Self-Centering Vise or Chuck for Irregular Round Stock?

Use a chuck when the part is genuinely round and needs to rotate true on a lathe or fourth axis. Use a self-centering vise when the stock is round-ish but the feature you're cutting needs to sit flat and still on a mill table, especially if the round profile is interrupted by flats, flanges, or an out-of-round forging surface.

What a chuck actually buys you

A three- or six-jaw chuck centers on the round OD and holds the part concentric to the spindle axis, which matters when turning, boring, or indexing around a fourth axis where true position relative to rotation is the whole point. Six-jaw and collet chucks grip more of the circumference and distort thin-walled round stock less than three-jaw.

But a chuck assumes the part is actually round where it's gripped. Feed it an out-of-round casting or a part with a flat milled into one side, and it either won't seat consistently or clamps unevenly and pulls the part off-axis.

What a self-centering vise buys you

A self-centering vise closes two jaws simultaneously toward a fixed centerline, so a round part lands centered on the vise's axis without indicating it in. That axis is fixed relative to the mill table rather than a rotating spindle.

That's the right tool when the part needs to sit flat and square for milling operations, slots, pockets, drilled holes, rather than needing to spin true. It also tolerates irregular round stock better than a chuck. Forging flash, a slightly oval profile, or stock with flats already machined in still centers reasonably well, because the jaws close symmetrically rather than depending on three or six points landing evenly.

The failure mode to watch for

The mistake is picking based on "the stock is round" instead of "what does the next operation need."

Round stock destined for turning needs a chuck. Round stock destined for milled features on its face or side needs to sit still and square, which a self-centering vise does better than fighting a chuck sideways on a mill table.

If a part starts round on a lathe and finishes with milled features, that's two separate workholding decisions. Chuck it for the turning ops, then transfer to a vise or soft jaws for the milling ops, rather than trying to make one fixture do both jobs.

When neither is quite right

Genuinely irregular stock, an as-cast part with a round boss but no other symmetry, often needs a custom soft-jaw profile machined to match the part rather than either a generic chuck or a generic self-centering vise.

That's more setup time up front. It's still the only way to get repeatable location on a shape without a true round reference to center against.