The first thing is to make sure the arm is absolutely parallel to the table from front to back. Once that setting is done, you won't need to turn the big black knob at all. Unless you just feel that you need more tension. As was mentioned, the white plastic lever may be worn out too. Follow the instructions in the manual. If you didn't get the manual with the saw, you can download it online. Once you get used to that saw, unless the previous owner wore it out, you will love it. My main suggestion is to spend another $100 on the Pegas clamp set. It might just prove to be the answer to all your problems. But start with leveling the arm first.