Bar Feeders Boost Turning Productivity
By Jim Destefani
One of the most cost-effective ways to obtain the benefits of automation is by adding a bar feeder to a CNC lathe or other bar machine. Costing anywhere from about $10,000 to $40,000 depending on configuration, the devices can add hours of untended operating time for part volumes of a few hundred to tens of thousands.
Bar feeders supply bar stock or shafts to turning centers and other bar machines. Bars as long as 12' (3.7 m) can be used, and most units can handle round, square, and hexagon shapes. Bar feeder units are offered as an add-on for a CNC turning machine or integrated with CNC machines from some manufacturers. Turning centers must be purchased with through-hole chucks to accommodate a bar feeder.
Bar feeder selection is based on several factors, including production quantities, the level of automation desired, bar diameter and length requirements, the part being machined, and the type of turning machine to be used with the unit.
There are two basic types of bar feeder systems: hydrodynamic and hydrostatic. The main difference lies in how the bar is supported.
Hydrodynamic systems contain the bar in a tube surrounded by pressurized oil that supports it when it is rotating. When spindle speed is zero, the bar rests on the feed tube. As speed increases, the bar produces increasing oil pressure that lifts it from the bottom of the tube and centers it in the tube to promote smooth feeding and minimize vibration.
In hydrodynamic systems, bars usually must be manually reloaded one at a time, and the feed-tube size must be changed to run a bar outside a relatively narrow diameter range. Hydrodynamic bar feeders use oil to push the piston that advances the bar and for hydrodynamic bar support. Oil encircles the spinning bar and pushes against it and against the feed tube ID to create a fluid bearing. This action centers the bar stock in the feed tube, and reduces noise and vibration. The hydrodynamic centering forces increase as bar rotational speed increases.
Hydrodynamic bar feeders offer low noise and vibration because of the oil-filled tube. Other pluses include high rotational speeds, automatic remnant retract, and good part length control.
An example of an economical, single-tube hydrodynamic bar feeder is the Mini-Rhinobar from Lexair Inc. (Lexington, KY). Designed for use with Swiss-type CNC lathes and smaller fixed-head turning machines, the unit can handle stock from 1/8 to 1" (3.2 - 41 mm) diam in 6 or 12' lengths.
Some hydrodynamic bar feeders feature a multitube design that allows a single operator to quickly change bar sizes. An example of this approach is the Super Hydrobar from LNS America (Cincinnati). The unit's patented modular design features guide tubes of various diameters arranged around a central axis, like the barrels of a Gatling gun. Each tube can accommodate a different bar diameter range.
The tubes are clamped between the unit's aluminum core and the halves of the feeder body, and each tube has its own piston and pusher assembly. In operation, the machine attendant decides which tube most closely matches the bar size to be run. The operator then rotates the drum to that tube, inserts an appropriately sized liner in the lathe spindle, loads the first bar, and feeds it into the spindle.
In a hydrostatic system, the bar rests in guide channels that close around the stock to hold it in place during machining. Hydrostatic bar feeders can usually accept a wide range of bar diameters without change. Hydrostatic bar feeders pump large volumes of oil into the guide channel to create support, while the bar pushing is operated by a servomotor.
Many bar feeders are designed to run 12' long bars. Material yields are usually better, and time lost to bar changing is minimized. But, feeders for 12' bars may add 16' (5 m) or more to the length of the machine. Another potential difficulty with use of long bars is whip and vibration at high spindle speeds if the bar is not straight.
Full-length bar feeders from FMB Machinery USA (W. Caldwell, NJ) minimize vibration from a bar that's less than straight using what the company calls adaptive speed control (ASC).
ASC allows CNC turning machines to operate at maximum possible spindle speed even with out-of-true material bars by automatically reducing spindle speed via the override function of the machine control. The adjustment depends on the degree of vibration, and the machine automatically returns to the set speed when vibration diminishes. Users can set the upper and lower limit of the vibration sensitivity, and check intervals and other parameters at the bar feeder control panel.
The capability is available on all the company's bar feeders, including the Miniturbo. Designed for use with Swiss-style and other smaller CNC turning machines, the unit can feed round, square or hexagonal bar from 3 to 26-mm diam and up to 4.2-m long.
Another possible answer to vibration--and tight floor space requirements, which are an issue in many shops--may be a spindle-length bar feeder. These are magazine-type units that hold multiple shorter bars and automatically feed them into the CNC lathe.
With a short-bar feeder, the entire bar is contained in the lathe spindle. The spindle liner is sized to fit the bar. Bar straightness is less important, and minimal bar end preparation is required.
Operation of spindle-length bar feeders is automatic after the operator loads bars into the unit. Spindle-length bar feeders also can generally run at higher speeds than 12' feeders, because vibration and whip is minimized with the shorter bars. Some spindle-length bar feeders also can handle larger bar diameters than full-length machines.
According to SMW Systems (Santa Fe Springs, CA), the short-bar magazine bar feeder has become pretty much the industry standard for single-spindle lathe applications. They work for both small-batch and volume production for relatively short workpieces. If the workpiece is long or the work material is costly, the added remnants produced by using short bars can be an economic issue. But for steel and aluminum workpieces, extra remnant cost is usually incidental compared with the production improvements of the magazine bar feeder, SMW says.
The current state of the art for magazine bar feeders is servo-driven pushing, which eliminates the need for a physical bar stop in the lathe turret. The system is accurate to within about ±0.004" (0.10 mm), compared with ±0.001" (0.025 mm) for pneumatic systems with a bar stop. Servo-driven feeding can save a couple of seconds per feed cycle, which can be significant depending on the type of part and production quantities.
SMW says its Spacesaver 2100 bar feeder is the only all-electric unit on the market. In addition to servo-driven pushing, it also uses electrically operated bar handling. This approach eliminates pneumatics and the attendant compressor. Pneumatics also tend to require more maintenance, the company says. Models are available to handle bars from 1/4 to 4" (6.4 - 102-mm) diam and up to 76" (1.9-m) long.
Bar feeders from IEMCA (St. Louis) save even more floor space with a patented double-pusher design. The company says using a "pre-pusher" to move the bar into the spindle and a conventional pusher from that point eliminates about 4' (1.2 m) off the length of its bar feeders. That means a feeder for 12' bars would have an overall length of about 14' (4.3 m), compared with 16 - 18' (5 - 5.5 m) for competitive models.
The double-pusher technology works for both full-length and short-bar feeders. The company's new Prodigy 547, for example, can handle 2.1-m bars and has an overall length of 8.5' (2.6 m). IEMCA says the intermediate bar length strikes a balance between spindle-length feeders and full-length units. The length of bar that's outside the lathe spindle is hydrodynamically balanced, so the unit can handle high-speed turning.
The feeder can handle bars from 5 to 51-mm diam. Capacity is 40, 10-mm diam bars, and the Prodigy is equipped with a new "top-down" loading feature which lets operators add bars to the magazine without interrupting the feed cycle.
The increasing use of bar feeders has caught the attention of machine builders as well. Some builders of turning equipment now are also supplying bar feeders that integrate with the machine control, hydraulics, and other systems, simplifying operation and maintenance.
Haas Automation (Oxnard, CA) manufactures its own magazine bar feeder, which integrates with the company's CNC lathes. The Servo Bar 300 bar feeder can handle bars to 3-1/8" (79-mm) diam and 60" (1.5-m) long. The servo-driven unit has a small 4.5 X 8' (1.4 X 2.4-m) footprint and runs directly from the Haas CNC. It can store a single layer of 60" bars up to 30" (0.75-m) wide.
According to Haas, direct integration with the CNC simplifies setup and operation. Users access a setup page in the machine CNC that prompts them for parameters such as part length, initial push length, minimum clamping length, and tool offsets, then select the needed program.
Tightly integrating the bar feeder with its Swiss-style turning centers is Marubeni Citizen Cincom (Allendale, NJ). The company's CAV bar feeders share the CNC, hydraulics, lubricating system, and electronics with the company's Swiss-style turning centers, reducing the number of components needed to run the unit and cutting cost and floor space requirements.
Bar feeder movements are programmed as an additional Z axis on the turning center's CNC. The fully enclosed units can handle round, square, or hexagonal stock in diameters from 0.078 to 3/4" (2 - 19 mm) and lengths to 13' (4 m).
Bar feeders from Tornos Technologies US Corp. (Brookfield, CT) are integrated with the company's Swiss-style turning machines, and have a design that is said to cut the time needed for bar size changes to 15 minutes.
Newest in the line is the SBF-216, designed for use on Deco 10a and 13a single-spindle automatics. In conventional bar feeders, a bar size change requires an operator to manually change each section of the guide channel. On large feeders, that can mean four to ten components with oil lines and other pieces.
Tornos' quick-change system--essentially a series of connecting guide blocks that operate as one unit--lets operators simply index the guide block to the proper size capacity. The unit also features a bar gripper mechanism between the bar feeder and the sliding headstock that provides a stable transition zone for bar ends regardless of bar diameter.
The SBF-216 accommodates bar sizes from 2 to 16 mm. Also available is the larger SBF-532, for bar diameters from 5 to 32 mm. The latter is designed for use with the company's Deco 20a and 26a machines.
Feeding the feeder to provide another layer of automation is the idea behind the Bundle Master model 610, a 6' (1.8-m) capacity bundle loader from J.F. Berns Co. Inc. (Cincinnati). The unit automatically loads a barfeeder or other device with bars from 1/4 to 3" (6.4 - 76-mm) diam and weighing up to a ton. The device will then supply the bars to a bar feeder at the required pace until the bundle is gone. It requires 115 VAC power, and is also available in 12 and 24' (3.7 and 7.4-m) versions.
This article was first published in the March 2004 edition of Manufacturing Engineering magazine.