I owe a big thanks to @BobKarnaugh and @procterc for their help on the Bridgeport tonight! Bob helped me square the vise (OK, let’s be honest - he did it). And the Bridgeport head was seriously out of tram (0.007" across a 1" sweep!) (EDIT: Across 2" sweep). Bob, Charles, and Sam re-trammed the head. (If someone knows Sam’s talk ID, please add it for me).
Thanks much, gentlemen!
How? 
I need to correct myself … it was across a 2" sweep.
Front to back …
I made some passes along the X axis with the shell mill and there was about 0.004" height difference between the front of the cut and the rear of the cut - and that wasn’t across the full width of the cutter. That’s how we noticed it was so bad.
But if you’re asking how it got that way, I have no idea. If someone was using a half-inch end mill they might not even notice the issue.
Its been a while since I’ve taught a bridgeport class, but we verify X, Y, and Z when we square the vice, and it wasn’t like that last time I taught it.
Now that we know it’s good, we’ll need to see if it goes out of tram again. It shouldn’t do that on its own.
Its been off a hair for some time. The head is held in place by friction and an adjustment screw, the possibility of the bolts loosening and the alignment going out exists.
It should probably checked periodically.
do we have a tramming ring or are we doing it another way? The method I know to check is to is a large ring and run a dial indicator on an across it circularly. I’m sure it can be done otherwise but I haven’t seen a ring around.
We have the correct tool with two dial indicators on it 4" or so apart.
This is what we have.
I believe that is Sam Mcfadden that really knew how the tram should be done. The proof is: a multiple sweep of the fly cutter was producing steps that could snag a finger nail, but after the tram the fly cutter sweep lines could be seen but no longer felt.