I finished squaring the block for my latest part, set up a stop, found0,0 and clamped the evolving part in the vise when I remembered that I should check the alignment of the vise. There was no reason to expect misalignment, but one never knows, and all the more so given that I recently shimmed the column.
The vise wasn't just in line, it was as close to dead nuts as I've seen. Ok, it _could_ have moved favorably, but, I think something else is at work.
Having the block in the vise, I indicated on the block, which I had clamped with 1.5 inches front to back on the vise. Normally, I stand a wide parallel in the vise such that it stands above the jaws enough to give the indicator a nice surface. In that orientation, the vise is squeezing a thin plate that would be "easy" to bend. A 1.5 inch deep beam, even Al, is going to be much more ridgid in bending.
Indicating off of a parallel, I began to suspect there was a "bump" in the middle/right the jaws, and have been meaning to dig into it and ask some questions. Now that I've seen how steady the indicator reading can be across the vise, I put the feed lever in neutral and started typing :)
The vise is a Kurt clone (H&H). Is there something simple that could be wrong? Some junk under one of the jaws comes to mind; my hunch is that you will have me start with removing the jaws and cleaning. What should I do to find and hopefully fix it? Or, is it unrealistic to expect a on-edge parallel to be flat when clamped in the vise? If that's the problem, should I make another "deep" block to use for aligning the vise? What material would be best?