Have build a gantry router 4000 x 2000 with linuxcnc 2.8 masterbranche, have challenge with interpolation movements on x and y,
have a Ferror of 0.04 on both axes, but a riple of about 0.1 to 0.3 on surface.
The surface issue should be mechanically. From the encoder point of view (from where the ferror is observed) until the tip of the tool is mostly mechanically. Can you post a few pictures of your Z axis setup?