Please see attached file. A small block with a thread in it. I want to trim the inside of the hole using the thread as the cutting tool but it will not work. Strangely though the thread can be cut by the blocks surface but not the other way round. Why is this?

Hi bigseb,
Your shapes are "open surfaces". There is not a cut that can be made by that open screw thread ..

Ok... meaning? What do I do to fix? Surely there is a way?

Hello,

Were you looking for something like this?

Sort of. I want to trim B with A, then trim C with B.

Hello,

So something more like this?

Thanks. I got it to work though. Cheers anyway!

In your model, the thread obeject is not joined properly. It has an open edge running as a spiral round the thread. If you redo the thread so that this doesn't happen, then slice of the ends with a trim, you can add planar ends to make a solid. This solid will then boolean from the block.