Hack saws are hand saws that are used to cut metal.
First, as with any material removal in metal, keep the blade and material well lubricated with cutting oil. This can be elliminated and is not as important with soft metals but is needed and will save your hacksaw blade with harder materials. They usually have finer teeth then wood hand saws and the blade should be put in such that the teeth point toward the user handle, not away from the user handle as in a wood saw. As the teeth are pointing toward the user, the cut into the metal is done on the pull stroke and not the push stroke. This allows a finer and more straight cut as the blade pressure during the cutting stroke is from the solid handle and not the tensioned far end. Finer teeth are used for harder metals and courser teeth used for softer metals like aluminum but both should still be metal cutting blades. Fine teeth can be used for hard and soft metals but course teeth should only be used for soft metals. Only apply slight pressure on the pull (cutting) stroke and no perssure at all on the push stroke. Do not force or excesssively bear down on the saw.
Most of the time after the piece is cut there will be a burr on the end of the material. This can be filed or sanded away.