March 2012 / Volume 64 / Issue 3|
Raising the Bar
By Kip Hanson
| 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
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But rattle tubes are mostly a thing of the past, having been replaced by far more effective solutions.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
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
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
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
But what about the rapid motion? “Our machines utilize high-acceleration headstock synchronization, with feed rates of 39 ips,” said Jim Burris, associate marketing director for IEMCA. “And our dual-bearing pusher design handles speeds up to 20,000 rpm.”
Put simply, this is one moving machine. As the lathe pulls the bar stock forward, the bar feeder’s pusher assembly follows it. Pull the bar back, such as when clearing chips during drilling, and the pusher backs off, all the while keeping a constant but light pressure on the end of the bar.Don’t Call Me Short
Hydrostatic bar feeders aren’t the only option for lights-out machining. A lower-cost option is a short-bar magazine loader. Like Goldilocks looking for the best porridge, a short bar might be “just right” for certain operations.
Courtesy of IEMCA USA
Where full-length bar feeders resemble a gymnast’s balance beam, short-bar loaders look more like a pommel horse. Bar-diameter capacities run from 3⁄16" to 2½ " or so, and maximum bar length is about 5 '. All short-bar loaders handle multiple bars, and, like their hydrostatic cousins, use a servodrive push mechanism to shove the bar stock into the lathe. The price point for most short-bar loaders lies somewhere between single-tube hydrodynamic bar feeders and magazine-fed hydrostatics.
Each short-bar loader builder promotes different selling points, but all agree on one thing—short-bar loaders use no oil and require cutting bar stock into spindle-length pieces. By containing the bar completely inside a spindle liner, everything moves together. The result is no noise and low vibration. The drawback is it takes time to cut the bars and there are three to four times as many material remnants as on a 12 ' bar feeder.
Both of these factors produce slightly higher operating costs. Despite this, short-bar loaders are a good solution for certain shops. They have a smaller footprint than full-length bar feeders, are less expensive and offer a viable alternative for shops performing high-volume, unattended production.Changeover Time
One other thing to consider when using a bar feeder is changeover. Historically, this has been a lengthy process for most bar feeders—including swapping out guide bushings, adjusting for different bar stock diameters and programming the programmable logic controller for the new part length.
Bar feeder builders attack this problem with different solutions, such as using quick-change, insert-style guide bushings and no-tool-required size adjustments. The result is that, for the most part, changeovers that once produced dripping guide tubes, oily floors and 30 minutes of downtime can now be accomplished cleanly and easily in just a few minutes.
Finally, look at programming. What was once a nonintuitive PLC interface has evolved into an intelligent device capable of storing part libraries, easy programming and even meaningful conversation with the lathe control. Also, an Ethernet interface option, said Edge Technologies’ Bauer, “allows good data transfer and programming directly from the lathe or from your desktop. You can even interface it to your iPhone. Within the next 3 years, Ethernet will be the standard interface on bar feeders and machine tools alike.” CTEAbout the Author: Kip Hanson is a manufacturing consultant and freelance writer. Contact him by phone at (520) 548-7328 or e-mail at email@example.com.
Shop discovers bar feeder learning curve
Courtesy of JF Berns
Courtesy of JF Berns
of the JF Berns Co. Inc. in Cincinnati in 1983, shortly after the debut of the first commercially available hydrodynamic bar feeder. While it started out making a single product line—spindle liners—the company now offers a broad line of lathe and bar feeder accessories, as well as custom products.
As the name implies, spindle liners are the first line of defense against bar whip and vibration, and are required in any bar feeding operation, regardless of the equipment type. According to Joe Berns Jr., who has taken the helm at the company, spindle liners are offered in steel or plastic, can be hardened to prevent chip pickup and are 0.02 " to 0.03 " larger than the bar stock diameter. “If the bar’s not exactly straight, you might need a little more space,” he said.
JF Berns spindle liners have evolved from simple O-ring and tube affairs into precise, quick-change devices. “Our quick-change adapter fits on the back end of the spindle,” said Joe Berns Jr. “The liner clicks into place inside the adapter; to remove the liner, just pull a pin and the liner slides right out.”
Another consideration for prospective bar feeder owners is the dreaded chamfer. Back in the day, when hydrodynamic bar feeders were a shop owner’s only option (aside from rattle tubes), chamfering the ends of bar stock was required work for all bar feeder operations. This was because the end of the bar sat in the V-shaped cup of the bar pusher, and a clean chamfer produced a smoother running bar. Unfortunately, bar chamfering was—and still is—performed on belt sanders, a haphazard proposition at best.
To solve this problem, JF Berns developed the Bar-Champ, a small machine that makes short work of bar chamfering. Since the introduction of magazine loaders, chamfering has become less prevalent, as all that is required is a clean, burr-free end. However, for those shops still using hydrodynamic bar feeders, the Bar-Champ is a nifty tool. And some bar feeder manufacturers still advise chamfering bars on all styles of bar feeders because, as Joe Berns Jr. explained, “the Bar-Champ makes the bar easier to slide into the spindle.”
Lastly, let’s say you can’t afford a bar feeder, but still want to run full-length bars. Or maybe you have to turn the ends of some really long parts. Don’t despair. For around $6,000, JF Berns offers an alternative to “real” bar feeders: its Top Load Sport series, which supports the bar along its entire length with a set of removable bushings every 6 " to 12 ". This eliminates bar whip and keeps the bar safely contained. And the Sport has a flip-open design, like the top of a metal bandage box, making bar changes easy. It requires much less room to load than end-loading models.
Cane Creek Cycling Components
JF Berns Co. Inc.
LNS America Inc.
CUTTING TOOL ENGINEERING Magazine is protected under U.S. and international copyright laws. Before reproducing anything from this Web site, call the Copyright Clearance Center Inc. |
at (978) 750-8400.