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Sorry taking so long to answer, work has been frantic lately. Yes the tab on the back of the platform that holds the long screw was rubbing against the back panel. Actually, looking closer, the head of the screw overhangs the tab by about 1/4 of a millimetre.

After eliminating the Z screw and the guide rails, the only thing left was the tab on the back of the build plate. When I inspected the back panel, I found some small marks made by the tab rubbing on the panel. This was after I reassembled everything so rather than take the build plate out, I sanded the back panel down and now I get silky smooth Z travel again.

I've had my UM2 for a couple of weeks and lately I'm having a problem with the bed sticking at about 12mm. It doesn't move for a couple of layers and then it drops to its proper position and continues normally. You can see the result in this picture I have removed the four screws from the 'nut' that drives the bed and it still sticks when I slide the bed up and down. Has anyone had this problem and more importantly how do I fix it? Thanks, Phil.