The initial position of the Z clipping planes is set based on the Z (depth) extent of the visible geometry - plus quite a bit extra to leave room for transformations. The plane positions can only be set via the Transformation Editor dialog.
Click the Graphics Window Transform icon on the Tools Icon Bar and select
Select →
The graphics display shows the relative positions of the front and back clipping planes (left and right vertical red lines) to the Z extent of all currently visible objects (white box).
Toggle the Float Z-Clip Planes with transform option on to have the Z-clip planes automatically adjust.
OR
Toggle the option off to manually adjust the Z-clip plane locations.
If the Float Z-clip Planes With Transform option is on, you can specify the minimum Z value that the Front clip plane can float to.
If the Float Z-Clip Planes with transform option is off, you can edit the plane positions either by dragging the red lines or by entering explicit values in the Front and Back text fields. Recall that the values represent the distance from the look-from point to the plane.
Place the mouse pointer over the desired plane marker and click the left mouse button.
Drag the marker left or right to the desired location. The Graphics Window will update as the marker is moved.
OR
Enter explicit values in the Front and/or Back text fields and press Return.
If the markers become difficult to manipulate due to changes, click the Redraw Z-Clip Area Above button to rescale the markers.
Each viewport maintains its own independent Z clipping planes. The operation described above will change the planes for the current viewport (as set by clicking in the desired viewport in the Graphics Window).
Note: Clicking the Reinitialize transforms icon on the Tools Icon Bar will reset the Z clipping planes of the current viewport based on the Z extent of all objects currently visible in that viewport.
You can also reset by clicking the Graphics window transforms icon and clicking Reset... to open the Reset Tools and Viewport(s) dialog. Then click the button.