What's a Practical Way to Align Two Faces of a Workpiece When Starting From Unsquared Stock?

Pick one reference face, indicate it true to the machine's X or Y axis, then machine a second face perpendicular to it before touching anything else. Everything downstream gets located off those two machined faces, never off the raw stock's edges.

Why the raw stock can't be the reference

Bar stock, plate, and castings all come with edges that are close to square but not actually square. "Close" isn't good enough once you're holding a real tolerance. If you set your work offset off an unsquared edge, every feature you cut inherits that error, and it shows up as a part that measures fine relative to itself but is rotated relative to the print's datum structure. The fix isn't a better indicator. It's not trusting the raw edge as a datum in the first place.

The standard sequence: indicate one face in the vise or on the table until it reads flat within tolerance, machine an adjacent face square to it in the same setup, then use that machined pair as the actual reference for the work offset. Once you've cut a true edge, you can trust it, because you made it true on this machine, in this fixture, right before you needed it.

Where people get it wrong

The common mistake is indicating the stock's original edge carefully, sometimes to a few thousandths, and treating that as good enough. Indicating repeatability isn't squareness to the machine axes. You can dial an edge in dead flat and it can still be rotated a degree off true if the raw edge was never square to begin with.

The other common miss is skipping the second face on parts where only one datum feature seems to matter. If the print calls a two-plane or three-plane datum structure, machining only one reference face and eyeballing the second leaves the part vulnerable to a rotational error. That error won't show up until final inspection, when it's expensive to fix.

What to actually do

Machine your primary and secondary reference faces in the same setup, off the same vise closure, before cutting anything the print actually cares about. Use those machined faces, not the raw stock edges, to zero the work offset for the rest of the job. If the part needs to flip for a second setup, machine a third reference surface while it's still in the first setup, so there's something trustworthy to index off after the flip.

This is standard first-operation work regardless of material or geometry.

It's also why a job with tight positional tolerances usually costs a setup more than one that doesn't. Someone has to build the datum structure before they can cut to it.