There are several ways to make the z axis zero.

1. Change value in the Properties Panel

The easiest way one can think of is to select all objects in the drawing and open the Properties Panel and change the z-axis value. But this is plausible if there are only few of objects whose z-axis are not zero. Because when you have selected all the objects, the properties panel only shows properties in common.

We can use Quick Select to select them by category and then change them.

Enter QSELECT then press Enter key or directly click the Quick Select button in the properties panel to open the dialog box, select “Entire drawing”, as shown in the following picture,

2. Use CHANGE Command

The change command can be used to change not only many objects, but properties and elevation of different kind of objects.

Enter CHANGE Command, press enter key, window select the objects to be changed, press Enter, input P, pres Enter, input E, press Enter, input 0, press Enter.

3. Flatten command

Flatten command is used to flatten objects, it creates 2D representation from a 3D model or we can use it to force the thickness and elevations of selected objects to 0.