Roundness Not Required

How would you machine this part? The answer seems obvious. The square part fits nicely in a vise.

Columns From: 3/1/2002 Modern Machine Shop, ,

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Peter Zelinski

How would you machine this part?

The answer seems obvious. The square part fits nicely in a vise. Clamp it there and let a machining center go to work. In other words, perform the same process through CNC that a toolmaker would perform on a manual machine.

Now look again. The way to machine this part only seems obvious. Instead of using a machining center, the shop that made this part used a lathe with rotary tool capability.

Some refer to these multitasking lathes as “turning centers” or “turn-mill machines.” These labels are unfortunate, because they imply that these machines are turning machines first and foremost. In fact, they are single-setup machines. Many perform turning, milling and drilling with equal proficiency.

A standard vertical machining center clamping a cube-shaped part in a vise needs a separate setup for every face of the part. Add an indexer, and perhaps four faces can be reached; a two-axis indexer brings the number to five. But in each of these cases where the part is rotated, making the maximum number of faces available may present a trade-off in terms of how much clamping force can be applied.

Contrast this with a well-equipped multitasking lathe. The machine can clamp the part securely in one chuck to machine some of the six faces, then transfer the part to an opposing chuck to reach all the other faces. The single-setup process not only minimizes human involvement, it also ensures feature-to-feature location accuracy across all the surfaces of the part.

Multitasking lathes have become common in shops today, but certain assumptions still limit their effectiveness. Disk- and cylinder-shaped parts are assigned to these machines when some orthogonal parts could also be produced efficiently. Also, personnel skilled with turning equipment are considered the best qualified for these machines when expertise with rotary-tool machines may be just as valuable.

One fitting label for the machines is already taken. If the term was still available, however, we could do worse than to use "machining centers" to refer to this class of equipment.

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