How To Cut The Glass Tube

If the glass tube is very thin, you can use a glass knife to draw a circle around the place that needs to be cut off, and then break it off gently.
If the glass tube is thick, you can use a file to file a groove around the place that needs to be cut off, then use a thick cotton thread dipped in gasoline or alcohol to wrap around the groove for several times, light it with fire and turn the glass tube to make the grooved place of the glass tube heated evenly. Put the glass tube into cold water immediately after the fire is extinguished, then the glass tube will snap off automatically.