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:
- Electronics. Machine tool electronics no longer fail because of coolant exposure, he says. Reliability is good and getting better.
- 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.
- Tooling. 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.
- Controls. These are far more comprehensive now, with error diagnostics built in.
Trends such as these increasingly make it possible for plants to rely on processes that arrange machining operations in a dedicated sequence, he says. Today, there is much less need for a plant to spend its resources on multiplicity.
Licon makes series production systems. The company supplies flexible CNC linear and rotary transfer machines for parts that are machined in large volumes—automotive components in particular. The point of Licon’s CNC technology is to capture the advantages of flexibility that machining centers can deliver, without the limitations that standard machining centers bring to volume production work. When series production is performed on machining centers, parts have to be unclamped and reclamped at each machine, adding to the variables that affect the consistency of the process.
Part of the benefit of serial production relates to tooling and fixturing. Parallel production requires the separate machines to carry redundant complements of fixtures and tools. This extra inventory imposes not just the extra purchase price, but also extra costs related to management and maintenance. When the process can assign specific operations to specific stations or machines, however, the fixtures can stay in place and the tools can be divided accordingly. The resulting process is cheaper, simpler and easier to troubleshoot.
Flexible transfer machines expand the benefits of series production, Mr. Koerner says. The potential benefits of series production on these machines include the following:
- Labor savings. The flexible transfer machine is a single machine tool that has many spindles. The part is typically clamped in a single pallet or fixture, moving from station to station automatically. Whereas a series of independent machines might require various operators, the transfer approach lets a single operator feed all of the spindles that the process requires.
- Cost and floor space. The capital expense of a transfer system is likely to be lower than that of a comparable line of machining centers. The floorspace requirement is almost certain to be lower.
- Productivity. Modern machining centers are capable of precise high speed cutting. However, 70 percent of the cycle time for a typical production part takes place outside of the cut. A flexible transfer machine can reduce this non-productive time partly in the same way that hard-tooled transfer lines did in the past—through short strokes that keep the tool close to the work.
- Support savings. Multiplicity of machine tools has the effect of multiplying the maintenance work. Chips have to be removed, coolant has to be refreshed and the hydraulics have to be replenished. The impact is often underappreciated, Mr. Koerner says. However, these operations can contribute to a high indirect labor cost for a process that has many machines.