Transfer lines get flexible

Manufacturing Engineering, Jan 1999 by Owen, Jean V

Traditional transfer lines, built for high volume and low flexibility, are the mainstay of high-volume manufacturing, but efficient use of dedicated equipment requires accurate forecasting of product changes and consumer demand.

"Engineering and constructing a dedicated line takes a long time," says Neil Coughlin, vice president, Kingsbury Corp. (Keene, NH). "If production is estimated at 200,000 and only reaches 50,000, automakers have spent a lot of money on capital equipment that's not used."

Ten or twenty years ago, US automakers built dedicated lines for all their products, says Bill Kalp, director, Detroit Operations, Ingersoll Milling Machine (Troy, MI). "If everybody wanted V-8s, the plant cranked up the V8 line. If they didn't, it sat there, underutilized. No automaker today can afford to make that kind of side-by-side investment. If the energy crisis hits again, and customers start demanding four cylinders, automakers want to ramp up that four-cylinder line and make the V-8 go away."

Jean-Paul Bugaud, machining systems manager at Renault Automation (Castres, France), says uncertainties in Europe over environmental policies affect demand there. The engine builders who are his traditional transfer-line customers, he says, know well that European roads are filled with diesel-powered cars because tax policies in Europe favor diesel fuel over gasoline. Such laws could be repealed tomorrow, and engine builders' large fixed investments could suddenly become nonproductive.

Global manufacturing is another wild card. As price wars escalate, OEMs and their suppliers shift components manufacturing and assembly to emerging markets to be cost-competitive or close to the consumer. There the politics of the host country may restrict their operations, and their estimates of demand may prove wildly overoptimistic, so that production fluctuations, become a way of life.

Speed and Modularity

Is there a high-production equivalent to the job shop's stable of CNC machines? Supporters of the high-speed or high-velocity machines introduced in the last decade, mostly builders of traditional transfer lines as well, say they offer CNC flexibility plus transfer-line levels of speed, at much lower equipment and engineering costs. Driven by linear motors or ballscrews, they make parts as standalones, in cells, or as stations on a transfer line.

Modular design allows a machining center to become part of a manufacturing cell. A manufacturer who needs to produce 50,000 units a year today and 100,000 units a year in two or three years can defer capital expenditure by using such cells to produce the parts now, then add increments, says Mark Tomlinson, vice president of engineering, Lamb Technicon Machining Systems (Warren, MI). "When production gets to 200,000 units, dedicated units will probably take over, and the cells will shift to making another product. Cells can also jack up the productivity of a dedicated line as volumes increase."

Ingersoll developed a linearmotor-driven high-velocity machine in the early '80s, worked for a decade with Ford Motor Co. to fine-tune the HVM, and then put HVM cells into Ford Motor Co. plants making cylinder heads and blocks for low-volume applications. Now Ingersoll is exploring the use of HVM modules as stations on a transfer line. Kalp says that once a high-speed module is available, "the next logical step is to redesign those modules into transfer lines, providing CNC flexibility right in the line."

Let's take a look at what this equipment can do and how it's being used today.

Standalones and Cells

Renault Automation's "Naga" U-shaped cells, named for the slow, placidly flowing Naga River in Japan, curve around the operator. They contain a limited number of machining centers, perhaps an assembly machine, and produce 100-400 parts per day, with cycle times from 10 to 2 min. All handling is manual, keeping costs low.

As production needs ratchet up, the cells can keep pace. Renault Automation believes optimal production for HSM machines is about 1200-1500 cylinder heads and about 1500-1800 gearbox casings per day in two shifts. Beyond those levels, the advice is to stick to simple layouts and double up the line.

Kingsbury's high-speed machining cell called Cyber-Cell 2, intended as part of a U-shaped cell, as a stand-alone unit, or as an in-line flexible transfer system, features an 18,000-rpm vertical spindle and axis travel speeds to 90 m/min. Linear motors provide the fast acceleration and deceleration rates needed for high production-with a 354-kg fixture and workpiece, axis acceleration on X is 1.2 g, on Y, 0.6 g, and on Z, 1.6 g. "It's well suited for making cylinder heads or transmission parts," says Coughlin. "You can put a high-speed machine in a line, transfer parts on a pallet from machine to machine, use horizontal or vertical spindles."

This latest version of the Cyber-Cell, shown at IMTS, is a narrow rectangle 1524 mm wide X 4972 mm long, with a 500mm^sup 2^ worktable. No special foundation is needed, so it moves easily from site to site. One US automaker will soon use the CyberCell as a stand-alone machine for a variety of drilling and milling operations, and Couglin expects others to follow.