I have built and currently print with my first FT-6 printer. At first the printer was way off and couldn't print a straight line repeat for any build. I soon found the issue and fix the problem. The hotend support caridge where it is machined and keyed together has slop in these notches joints. I fixed the issue with a wedge screw thus locking all parts solidly ridgid.
Support sugestion: Taper mill the locking tab on the top and bottom plates by 5 degrees on each side of each tab. When the side plates are screwed down to create the caridge of the hotend they will interlock with a mechanical slop free joining. The printer will be bound tight.