One of the axioms, or sayings, of production is that no two objects are ever made exactly alike. In fact , the variation concept is a law of nature because no two natural items in any category are the same. The variation may be quite large and easily noticeable, such as the height of human beings, or the variation may be very small, such as the weights of fiber-tipped pens or the shapes of snowflakes. When variations are very small, it may appear that items are identical; however, precision instruments will show differences. If two items appear to have the same measurement, it is due to the limits of measuring instruments. As measuring instruments have become more refined, variation has continued to exist; only the increment of variation has changed. The ability to measure variation is necessary before it can be controlled.
There are three types of variations in piece part production:
- Within - piece, variation is illustrated by the surface roughness of a piece, wherein one portion of the surface is rougher than another portion.
- Piece to piece variation occurs among pieces produced at the same time. Thus, the light intensity of four consecutive light bulbs produced from a machine will be different.
- Time to time variation is illustrated by the difference in product produced at different times of the day. Thus, product produced in the early morning is different from that produced later in the day, or as a cutting tool wears, the cutting characteristics change.
Categories of variation for other types of processes such as a continuous and batch are not exactly the same; however, the concept is similar.
- Variation is present in every process due to a combination of the equipment, materials, environment, and operator. The first source of variation is the equipment. This source includes tool wear, machine vibration, work holding - device positioning, and hydraulic and electrical fluctuations. When all these variations are put together, there is certain capability or precision within which the equipment operates. Even supposedly identical machines will have different capabilities. This fact becomes a very important consideration when scheduling the manufacture of critical parts.
- The second source of variations is the material. Because variation occurs in the finished product, it must also occur in the raw material (which was someone else's finished product). Such quality characteristics as tensile strength, ductility, thickness, porosity, and moisture content can be expected to contribute to the overall variation in the final product.
- A third source of variation is the environment. Temperature, light, radiation, particle size, pressure, and humidity all can contribute to variation in the product. In order to control environmental variations, products are sometimes manufactured in white rooms. Experiments are conducted in outer space to learn more about the effect of the environment on product variation.
- A fourth source is the operator. This source of variation includes the method by which the operator performs the operation. The operator's physical and emotional well-being also contribute to the variation. A cut finger, a twisted ankle, a personal problem, or a headache can make an operator's quality performance vary. An operator's lack of understanding of equipment and material variations due to lack of training may lead to frequent machine adjustments, thereby compounding the variability. As our equipment has become more automated, the operator's effect on variation has lessened.
The preceding four sources account for the true variation. There is also a reported variation, which is due to the inspection activity. Faulty inspection equipment, the incorrect application of quality standard, or too heavy a pressure on a micrometer can be the cause of the incorrect reporting of variation. In general, variation due to inspection should be one-tenth of the four other sources of variations. Note that three of these sources are present in the inspection activity - an inspector or appraiser, inspection equipment, and the environment. Below picture shows hierarchy of varitation categories.