I have an idea for a spear, I want to make a spear with a nice decorated main shaft and multiple foreshafts with diferent heads that will screw into it so. think of it as a multi spear with interchangable heads for diferent purposes such as hunting, fishing, and battle. let me know what you think and it would be nice if you guys could post some pics of diferent spear heads, harpoons, and other things you think will help

You will have lots of prior examples to give you guidance on the matter.  Many, many cultures use spears with interchangeable foreshafts socketed into a more permanent main shaft.   Paleoarts posted pictures of some of his magic recently when he worked on a socketed spear.  In truth, his advice would be much greater than anything I have to say.

But I don't think having a literal screw joint would be a good idea.  Finding a tapping screw to put the inside threads on the socket would be easy enough, though expensive if you have to buy the tapping screw.  Putting the outside threads on that which is to be socketed would be extremely hard if you don't have access a lathe geared and timed to cut threads.  You would be reduced to filing the threads out by hand, and good luck getting the two even enough to mate properly.  

You would need to use a very dense crossgrain wood if the threads are to last, not the best stuff for a spear, but great for hand carving.  Also, the grooves of the threads would encourage the wood to split when stressed.  In my untried opinion, you would be better off with a simple friction joint.

for the screw I would simply turn the main female form on the metal lathe with multiple male forms to attatch to the forshafts and tap them by hand. there is no need to file the threads they make tap and die sets for threading metal (both male and female forms) but i also have another idea for attachment which would be a mortise and tenon joint and a bone or antler pin through it which could be pushed out to change foreshafts