Hi Koi, there's a tutorial here:

http://moi3d.com/forum/index.php?webtag=MOI&msg=490.36

> So firstly: is it more easy to build a screw with as a boolean UNION with shaft and
> thread? OR with boolean DIFFERENCE (build solid cylinder and bool. diff. the
> negative of the thread)?

Boolean union is probably the easiest.

But don't construct the thread so that it's just barely skimming over the same surface area as the cylinder, make it push through into it a little bit.

See the above link for some more description of that. It's difficult to boolean pieces that skim over some overlapping surface area instead of pushing through each other a little bit to make a clean distinct intersection.

Some other tutorials linked to above in this same discussion thread:
http://moi3d.com/forum/index.php?webtag=MOI&msg=10605.2

- Michael