Manufacturers in the 1940s and '50s knew something about machining technology. The hard-tooled transfer lines that machined automotive components were fast, easy to attend and easy to control. Short tool strokes kept out-of-cut motion to a minimum; one operator could feed many spindles; and the series arrangement of machining operations made it easy to diagnose which station was the source of any problems.

Of course, those hard-tooled lines also had drawbacks. One was that a problem with any component of the line could shut down all of the machining, and from there shut down all of production. Maintenance and repair personnel had to be kept on staff and continually ready.

The appearance of relatively low-cost CNC technology made it possible to replace these hard-tooled lines with more flexible and adaptable production strategies. In the 1990s, automotive suppliers responded to shrinking production runs and the need to reconfigure their processes more frequently by performing their machining on standard CNC machining centers, usually working in parallel.

The redundancy of this parallel approach reduced the pressure for quick repair. If one machining center went down, others could continue production. Thus, the owners of these machines could afford to rely on the machine tool suppliers for service rather than their own personnel. Series production was replaced by this theory of "multiplicity," in which independent machines do identical work. Many manufacturers still favor this approach.

But maybe they shouldn't. Lothar Koerner, sales director with machine tool supplier Licon (Ann Arbor, Michigan), says it may be time to rethink the redundancy. In many ways, the case for series production has improved.

He cites these trends:

1. Electronics. Machine tool electronics no longer fail because of coolant exposure, he says. Reliability is good and getting better.

2. Lubrication. Historically, this has been the major cause of failure on production equipment, but lubrication is now regulated by automatic systems. Feedback ensures that lubrication is well distributed.

3. Fooling. Modern tool materials deliver high performance across long tool lives. A PCD tool in aluminum may last for 100,000 parts. The need to frequently replace tools may have held up series production in the past, but tool replacement does not have to occur as frequently today.

4. Controls. These are far more comprehensive now, with error diagnostics built in.