Binding or bound up?
Most common is that the quill goes up too far and binds up. Bridgeport, on later machines, put a rubber disk on top of the quill but this binds up as well.
I usually take a pair of channel locks and move the motor pulley such that the front part of the Z belt goes into the quill area. This should free it up.
Remember that the stepping motor BOSS machines need .200 inches of decel or else they will mis-position.
Rare, but seen is that with a lot of Z work, the chrome on the quill can peel and bind up the quill VERY tight.
George
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)