Planar surface: Opposite flat surfaces of a rectangular solid need to be parallel to each other. Cylindrical surface: A cylinder appears round when looking at its end, but when held upright, its sides must be parallel for the cylinder to function properly. Parallelism also needs to be measured on the interior cylindrical surfaces of drilled holes.
In technical drawing, the datum is marked with a capital letter in a square box called the datum feature symbol. All other surfaces of the tested item that are parallel to that surface are marked with the same letter for reference. In machining parts, it is common to define three datum surfaces, for reference in all three dimensions of the solid object. These will be labeled A, B and C. [3] X Research source
The level of precision that you need in measuring the parallelism will factor into just how clean the datum must be.
For example, you might require two surfaces to be parallel to within a quarter inch. This means that the measurement at one end of the surface may be a quarter inch or less from the measurement at the other end of the surface, for the surfaces to be called parallel. If the difference is more than one quarter inch, then it fails parallelism.
For example, a standard gage comparator will have a flat base. This will serve as the surface plate. Set your object on this base to begin the measurement.
For low precision, low-tech measurements, you can use a simple ruler. Measure the distance from the datum to the opposite surface of the object at multiple points and compare these measurements.
For more high quality testing, precise instruments exist that can measure to within a millionth of an inch. Some of these operate with a dial indicator arm that you pass over the tested surface. As you pass the dial indicator arm over the surface of the object, it will register any variations. At the highest level of precision, optical interference technology can measure parallelism extremely precisely. A special kind of glass lens is placed on the flat surface whose parallelism is to be tested. A laser light with known frequency is then passed through the lens. If the surface is precisely parallel, the laser passes through with a predictable wavelength pattern. If the surface is not parallel, the wavelength pattern is disrupted. [7] X Research source
For example, if a particular part is supposed to be 3 cm. thick, with a parallelism tolerance of 0. 02 cm. , you need to review the measurements that you collected. Acceptable measurements will fall between 2. 98 and 3. 02 cm. If you find measurements outside this range, you need to conclude that the surfaces are not parallel. [9] X Research source
