Beyond Tweaking

When vertical machining centers took the place of manual machines in short-run production, some of the machining knowledge that used to be applied at the machine tool began to be applied somewhere else—at a programmer's desk. Now, with horizontal machining centers replacing VMCs for many of these same applications, another relocation of process knowledge is taking place.

Columns From: 10/1/2002 Modern Machine Shop, ,

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Peter Zelinski

When vertical machining centers took the place of manual machines in short-run production, some of the machining knowledge that used to be applied at the machine tool began to be applied somewhere else—at a programmer's desk. Now, with horizontal machining centers replacing VMCs for many of these same applications, another relocation of process knowledge is taking place. Specifically, many shops are breaking away from their reliance on tweaking.

A VMC typically has a skilled operator standing by. But an HMC, with its capacity to hold more work, is at its best when allowed to run a long series of parts without interruption. The processes for these parts are engineered so the machine can run reliably without intervention. Factors the process considers include repeatable part location, predictable tool wear (or monitoring of the tool) and optimized speeds and feeds. With such a process in place, it's not enough to say the operator can leave the machine alone—the operator should leave the machine alone.

"Tweaking"—that is, adjusting the speed and feed rate override knobs or otherwise changing the process based on snap decisions at the machine—is a way of settling for less. If the process was engineered for optimum performance to begin with, then tweaking amounts to reducing the efficiency of that process in response to some perceived change. The better response would be to stop the process altogether and fix whatever has gone wrong.

If 20 minutes of downtime can correct a change in the process, that's preferable to tweaking the process so it's 95 percent efficient and letting it run that way for 8 hours. And if 8 hours of downtime can correct a change, that's better than running at 95 percent for a few days, followed by 85 percent for a few days after that when another change has led to further tweaking.

Once the expectation that operators will intervene in the process has been taken away, a skilled operator is no longer needed near each machine. The shop that runs well-engineered processes can afford to centralize its process knowledge in the minds of just a few troubleshooters, whose job it is to respond whenever a given process is no longer performing as expected.

A number of shops have been running this way for some time, many of them seeing this approach as a response to the declining availability of employees skilled in metalworking. In today's market, in fact, engineering the process and leaving it to run without tweaking may be the most efficient way to use not just a horizontal, but a vertical as well.

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