Tapping Top Productivity: Medical Manufacturing

Courtesy of Both photos: Methods Machine Tools

The Med Cell configuration of the Fanuc RoboDrill uses a 6-axis robot and features a custom drop-trunnion rotary table. See description below.

Machine tool builders discuss the development and evolution of drill/tap machines.

About 30 years ago, development began on a special breed of CNC machine tools called tapping centers, or drill/tap machines. With 30-taper spindles and generally less than 10 hp, the machines were simple, compact and lean. Engineered primarily for light- to medium-duty work in the Z-axis plane, the machines were used primarily for high-volume drilling and tapping. Their lightweight components facilitated fast rapid traverse and high feed rates.

Yamazen Inc., Schaumburg, Ill., distributes the pioneering Brother line of drill/tap machines. Jim Hansen, Yamazen senior vice president, said Brother first developed TC (tapping center) products in the 1980s for internal use in the manufacture of sewing machines, typewriters and other products. Later versions of the machines were sold to the general manufacturing marketplace.

“There are certain things you can do with the 30-taper platform that you can’t do with a 40- or 50-taper spindle,” Hansen said. “Because the spindle has a smaller mass, it can accelerate quicker. Machines of this style were originally called tapping centers, but as their function has expanded, people often just changed the vernacular to 30-taper machines.” As the machines evolved, their capabilities grew to include increasingly heavier drilling and tapping, moderate milling, additional axes and automation.

Because of volume requirements, automotive and consumer electronics were traditionally the largest markets for drill/tap machines. “But that’s changed,” Hansen said. “We sell them to virtually every industry. The market has shifted from mostly high-volume applications to general-purpose machining as well. The difference between drill/tap machines and VMCs has blurred significantly.”

One machine that incorporates characteristics of vertical machining centers and drill/tap machines is the Fanuc RoboDrill machining center, imported exclusively into the U.S. by Methods Machine Tools Inc., Sudbury, Mass. Steve Bond, national sales manager for RoboDrill, RoboCut and EDM products, traced the RoboDrill’s origin to the Fanuc (then Fujitsu) Drill Mate, developed in the early 1980s. “That machine was strictly for drilling and tapping,” Bond said. “It was basically a manual machine to which Fanuc added a CNC.” The Drill Mate had turret-style tool management. “It would just rotate a tool into position, then drill the holes,” Bond said. “The machines were so reliable that customers would challenge them to do different things.”

Courtesy of Both images: Brother

To speed workhandling by reducing the noncutting time when exchanging pallets, the TC-R2B tapping center from Brother has a servodriven Quick Table that rotates 180°, like a Lazy Susan (top) that permits workpiece changes in 2.3 to 3.4 seconds.

For the RoboDrill that followed the Drill Mate, Fanuc developed faster controllers that enabled rapid X- and Y-axis (milling) movement. Further control improvement permitted 3-D contouring. Spindle updates, including the addition of dual-contact (BIG Plus) spindles, enabled the machines to handle heavier machining and the greater side loads of milling. Fourth-axis tables permitted part rotation, increasing the number of operations that could be performed in one chucking. Various options and configurations are available, including a Job Shop Cell that performs 4-axis machining and, with workhandling via a 6-axis Fanuc 200iC robot, facilitates unattended operation in job shops. A Med Cell configuration, also utilizing a 6-axis robot, features a custom drop-trunnion rotary table to permit 5-axis machining.

Bond noted that more than 75 percent of the several hundred RoboDrills Methods Machine Tools sells yearly are shipped with performance-enhancing options and upgrades, such as fourth-axis capability or robotic automation. As a result, he said, the machines are essentially compact, fast machining centers.

New Approach

In contrast to machines that evolved from legacy drill/tap machines, the DT-1, a drill/tap machine with full milling capabilities, represents a clean-sheet-of-paper approach. Introduced in 2009 by Haas Automation Inc., Oxnard, Calif., the machine has a direct-drive, 15-hp, 12,000-rpm spindle. Scott Rathburn, marketing product manager, said the main development driver for the DT-1 was user interest in a machine with a compact footprint and high-speed capability.

A key design element is chip evacuation from the rear of the machine. This feature is aimed at shops using drill/tap machines in high-volume production, sometimes putting as many as 100 side by side in rows. That arrangement is typically seen in Asia, Rathburn said, in the manufacture of consumer electronics and automotive parts. To enable tight side-by-side machine placement, chips must be disposed behind the machines, not the sides. For the same reason, the DT-1’s door does not protrude from the side of the machine when open, so the machines can be placed close together.

Developers seeking to upgrade drill/tap machines face a dichotomy between versatility and performance. “We can make a machine incredibly fast if we limit it to doing one thing in the most productive way possible,” said Yamazen’s Hansen. “If we start to broaden the machine’s capabilities, typically we have to make some compromise in performance to increase versatility.”

Courtesy of Haas Automation

The Haas Automation DT-1 drill/tap milling machine features a 30-taper, 12,000-rpm spindle.

Toolchanging alternatives illustrate Hansen’s point. The first tapping centers featured drum-style toolchangers, which, according to Hansen, are the fastest way to present a tool to the workpiece. The limitation of the drum-style changer is the number of tools the drum can hold. “The maximum on our drum-style machines is 21 tools,” he said.

As the cutting capability and the types of work drill/tap machines could handle increased, so did the need for tool selection. Making more tools available, however, can slow tool changes. For example, the 40-tool magazine of Brother’s TC-32BN enables machining of various workpiece features. Tools are switched with a twin-arm changer, which is somewhat slower than a drum-style changer. The machine’s chip-to-chip tool-change time is 2.0 seconds compared to the 1.6 second chip-to-chip tool-change time for the Brother TC-S2DN drum-style-changer machine.

On the other hand, Brother’s TC-20B machine demonstrates the advantages of specialized designs. Focused on speed, the 15-taper machine has low-mass components that exhibit minimal inertia, and its high-speed, compact toolchanger permits chip-to-chip times as low as 1.1 seconds when using small tools.

Regarding ongoing development, Hansen said, “Tapping centers are morphing into what I call near-net-shape machining centers. Through the use of larger tools and with more horsepower, they are becoming more efficient in removing higher volumes of material.”

Machine efficiency has improved via upgrades to a variety of machine components. The changes include advances in spindle technology such as dual-contact spindles to maximize rigidity, and through-spindle coolant.

Perhaps the greatest single productivity improvement is increased horsepower. “Back in the early days of TCs, standard power specifications for a typical tapping center would be 5 to 7.5 hp,” Hansen said, adding that today’s products have 13.5 hp.

The increases continue. In August 2011, Brother is introducing new, higher-powered models of existing machines. Called TC-S2DN High Torque machines, the new models boost the cutting capacity of Brother’s TC-S2DN machine offering. Peak machine horsepower grows from to 35 hp, and torque to 68 ft.-lbs.

Hansen said the maximum recommended drill diameter in carbon steel with the standard TC-S2DN machine is 25mm, while with the new TC-S2DN High Torque machine that maximum diameter increases to 30mm. Similarly, with the standard machine, M16-size taps are the maximum size recommended for carbon steel; the high-torque models can handle M27 taps. Despite the improved capabilities, the machine footprint is similar to prior models, Hansen said, and the new high-torque versions retain the prior models’ fast rapid traverse, feed rates and tool change times.

New Applications

As drill/tap machines gain in capability, end users are finding new ways to apply them. Said Haas’ Rathburn, “We are always surprised by what our customers do with DT-1 machines.”

New consumer electronics applications, for example, include high-speed milling of parts from aluminum billet. Users take advantage of the machines’ ability to produce fine surface finishes by cutting at high speeds and light DOCs. Rathburn noted that some shops exploit the machines’ speed and precision in making small components for the telecom industry, while others are drilling, tapping and spotfacing near-net-shape castings for automotive parts.

Although the machines are popular for high-volume applications, Rathburn said, “There are job shops that use them as well.” (See sidebar below.) The machines are versatile, but they may not be the primary machine in a job shop, where the mix of product size and material may vary. However, the DT-1 and machines like it can be very effective in handling secondary operations. “You could easily use a VMC to do rough hogging, then move the part into the DT-1 for drill and tap and finish work,” Rathburn said.

The DT-1 machines employ a scaled-down version of the same rigid spindle design used in Haas’ 40- and 50-taper VMCs. The DT-1 machines do not have quite the milling capacity of those larger machines. “But they are full 3-axis machines that can be upgraded to 4th- and 5th-axis capability,” Rathburn said.

Saving Time

In the quest to reduce cycle times, reduction of noncutting time is as important as speeding up cutting operations. Said Yamazen’s Hansen, “Assuming we can push a tool to its maximum cutting capability, the only machining variable becomes noncutting time, including tool-change and workchange times.”

To reduce the time picking up and exchanging pallets, Brother developed a servodriven Quick Table that rotates 180°, like a Lazy Susan. The system can execute a workpiece change in 2.3 to 3.4 seconds, according to Brother. Because the system is servodriven, it can change the work simultaneously with toolchanger or axis movement.

Exchanging workpieces raises issues of repeatability, especially in high-precision applications such as 5-axis medical parts machining. According to Methods’ Bond, successful 5-axis machining depends largely on accurate and repeatable location of the parts being machined.

To maximize repeatability, the RoboDrill Med Cell incorporates a zero-point pallet system employing 70mm-square pallets like those used in an EDM. The workholding pallets have a centrally located drawbar or stud drawn into a machine-mounted chuck. The system provides consistent positioning and repeatability within a few microns. Workpieces are mounted to the pallet and the robot fits the pallet into a chuck on the machine. After the part is machined, the robot removes the pallet and replaces it with another. The zero-point pallet locking system assures consistent location of each succeeding part. (See sidebar below.)