117 Impacts7. Impacts of Automation on of Precision Au Alkan Donmez, Johannes A. Soons 7.4 Cost and Benefits of Precision................ 119 Automation has significant impacts on the econ- omy and the development and use of technology. 7.5 Measures of Precision ........................... 120 In this chapter, the impacts of automation on precision, which also directly influences science, 7.6 Factors That Affect Precision .................. 120 technology, and the economy, are discussed. As 7.7 Specific Examples and Applications automation enables improved precision, precision in Discrete Part Manufacturing .............. 121 also improves automation. 7.7.1 Evolution of Numerical Control Following the definition of precision and the and Its Effects on Machine Tools factors affecting it, the relationship between pre- and Precision .............................. 121 cision and automation is described. This chapter 7.7.2 Enablers to Improve Precision concludes with specific examples of how automa- of Motion .................................... 122 tion has improved the precision of manufacturing 7.7.3 Modeling and Predicting processes and manufactured products over the last Machine Behavior and Machining .. 122 decades. 7.7.4 Correcting Machine Errors.............. 122 7.7.5 Closed-Loop Machining (Automation-Enabled Precision) .... 123 7.1 What Is Precision? ................................ 117 7.7.6 Smart Machining.......................... 124 7.2 Precision as an Enabler of Automation ... 118 7.8 Conclusions and Future Trends .............. 124 7.3 Automation as an Enabler of Precision ... 119 References .................................................. 125 7.1 What Is Precision? Precision is the closeness of agreement between a se- In this definition, the specified conditions describe ries of individual measurements, values or results. For whether precision is associated with the repeatabil- amanufacturingprocess,precisiondescribeshowwell ity or the reproducibility of the measurement process. Part A the process is capable of producing products with iden- Repeatability is the closeness of agreement between re- tical properties. The properties of interest can be the sults of successive measurements of the same quantity dimensions of the product, its shape, surface finish, carried out under the same conditions. These repeata- 7 color, weight, etc. For a device or instrument, precision bility conditions include the measurement procedure, describes the invariance of its output when operated observer, instrument, environment, etc. Reproducibil- with the same set of inputs. Measurement precision is ity is the closeness of the agreement between results of defined by the International Vocabulary of Metrology measurements carried out under changed measurement as the [7.1]: conditions. In computer science and mathematics, pre- cision is often defined as a measure of the level of detail ...closeness of agreement between indications ob- of a numerical quantity. This is usually expressed as the tained by replicate measurements on the same or number of bits or decimal digits used to describe the similar objects under specified conditions. quantity. In other areas, this aspect of precision is re- Springer Handbook of Automation Nof (Ed.) • ! Springer 2009 1 118 Part A Development and Impacts of Automation ferred to as resolution: the degree to which nearly equal with the true value due to the infinite amount of infor- values of a quantity can be discriminated, the smallest mation required to describe the measurand completely. measurable change in a quantity or the smallest con- To the extent that it leaves room for interpretation, the trolled change in an output. incomplete definition of the measurand introduces un- Precision is a necessary but not sufficient condition certainty in the result of a measurement, which may for accuracy. Accuracy is defined as the closeness of or may not be significant relative to the accuracy re- the agreement between a result and its true or intended quired of the measurement; for example, suppose the value. For a manufacturing process, accuracy describes measurand is the thickness of a sheet of metal. If this the closeness of agreement between the properties of thickness is measured using a micrometer caliper, the the manufactured products and the properties defined result of the measurement may be called the best esti- in the product design. For a measurement, accuracy is mate of the true value (true in the sense that it satisfies the closeness of the agreement between the result of the definition of the measurand.) However, had the mi- the measurement and a true value of the measurand – crometer caliper been applied to a different part of the the quantity to be measured [7.1]. Accuracy is affected sheet of material, the realized quantity would be differ- by both precision and bias. An instrument with an in- ent, with a different true value [7.2]. Thus the lack of correct calibration table can be precise, but it would information about where the thickness is defined intro- not be accurate. A challenge with the definition of ac- duces an uncertainty in the true value.Atsomelevel, curacy is that the true value is a theoretical concept. every measurand or product design has such an intrinsic In practice, there is a level of uncertainty associated uncertainty. 7.2 Precision as an Enabler of Automation Historically, precision is closely linked to automa- quette de Gribeauval promoted the use of standardized tion through the concept of parts interchangeability. parts for key military equipment such as gun carriages In more recent times, it can be seen as a key en- and muskets. He realized that interchangeable parts abler of lean manufacturing practices. Interchangeable would enable faster and more efficient manufacturing, parts are parts that conform to a set of specifications while facilitating repairs in the field. The development that ensure that they can substitute each other. The was enabled by the introduction of two-dimensional concept of interchangeable parts radically changed the mechanical drawings, providing a more accurate ex- manufacturing system used in the first phase of the pression of design intent, and increasingly accurate Industrial Revolution, the English system of manu- gauges and templates (jigs), reducing the craftsman’s facturing. The English system of manufacturing was room for deviations while allowing for lower skilled based on the traditional artisan approach to making labor. In 1778, master gunsmith Honoré Blanc pro- aproduct.Typically,askilledcraftsmanwouldmanu- duced the first set of musket locks completely made facture an individual product from start to finish before from interchangeable parts. He demonstrated that the moving onto the next product. For products consisting locks could be assembled from parts selected at random. Part A of multiple parts, the parts were modeled, hand-fitted, Blanc understood the need for a hierarchy in measure- and reworked to fit their counterparts. The craftsmen ment standards through the use of working templates had to be highly skilled, there was no automation, and for the various pieces of the lock and master copies to 7.2 production was slow. Moreover, parts were not inter- enable the reconstruction of the working templates in changeable. If a product failed, the entire product had to the case of loss or wear [7.3]. The use of semiskilled be sent to an expert craftsman to make custom repairs, labor led to strong resistance from both craftsmen and including fabrication of replacement parts that would fit the government, fearful of the growing independence their counterparts. of manufacturers. In 1806, the French government re- Pioneering work on interchangeable parts occurred verted back to the old system, using the argument that in the printing industry (movable precision type), clock workers who do not function as a whole cannot produce and watch industry (toothed gear wheels), and ar- harmonious products. mories (pulley blocks and muskets) [7.3]. In the mid Thomas Jefferson, a friend of Blanc, promoted to late 18th century, French General Jean Baptiste Va- the new approach in the USA. Here the ideas led to Springer Handbook of Automation 1 Nof (Ed.) • ! Springer 2009 Impacts of Automation on Precision 7.4 Cost and Benefits of Precision 119 the American system of manufacturing. The Ameri- Precision remains a key requirement for automa- can system of manufacturing is characterized by the tion. Precision eliminates fitting and rework, enabling sequential application of specialized machinery and automated assembly of parts produced across the globe. templates (jigs) to make large quantities of identical Precision improves agility by increasing the range of parts manufactured to a tolerance (see, e.g., [7.4]). tasks that unattended manufacturing equipment can Interchangeable parts allow the separation of parts pro- accomplish, while reducing the cost and time spent duction from assembly, enabling the development of on production trials and incremental process improve- the assembly line. The use of standardized parts fur- ments. Modern manufacturing principles such as lean thermore facilitated the replacement of skilled labor manufacturing, agile manufacturing, just-in-time manu- and hand tools with specialized machinery, resulting facturing, and zero-defect manufacturing cannot exist in the economical and fast production of accurate without manufacturing processes that are precise and parts. well characterized. The American system of manufacturing cannot ex- Automated agile