What's the Most Reliable Way to Square an Out-of-Square Blank When the Vise Is Already True?

Indicate the actual stock surface, not the vise jaws, and cut a reference face off the blank before you trust anything downstream. A vise that's dead true to the machine axes doesn't help you if the material clamped in it isn't square to itself. The vise squares the jaws, not the part.

Why a true vise doesn't solve this

Rolled or sheared bar stock, plate cut on a manual saw, and stock that's sat warped in a rack all have the same problem: the outside faces aren't necessarily parallel or perpendicular to each other. Clamp that in a perfectly square vise and you get a perfectly square vise holding a crooked part.

The machine will faithfully cut relative to its own axes. That's exactly what produces the classic complaint of "it indicated fine but cut out of square." The operator indicated the vise or a clamped face, not the geometry that actually matters for the finished part.

The actual sequence

Start by indicating the stock itself, sweeping along the face you plan to reference, not just checking that the vise jaw reads zero.

If the raw face is out more than you can live with, don't try to shim it true in the vise. Machine a clean reference face first, in a light pass, and use that face as your real datum for every operation after.

Once you have one true reference face, every subsequent setup references off it, either by re-clamping against it directly or by using it to zero a work offset. That's the difference between square to the vise and square to the part: you're building your own datum instead of assuming the raw material already has one.

For stock that's twisted rather than just off-parallel, a single reference face on one end isn't enough. You need to check squareness at both ends independently, because a part can read true at one point and be rotated a couple thousandths at the other end.

Where people get burned

The failure that generates the most confused questions is a part that indicates perfectly in the vise and still cuts out of square relative to the machine. That's almost always because the indicator checked the vise or a clamped face rather than a true reference on the workpiece.

A vise being true to the machine tells you the holding fixture is good. It tells you nothing about whether the material you dropped into it is square to itself.

If you're running a batch of the same rough stock repeatedly, machine a reference face on the first few pieces and check how consistent the raw material actually is. Stock that comes in consistently square doesn't need this treatment every time. Stock that varies piece to piece does, and skipping that check is how a whole day's production ends up out of tolerance in the same direction.

Square the part to itself first, with an actual machined reference, before you trust anything the vise tells you.