NUGGET S™ TECHNICAL APPLICATION BRIEF Introduction to SPC and Weld Checkers ® and Monitors

BACKGROUND Statistical Process Control (SPC) and Statistical Quality Control (SQC), are the buzzwords for manufacturing 474 • ) Frequency Distribution today. Most large manufacturers and s ••

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Weld Checkers and monitors are SPC ••• 3σ = •• devices. What is SPC and how does it X • relate to the Weld Checkers and 426 Monitors and the Resistance Welding process? SPC is a method which uses the concepts of probability and statistics to determine a normal and Frequency Distribution- an arrangement of data showing the number of times that a value acceptable pattern of variation in a has occurred. process. The process is then monitored in order to identify Normal Distribution- a predictable tendency for data to form a “bell curve” frequency unacceptable variations and to distribution. eliminate the causes. Weld Checkers Sample Size (n)- number of observations in a group of data. A minimum sample size of 100 is and monitors measure voltage advised when collecting Weld Sentry data, some customers use 200. and/or current during the resistance welding process. Their basic Mean (Average, X)- for a group of data, the sum of observations divided by the number of functions are to provide data to be observations. used in statistical determinations O Range, R- difference between the largest and the smallest values in a group of data, amount of and to monitor the ongoing process dispersion. and alert the user to unacceptable 3 U ( X + 3σ ) variations in a voltage or current- Standard Deviation (Sigma, σ)- measures the uncertainty of the data to spread out from the based parameter. mean. C ( X = average ) STATISTICAL DEFINITIONS PROCESS Certain accepted statistical patterns Some of the process components of resistance welding include development of weld determine the allowable amount of schedules, tooling and fixturing, equipment maintenance, proper materials, and operator σ variation in a process. The graph training. It is expected that any of these could vary to a slight degree. N o two parts a (r eX e+v e3r ) above illustrates how data on weld exactly the same even if they are of acceptable quality. However, process variations such as pull strengths is used. power fluctuations, misalignment of parts, electrode wear, material differences, or operator error could cause serious variation and produce unacceptable parts. When this variation is 5 10 15 discovered, the “assignable cause” must be determined and remedied.

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