Any of the following may cause saw blades to break:
The saw blade is threaded incorrectly. When you pluck the blade, it should make a “pling” sound. For step-by-step instructions on threading a saw blade into a saw frame correctly, please follow the Setting Up the Saw Blade in the Saw Frame Technique
Sufficient cut lube isn’t used. Cut lube is an essential element to the sawing process. It prevents the saw from sticking and enables a smooth cut. The saw isn’t held at the right angle. Hold the saw frame lightly in your hand while moving the saw frame up and down, keeping the blade at a 90-degree angle to the metal. Move forward in the metal while following your design. If the saw blade is not kept at a 90-degree angle, it will break. Remember that the blade is making a channel that is only as wide as the blade.
Cutting a curve with the blade. Sometimes a blade may snap when it is cutting around a curve or corner. Breakage can occur when you push the blade instead of pushing the metal. Refer to Steps 11 and 12 in the Sawing a Design Out of Sheet Metal Technique