My God! My machine is sagging under its own weight!

 I happened to put a precision level on the mill table because I needed a level surface to check a different, cheaper level, and I thought I had leveled the mill; Ghu knows, I pounded enough wedges under it to do so.

 But the level showed off by two graduations; um, I don't remember what the graduations mean, frankly.  Much less then a degree of angle, I remember that much.  There's a card in the tool chest that has the info, but I am thirty feet away inside the house, and with my hip/leg pain of late, I dislike changing positions.  On the same surface, the bubble in the cheap level I wanted to check just disappeared.  The precision machinist level from Starrett is more sensitive by a couple orders of magnitude, than the no-name woodworker's level.  But I digress.

 I thought I'd leveled the mill table, so I moved the level to the middle of the table.  There, it was level.  Moving the level to the other end, I find the table is out of level in the other direction.  Could that massive hunk of cast iron be bent and/or bending under its own weight?!?  NO WAI. 😱

 I moved the table to one end and measured the center again, and it's close to level but not as perfect as when the table was centered.  The far end was worse.  Same-same when I move the table to the other end. 

At this point, a picture began to develop.

 Number Uno, the table really does sag under its own weight, and that's normal!  A bit of research reveals that they sag when they are new.  If Bridgeport made it 2X or 3X as thick, it would still sag, just 2X or 3X less.  To take the sag out down to a tenth or so, would require a nine inch wide table to also become nine inches thick.  At which point, it would weigh as much as the rest of the machine.

 Numerus Secundus
 More importantly, that sag doesn't matter, because that isn't where one is machining.  The saddle holds the table up to the right height as the table moves under the spindle.  The only part that really matters (all other things being correct) is that the table not tilt on the saddle.

 The friend I bought the mill from told me something about how the saddle under the table is the actual flat reference you are milling with, ie; the saddle ways - it's not the table.  I don't remember what we were talking about, but I now perceive I did not think through all of the implications that I originally got from his advice.  Sometimes I learn quick.  Sometimes I learn slow.

 It also appears that my table does tilt - slightly - when extended, which I believe is a loose gib problem.  And that does need fixing because the whole table is tilting, work and all, under the cutter.  That's bad if it is significant, but fortunately that's a big if.  I think the saddle gib can be adjusted without taking anything apart.  I'll look during my second smoke trip of the morning.  It is possible my gib is worn unevenly and if tightened, will bind, making it harder to move the table (rough on the power table feed I am fixing) if I try to tighten it.  It is possible it was loose for whatever reason and just needs a snug.

 It is possible I will do the math off the level's reading and think to myself, "I can live with that".  Time will tell.

 But man!  This just goes to show there is more than one way to measure something.  If I had actually set out to measure the sag of my table this morning, I would have wondered what and how I would mount my tenths indicator on.  I still can't think of a way. 

 My reference point would of necessity be the spindle nose.  How could I attach something there (like an enormous Indicol?) and then hang it two feet out to pick up the end of the table?  I'd have to build a gantry/truss structure so it wouldn't weigh much, it has to be rigid in two dimensions, and uh, yeah, I'm not making that unless I'm gonna use it more than once. 

 But drop a level on the table and with a bit of math, I get the answer without having to make a one-time, single-purpose fixture I'll never use again.  The reader is referred to the Shop Math section of your copy of Machinery's Handbook. ;)

"When will I ever need any of this weird math?
I'm not going to become an engineer, I want to work
with my hands, maybe be a machinist or an electrician!"

 ~≈{👁}≈~