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Bend a test pipe at a 90-degree angle. Find the place where the bend in the pipe starts. Mark the ends of the bend with a permanent marker. Lay the pipe against the square again to find the length of the pipe in the bend. Find the place on your bending die where the bend begins.
Not only will this familiarize you with how much force you need to apply to operate your bender, but this pipe will serve as a reference for future bends. To check the angle of your pipe, lay it against a carpenter's square with the outer bend facing the corner of the square. Both ends of the pipe should just touch the square's sides and run parallel to them. You should see or feel a small flat spot or distortion at the place where the bend starts and where it ends. Draw the line completely around the pipe. Note the place on each side of the square where the pipe's markings touch. These should be the same distance from the inside corner of the square. Add these lengths together. If the markings on each end of the pipe bend touch the square at 6 inches (15 cm) from the inside corner of the square, the total length of the bent section of the pipe is 12 inches (30 cm). Place the bent tube back in your bender with the die used to bend it and note where on the die the mark on the pipe lines up. Mark this place with a dot of paint or by notching the metal with a file.   If you have more than one die (for different diameters of pipe), make a test bend for each die, as each diameter will require a different amount of metal to make a 90-degree bend. Once you know how much pipe is needed to form the bend, you can calculate how long a piece of pipe you need by adding this figure (called the bend deduction) to the vertical and horizontal lengths of the pipe.