Mount a switch on the underside of the gantry back plate at each end.

A piece of aluminum angle mounted to the lower parts of the z-axis carriage will trigger the switches.

The switches need to be placed so that they trigger just before the machine would reach the physical limit.

Two switches are mounted to the bottom of the gantry. Notice how the switches are wired in series so that a single pair returns to the breakout board.

A piece of aluminum angle is mounted to each frame end to trigger the switches.

Hiding the switches like this will protect them from being clogged by dust.

The z-axis only needs a positive limit switch, because if you reach the negative limit, you will probably have already crashed your tool. A nail or small screws is used to trigger the switch.