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ISYE 315 DISCUSSION 8 (Ch. 13 SPC and Process Capability) Spring 2018 Statistical Process Control (SPC) Variable Control Charts Term Definition SPC Charts SPC charts are used to monitor a product’s key quality characteristics. We can compare current performance to process history to detect special causes and to determine if the process is stable and in statistical control. If we see an outlier sample, we can investigate to see why it happened, related trends, how to prevent the anomaly, etc. This will allow for improved economic operation. Special Cause (assignable) Variation The variation can be pinpointed to one cause that can be corrected at the machine-level Makes up about 15% of quality problems Ex. local faults, broken tool, tool wear, changes in raw material, and machine slippage Think of special causes happening at the machine level, or at a very specific part of operations Common Cause (random) Variation The variation effects all the output quality and requires a systematic change Makes up about 85% of quality problems Ex. system faults, poor tool maintenance, poor supervision, poor training, inappropriate work methods, poor workstation design Think of common causes happening at a system level - there is a larger scale issues and possible need for system redesign Variable Charts ( X and R and moving range) Used for variable data (any value in a large range that is expressed as a number (a specific measurement… ex: height, weight Equations: Monitoring Process Mean Monitoring Process Standard Deviation 0 Estimated by X and 0 estimated by 2 d R X - R charts R D UCL R D LCL R A X UCL R A X LCL R R X X 4 3 2 2 Moving Range Chart (Use when a single measurement is available – no rational subgroups of size n) X_R m charts m Rm m Rm m X m X R D UCL R D LCL R E X UCL R E X LCL 4 3 2 2 1
ISYE 315 DISCUSSION 8 (Ch. 13 SPC and Process Capability) Spring 2018 Problem 1: A forging process produces Piston rings for an automotive engine. We wish to establish statistical control of the inside diameter of the rings manufactured by this process using x and R charts. 25 samples, each sample of size 5, have been taken when we think the process is in control. The inside diameter measurement from these samples are given below. 25 1 028 . 1850 i x 25 1 581 . 0 i R 25 1 2350 . 0 i S The specification limits on this piston ring are 74.000 0.05 mm. (a) What are the control limits for R X __ chart? (b) What is the process standard deviation? 2 =
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ISYE 315 DISCUSSION 8 (Ch. 13 SPC and Process Capability) Spring 2018

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