Raising the Bar

Bar feeders, once known as slow and inefficient ‘rattle tubes,’ are now high-speed, automated units.

Ask any hand screw or screw machine operator the meaning of the term “rattle tube” and you’ll likely get an earful. Of course, you’ll probably have to shout, because he’ll be hard of hearing.

Rattle tubes, or pneumatic or pulley-drive bar feeders, are great at pushing bar stock into the back end of a lathe or screw machine, but they’re also noisy, dangerous and prone to breakdown.

Courtesy of Edge Technologies

Quick-change channel liner on an FMB Minimag small-bar loader from Edge Technologies.

The reason rattle tubes get such a bad rap is that, even though the bar stock is contained inside a tube, it’s still left unsupported along its entire length. As the bar spins, centrifugal force pushes it against the side of the tube, which not only makes a racket but creates vibration—never a good thing when machining.

But rattle tubes are mostly a thing of the past, having been replaced by far more effective solutions. Hydrostatic and hydrodynamic bar feeders and short-bar loaders are elegant, reliable alternatives.

Dynamic Situation

Say your boss has grown tired of hearing the airplane-like roar of that 1 “-dia. bar stock spinning at 2,000 rpm and wants you to get more production out of a new CNC lathe, or your customers are complaining about poor part surface finish because you can’t get decent speed on your bar feeder and lathe without setting off car alarms. What can you do?

Tell your boss to get out his checkbook. Even a low-cost hydrodynamic bar feeder is $8,000, not to mention the cost of installation, spindle liners, guide bushings and an interface to the machine tool.

However, the expense will be justified by the equipment’s ability to increase machine output, enable cutting tools to impart finer surface finishes and improve tool life—all because a bar feeder lets you achieve the correct cutting speed for the workpiece material. Better yet, you won’t need hearing aids later in life.

Hydrodynamic bar feeders work by surrounding the bar stock in oil, typically a viscous hydraulic fluid. The unit has a pump that circulates oil continuously through the bar feeding system, supporting the bar and damping its vibration. To contain and direct the oil, and because there’s a limit to the amount of oil that can be pumped, the ID of the bar feeder is reduced with a tube, or liner, slightly larger than the bar stock. This tube is similar to the one lining the machine spindle, but much longer. Each liner covers a small range of bar sizes and provides ¹⁄₃₂ ” to ³⁄₈” of bar clearance.

Courtesy of LNS America

The LNS Sprint 542 bar feeder changes over ¼ “- to 1⁵⁄₈“-dia. bar stock in less than 8 minutes. minutes.

Gravity pulls the bar to the bottom of the liner when the spindle is idle. When the pump is activated, the gap between bar and liner fills with oil, the bar starts to spin and the hydrodynamic effect kicks in as the moving oil surrounds and centers the bar in the liner.

Damien Wenisch, director of technology and national bar feed product manager at LNS America Inc., Cincinnati, offers an analogy. “It’s a little like your car’s tires on wet pavement,” he said. “When you’re at a stoplight, the tires rest on the roadway. Hit the gas too hard and, suddenly, you’re hydroplaning.”

That’s a bad thing when driving, but a good thing when operating a hydrodynamic bar feeder. “As the bar speed increases, the hydrodynamic effect continues to grow until the bar is centered and you’ve reached equilibrium,” Wenisch continued. “However, this effect is only good up to a gap of 5mm.”

Not Really Static

Hydrodynamic bar feeders have been around for more 30 years and are a favorite among shops with tight budgets.

At the next level are hydrostatic bar feeders, which operate at faster speeds and self-feed. Instead of using the tube-inside-a-tube approach as in a hydrodynamic bar feeder, they employ a series of guide channels, or bushings, resembling clamshells or fat C-clamps. These bushings run the length of the bar feeder, one after another. The bar stock rests inside these bushings, which cradle the material with minimal clearance, and hydraulic fluid is forced into each bushing under high pressure, surrounding the material in an oil film.

Courtesy of JF Berns

Bundles of chamfered bar stock, ready to load in a bar feeder. The bars were chamfered by the Bar-Champ machine from JF Berns.

Wenisch explained that this oil film operates under the same principle as machine tool spindles, except on a larger scale. “The ideal gap is 1mm. This is where we see the best hydrostatic effect,” he said. “To achieve this, we maintain high pressure on the oil film to overcome the centrifugal force of the bar, which is constantly trying to push the oil away.”

To meet the need for a precise gap between bushing and bar stock, every bar size requires its own set of guide inserts. LNS America provides a complete set of bushings with each of its hydrostatic bar feeders, covering the full range of bar stock sizes.

The result is a bar feeder capable of “optimal rpm,” Wenisch said. “Hydrostatic bar feeders allow you to achieve better surface finishes, superior tool life and tighter tolerances on turned parts.”

Equally important is their ability to help produce more parts per shift and without operator intervention, because most hydrostatic bar feeders are magazine-fed. According to Rick Bauer, national sales and operations manager for Edge Technologies, St. Louis, “Unattended operation is a huge part of the future, but it’s something you can’t achieve without good bar feeds. Our products are enabling American manufacturers to go lights out.”

Edge Technologies markets its own products and distributes products from German manufacturer FMB Machinery. It offers entry-level bar feeders as well as “Cadillac” versions that can accept a bundle of material. A $300,000 mill/turn center might require a top-of-the-line bar feeder, but, for a basic 2-axis lathe with lights-out production needs, a simpler and less expensive bar feeder may be more appropriate.

Various hydrostatic bar feeders can accommodate bar stock lengths from 4 ‘ to 24 ‘ and diameters from the size of spaghetti noodles to family-sized soup cans. They cost from $15,000 to $50,000 or more.

That may seem like a lot to spend when you’ve just dropped $70,000 on a CNC lathe, but part volume, tolerance and surface finish requirements may require it. And keep in mind that, contrary to what you might think, when it comes to hydrostatic bar feeders, bigger doesn’t necessarily mean more expensive.

Keeping Up With the Swiss

Hydrostatic bar feeders are also available for Swiss-style machines. You might think that feeding very small bar stock into a Swiss-style machine would be no big deal—after all, those old pulley-fed bar feeders did an OK job of it. But that’s not the case.

With small material, the slightest amount of grime or packing oil on the bar stock can gum up the best bar feeders, making automatic loading a challenge. There’s also the back-and-forth motion of the material to deal with. Picture a threading operation on a Swiss-style CNC—the bar stock will be flying back and forth like a lumberjack sawing a log. How does the bar feeder cope?

Courtesy of Edge Technologies

The Rebel short-bar loader from Edge Technologies is a lower-cost option for budget-minded shops than a full-length bar feeder.

On bar stock more than ¼ ” in diameter, the material is simply laid out in neat rows across the part of the magazine that serves as the waiting area. Once the first bar is consumed, small feed fingers grab and lift the next bar and roll it into the business side of the bar feeder.

Go much below ¼ ” in diameter, however, and you’ll start to have problems. Small bar stock is too light, and it’s too easy to grab two bars instead of one, which would ruin everyone’s day. “Working with small stock is very difficult,” Bauer said. “It takes a lot of finesse and patience.”

Even with clean, well-prepared bars, large-bar pickup methods are prone to failure when Swiss machining. To counter this, bar feeder manufacturers have developed the “walking beam” for small bar stock. Shaped like a saw blade, the walking beam holds each piece of bar stock separate from its neighbors. When it’s go-time, the beam “walks” the material into the bar feeder bushing before grabbing the next piece.

“This technology guarantees 100 percent loading,” Bauer said. Better yet, the same bar feeder loading ¹⁄₈“-dia. stock today can be used for ³⁄₄“-dia. stock tomorrow. “Just drop the walking beam out of the way, and you’re ready to go.”

The next problem on a Swiss-style machine is how to feed material the size of a spaghetti noodle. Unless you have a bar feeder specially designed for the rapid back-and-forth motion of a Swiss-style machine, you’ll end up with a metal pretzel.

Solutions are available, however. One is from IEMCA USA, Charlotte, N.C. Similar to the Edge Technologies’ approach, IEMCA uses a walking beam to manage small material in its bar feeders.

Courtesy of Edge Technologies