How Do You Check Whether an Undersized Bore Can Be Salvaged Instead of Scrapped?

It depends on how much material is missing and what the bore has to do. A few thousandths under on a bore that's about to take a bushing or bearing with fit tolerance to spare is usually recoverable. A bore that's undersized relative to a tight interference fit, or carries a true position and roundness callout on the print, often isn't. Measure before you decide, don't guess.

Start with what the print actually requires

Pull the tolerance band off the drawing and figure out where in that band you actually are, not just whether you're under. A bore machined 0.05 mm under a ±0.01 mm callout is scrap. The same 0.05 mm under a bore with 0.2 mm of tolerance to work with might not even be a problem, once you finish measuring it properly with a bore gauge or CMM rather than trusting the in-process reading that flagged it.

Check roundness and taper along the length too.

A bore undersized because of a straightforward offset error is a different situation than one undersized because the boring bar deflected and left it tapered or oval. The second case usually isn't salvageable by re-boring alone, because the surface you'd be cutting from isn't uniform to begin with.

Recovery options

If there's still room in the tolerance band, a second, lighter pass with a fresh wear offset can bring it in. This is the most common fix, and it works as long as the bore hasn't gone tapered.

If the bore ends up outside print tolerance but the assembly can accept a design change, opening the bore further and moving to an oversized bushing, dowel, or bearing is a legitimate engineering fix, not a defect. It's standard practice for interference fits, and it's exactly why oversized dowel pins and step bushings exist as stock items. It only works if the mating part can absorb the change.

If neither applies, it's scrap. The tolerance is genuinely tight, the part can't take a design change, and there's no material left to remove without breaking through a wall or a feature. Forcing a fix into a part with no tolerance room and no material margin usually costs more in re-machining time than starting a new blank.

What to actually do at the machine

Measure with a real instrument before deciding anything. If you're still inside the tolerance band, take a light finishing pass with a wear offset correction rather than a full geometry offset change, since a geometry change affects every future part run on that tool. If you're out of the band, stop and get engineering sign-off on a design change before cutting further.

A tight bore-to-shaft or bore-to-bushing fit is one of the more common places a first article inspection catches a problem before it becomes ten scrapped parts instead of one.