There are several reasons why the saw blade of a paper cutting machine vibrates during cutting. Some of the main causes of blade vibration are listed below:

1. Blade Imbalance: Blade imbalance due to wear and tear of the blade or incorrect installation can cause the blade to vibrate during operation leading to inaccurate and rough cuts.

2. Inappropriate Blade Design: The design of the blade can also be a factor in blade vibration. If the blade is not adequately designed or does not match the specifications of the material being cut, it can cause the blade to vibrate during cutting, leading to poor quality cuts and uneven edges.

3. Blade Quality: Poor blade quality can cause vibration during cutting. If the blade is made of inferior quality material or is not sharp enough, it cannot cut efficiently, resulting in vibration and poor quality cuts.

4. Cutting Material: The properties of the material being cut can also cause the blade to vibrate. If the material is too hard, the blade may not be able to cut through it smoothly, causing vibration. Similarly, if the material is too thin or flimsy, it can cause the blade to shake while cutting, leading to imprecise and rough cuts.

5. Machine Setting: The settings of the cutting machine can also contribute to blade vibration. If the speed at which the blade cuts is too high or the pressure applied on the blade is insufficient, it can cause vibration during cutting.

In conclusion, blade vibration during cutting in paper cutting machines can be caused by a variety of factors. However, proper maintenance of the machine and the blade, using appropriate blade design, high-quality blades, and correctly setting the machine parameters, can significantly reduce blade vibration and improve the quality of cuts.