Reliable Speed for Production Machining

Makers of high-volume machining centers find ways to improve production time without sacrificing uptime.

First thing’s first: Each of the machine tool makers I spoke to about improvements in high-volume automotive machining wanted to talk about speed and flexibility—but in each case, they made clear that priority number-one for their customers is reliability.

“It’s not abnormal for a given machining center in automotive production to see upwards of 2-million tool changes in a given year,” Makino Inc. Product Line Manager, John Enberger, says. “We understand, based on significant customer feedback, that the fastest machine in the world, if it’s not reliable, is not viable for production.
 

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Makino has a significant number of machines at suppliers. As they have tailored their capacity to meet demand, Enberger says, “What that means then is that there is a very heavy reliance on the reliability of the equipment to be up and running for the planned period of time.” 
 

Enberger used features of the tool changer in Makino’s nx series horizontal machining centers to illustrate how reliability is designed in. There are  standard automatic lubrication inside the tool changer mechanism; a simple, robust single cam-driven system to execute the physical tool change itself; and “something people don’t really think about,” he noted: the shutter door that gives access from the machining area to the tool magazine opens and closes by way of a servomotor-driven ball screw rather than via a lower-cost, less-reliable pneumatic cylinder. 
“That tool changer is just one system on a machine—but if any system on a machine is broken, it’s not going to operate.”

Fewer Setups for Better Throughput
Makino’s newest offering is the a500Z horizontal machining center (HMC), designed to increase throughput by reducing the number of setups needed to complete a part. The a500Z takes the basic design of the X,Y,Z axes of the company’s nx HMC and combines it with a design that’s translated in from Makino’s d200Z and d800Z vertical machining centers. 

“It’s the technologies of our 4-axis horizontals, married with the 4th and 5th-axis designs from our high-performance verticals,” Enberger explains. The factory-built, fully integrated ‘Z-type’ fifth axis helps accommodate complex part geometries in a minimum number of workholding setups and machining operations. The novel kinematic arrangement of the a500Z “minimizes the force path lengths both through the spindle as well as through the workpiece, efficiently transferring both cutting loads and reactive forces into the machine’s three-point leveled bed,” maximizing both metal removal rates and perishable tool life.

The a500Z accommodates workpieces of up to 630 × 500 mm high, with payload weights up to 400 kg. The X and Y strokes are 730 mm and 750 mm respectively, with the Z-stroke ranging from 500 to700 mm, depending on the rotary axes positioning. Both rotary tables are of the direct drive type, with unlimited 360° movement on the B-axis, and 180° on the slant-style C-axis unit. All axes on the a500Z have closed-loop active temperature control standard.

The platform is designed for highly efficient machine movements, with rates for all linear axes of 60 m/min, and a B-axis rotation rapid rate of 45,000 degrees per minute, according to Enberger.

Reliability and Throughput Pay Off
Heller Machine Tools VP of Sales, Steve Pegram, says that the cost of making a machining center robust and accurate may result in a high sticker price, but it pays off—even in the short term. He cites, for example, the quality and size of the castings they use to build a machine, the types of motors, drives and control system: “All of these factors make a difference in quality and reliability. Even though in general, our equipment has a higher upfront cost, because of its reliability and capabilities, the ROI comes in a shorter period.”

Pegram says that in his experience, low-mix, high-volume production customers prefer standalone horizontal machines as part of a linear cell design. “If you take the production of, say, heads and blocks, most aluminum cylinder blocks need five or six processes, and we’d split those processes between individual machines. When you do multiple operations on different machines, that’s where you get your improved tact time from. 

“The beauty of these lines is, if you want to change to a different kind of part, all you have to do is change the fixtures,  the tooling and the holding mechanism on the gantry system. So, we might build a production line that’s capable of building three or four different kinds of engines.” 

Heller recently introduced its new HF series of horizontal machining centers. The series of two machines features three linear axes in X, Y and Z and two rotary axes in A and B integrated into a rotary table on a trunnion. 

“The HF machines are designed for 5-sided machining and simultaneous 5-axis machining,” Pegram says. They may alternatively be equipped with a lift-and-rotate pallet changer for series 5-sided production. He contrasts the HF concept with conventional 5-axis machining centers, noting that the former “is not only based on single-part clamping but provides the possibility of multiple clamping or the clamping of very large components such as transmission cases using ‘window-type’ fixtures.” 

The HF 5500 offers a work area of 900 × 950 ×900 mm; the smaller HF 3500 has a work envelope of 710 × 750 × 710 mm. Users can specify from a range of four spindle packages based on the material they are to process. Spindle speeds up to 18000 rpm and torque up to 354 Nm are available.

Speed in a Small Footprint
Chiron America Inc. VP of Sales, Mike Defer, says that North American customers want a CNC machine that can cover a whole gamut of parts while also maintain high production levels. “Even though the transfer lines used a generation ago lacked flexibility, they were stable and reliable—and that’s the standard the CNC machines of today are being held to,” he says. 

“The simple in-and-out of the transfer systems, bringing in a gang of drills, was quite reliable,” he notes. “If you ever had to get in there and repair a transfer line, it was challenging, but day in, day out, it was a stable process.”

The challenge for companies such as Chiron America is to maximize the stability of the CNC process. “Meantime between failures, meantime to repair, we’ve got to be at the top of the line,” he says. 

As machine reliability has improved and the value of a small machine footprint has risen in recent years, Defer has seen a trend toward more use of vertical machining centers in the high-production market. “Vertical CNCs allow smaller footprints. All across North America, floor space is becoming more at a premium,” he says.

Defer has also seen the same trend as other CNC machine builders of higher acceptance of multitasking capabilities in this high-volume sector. “This has been the biggest evolution in the last few years. We’re seeing it being more and more accepted in the North American market now.” He has seen the trend in Europe for around 10 years. 

“With our Stama brand, we’re doing milling and turning in the same machine, and able to maintain a critical relation between the turned and milled features without refixturing. There’s increasing pressure to get more and more done in one chucking,” he notes.

Reducing the number of setups is one way to increase throughput; another is, of course, to move more quickly. Defer says that the company is seeing more and more demand for speed. 

“In machining aluminum, for example, we embrace that." The DZ08 FX Precision+ is a double-spindle vertical with five simultaneously controlled axes and contact-free linear direct drives in the X,Y Z axes. These drives bring the spindles faster to their feed speeds and increase path speeds.

“With more and larger automotive structural parts being made from aluminum for lightweighting purposes, there are longer travels between cuts or between the part and the tool-changer. We’re speeding up the machine’s travels to minimize those travel times,” Defer says.

Compared to the previous model, this machining center produces aluminum components 20 to 50 percent faster, Defer says. It features two dynamic 14-kW milling spindles, capable of accelerating to 40,000 rpm in 1.9 seconds. In rapid feed, they reach a speed of 75 m/min (in X,Y) and 100 m/min (in Z). The results are chip-to-chip times of about 3.0 seconds.