Sometimes it pays to work backwards

Nov. 1, 2000


Instead of a vertical gantry design, Makino went with a horizontal configuration for its MAG4 aerospace MC.

Shops can create MAG4 cells that include automated pallet handling and an elevator system for vertically storing stock material.

Instead of first building a machine and hoping it meets the needs of customers, Makino worked backwards to develop a not-sotraditional machine tool specifically for the aerospace industry. The Mason, Ohio, builder started out by interviewing various shops involved in aerospace, asking about such things as most-used tool sizes, speed and power requirements, and, most importantly, the specifics of part machining. So before it even fired up its design software, Makino had a pretty good idea of what would and wouldn't work.

What aerospace shops wanted was a machine geared for high productivity to reduce both cut and non-cut times. Also, one with a modular design providing flexibility by being field-upgradable. All this Makino could supply. However, what wouldn't work for its dream machine was a vertical-gantry configuration.

The new machine, the MAG4, is a five-axis horizontal machining center that cuts large, monolithic, aluminum aerospace components such as wing-boxes, fuselage sections, and rudders. These parts are most commonly machined on gantry-type machines with multiple spindles, but, according to Makino, this approach is somewhat labor intensive for operators. They can spend excessive time on top of machine tables sweeping chips, spraying off coolant, and loading/unloading parts. A horizontal configuration, says Makino, makes it possible to develop machine features that not only minimize these tasks but also maximize spindle usage for high productivity.

Reducing cut time
Makino reduces MAG4 cut times by zeroing in on both high-speed and high-power machining. It accomplishes these things by using a high-speed spindle tough enough for roughing operations, a special C/A-axis articulating head, and feedrates matched specifically to aerospace-type machining.

The MAG4's spindle is built to 40,000-rpm parameters, but Makino limits it to 30,000 to en-sure performance. Spindle bearing life is long — at 30,000 rpm, Makino estimates 9,000 hr. "That's 3 the life of similar 30,000-rpm spindles," says Steven Colston of Makino's Aerospace Group.

For consistent power and torque, the spindle packs 60 kW, and its main bearing I.D. is 75 mm, which is actually larger than the machine's HSK 63 tool interface size. There are five bearings in the spindle: two in the front, which are lower bearings; one set of roller bearings in the rear; and two angular-contact bearings.

Makino improved upon its successful roller bearing-type spindle design for the MAG4. What it came up with is a hybrid between a cylindrical roller and a ball bearing. Ceramic material covers the roller element, and the race is made from a special steel that minimizes heat. Also to control heat, the machine includes both an inner and outer cooling system.

MAG4 spindles are compact, measuring 150 mm in diameter and 200-mm long. This small package makes for quick and easy maintenance because, according to Colston, high-speed spindle bearings require frequent changes. With Makino's system, shops can exchange spindles themselves. It's basically a plug-and-play design in which the spindle's cartridge simply plugs into the housing.

Another, more important reason for the compactness of the spindle is to accommodate the MAG4's articulating head. This head maximizes cut times because it rotates infinitely in the C axis. Typical heads do rotate 360°, but at some point, the operator must stop to unwind cables or wires that connect the head to its power source. Makino's head, on the other hand, delivers unobstructed rotation. It accom-this through a slip-ring design that transfers 400 V to the head without using external connections.

The head's A axis, which is dual-gear driven, moves in a wrist action. Makino extends its reach by adding an extra 10° beyond 90° in each direction to ±100°.

Makino determined the MAG4's feedrates based on two popular machining approaches used by shops producing aerospace components. The first is high-speed cutting, which is light radial or axial depths-of-cut, while the second, more popular approach, is maximum-force cutting. Aerospace shops use this technique, which is taking heavy radial and axial depths-of-cut, to hog out large amounts of material from huge monolithic parts.

To handle high-speed cutting, the MAG4's X, Y, and Z axes rapid traverse and cut at a feedrate of 40 m/min. According to Colston, this is more than enough to also handle maximum-force cutting, which usually requires about a 20-m/min cutting feedrate. plishes

In addition to feedrates, Makino again relied on information from the aerospace shops to decide on the best acceleration/deceleration rate for reducing cut times on the MAG4. According to Colston, these shops typically use end mill diameters around 25 mm, at 117 kg of force, with cutting torque of 1.5 kg

of force. Makino matched these parameters to a 0.5-G acceleration/deceleration rate. Why not faster? Because, as Colston explains, "increasing the acceleration/deceleration rate from 0.5 G to 0.6 G for milling a standard pocket results in diminishing returns as far as cycle-time reduction is concerned. So, why go with a faster, unnecessary rate and pass the added expense along to the customer when it re-ally doesn't present a significant benefit?" In addition, he says, if that rate could increase to as much as 2.5 G, it would not significantly reduce cycle times.

MAG4 machine components efficiently and accurately move at these rocketlike feedrates and acceleration/deceleration rates thanks to another Makino hybrid design that combines linear-motor and ballscrew technologies. Dual ballscrews are on the Y and Z axes, but for long X-axis travels, Makino uses linear motors. It does this because ballscrews, beyond a certain length, do not operate effectively, says Colston.

The linear motors are on the upper and lower portions of the carrier unit, with the primary and secondary magnets mounted overhanging at angles. The tracking force of the linear motor pulls up, while the weight of the moving mass pulls down. This setup is the same at both the top and bottom, balancing the system. The overhanging design protects the system from contamination. Also for less interference during high-speed machine movement, Makino minimizes its use of covers, cables, belts, hoses, and so forth. Instead, it opts for such things as wipers and air seals.

Reducing non-cut time and adding flexibility
The biggest chunk of non-cut time for any machine tool is part loading and unloading. Makino reduces, if not eliminates, this noncut time by keeping the MAG4's spindle working during part setups. Once a raw part moves into the machine and a finished one moves out, operators can then clear away chips and coolant, un-load the part, and set up for the next one, all while the machine is running. This is possible by incorporating a work indexer, which operates much the same way as a typical HMC pallet changer.

The unit indexes 180° and accommodates different types of workfaces such as two and three-sided or square. Pallets for the system are as long as the machine's X-axis travel and ideal for the long thin-plate parts commonly run at aerospace shops. Also for such parts, shops can choose different fixturing such as vacuum or tapped holes.

Servomotors and induction gears drive the work indexer from both sides. Hydraulic cylinders lift pallets, while tapered cones precisely locate them onto the machine table. In addition, the MAG4 receives pallets from overhead.

This arrangement, says Colston, makes for a smaller machine footprint as compared to other automated pallet systems. A carrier mechanism picks up the raw material, brings it to the work indexer, rotates the piece 90°, and unloads. Along with this, shops can install an elevator system for vertically storing stock material.

To increase user flexibility, Makino went with a modular design for the MAG4. This lets shops increase X-axis table length by 6-m increments without having to remove the machine from the shop floor. The smallest MAG4 table size is 4 m, and that is field-upgradable to 16 m. The machine's other travel lengths are 2 m in Y and 0.7 m in Z.

Also to make things easier for the customer, Makino wires MAG4s using an AS interface, converter, and modules that plug into a common cable running throughout the machine. This cuts down on the time spent conducting diagnostics and wiring the machine. It is also easier for shops to trace problems because there is no mass tangle of wires. All the machine has is a bus-line cable and a power-assist cable, and the modules simply clip into place without the use of tools.