I spent a few weeks one summer doing chores for an organ restorer in New Hampshire when I was in high school. He gave me an eight foot wooden pipe that we'd taken out of an old organ, so I got to study it pretty well. I used to make little organ pipes from shirt cardboard and Elmer's glue modeled after this wooden one. They worked perfectly well, and have an amazingly good tone considering they were cardboard. Simple as could be to make. They're just very long whistles.
Wooden ones are the easiest because they are square in cross section.
Metal pipes are the exact same priciple, of course, but require you to be a proficient metal worker.
Reed pipes work on a different principle, and are a bit more complicated to make, although I managed to make one a few months ago. (It took quite a bit of googling to find a good explanation of how reed pipes work.) I also, a few months ago, made a nice, small wooden pipe. It turned out very well and has a very nice tone.
I made so many of the cardboard ones that I now just have a "feel" for how large to make the orifices and slits where the air is directed against the "tongue", and how much to push the tounge down into the air stream. Unfortunately I have no capacity to make or post graphics or I could give you a measured drawing of a basic pipe.
Basically, I score three parrallel lines into the cardboard an inch apart with an exacto knife leaving an inch strip on either side of the outer lines. The score marks let you fold the cardboard sharply.
Down toward one end I cut the horizontal orifice, maybe about an eighth inch by 3/4 inches wide. Before you glue the rectangle shut you have to add the piece that guides the air onto the "tongue". This is a simple piece of more cardboard that is bent at about a 45 degree angle. I make slits from the outer edges of the "tongue" up toward the direction of the top of the pipe about 3/4 inches long so that this tongue can be pushed ever so slightly into the air stream.
If you practise making some of these toy shirt cardboard pipes you'll know everything you need to know about configuring pipes of wood. They're just whistles, as I said.
These are about as small as you want to make them. less than an inch on a side and they get squeaky and touchy.
The rules about pitch to length of the pipe don't work in practise. There is a secondary phenomenon that comes into play where you have a certain amount of length added to the actual length of the pipe by the "plug" of air that is formed at the other end (top) of the pipe when you sound it.
Each pipe, of course, has to have some provision for tuning
it. This can be a sleeve that can be raised or lowered to change the length, or you make it longer than you know it needs to be, and have a sliding element on the back top of the pipe (where it won't distract from appearance) that can be adjusted up or down. Part of the back wall of the pipe would be cut away: that's all it takes to change the pitch - it doesn't have to be cut away all around.
I found a site once where a guy had made a complete working organ from scratch out of wood just for the experience of it. It was small and had only one rank. He had a sound bite and it sounded quite nice to me.
So, it can be, and has been, done. Alot of work, though.
