Yes and front to back.
Start with a gauge block or something in the vise. Twist,shim whatever to make somewhat flat.
Put on the indicol out a ways to the side with the DTI. Put the machine spindle in neutral.
Move left or right and zero on the block. Make a dot on the block with a sharpie where you made zero.
Move the table to the other side and swing the spindle over the mark. Do not use the arm of the indicol to turn it or you will upset it.
Twist the head to get both to match. Take out half the error and go back to the other side. Rinse and repeat.
Now move the table in or out. Pick up the same point on the block at 6 or 12 o'clock. Move the other direction and pick up.
Adjust the nod to get zero difference as you did with the side to side.
Go back and check the tilt and nod at 12, 3, 6, 9 o'clock moving the table axis to the dot on your gauge block turning the spindle to each position.
Now you are aligned square to the X/Y table motion. Shim and or twist the vise so that it now runs true.
Yes this is a lot of work but eliminates table top or vise errors.
To get really picky now Z. Using the indicol indicate a pin or better a tooling ball at a marked height.
Move the knee say 6 inches. Indicate it again. This is Z on the knee not straight. You can make a correction table to comp for this when you move the knee.
(confusing, maybe I should learn how to make a video)
ok, thanks for the clarification, thats what i thought you were describing, this is the exact procedure i use to correct tram on bridgemills, our specification on a new machine is less than 10micron per 500mm. just for a bridgeport because you have easy adjustment and often work will often fit in a vise, i only concern myself with dealing with an 8" swing. imo anymore is deep into diminishing returns, if you barely push on the head you can see an indicator move a couple thou. what you are looking at on the z is not straightness, you are looking at parallelism error between the quill travel and the knee travel. this is also part of my geometry correction on a bridgemill because our machines also have 2 axis of vertical motion, i will correct parallelism in 2 planes between the z (spindle ram) and w (bridge motion) in relation to x, and also in relation to y. if you have a linear error in your travel, its straight, its just not parallel