Hi Martin, the result that you have at the end there is not a solid - it's a bunch of separate surface pieces. So that's why it's not behaving like a solid, because it is indeed not one...

It looks like your original sweep in this case is done with an open curve:




It can be ok in some situations to use a surface as a cutting object, but only if it fully divides the solid - your cutting surface in this case does not fully divide the solid at the very top and bottom spots I don't think. So it would be better to form a solid as the cutting object in this case instead.

You can make a solid for the cutting object by drawing in a line segment on the ends of that open "V" curve and then select the V and the line and use Join to make a closed triangle profile curve. Then the sweep result of that will be a solid.


Also you'll probably be better off making the cylinder be slightly shorter than the cutting object rather than having the cutting object stop right where you have it currently since that makes a sort of teeny little notch at the end.

See the attached 3DM file for an example - here I've shortened the cylinder so that the cutting object runs cleanly off the end, if you wanted the same sized cylinder as before you would probably instead want to make the helix path slightly longer instead.


At some point in the future I could see making a "helix cut" type plug-in that would automate this particular type of thread type stuff, but that probably won't happen anytime too soon though.

You can probably find a lot of various premade standard parts like this in IGES or STEP format so that you wouldn't need to model it at all though - I'd recommend checking out http://grabcad.com, and also see the links here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=1826.1
http://moi3d.com/forum/index.php?webtag=MOI&msg=3505.3

- Michael