Measure it twice because you can only tool it once
Once engineering has satisfied the product criteria--the product must be tested. In general, this requires a physical prototype be built. The prototype may have mechanical parts that are created subtractively, that is via cutting processes like milling and turning. The parts could also be created additively, that is via a "rapid prototyping" process that selectively deposits material layer-by-layer or vector-by-vector. The prototype may also have other parts and assemblies, such as electronics, sensors, actuators and the like.