I mainly mill copper fr4....for clamps I just use ali strips ....about the same length as the copper board (therefore 160mm long) and about 40mm wide (the ali is 3mm thick). So basically one long clamp either side of the copper board...I have my CNCUSB tool sensor wire permanently connected to one of these clamps...& becuase it is ali, when I clamp the board down it makes electrical contact with the copper FR4 surface - which means I can then use the FR4 copper surface itself to touch off for Z auto tool zeroing.