I do not find it necessary, nor indeed within my capacity, to explain, in English, to me a foreign language, the workings of an organ pipe. There are many good Web pages on the subject. Let me just mention Wikipedia, or How the flue pipe speaks.
Instead I shall describe my method and the various decisions I had to make along the way. Soon after I started seriously to plan my pipe production, It became clear that a multitude of things had to be taken into consideration.
The most common material of which organ pipes are made is an alloy of tin and lead. This was never a serious option in my case. In these early days of my project, the fact that organ pipes could indeed be made of paper, had not come before my eyes. I think I might well have considered it, but in the end I was left with wood as the only manageable way to go. I had some lengths of mahogany, 9x50 mm in diameter, which I used for my first pipe. But it was clear that this would suffice for only very few pipes, and I was not sure if I could afford to buy quality wood for 50 - 60 pipes.
Then I came across Phil Radford's video on Youtube, where he shows how to make pipes out of MDF (Medium density fibre). MDF, for those who do not know, is made from wood fibres by applying pressure and heat, is strong and uniform in texture, universally available in various thicknesses and reasonable in price. I decided to make all my pipes of MDF.
Now, one might ask if this would give the same sound quality as traditional materials. I have it from reliable sources that it has been found extremely difficult to show that materials make any difference in tone quality as far as organ pipes are concerned.
The width of a pipe has influence on the tone quality. Narrrow pipes are richer in harmonics. The relationship between windsheet, languid and mouth opening is critical, among other things not mentioned here. Wind pressure is also critical. The pitch is determined by the length of the pipe body. On the Internet one can find tables giving all the critical dimensions of a rank of pipes. I had tables from Raphi Giangiulio and Johan Liljencrants, Excel tables that could be adjusted for various parameters according to your needs. The general rule is that each pipe is different, in length obviously, but also the width, mouth opening, windsheet and wall thickness are scaled.
I reasoned that it would be too cumbersome for me to closely follow these recommendations. From Phil Radford I learned that even a whole octave in the same width was possible, and that is how I started. Soon my plan developedn into making 6 - 7 pipes in the same width, going for the average width of that particular range in the tables. Wall thickness is 4 mm for the smallest twelve, 6 mm for the 20 middle ones and 9 mm for the 19 at the bottom of the spectrum. I also made my pipes with a fixed upper lip (see Building pipes illustrated). This decision saved me some time and effort, but in hindsight, an easily adjustable upper lip would come in handy when voicing the instrument.
Anyway, these decisions made it possible for me to plan the MDF dimensions needed and have it cut at the retailers. At this point in time I had not decided how many pipes I would build, but gradually I came down to the number of 51, from c (131hz)to d (2349hz). To save space, and MDF, I decided to have the bottom octave stopped, that means the pipes are roughly the same length as the next octave above, but wider.
I started making pipes of the middle range, not too small and not too big. My worst problem at that time was to get the windsheet right, both smooth and precise in thickness. I did this by cutting, filing or sanding off the languid until the opening was right, and then I glued the lower lip in place. Lacking precision tools this proved very time consuming and inaccurate. Later, I took up the method of using a gasket, or shim, of paper or cardboard to determine the windsheet, and I also took to fixing the lower lip with 2 screws. This is a much better method, unless you have a precision router to trim the languid.
The open pipes have a tuning slot cut down into the front side, partly covered with a tuning slide. I made six pipes with real slides and guiding rails glued on the pipes. I did not like this method and tried to find an easier way to hold the slide. I had som 1 mm steel wire that I used to make pallet springs. It was indeed possible to make springs that would hold the slides in place, a much quicker method, and really neat to look at. Examples of the two types of springs I developed are in the illustrations section. For the 4 - 5 smallest pipes I made tuning slides of thin tin-plate, formed to fit around the pipes at the top.
The stopped pipes are tuned by nudging the stoppers in and out to vary the internal length of the pipe. The stoppers are made of 20 mm thick wood, with rounded edges and padded with leather to make them air tight.
Before the final assembly, the inside of a pipe has to be varnished. I ignored strong advice to use polyurethane varnish. The strong smell is unbearable in a home, so I have used water based acrylic varnish throughout. The classical method, dipping the pipes in warm glue, was not even considered.