I have about 6 hours on my machine now doing simple stuff. When I started a complex V Carve project I ran into problems with smoothness of the finished piece. Which led me to start figuring out what was going on and here is what I did to fix the problem.
I started looking at the X Axis Gantry with a square with the table and found that I was out of square between 1/8 and 3/16 at the bottom of the front gantry which indicated that the weight of the DeWalt 611 had tilted the maker slide down in the front.
The first thing I did was to stiffen the gantry by using #petesworld suggestion by using a 3/4" and 1" aluminum plates across the x axis with 6) 4mm x 50mm screws holding the plates to the slide.
After completing this mod I found my gantry was much stiffer but I still could not get the Z axis square to the table. What I found was that in the y axis plates just the rear x axis slide has slots for adjustment and not the front. So I drilled out the 4 holes in y axis plates for the front x axis slide to 7/32. This gave me enough adjustment to make the z axis square to the table. I also added 5mm lock washers to all 8 screws holding the x axis gantry to the y axis plates which I hope will keep the gantry from twisting over time.
I ran a test V carve after making these modifications and all of the problems I had were corrected. I hope this could help someone else save some frustration.