If we agree that the purpose of limit switches is safety and prevents the machine from exceeding its working area - why then is it not required to have limit switches in both ends of each of the axes? (ok Z is a little different).
This is because they use a target at both ends ie the switch will travel with the gantry and so will hit the target at the end of travel in either direction.
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Re point 3 It is the homing that give the ref points for the soft limits.
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re point 4 After you have homed the Z you then move Z down to a ref point (say top of work piece) and then Zero it on the dro so now the machine or should I say the controller knows where zero is and can work out the depth of cut.