Earlier Axiom 20 machines may need their belt tensioners replaced if experiencing an  inconsistent first layer height. A tighter belt forces the nozzle to print closer to the bed at the center of the build plate and farther toward the sides. 

This replacement part will alleviate the excess tension and allow for more consistent adhesion and overall printing.

Below is a link to the video that instructs you how to do this.

Slice with Apex standard settings on ABS.

Change Axiom 20 Belt Tensioner