I used the same drawing Nick Chrsitodoulou posted above to manufacture two clamps to help in gluing down the bridge on a guitar I had just put a new top on. They worked fine. I'm not sure what you mean by "cam slipping". The only issue I had was that the clamp arm that the cam is on would want to slide up the metal bar. I was able to solve this by putting the clamp in place, pushing down slightly on the arm,where the metal arm goes through the back of the arm. This helps the cam arm to lock onto the bar when you pull the cam lever to clamp the work. Once the cam levers were rotated, NOTHING moved. Hope this helps

Al