A harder material will cut or scratch a softer material. Therefore the cutting tool must always be harder than the stock. Different metals, such as iron, wrought iron, steel, copper, brass, etc., possess different grades of hardness. The harder the metal, the greater the friction, the quicker the tool is worn away, and the greater is the strain on the machine. For these reasons it is necessary to have different cutting speeds for different metals. If this speed is exceeded or if too great a cut is removed, the heat will be correspondingly greater. This heat may be sufficient to take the temper out of the steel or warp the stock. Therefore it is very important to know the best cutting speed and feed in all cases, so that the maximum output of the machine can be obtained.