Managing multi-axis manufacturing

As composite parts have become more common on commercial aircraft, especially with the advent of the Boeing 787, the need is great to mass produce parts for aircraft with significant portions of their airframes built from carbon composites. Much early computer numerically controlled (CNC) machinery served short-run military and space programs where cost per part was too high to be acceptable in the manufacture of commercial aircraft. But machine manufacturers increasingly have been asked to build machines that could be involved in routing mold tools, and trimming and finishing demolded parts for hundreds of aircraft — including not only flaps, stringers and stiffeners, but wingskins, wingboxes and fuselage barrels. Yet the machines must do so with the same care and attention to detail the parts once received during manual operations performed by highly trained technicians. The resulting technology is complex, sophisticated and can be quite large as well.

Leveling up

Today there are three levels of CNC machinery, says Dave Steranko, president of Anderson America (Charlotte, N.C.), the U.S. subsidiary of the Taipei, Taiwan-based Anderson Group. “There is the ‘jobber’ machine, a small-configuration, single-spindle unit. Then there are the heavy, larger production machines that can handle a wide range of products.” The latter have wide “use function,” that is, they can process a wide variety of workpiece shapes and sizes. These machines, say Steranko, “don’t have ‘sprint speed,’ but they are, in the long term, less expensive to operate because the use-cost (cost of the machine amortized over program years less any cost recovered when a machine is sold off) is lower.” The third type is the customized or purpose-built machine. “This machine is typically built for a particular job,” Steranko explains. It is ultimately a less complex machine, designed to operate within a narrower window of size/shape variation and, therefore, capable of greater production speed than the “production” machines, without sacrificing accuracy. Large manufacturers go for these because their use-cost also can be smaller, in the long run, than a jobber machine.

A recent HPC survey of CNC machinery trends turned up the following diverse group of automated systems in the production and custom categories that illustrate several paths that machine manufacturers are taking to meet customer expectations for part quality, production speed and optimum use cost. Doubling capabilities A common feature on newer large-item machining equipment is twin-part processing. These machines are equipped with identical fixturing systems that permit one part to be machined while part offloading/loading and other functions take place on the other. The additional cost of the second processing station ultimately is offset by reducing machine downtime. CMS North America Inc.’s (Caledonia, Mich.) Ares Series CNC machining centers, for example, have twin shuttle tables available as an option. Work areas range from 12 ft by 4.5 ft (360 cm by 135 cm) up to 19.5 ft by 7.7 ft (585 cm by 231 cm) with multiple zone configurations for either pendular production (that is, work done by a spindle suspended over the work space and capable of moving between zones) or single-work area production. Its liquid-cooled spindles, 8 hp or 16 hp, have fast tool-change capability. The machines have Fanuc or OSAI numerical controls with PC Office front ends ease of use and Ethernet connection.

When CMS was approached by Brookhouse Composites (the largest independent composites manufacturer in the U.K.) to quote on a machining center to provide 5-axis machining of a complex carbon composite part, CMS proposed a fully enclosed Ares 3626 APC 5-axis machining center with a 12-kW, 24,000-rpm spindle and twin 1,560 mm by 2,020 mm (61.4 inches by 79.5 inches) vacuum tables that can be electronically linked to function as a single 3,120-mm by 2,020-mm (122.8-inch by 79.5-inch) table, enabling either tandem or single table work. The system shuttles tables out of the working zone for load/unload operations. Automatic doors with interlock ensure that the operator is completely protected from the cutting area. This machine has a 3.6m x-axis by 2.6m y-axis by 1.2m z-axis (11.8-ft by 8.5-ft by 3.9-ft) processing envelope. Machine slideways and electrical components are protected against the ingress of carbon dust, and its high-velocity dust extraction system features a collection hood that rotates with the head’s axis movements.

In a variation on the “twin” theme, machining centers also can include not only two workstations but two or more spindles as well. When Hindustan Aeronautics Ltd. (Bangalore, India), a huge aircraft manufacturer whose current projects include the uplock box assembly for the Boeing 777, overwing exit doors for the Boeing 757 and bulk cargo door for the Boeing 767, needed CNC machining centers with the capability to machine identical or mirrored aerospace structural components, CMS responded with a gantry-based Poseidon series machine. The Poseidon’s TWIN ram each house a 30 kW, 70 Nm 2-axis spindle with a high feed rate and acceleration, suited to high-volume materials removal. The Poseidon is particularly suited for producing patterns and prototypes and trimming large aluminum or composite components. Work areas range from 9.8 ft by 8.5 ft by 4.3 ft (294 cm by 255 cm by 129 cm) up to 100 ft by 28 ft by 13.1 ft (3,000 cm by 840 cm by 393 cm) with even longer work areas available upon request. The spindle capacity is 16 hp (standard) or 20.1 hp and 37.5 hp (optional).

Another key trend is multitasking. Dan Wiggin, president of Hendrick (Salem, Mass.) notes high interest in multi-axis machining centers with automated tool changing systems that enable a single machine to perform cutting, trimming and drilling functions. Hendrick’s gantry-based 50ZSV, a heavy-duty, large format 5-axis CNC router, is available with a wide range of table sizes and z-axis strokes. Its fixed vacuum table and moving gantry head configuration can handle extremely long workpieces. By contrast, Hendrick’s 60ZXSV 5-axis router features a moving vacuum table and fixed gantry, with either one or two tables. Both machines can be fitted with 4- and 5-axis saw heads, 4-axis planer heads and multiple (two or more) head configurations. Automatic tool changers are offered as an option. The company also regularly builds custom machines for specialized or part-specific production scenarios.

Automated tool changing, although still an option rather than a standard feature on most machines, is commonplace today. Anderson America offers custom-built add-ons for machines, including tool magazines for multi-axis machining centers that hold from eight to as many as 32 tools. The company’s EXXACT Plus Duo Series’ top model offers twin-table capability and features four spindles. It has two 10-hp, air-cooled Andi HSK 63F routing spindles that are capable of a maximum 24,000 rpm, each with a 10-tool toolchange carousel. Two additional spindles handle drilling and two-way end boring and also accommodate grooving saws. The spindles are mounted over two 1,840 mm by 1,600 mm vacuum tables (72.4 inches by 63 inches). Maximum processing speeds in the x/y axes are 3,150 inches/min (80 m/min) and in the z axis, 787 inches/min (20 m/min). Its x/y/z working envelope is 4,400 mm by 1,775 mm by 350 mm (173 inches by 70 inches by 13.7 inches).

CMS North America offers auto toolchange as an option on its 5-axis Poseidon and Ares CNC machining centers. Its Poseidon machining centers can come equipped with liquid-cooled automatic toolchanging racks with tool-holding stations designed to store and interchange from eight to as many as 40 different tools.

Quintax CNC Routers (Stow, Ohio) offers its Quintax Dual Carriage 5-axis Router. It features two identical vacuum tables that can be served by its 5-axis head, with a part being cut or drilled while another is prepped for processing on the other table. However, the machine includes what the manufacturer calls a “tag-along” 3-axis spindle that is capable of duplicating the operation of the primary 5-axis spindle in cases where users desire to double output by processing parts on both tables simultaneously. Machine features include two 5 ft by 5 ft (1.5m by 1.5m) tables, a primary carriage with a 5-axis, 10-hp spindle and a secondary carriage fitted with a 3-axis, 7-hp spindle. Both spindles are rated to 24,000 rpm. The machine is available with one or two eight-station, rotary traveling toolchangers.

Bridging the CAD/CAM gap

Hendrick’s Wiggin says that a positive trend in the production machine market is the vastly reduced expense of programming the machine software for each individual part. Historically, one of the most significant challenges for manufacturers of 4- and 5-axis trimmers and routers has been the complexity, and therefore the great expense, of programming. Wiggin points out that two methods have been developed to mitigate cost. The first applies the concept of reverse engineering. A master model or prototype is placed within the machine’s operating envelope, and the head traces over the part surface or trim outline while the machine inputs the coordinates into the software. These become the numerical code for the tool path. “The machine essentially becomes its own digitizer,” says Wiggin, “enabling the machine to write its own program.” Third-party providers of laser digitizing equipment provide the same capability without involving the machine, which keeps the machine running.

Less time consuming, however, are solutions offered today by third-party software providers, who write programs designed to bridge the gap between CAD design programs and computer-aided manufacturing (CAM) programs. The software reads the CAD data on the part geometry and then, in the CAM software, it develops the machine tool paths, and a postprocessor converts the path into numerical code to operate the machine. “This third-party software has, until very recently, been very expensive for the 4- and 5-axis machinery because of the complexity of the tool path information,” says Wiggin. “However, quite recently pricing has come down to where it’s affordable even for the small shops.”

On the more expensive machinery, machine builders often include “bridge” software in the package. CMS North America’s Ares 3626 system, for example, comes with software that is capable of converting into machine code the customer-supplied models generated in CATIA CAD software (supplied by Paris, France-based Dassault Systémes). Similarly, Quintax CNC Routers maintains a business alliance partnership with Mastercam (Tolland, Conn.), a supplier of CAD/CAM “bridge” software for multi-axis systems.

Custom, part-specific, purpose-built

As composites enable aerospace manufacturers to implement parts consolidation strategies, the size of primary aircraft structures continues to increase. The massive tools needed to fabricate these parts require milling machines with enormous work envelopes yet great accuracy over the huge distances that spindle heads must travel. Forest-Liné, (Clifton, N.J.) and its subsidiary, Canada-based Liné-Machines (formerly Henri Liné Machines), supply some very large gantry machining centers that are custom built for machining mold surfaces. Forest-Liné manufacturer’s rep Carroll Grant reports that the Liné-Machines Powermill V is the latest generation of large, gantry-based milling machines specifically designed to mill, drill, bore and tap components made of aluminum, composites and Invar materials. A typical Powermill envelope is 60 ft (x axis) by 20 ft (y axis) and 7 ft of travel on the z axis (1800 cm by 600 cm by 210 cm). To efficiently form the large tool surfaces, the system employs linear drives on the main axes (x, y and z) that can move the milling head at speeds up to 2,000 inch/min (5,080 cm/min) with 0.5G of axis acceleration. The lightweight and rigid design of the gantry’s mobile structure reportedly allows smooth movement that enables close-tolerance accuracy and good surface finish while machining fine details at a high feed rate. The company maintains that Powermill can achieve twice the metal removal rate of conventional gantry milling machines and reports that customers who use this equipment have machined Invar at speeds of 1,200 inch/min (3,048 cm/min).

Forest-Liné’s European operation (Paris, France) builds a similar machine, the Linear Minumac, with linear drives on the gantry’s x, y and z axes, capable of spindle speeds as high as 30,000 rpm.

Mike Reese of Ingersoll Machine Tools Inc. (Rockford, Ill.) cites as an example of its capabilities an Ingersoll Gantry Machine that was manufactured for Mitsubishi Heavy Industries (Tokyo, Japan) to shape Invar layup tools for a Boeing 787 wingskin. The Mastercenter-Vertical from Ingersoll is available as a fixed rail, adjustable rail or open side version. Complex 5-axis sculpturing cuts are performed by the Masterhead, the machine’s nutating spindle unit, which means the spindle has a slight, intentionally irregular off-axis motion during travel, resulting in a smoother workpiece surface.

Masterhead was developed to perform a full range of routing functions on five sides of a workpiece in a single setup. The machine is provided with a broad choice of automatically changed spindle units, including various straight, angular, universal and gimbal configurations as well as a 2-axis CNC spindle unit for 5-axis contouring that permits the cutting tool to be positioned at any angle to the workpiece.

Cincinnati Machine (Hebron, Ky.) has expanded its HyperMach series of High Rail Linear Profilers by adding a drilling and trimming platform. The machine reportedly meets the speed and accuracy demands for composites or metals. In a system dedicated to automated hole drilling in, for example, aircraft wings, forward fuselage elements and other large aircraft sections, the equipment is available in single- and dual-head configurations. The dual head configuration’s spindles each are accompanied by a large-capacity, automated toolchanger, and each head features a Siemens 840D controller. Each of the system’s two gantries will extend to the entire dimensions of the component. A pressure foot is provided to enable precision countersinking of holes. In operation, fixtures are brought to the machine via an automated transport system. A special device marks centerlines, an important function in proving out a part program. A dust collection system prevents airborne dust and debris from interfering with machine operation.

Purpose-built machinery is, by definition, highly specialized. But Northrop Grumman (El Segundo, Calif.) recently turned to Spanish design and manufacturing firm MTorres (Torres de Elorz, Spain and Santa Ana, Calif.) to develop an automated system for drilling a single part, the F/A-18 E/F jet fighter’s vertical stabilizer. The resulting vertical automated drilling (VAD) system is a complete, turnkey, CNC-controlled manufacturing cell that drills and countersinks more than 1,000 fastener holes of various diameters in the part. The system also performs several other fabrication processes, previously done in separate operations, into one line for much faster overall production (see “Related Content,” at left).

The machine includes two drilling heads that can drill in unison, or alternatively each can be programmed to work independently. The design for each drill head incorporates tilt, rotary and feed or drive axes of motion in addition to vertical and horizontal (lengthwise) motion along the machine’s support frame. Each drill head can achieve speeds of 1,016 cm/min (400 inches/min) in the drive axis and up to 3,048 cm/min (1,200 inches/min) along the x axis, that is, parallel to the work cell, as it moves from hole to hole.

Special Section: Purpose-built laminators

Part-specific construction is not limited to CNC routing and drilling machinery. Builders of tape laying equipment are trending toward “purpose built” equipment as well.

Accudyne Systems (Newark, Del.) doesn’t have a line of “standard” machines. Instead, the company focuses on development of one-of-a-kind equipment and processes. The company observed a market need for what it calls “part-specific” custom-automated equipment to fill a gap that it believes is left open by the large tape- and tow-placement equipment manufacturers, says Accudyne’s John Melilli. The company’s approach to design is to target machines to the various small- to medium-sized composite parts that will be built on emerging commercial aircraft programs. The large quantity of each part that will be required justifies the cost of a specialized machine that is dedicated to fabrication of each part type.

Accudyne recently completed a stringer laminator with a stationary vacuum table and moving head assembly, designed to build long thermoset laminates for the aircraft industry. The machine forms 12-inch/30-cm wide laminates up to 36 ft/11m long. The laminates are used to make stringers, which are hot-drape-formed in a secondary process. The carriage holds three different rolls of thermoset prepreg fabric and two film rolls, and it moves 50 ft/15m under closed-loop servo control. Material heads are individually controlled and include position and tension control of the web, transverse cutting, and heating to ensure tack. Cutting of the uni tape is accomplished by using a stylus cutter, without cutting through the backing paper. The system reportedly has demonstrated laydown speeds of 100 ft/min (30.4 m/min) with edge alignment of ±0.010 inch (0.0254 cm) and add/cut accuracy of ±0.030 inch (0.0762 cm). The laminator also features a turret loading system to automate material changeover.

Likewise, Automated Dynamics (Schenectady, N.Y.) recently delivered a large gantry fiber placement machine specifically designed to layup spars for wind-turbine blades. The original hand layup process used for the part was unacceptable for long-term production with more advanced material systems, says the company’s Jim Martin. Reportedly, it can produce spars up to 50m/164 ft in length. The creel carries 12 unidirectional thermoset prepreg spools, each weighing as much as 750 kg/1,650 lb. The entire creel and head assembly, weighing 5,000 kg (11,000 lb), is picked up by and rides with the gantry, much like a toolchanger. Nevertheless, this heavy assembly reportedly can traverse at a rapid 1 m/sec (3.3 ft/sec) and process at 0.5 m/sec (1.65 ft/sec). The gantry is wide enough to bridge two tools. The gantry traverses from the root to the tip of one blade spar tool, placing six 31-mm (1.24-inch) tapes in one direction, then it places six more as it travels back in the other direction. Each prepreg tape is individually fed, clamped and cut, providing for precise placements of complex ply edges. The head is mirror-imaged, so the tapes feed in from both sides but share the same central compaction roller at the part surface. In this manner, the machine achieves bidirectional processing without rotating the massive creel/head assembly. When empty, the creel and head are jettisoned and replaced with another creel/head assembly, resulting in less than 30 minutes of downtime. When the placement operation is complete on one spar tool, the head moves laterally to the other station to continue the placement operation on the other tool. The first tool then can be vacuum bagged and oven-cured in place. The machine is programmed and controlled through Automated Dynamics’ proprietary FPM offline software, FPS online software. The customer imports solid model data generated in CATIA CAD software (Dassault Systémes, Paris France) to FPM, and the programmer then generates the fiber paths by interacting with a series of menus and tables.