[Dan McCann]

They’re the unsung workhorses of microtechnology. Unseen—and, if working properly, unheard—micro ball bearings pervade the microworld. They’re in dental and surgical drills, aerospace and aircraft instrumentation, fax and copying machines, and an array of emerging technologies.

According to manufacturers, making and assembling the major bearing components—the inner and outer rings and incorporating micro balls—presents an array of challenges.

For one, “the finishes are a major factor,” said Nancy Cogliandro, marketing director for Pacamor Kubar Bearings (PKB), Troy, N.Y., which makes bearings with ODs from 0.0400" to 0.875". “So if you have some debris in the raceway, like a piece of hair, it would be like hitting a telephone pole with your car.”

Makers of micro ball bearings follow exacting standards throughout the entire manufacturing process, paying special attention to polishing and assembly to ensure pristine finishes and precision tolerances.

At PKB, that process begins with soft machining of 440-C stainless steel round bar stock in automatic screw machines. The rings are turned to rough dimensions, maintaining tolerances within ±0.001".

The rings are then heat-treated and worked to a hardness of 58 to 62 HRC. The heat treating stabilizes the rings, which enables the subsequent grinding process to achieve the required tolerances.

PKB workers use Gardner face grinders to remove excess material on the ring faces. With a Norton face lapping machine, they then further grind the rings to achieve the final dimension with width and parallelism tolerances that meet ABEC-7 specifications. (The American Bearing Engineering Committee rates bearing precision on a scale of odd numbers from 1 through 9; the higher the number the tighter the tolerance.)

Next, with through-feed Cincinnati centerless grinders, workers grind the ODs of the inner and outer rings, using the ring faces as a reference.

Afterward, the rings undergo a series of finishing operations to remove grinding burrs and sharp edges. They’re processed in a Sinto barrel-style tumbler running silicon-carbide media and burnishing compound. “It basically agitates the media onto the surfaces of the rings, removing sharp edges and shining them,” said Ed Osta, PKB’s executive vice president. “It’s a 3-day process to get the finish we’re looking for.”

Using the finished ODs and ring faces as a reference, the bores and inner and outer raceways are finish-ground, maintaining strict tolerances. Bore diameters are held to within 0.0002" of nominal, roundness of 0.000050" and taper within 0.0001". Raceway diameters are held to tolerances of ±0.0002" and roundness to within 0.000050".

Next, honing and super-polishing machines remove any asperities in the inner and outer raceways left by grinding. PKB’s specs call for a raceway finish of 2μin. Ra or better.

The completed rings are then sent to passivation to remove any free iron from the parts. During the process, a chemical reaction occurs to produce a chromium-oxide coating on the rings, which prevents further chemical reactions such as oxidation.

When it comes to assembling the micro ball bearings, machinery takes a back seat to skillful handling. “The bearing business technology is probably 70 or 80 years old,” said Osta, adding that “the bearings [still] have to be hand assembled. There really haven’t been a lot of advances in the assembly of small bearings.

“Now, the assembly of automotive bearings can be automated because of their larger size,” he said. “But some of our balls are probably smaller than the head of a Bic pen. You’re trying to put together a bearing with a 0.125" OD and you can hardly see the balls. When you get oil on these parts, they are so light the oil could carry them away, so there’s no way you could run them through a machine to assemble.” Hand assembly is also efficacious because of the variety of customers’ unique requirements, such as special retainers, radial plays and lubricants.

The first step in assembly is to gauge and match the inner and outer rings with the corresponding balls to ensure specified radial plays. The bearings are then cleaned with a closed-loop vapor degreasing system in a Class 10,000 clean room. Then, lubrication and final assembly take place under Class 100 laminar-flow work benches.

“We assemble and keep filtered lubes and different types of plastic sealing bags that are heat-sealed to keep debris out,” said Cogliandro. “Depending on the bearing’s application, sometimes we have to nitrogen-backfill [the bags] to make sure that even oxygen is kept out.”

The entire manufacturing process takes between 8 and 12 weeks. “We focus only on the miniature ball bearing market,” said Cogliandro. “We know the machining and assembly and what the bearing can withstand in different environments. If a [ball bearing] design engineer wants to talk about a special lubricant or design, or how far you can push the envelope of a [specific] bearing, that’s what our guys know.” µ

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About the author: Daniel McCann is senior editor of MICROmanufacuring. Telephone: (847) 714-0177. E-mail: dmccann@jwr.com



This Timken S100 radial open bearing, generally used in guidance applications, has a 0.1" OD, a 0.025" bore and a full complement of seven 0.025"-diameter balls. Photo courtesy The Timken Co.

Making the balls

Many bearing manufacturers obtain the balls for their products from other companies. “[Making bearing balls] is an entirely different procedure than making bearings,” said Ed Osta, executive vice president of Pacamore Kubar Bearings. “It’s a very unique process, and it’s a huge capital investment.”

One of the manufacturers of balls used by Precision Bearing Co., Northbrook, Ill., is Tsubaki Nakashima Co. Ltd., Nara, Japan. The process starts with “cutting a wire rod [Si-52100 steel] to length, slightly larger than the volume of the finished ball,” said Joe Andalman, Precision Bearing’s founder.

“The cut slug is then cold-forged in two steel-carbide dyes to produce a cold-headed ball,” he continued. “That leaves an equator on it because you have two female dies.” The balls then move to a flashing machine that removes the equator and gives the balls a rough finish. Next, they’re hardened in a furnace to ensure strength and durability.

After grinding the balls to approximate size, said Andalman, lapping machines are used to impart a fine finish as the balls are polished to exact size. (Balls are graded in terms of roundness, or sphericity. Each grade represents a 0.000001" deviation from a perfect spherical form. So, a Grade 5 bearing deviates 0.000005" and Grade 25 deviates 0.000025".) The balls are then cleaned in a supersonic bath, inspected, sorted, tested and packaged.

—D. McCann


Pacamor Kubar’s raceway measurement operation ensures proper radial play. Here, raceways are in a rotating bowl prior to being measured. Photo courtesy Pacamor Kubar Bearings.

Micro vs. macro bearings

Compared to making conventionally sized bearings, producing microbearings invariably entails some knotty engineering adjustments. “There are inherent challenges with microbearing manufacturing compared with the larger variety,” said Mark Mcilrath, chief engineer of the aerospace division at The Timken Co., Canton, Ohio, which has made bearings since 1899. “I can’t get into our proprietary activities, but we do have a lot of engineers whose sole responsibility is to find ingenious [solutions to the following problems]:

• Workholding. “Traditionally in larger manufacturing, parts are held in machine tools by several methods: magnetic drivers, collets and chucks, for example” said Mcilrath. Yet such solutions don’t usually work in micro applications because of the reduced work area, which can be less than 0.025". “For instance,” Mcilrath continued, “if one is trying to use magnetic drives to hold a workpiece in place while grinding, that magnetic force imparts a pressure on the workpiece. But since there’s so little surface area, that doesn’t create a lot of friction between the driver and the part. So it’s hard to hold the part in place using a magnetic field.”
• Automated handling. While manufacturers of larger parts might use gravity to transport parts from machine to machine, that’s not an alternative for small-massed microbearings. “Now you need a man or a mechanism to intercede in that operation and provide [a way] to move parts into place reliably,” said Mcilrath.
• Mass finishing and deburring. In conventionally sized bearings, hard media of various shapes often are used to deburr, said Mcilrath. But with microbearings, manufacturers have to find a media small enough to get inside parts’ holes and with sufficient mass to do the job. “You’ve got to find ways to remove burrs,” said Mcilrath, “especially in internal surfaces that are hard to get at, without using a traditional method like mass tumbling.”

—D. McCann


Contributors

Pacamor Kubar Bearings
(866) 902-4998
www.pacamor.com

Precision Bearing Co.
(847) 559-9961
www.precisionbearingco.com

The Timken Co.
(330) 438-3000
www.timken.com