The 3-2-1 locating principle is one of those foundational fixture-design ideas that every mechanical engineer eventually bumps into — six points constrain six degrees of freedom, and if you get the arrangement wrong, your part rocks, spins, or measures differently every time you clamp it. The flat-part version is easy to visualize: three points on the primary datum, two on the secondary, one on the tertiary. But cylindrical parts are where it gets interesting, because you can't just plop a shaft on three pins and call it constrained.

This video walks through the specific fixturing hardware engineers actually reach for when the workpiece is round: V-blocks for cradling shafts along their axis, mandrels for locating from an internal bore, collets for concentric gripping, and diamond pins — the clever asymmetric locator that constrains rotation without over-constraining a second hole's positional tolerance. That last one in particular is a detail most GD&T primers gloss over, and understanding why a diamond pin exists (rather than a second round pin) is a genuine "aha" moment about how tolerance stack-ups drive fixture geometry.

It's a small channel with a companion flat-part video published the same morning, so watching them as a pair gives you the full picture of how the same principle adapts across geometries.