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Precision With/Without Guide Bushing

Jim Lorincz
By Jim Lorincz Contributing Editor, SME Media

Technology extends the productivity of CNC automatic lathes

Last year’s surge in medical machining and firearms manufacturing could well be joined or even eclipsed by this years’ reemergence of production for applications in the automotive, aerospace, electronics, and hydraulics industries, generating increased interest in Swiss-style machining. This isn’t news. But what may be surprising is that the venerable, tried and true Swiss automatic CNC lathe coninues to evolve, adding bells and whistles where needed, or conversely stripping one—like a guide bushing—away to maximize its efficiency in machining parts complete. Without a guide bushing, the resulting machine, generally of fixed-headstock design, is one that can produce about 70% of the parts under 20-mm diam that are processed on Swiss–style machines with the advantages of not having to use ground bar stock and being able to use other bar shapes.

That’s just one of the trends in Swiss-style machining encompassing a range of machine offerings that reach from lower-end machines priced just under $100K to machines that, when fully tooled, can top $400K. Machine tool builders are especially effective at developing their latest machine offerings with advanced technologies that carve out machining advantages for manufacturers in industries as diverse as medical, dental, electronics, hydraulics, and aerospace, to mention the most obvious.

Trends in machine technology include bringing multiple tools to the cut simultaneously using multiple turrets, or quick-change gang-tooling slides. Using either turrets or gangstyle tooling, Swiss machines can be tooled up with enough tools to accomplish untended machining on even the most complex parts,or on the toughest materials, where multiples of tools can reduce the wear and tear on single tools that would have to be replaced often.

Parts are readily machined complete in one fixturing, eliminating secondary operations and potential error stackup resulting from multiple setups on multiple machines. Programming software is generally available from machine builders, with more complicated part programming being done using Esprit software from DP Technology or PartMaker software from Delcam.

In Marshall Manufacturing's Swiss cell, one Fanuc robot tends the four Citizen Cincom Swiss machines. Another Fanuc robot handles parts cleaning, inspection, and final hand off to shipping.

For one contract manufacturer, Marshall Manufacturing (Minneapolis), a cell of four robot-fed Swiss machines has expanded its already formidable medical-device machining capability. Marshall Manufacturing is an ISO-certified manufacturer that, in addition to Swiss machining, offers CNC turning, milling, custom bending of small-diameter tubing and wire, and metal forming.

“Medical component designers have a new and better option for sourcing their families of parts, large and miniature,” explains Marshall Manufacturing’s Tom Plantenberg. “For the right part project, this system is extremely fast and provides the desired quality at a competitive price.”

Marshall’s cell typifies the advantages of Swiss in machining precision-engineered parts. Once a process is created, the cell is designed to minimize changeover between parts. Process standardization for pre-setting cutting tools, customdesigned pick-off and ejection tooling, and CAM programming allow seamless changeover from one part to the next. In addition, the ability to switch over quickly between different longer-running jobs allows lower inventory levels while providing delivery-schedule flexibility.

Medical parts machining benefits from simultaneous machining with multiple tools in the cut on the new Traub TNL 18 sliding headstock automatic lathe and the TNL 18P production variant.

The system Plantenberg refers to is a cell comprising four Citizen Cincom L20 Swiss machines from Marubeni Citizen Cincom Inc. (Allendale, NJ), two robots from Fanuc Robotics America (Rochester Hills, MI), a parts-cleaning station, and an inspection system that is capable of producing precision families of different medical components without operator intervention. Prior to having the cell with robotic systems engineered and integrated by Productivity Inc. (Minneapolis), Marshall ran its Swiss machines untended. The types of parts that were run untended in the single machine were parts under 4″ (102 mm) in length with several operations.

In Marshall’s cell, one Fanuc M16iB 20T gantry robot tends the four Swiss machines. A Fanuc LR Mate 2008C robot handles parts cleaning and inspection, and final hand-off to shipping. Each Swiss machine is equipped with customized parts drop-offs, which have been modified with a custom parts catcher for staging the machined part prior to robotic pickup and automatic bar feeders.

The range of medical parts that Marshall produces in the cell includes catheters and surgical tools, urological devices (surgical needles), orthopedic devices (bone screws and implants, minimally invasive surgical devices like laparoscopic instruments, diagnostic devices, woundcare devices (suture anchors and clips), and dental implants and instruments.

The Swiss-based cell is designed to produce a wide range of feature-generating techniques. In addition to milling and turning, the machines in the system can broach, polygon mill, hone, knurl, burnish, hob and perform thread whirling and thread rolling. Part diameters range from 0.050 to 0.750″ (1.3–19 mm) in lengths to 10″ (254 mm). The cell is capable of processing parts in medium-to-high volumes from 20,000 to 500,000 pieces annually.

“Gang-tool-style Swiss machines emerged as a low-cost alternative to twin-turret technology 20 years or so ago. They offered an effective way to reduce changeover time, increase machining speeds, reduce cycle times, and provide the maximum flexibility with the least amount of investment,” explains Dan Murphy, regional manager, REM Sales (Windsor, CT). “Today’s gang-tool machines can be tooled up to machine complex parts with a lot of features and families of parts with similar requirements,” Murphy explains.

REM Sales has introduced two new Tsugami 20-mm gang-style models, the seven-axis S205 and eight-axis S206, aimed at the medical, automotive, and hydraulics industries, among others. The S206 has the same capabilities as the S205, with added backworking Y-axis movement. The machines can be used with an electronic guide bushing driven by an integral motor, or as a fixed-headstock machine. “The advantages of the fixed headstock for machining the large percentage of parts that are under 3x D long include reducing the remnant to a few inches, and the ability to achieve tight tolerances without using ground bar stock,” says Murphy. “The guide bushing on a Swiss machine provides the support for long, slender parts greater than 3x D in length, but a remnant from 6 to 8″ long wastes as much as 5% of the bar.”

In Swiss machining, the tiniest parts can be machined accurately and efficiently.

“Tsugami’s software is built around the requirements of a specific machine, easing the transition to Swiss machining for users who are familiar with cam screw machines, CNC lathes, or some other method of making parts,” says Murphy. “The software, which can handle about 90% of parts, simplifies programming by picking the correct processing sequence of operations for the user. He doesn’t have to know how to process a part on a Swiss machine. Typically, I can sit down with a drawing and generate the parts and graphically prove the program out in 20 min, and generate both required programs. A two-path control permits true simultaneous operation of the main and subspindle, reducing cycle times. The Fanuc 31i-A control is standard on both models.”

INDEX Traub has introduced the Traub TNL 18 sliding- headstock automatic lathe and its variant, the TNL 18P production machine. “Both models can be changed over in minutes to produce precision parts with or without guide bushing for long or short parts from bar up to 20-mm diam, with up to three tools on the part at the same time,” explains Thomas Burmeister, proposal engineer for multispindle and sliding headstock machines, INDEX Corp. (Indianapolis).

“There are advantages to machining without the guide bushing,” says Burmeister, “sometimes you get better precision, because you can only go up one quality better than the bar stock. If you use the TNL 18P as a fixed-headstock machine, you don’t have to worry about the bar stock. Your precision is dictated by the main spindle. This also allows you to use hexagonal material or other profiled barstock, something that is not possible with the guide bushing.”

Medical parts produced in the Marshall Swiss cell include catheters and surgical tools, orthopedic devices and implants, instruments, diagnostic devices, and dental implants.

The main difference between the seven-axis TNL 18 and five-axis TNL 18P is that the latter (P model) is designed for production. Simultaneous machining with multiple tools achieves high productivity levels for both machines, allowing up to three tools to be used simultaneously on two spindles. The TNL 18P production machine differs from the TNL 18 in that the Y-axis motion of the upper turret is mapped by interpolation of the X/C/H axes, permitting contour milling, and off-center, axially parallel drilling.

The swiveling subspindle is fitted to an X/Y/Z crossslide, which simultaneously also carries the bottom tool turret. Both models feature a backworking attachment that can handle seven toolholders, of which three can be driven and four stations are provided with internal coolant supply. Both machines also use a new Traub high-precision mounting of toolholders in the turret. Toolholders are seated deeper in the turret, which results in less leverage effect and higher stiffness.

The ZPS Swiss machine from ZPS America (Indianapolis) covers the range from 13 to 32-mm diam on a 4-t machine base for rigidity in continuous or series production of precision-turned parts. “As a member of the Tajmac Group, we have our own foundry that enables producing a machine with the rigidity that is required when machining even the toughest materials at 10,000 rpm at high feed rates,” explains Olaf Tessarzyk, managing partner and president.

“Stability and equal distribution of temperature are especially important for the accuracy and quality of surface finish of precision parts, when machining with up to four tools cutting simultaneously,” says Tessarzyk. “The sliding headstock provides up to a 250-mm stroke that enables part machining in succession at one collet clamping, as well as machining long parts with small diam.”

Markets that are targeted for the ZPS Swiss include medical, hydraulics, and aerospace, and those that use tough materials like titanium, Waspaloy, and chrome moly. “You can do pinch turning from both sides, mill flats on all sides, and handle parts like long steering shafts, valves, connectors, surgical instruments, implants, and shanks,” says Tessarzyk.

The ZPS Swiss has a 23-tool rackstyle station with 14 possible driven stations, enabling it to finish complex parts in one clamping. The 7.5-hp (5.6-kW) main and subspindle are identical, increasing machining capacity in rear-slide operations, and enabling handling of parts from 110 to 250-mm long through the subspindle.

The ZPS Swiss machine can run both ground and unground bar stock. “The machine has a pneumatically actuated guide bushing that keeps the spindle face/base in position and avoids twisting in the material when doing heavy milling completely in the Y axis. Or it can be unclamped for machining as a fixed-headstock machine,” says Tessarzyk. “Programming on a Swiss machine, which, in general, is more difficult than programming a six-axis CNC machine, is simplified by ZPS Plus programming software that is standard on the machine. ZPS Plus simplifies programming of the Z axis and the guidebushing relationship, offers complete simulation of the program, and facilitates importing CAD files.”

The ECAS-20T from Star CNC Machine Tool Corp. (Roslyn Heights, NY) is a 20-mm capacity Swiss-style automatic lathe designed for machining complicated parts with a variety of secondary operations. The 12-axis machine is designed with three identical, fully independent turrets that allow three tools to work simultaneously in the cut at any time. Two turrets are used for the main side operations, and the third turret is used for overlapping back work on the sub spindle.

The ZPS Swiss machine can run both ground and unground bar stock to produce these parts and more.

Each turret has eight tool positions with Y axis, and each position can accommodate from one to three tools in each station. Every station can use driven tools for any secondary operation including angular drilling/milling and thread whirling. The third Z axis on the lower turret allows simultaneous machining with opposing tools using independent moves for variety of machining including secondary operation such as 3-D machining.

With the availability of 50+ tools, the ECAS-20T machine is well suited for precision machining of complicated medical components, aircraft parts and those components made out of tough materials requiring rigidity and lot of tooling. Having three turrets with so many tool positions allows double tooling, which can be advantageous in production environment for longer running hours without tool changes.

This machine uses Star’s Quick-Change Tool System. Tools are preset off the machine, and holders are installed or swapped quickly, using one clamping bolt. Production volumes can range from low, medium to high production. A number of parts can be set up in the machine, and it may take nothing more than change of program and collets to produce different parts.

Two sliding-headstock machines that are designed to be used without a guide bushing are the seven-axis Cyclone 25/32 CS gang-style machines from Ganesh Machinery (Chatsworth, CA), The benefits are said to be faster setups, greater workpiece concentricity, and superior cutting rigidity compared with conventional Swiss-style lathes. The Cyclone 25 and 32-mm machines have a total of 27 tool stations to rough and finishmachine a feature on the workpiece. Conventional Swiss machines are limited by the length of the guide bushing and must microfeed cuts to make the rough and finish cut with one tool in one pass. These advantages are particularly evident in processing shorter workpieces with lower length-to-diameter ratios.

The main spindle of the Ganesh Cyclone 25/32 CS lathes uses 18 gangtools, comprising a bank of four live cross tools, a bank of three axial live tools, six turning tool stations, and five ID tool stations. Additional cutting tools can be used in machining by double-tooling the ID tool stations with dual-insert boring bar holders for rough and finish boring, boring and turning, and boring and threading, for example. The counter-spindle provides nine tools for machining the second side of the workpiece.

This article was first published in the May 2010 edition of Manufacturing Engineering magazine.

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