Thanks for your reply cotty - I am gobsmacked that I have to worry about thousandths of a millimetre! Got the test piece to work - now just have to apply it to the model…

Thanks again

Hi all - I've used sketchup a while but I'm no expert. this is frustrating me. I have part of a model that refuses to let me close a face. I pasted the relevant bit into the attached. It seems to me that if all the edges of this cuboid are on axis - and if you colour them, they are - and there are no stray ends (I used Stray Lines - there are none, and besides, if there were, the side faces wouldn't close), then the top face must be coplanar and close. But it doesn't. And when I draw 2 diagonals, they don't seem to touch each other, as I can't select a one-quarter triangle, only two overlapping one-half triangles, and the fourth quarter won't even fill.
If anyone can explain this before I throw either the computer or myself out of the window I would appreciate it hugely.
Thanks
Alastair

cube.skp