Have you tried to create your own coordinate system???
Insert-->reference geometry-->coordinate system. Choose the original origin as origin-->Front plane as X Axis direction reference and Bottom plane as Z Axis direction reference and there you have coordinate system which should be suitable for machining. Save the empty part with the coordinate system you just created as a template and you have it allways available.
Insert-->reference geometry-->coordinate system. Choose the original origin as origin-->Front plane as X Axis direction reference and Bottom plane as Z Axis direction reference and there you have coordinate system which should be suitable for machining. Save the empty part with the coordinate system you just created as a template and you have it allways available.
Actually, in all the other CAD systems (i.e. Unigraphics, AutoCAD), the Z-Axis points away from the Top Plane. Z represents Altitude.