ORIGINAL: Giles300-72
If you replace a course thread screw with a fine thread screw it will mess up the threads. it will work but you would never be able to put a course thread back in it.
Having done it myself, I have to disagree...
ORIGINAL: Giles300-72
In plastic the fine threads would make their own threads and would hold. My suggestion would be to look into finding the correct screw. If you don't believe me go to the hardware store and take a course thread bolt and try and put it in a fine thread nut. It won't work.
If you try to put a fine thread bolt into a coarse thread nut it will not work either, but this is because they are metal. Because the machine screw has a finer thread (and normally fractionally less diameter of thread, as they don't need to 'tap'), it does less damage the wall of the hole, it is no problem to have both threads, but if one of them gets even a tiny bit cross threaded, then you will have problems. I used to use the machine screw for one day or two until I could get an original coarse screw back in there, and I never had any problems getting the coarse screw to hold again, because the thread is much coarser and sharper. Of course, it does have a lot to do with the elasticity and quality of the plastic.
ORIGINAL: Giles300-72
but if you keep going back and forth from fine to corse then eventually nothing will hold.
Agreed...
ORIGINAL: Giles300-72
On the other hand if you are keeping the same thread pitch and just going from a tapping to a machine screw then no it want hurt anything. The reason a machine screw would hold better is because the threads go all the way to the bottom of the bolt versus a tapping screw the threads come to a point.
An extra 1 or half mm of thread on a 6-8mm screw really does nothing, machine screws have much finer threads, therefore more threads in the hole, that's why they hold better. They are designed for threaded metal holes. The tappers are designed to dig into plastic or wood and that's why they have coarse, sharp threads, you won't find a genuine machine screw with the same thread as a tapper, it wouldn't be feasible due to the angle of the thread.