This dual hotend mounting plate bolts directly to the existing hotend mounting plate on the Rostock MAX. It mounts the two hotends some 25mm apart (center-to-center) and holds them perpendicular to the build plate. The left hotend is offset from center -12.5mm on X and 0 on Y. The right hotend is offset 12.5mm on X and 0 on Y.
The hotends mounting holes are sized for seemeCNC hotends (appx. 11mm); the tolerances of the holes are "slip fit." The hotends are held in place with their normal nuts. The flats at the top of the mounting holes extend just above (0.2mm) the existing hotend mounting plate, allowing the nuts to be tightened on the hotends securely.
The second photo shows the first test print for the mounting plate while the print was in progress. Minor software correction of the X and Y offsets (+.1mm X axis and -.3mm Y axis on the Second extruder) achieved perfect registration. The "drool" that you see in the print was pretty well resolved by adjusting retraction when an extruder was taken out of active print mode (this is a slicer settings issue).
This is a very simple and rigid print. I used Cura 14.01 to slice the .stl file. The print was done with a .5 nozzle. Layers are .2mm. The shell and top/bottom are all .6mm thick. I used a 30% fill density. The first two layers were printed at 240°. The remaining layers were printed at 230°.