On one of my machines I pulled of the pair of lock down vises I normally
have on the table so I could mount some custom fixtures to machine a
mold that was almost the full work envelope of the machine.

After I finished I had to put my pair of vise back on, align them and
align them to each other. Not that long ago (a few years atleast) that
would have been a daunting task. I have gotten to the point that I
expect them to be within the range of my test indicator on the first
try... and they almost were, but getting the vises within 50 thousandths
is not the goal of course. LOL

I quickly got the first squared up on the table, and proceeded to the
second. Of course I tighten the hold down clamps they shift a few
thousandths. Lightly tapping with a hammer and going back and forth as
I bring the bolts down as far as I dare tighten them without to much
fear of damaging the table gets me within about a thousandth across 2
six inch vise with a 3 inch gap between them.

One vise. No problem. five minutes any day of the week, and much
faster on a good day. 2 vises... not a "problem," but tedious. Maybe
20-30 minutes. Maybe longer. I didn't really pay attention to how long
it took. Probably also not all that necessary as I use soft jaws with a
premachined step on the bottom so I can fully face them square to the
travel of the machine. Then I add a step in the top of the jaws, or in
this case take a skim cut on the step that is already there. I can use
the step for a high hold or for thinner parts, and use the entire jaw
when necessary.

Anyway, none of that really matters. Just background until I get to the
point. The point is even clamped down as hard as I dare the vises still
can move with only a modestly firm tap with a hammer. Its only a few
tenths to a couple thou depending on the blow, but it does move. I know
a hammer blow delivers a deceptively large amount of force, but still.
It made me wonder how easily those vises really move, and if there was
something I could do or even needed to do to more firmly affix them in
place. I tend to mount a vise or in this case two vises and only remove
them if I have to. Often even temporary fixture plates are just mounted
in the vises. The two I just remounted on the table were last off the
machine over a year ago. I've cut a lot of parts in those vises. Maybe
hundreds.

So what kind of "normal" cutting fores might cause those vises to move.
I put the word normal in quotes, not because I wanted only include safe
cuts that work every time, but also things that can happen, but maybe
shouldn't. I know a crash with a fly cutter could do as much damage as
a hammer. How abut a 1/2 inch mill entering a piece of hard steel or a
face mill banging on the edge of a work piece.

I know about the sheet of paper trick (it does work really well), but I
cut 95% aluminum with flood on that machine and having something between
the vise and the table that can not just hold but wick moisture is a bit
disconcerting. The only other thing I could think of was to add some
stop blocks bolted to the table up against the vise.