and are the same grade as the ones machine tool builders use for their spindles. According to the company, the tables have a dual-bearing system, where two sets of bearings at both ends of the body provide triple-contact support, and the largest angular contact bearing is underneath the table face to maximize bearing support.
The alternative to a dual-bearing system is a single-row, center-bearing design, where, as the name implies, one row of bearings is in the center of the spindle. This design tends to enable more spindle tipping, or leaning, but HPI-Pioneer’s Flick noted it can be engineered to provide the required support.
Courtesy of Koma Precision
HPI-Pioneer repairs and replaces worn bearings from its rotary tables, as well as from its competitors. “A good table will go 5 to 10 years before it needs work, just like a machine,” Flick said. “With a low-end table, you’re doing it annually. It’s that dramatic of a difference.”
After L.C.M. purchases ultrahigh-grade bearings, the company frequently reworks the new bearing assemblies to reduce the runout and improve the load-carrying capacity, ITI Tooling’s Bickham pointed out.
Similar to a braking system, a high-quality bearing configuration effectively supports the workpiece load and machining pressure in the axial and radial directions. This proves beneficial when machining on a rotary table that’s not adequately matched to the application. “Everyone wants to buy the least expensive table they can and push it to its limits rather than going the next size up,” said CNC Indexing’s Schwarz.
Put It on a Pallet
When high-volume manufacturers and those performing lights-out machining use rotary tables, they often palletize the tables. In this scenario, the main consideration is the cable management system, which enables a palletized table to shuttle in and out of the machine tool without disconnecting cables while keeping them clear of the machining process, according to Schwarz. It might involve running the cables through the middle of the pallet to some sort of track system or incorporating a spring-loaded arrangement that hangs from the upper sheet metal in a machine and allows cables to extend and retract.
Most end users are quite opposed to having to repeatedly plug in and unplug a rotary table, he said. As a result, the role of a cable management system, which machine tool builders and third-party installation companies provide, is critical.
Courtesy of Koma Precision
Koma’s Arnestad added that a cable management system ensures a rotary table is always connected to the machine without operator intervention, which extends connector life. “If you don’t have good cable management, the connections can fail due to abrasion or having the cables flex too much,” he said. “How many times can you plug and unplug a multiple-pin connection before somebody gets a chip underneath the unit, tries to force it on and it leaks and you have problems?”
To enable a plug-and-play arrangement, ITI Tooling’s Bickham noted L.C.M. supplies its rotary tables with pallet receivers.
Going cheap across the board with rotary tables pays initial dividends, but part manufacturers that skimp on quality may miss enhancing the capabilities of their machine tools—even entry-level ones. “If you put the right options on your rotary table,” Frick said, “you can make an inexpensive machine perform as well as a midrange machine.” CTE
About the Author: Alan Richter is editor of CTE. He joined the publication in 2000. Contact him at (847) 714-0175 or firstname.lastname@example.org.
Courtesy of ITI Tooling
Applications for direct-drive rotary tables
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