craftsman,
I had the same thoughts when I bought a used Bridgeport. If I machined
a flat plate and measured from the mill head to the plate. I saw no
deviation, but placing a straight edge on it, I could see light under
the edge when placed across the plate, and when placed lengthways, it
rocked in the middle. Upon examination, I could also see wear on the
ways( the frosting was missing at the ends of the saddle). What I did
to understand what was going on was to place a surface plate on the
table and measure the distance between the top of the plate and the
head on 1" squares, ie measure each inch along the length of the plate,
move it 1" crossfeed and repeat the 1" measurements along that length.
What I found on plotting the data was that because of the wear on the
ways, the longitudinal ways were worn curved, with the ends of the
saddle worn down more than in the center. On the crossfeed, the ways
were worn dished. Using a 12" x 18" surface plate, from the center
of the plate to either end, 9", the ends of the plate were 0.004"
closer to the head than in the middle because of the wear. Crossfeed
showed 0.002" further from head on front and rear of plate than in
middle.
One thing you might do is take a surface plate and place it upside down
on the ground surface of your table and see if it rocks. If it does
not, put some marking blue on the plate and run the plate on the ground
surface and see where the blue transfers to the table, in the center,
or on the 4 corners, or?? That should give you an idea as to the true
flatness of the table surface.