Tool up for power milling

Manufacturing Engineering, Mar 1994 by Noaker, Paula M

Few people have Albert Ferrara's tough, five-axis contouring problem. The machine operator at Boeing Defense & Space Group (Kent, WA) (see lead photo) must mill a titanium panel for the Air Force's F-22 aft fuselage. When completed, the part is about 2' (0.6 m) wide x 3' (0.9 m) long and has an average thickness of 0.1" (2.5-mm). The work material is difficult to machine; the part geometry complex; the required CNC program long. Many, however, share his processing goals -- to improve CNC milling productivity, and that isn't as easy as it sounds.

The most common machining process can also be the most demanding. The following are characteristic of many CNC milling applications:

*Complex Programs. Cutting a mold for an automotive quarter panel with a 25.4-mm-diameter flat-end milling cutter can require almost 28,000 tool moves. During their research into three and five-axis machining of sculptured surfaces, Robert Jerard and Susan Li at the University of New Hampshire programmed the quarter panel test part, described graphically by 37 surface patches, in 34 minutes on a Silicon Graphics Indigo R4000 workstation. Most conventional computer-aided manufacturing (CAM) systems aren't so fast.

*High forces acting on the machine frame. During high-speed machining of a pocketed aircraft part, a tool must pass through a corner about once every 100 mm of travel. According to Professor J. Tlusty at the Machine Tool Research Center, University of Florida (Gainesville, FL), accelerations on typical milling machines are about 2 m/sec sup 2 . Passing through a corner with a feed rate of 0.17 misec takes about 0.5-1.0 sec; traversing between corners 0.6 sec. Therefore, Tlusty estimates cornering accounts for 38-54% of cutting time. With this in mind, machine designers should minimize mass of the machine's moving structures and draw the highest torque possible from the servomotors, especially to handle tight-tolerance machining. Tlusty is currently designing a new three-axis horizontal milling machine with lightweight X and Y-axis motion components, features machine tool builders often don't pay enough attention to.

To optimize CNC milling, you must work with its basic building blocks: the machine tool (often a CNC machining center), programming, and tool management. Your machining application will only be as efficient as the weakest of the three components.

SMART MACHINE MOVES

Machine tool builders now offer buyers a somewhat bewildering array of CNC machining center configurations. Consider the options for both one-setup five-sided machining and five-axis complex contouring.

One way is to use a table-on-a-table construction, where a rotary-tilt table adds a fourth and fifth axis of motion to a conventional three-axis vertical machining center (VMC). Several large and small-VMC builders use the technique. "You give up some rigidity for added flexibility," says Yusuf Venjara, general manager, engineering, Hitachi Seiki USA Inc. (Congers, NY), "but the tradeoff is usually minimal for smaller parts. Moreover, users can reduce cycle times by machining five sides of a part in a setup, as well as doing some limited contouring."

Hitachi Seiki's VK Series VMC has several design features tailored for high-speed milling. A long-nose spindle provides good clearance at the part. There also is 5-sec tool-to-tool swing-type automatic tool change and 20-tool storage expandable to 120 tools. The no-overhang tables edges never move beyond the bed to improve rigidity. In addition, the spindle never moves outside the table dimensions. Positioning accuracy is +/- 0.0003" (0.008 mm), and repeatability is +/- 0.00008" (0.0020 mm).

"Thermal distortion also can be a problem during high-speed milling," says Venjara. "The VK design, however, relies on pretensioned ballscrews anchored at both ends and centered between the guideways to eliminate thermal effects on X, Y and Z axes movements. High-accuracy linear motion guideways have zero-clearance bearings with automatic lubrication to maintain accuracy over a wide temperature range."

Another example of a table-on-a-table configuration is the VF-4 VMC from Haas Automation (Chatsworth, CA), which has a standard fourth axis and provides a fifth-axis option using a rotary table. Unlike other VMCs in its price range, this machine provides expanded travels of 50 x 20 x 25" (1270 x 508 x 635 mm) to accommodate large parts, fixtures, and tools. Options include a chip conveyor and a Fanuc TM compatible 32-bit control with up to 1800' of expanded memory and user-definable macros.

Maho Machine Tool Corp. (Naugatuck, CT) provides five-axis simultaneous processing on its dual-spindle universal machines without stacking a table on a table, says s company vice president Mal Sudhakar. Its C-series machine, for example, uses built-in rotary tables -- either an NC rotary table, two-axis NC table, or NC rotary/tilt table. Some models also provide fifth-axis simultaneous processing using a built-in rotary table and a C-axis vertical head with programmable contouring of +/- 60 deg left to right. A built-in encoder provides angular measurement resolution of 0.001 deg.