A saw is a very valuable tool in the woods. It is easy to use without much experience, and it can cut down a tree much faster and with more precision than an axe. An unavoidable characteristic of a wood cutting saw is that the longer the blade, the more efficiently it works. While a small folding saw such as a Kershaw or Bahco Laplander is a useful tool, ideally, a large blade would be carried.
The downside to a larger saw of course is the weight. A 24 in or a 30 in saw blade requires a bulky and heavy frame. Few of us can afford to carry such weight.
One solution to the problem is to only carry the blade, and construct the saw from materials you find in the woods. The most common ways are the bow saw and the buck saw. Here is how to make the latter.
You will need three strong pieces of wood. One of them should be the length of the blade, the other two should be 2/3 of the length of the blade. You will also need a small thin stick and a length of rope at least twice the length of the blade.
Take the two short sticks, and using your knife, gauge out a conical hole in each piece of wood. If you are using a 24 in blade, the hole should be at least 10 in away from one of the ends.
Take the end from which you measured the distance to the hole, and split it with your knife. Make sure the split is perfectly aligned with the hole that you gauged out.
Take the side that you just split, and make a small indentation opposite to the gauged out hole, about half an inch from the bottom.
Insert the blade in the split part of the stick, and secure it with a bolt or a stick (the bolt goes through the hole of the blade). The bolt should rest in the indentation you just made.
The gauged out holes should face each other.
Now take the long stick, and sharpen the two ends so they fit in the gauged out holes in the short pieces. Shorten the stick as necessary so that when it is placed in the holes, the two short sticks are parallel to each other.
Take the rope and wrap it around the end of the short sticks, opposite to the blade. There should be at least one wrap around of the rope, so there are at least two strands of rope between the sticks. Now insert a small stick between the strands of rope and begin twisting it until you get the desired tension. When done, the small stick will just rest on the long center stick.
The tension created by the rope will pull the blade, making it stiff, and allowing for use of the saw.
The reason for why it was important to space the gauged out holes away from the edge of the sticks is so that the saw can be used more efficiently. For example, with a 24 in blade, you can theoretically cut through a 20 in piece of wood (leaving the other 4 in of the blade for the motion). If however, you had placed the long stick of the buck saw 5 in away from the blade, even if you cut from opposite sides of the piece of wood, you would only be able to get in 10 in, before the long center stick got in the way. By placing it 10 in away from the blade and cutting from both sides of the piece of wood, you can now get in 20 in. You can do the same math for any length blade.