If I am right, you are referring to the quill motion in the mill head. On the older style machines there are a couple of things to keep properly adjusted.
1. Be sure you have good spring return tension on the quill. Test it by putting the assembly in drill press mode, bringing the quill all the way down and letting go of the handle- it should snap back up quickly and solidly. If not, then you need to adjust the return spring.
2. If you have good spring tension and still have a lot of free play in the fine feed handle, watch the handle and check if it is moving back and forth horizontally. If yes, then the bushings on each side of the worm gear have most likely come loose. A common problem is heavy milling force will cause the bushings to push away and put a groove where the set screw dimple was. If this is the case, you can rotate the bushing a bit to a fresh spot and drill a new dimple for the set screw. Shoptask also offers a torrington bearing upgrade to eliminate this issue.