thanks tony, i saw the bracing in the pictures you posted, i`m thinking the ideal shape for the top of the frame would be a triangular form so the members are in tension rather than flex but that`s armchair engineering. i better ask first.
i`m worried about the casting of the base of the jack i`m planning to use not being strong enough to cope with any side load if there is misalignment while pressing something. i`ve used a different design in which instead of drilling holes for adjusting the height of the "table" there are tabs welded to the frame, looks easier and faster to make although a bit clumsier to operate.
i`ve experienced the chattering you mention and thought it was flexing within the pieces but makes much more sense what you`re saying, flexing of the press frame.
i`ll wait for your reply and thanks for taking your time to answer
pete