Guide Step fix

I had issue with the original step file under Freecad: the Guide thread was not watertight. As such it was then giving me error when exporting to STL.

This one fixed the thread part. But I also have to redo the thread completely.
Additionally I put a Hole for an M8 screw to kept it up.

This one fit perfectly a Dremel 3000, for which the thread is a M19x2 (found info [here](https://bbs.homeshopmachinist.net/forum/general/33057-dremel-anyone-know-the-thread#post632237) )

By the way, I also find this [remix on printables](https://www.printables.com/model/64586-rod-tube-and-screw-cutter-for-dremel)
- the base is enlarged for bigger dremel (I have the same problem with original base and dremel 3000)
- the screw cutter is well thought, and include a T-slot + guide for screw: that's what I used on the picture.