Relative Speed of Different Axes

Hi all,

I'm embarking on my second gantry router design & build and would like to request your opinions and lessons learned on the speed of each axis relative to the others.

Background: this is a three axis gantry machine with two drive motors on the long (X) axis (total of 4 motors). Haven't decided just how long X will be yet but Y will allow cutting edge to edge on a full sheet of MDF (49" min). Both X and Y will be rack and pinion. Z will be a 15mm ground ballscrew with 5mm pitch and ~8" of travel. I want the new machine capable of high quality 3D carving for commercial signage work. I'm shooting for reliable rapids in the 250-300ipm range on all 3 axes.

Question: is there any reason to make Z move faster than X & Y? My gut tells me the machine needs Z to move faster than the horizontal axes in deep relief carving situations to keep overall velocity up close to the intended feed rate.

Thoughts? The answer to this question will determine the gear ratios I need between the servos and the ballscrew/pinion gears.

Thanks as always,

Tom