On 9/12/2019 4:33 AM, David Billington wrote:
On 11/09/2019 23:11, Bob La Londe wrote:
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.Â* Not that long ago (a few years atleast)
that would have been a daunting task.Â* 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.Â* LOL

I quickly got the first squared up on the table, and proceeded to the
second.Â* Of course I tighten the hold down clamps they shift a few
thousandths.Â* 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.Â* No problem.Â* 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.Â* Maybe longer.Â* I didn't really pay attention to how
long it took.Â* 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.Â* 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.Â* 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.Â* Just background until I get to
the point.Â* 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.Â* Its only
a few tenths to a couple thou depending on the blow, but it does
move.Â* 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.Â* Often even
temporary fixture plates are just mounted in the vises.Â* The two I
just remounted on the table were last off the machine over a year
ago.Â* I've cut a lot of parts in those vises.Â* 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.Â* I know a crash with a fly cutter could
do as much damage as a hammer.Â* 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.Â* The only other thing I could think
of was to add some stop blocks bolted to the table up against the vise.


I'm wondering if your clamps may have the threads bottoming out so not
providing the full clamp load to the vice, I've seen it before. I once
did a job with the vice clamp bolts done up finger tight as I had
forgotten to use the spanner to fully tighten them, it did eventually
shift but I did a surprising amount of work before it did.



They certainly didn't bottom out in the table, but I suppose they could
be close in the nut(s). I'll see if I can get a hook tool in there to
see how deep the ends of the bolts are.