You can't put the shaft directly against the turntable. You have to design a mount that holds the motor with a rubber wheel from and RC car (hobby store or online) on the shaft of the encoder so the plate with the hole in it that the encoder mounts on has the rubber wheel installed on the shaft after the encoder nut is tighted on the threaded part. The metal plate is a 1 inch wide strip of aluminum or thin steel mounted flush with the bottom coming out perpendicular to the table and then up and over with a right angle bend in toward the table so the encoder goes into the plate from the bottom into the hole with the shaft point straight up
(perpendicular to the floor) and the nut screwed on from the top and then the rubber wheel mounted on the shaft so the wheel is in the same horizontal plane as the surface of the table and the encoder is below the surface of the table and to the side , shaft up.