Cold rolling and Hot rolling are the effective methods to reduce plate thickness. These are not subtractive ways of reducing plate thickness. Otherwise, we can take machining through shaping, planning and milling. But it will increase the cost of the process.
There are some known working processes, among are the hot and cold pressing, rolling & forging depending on the size and shapes ( complex or simple). Light metals are easily machined out by milling or on the lathe, in addition to these, grinding by different methods are also available. Check the common available literatures on Manufacturing processes, Mechanical or Production metallurgy.