Statistical Process Control

Transcription

1 Statistical Process Control Applying SPC Student Guide

2 First Edition (July 1992), Second Edition (July 1995), Third Edition (December 2000) Fourth Edition (February 2003) Please address any reader comments to The Quality Group, 6059 Boylston Drive, Suite 250, Atlanta, GA The Quality Group may use or distribute any of the information you supply in any way it believes appropriate without incurring any obligation. You may, of course, continue to use the information you supply. Copyright 1992, All rights reserved.

3 Table of Contents Applying SPC 1 Selecting Parameters 3 Review of Control Chart Benefits 5 Overview of the SPC Implementation Process 6 Take a New Look at the Process: Prepare to Collect Data 9 Words of Advice 21 Activity 1 22 Activity 2 25 Types of Control Charts 29 Control Chart Review 32 Parts of Charts Review 34 Chart Rules Review 36 Variables and Attribute Data Review 39 Types of Charts 40 Activity 1 52 Activity 2 56 Setting Up and Using a Control Chart 61 Calculator Review 64 Calculations on the C chart 67 Calculations on the X-bar and R chart 75 Using a Control Chart: Interpretation and Action 96 Identifying Problems and Causes: Control Chart Patterns 98 Identifying Problems and Causes: Pareto Charts and Fishbone Diagrams 103 Implementing and Sustaining Action 105 Activity Glossary 112

4 Fight Varitaion: The SPC Motto

5 Applying SPC Contents This Course Group contains three courses. Selecting Parameters In this unit, you will see how an actual SPC team decided which parameters to control in their process and what tools helped them make that decision. Types of Control Charts You will learn the characteristics of two different control charts and how they are used. Setting Up and Using a Control Chart You will actually calculate control limits for two types of control charts by following the step-by-step directions given in the interactive training and Student Guide. In this unit, you will be required to perform calculations, create control charts by hand, and refer to data provided for your use. You will need some sharp pencils, a calculator, a straight edge or ruler for drawing straight lines. Based on patterns on control charts, you'll learn to interpret a control chart and determine if and when you should adjust your process. You will also see the role your team can play in reducing variation by using data from control charts. Applying SPC Selecting Parameters Page 1

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7 Selecting Parameters Highlights The SPC Implementation Process is designed to get you and your SPC team started using SPC in your process. To get the full benefit of using control charts, you and your team have to decide how SPC will be used in your process. This is not an overnight task. The effort to make your process more stable requires teamwork and commitment. The SPC Implementation Process consists of three separate phases: Collect Data, Control Special Causes, Improve Capability. In the Collect Data phase, you and your SPC team will identify what parameters to control and establish a plan for collecting the data from those parameters. In the phase where you Control Special Causes of variation, you and your SPC team will calculate control limits from the data, and you will identify special causes of variation. In the Improve Capability phase, you will reduce the common causes of variation. Your SPC team will work with others over a longer time frame to improve the overall efficiency of the process. In this course, Selecting Parameters, we will focus on the first phase of the SPC Implementation Process -- Collect Data. Applying SPC Selecting Parameters Page 3

8 Parameters that you want to control will be identified. You'll identify those parameters that should be controlled first. Your knowledge of the process and advice from your engineer and expert will help. There are also tools to help you and your SPC team pick the first parameters you want to measure. Pareto charts, fishbone diagrams, and block diagrams (process maps), among other things, will allow you to see your process in a new way. You'll learn about the Control and Capability Phases of the SPC Implementation Process in the next courses. Terms Defined in the Glossary Pareto chart Fishbone diagram Statistically designed experiment Experiment Parameter SPC expert SPC driver Page 4 Applying SPC Selecting Parameters

9 Content Review Review of Control Chart Benefits Control Chart Benefits The benefits of using control charts are that they: Allow operators to identify problems and monitor solutions when implemented. Allow operators to take action on process spreads that are not capable, not centered, or are changing. Give operators an opportunity to take a larger role in improving the process. Allow operators to understand when investigation and action should and should not be undertaken. While special causes should always be investigated, common causes should not be. Provide an objective means of communicating to others in meetings regarding the status of a process, as well as the progress in making improvements. Provide a rational basis for asking for an engineering change to the process or for new equipment. Applying SPC Selecting Parameters Page 5

10 Overview of the SPC Implementation Process Overview The implementation process occurs through a variety of people -- management, the SPC team, operators, engineers, SPC experts, and SPC drivers -- over the course of several meetings and even several years. You will recall that the SPC Implementation Process is made up of three different phases: 1. Collect data from the process. 2. Control special causes of variation. 3. Improve capability by reducing common cause variation. Page 6 Applying SPC Selecting Parameters

11 Now let's take a quick look at these steps one by one. Collect data from the process Take a new look at the process, and decide "What data can we collect?" and "What will it tell us about what contributes to quality?" Three different ways of looking at the process are: Block diagrams Pareto charts Fishbone diagrams The purpose of this step is to understand your process in terms of its relationship to other operations, as well as the processes upstream and downstream. For many operators, this may be a new way of looking at how the process fits in. Identify the potential things to control (called "parameters") by deciding what has the most effect on quality and cost. Take the parameters that can potentially be used to monitor the process and put them in order of priority. Select the top parameters, classify the types of data, and set up a plan to collect the data. Control special causes Use the data to calculate the control limits. Plot data on control charts at the workstation. Identify the special causes of variation and take action to eliminate them. Applying SPC Selecting Parameters Page 7

12 Improve capability Improving capability by reducing the common cause variation is a process that usually occurs in stages over a long period of time: Monitor common cause variation on a control chart and --with involvement and support from the SPC team -- investigate how to narrow the spread of common cause variation. Note that this is a long-term and complex process. It involves the cooperation of other people and may result in the redesign of the process, or getting new equipment. This process is also complex for the following three reasons: You can't tell which single step or element in the process has the most impact on common causes. A statistically designed experiment is probably needed to find the process setting that will minimize common cause variation. A breakthrough in technology, such as a new tool of some kind, may be needed to reduce common cause variation. Reducing common cause variation typically requires the combined effort of lots of people over a long period of time. Now let's take a closer look at the first step of the SPC Implementation Process. Page 8 Applying SPC Selecting Parameters

13 Take a New Look at the Process: Prepare to Collect Data There are several things to keep in mind as you and your SPC team select the parameters and prepare to collect data. They are: Step 1: Construct a block diagram of the process. A block diagram is a simple form of a process map. Step 2: Use Pareto charts and fishbone diagrams. Step 3: Identify key process parameters. Step 4: Select key parameters to monitor. Step 5: Classify data and identify the type of chart to use. Step 6: Collect initial data. Applying SPC Selecting Parameters Page 9

14 Step 1: Construct a block diagram of the process Let's look at each of them in some detail. Even though you may be quite familiar with your process, it is helpful to take a step back and take a new look at what the process is doing. Some ways to help you do that include: Draw a block diagram that illustrates your process. Roughly sketch the process showing the inputs, the various operations in the process, and the results. Talk it over with the other team members. Ask the others what they think about the process and where it can be improved. Agree on the "hot spots" -- the causes of frequent trouble. Make a list of the kinds of problems you and the others have experienced over and over again. Step 2: Use Pareto charts and fishbone diagrams As you can guess, one process may have quite a few parameters that you could control and chart. To sort out which ones are important, your SPC team could use Pareto charts and fishbone diagrams. These are pictures that help you understand the importance of different parts of the process. Page 10 Applying SPC Selecting Parameters

15 Pareto chart The Pareto chart is a bar chart. Each bar can stand for one type of defect or problem, and they are arranged in order, from the problem or defect that happens most often to the one that happens least often. This highlights specific problem areas. When this information has been collected and diagrammed, the SPC team can decide where to concentrate its efforts -- for example, going after the most common defect first. Typically, a few causes account for most of the variation. The Pareto chart concentrates efforts on the troublemakers -- the few that are causing most of the problems. Here is an example of how to make a Pareto chart. The data that follow are the reasons people have given for not going to your restaurant: Customer Complaints -- January to June 32 Couldn't park, had to walk 3 Stew too spicy 21 Angry waiters 5 Burned cookies 17 Parked, got towed 18 Cookies not sweet 12 Waiters ignored me Applying SPC Selecting Parameters Page 11

16 First, the problems have to be sorted into categories, such as: Parking problems (49 complaints) Rude waiters (33 complaints) Cooking problems (26 complaints) The Pareto chart for this data would look like this: Number of complaints Parking problems Rude waiters Cooking problems Type of customer complaint The horizontal axis (labeled "Type of customer complaint") lists the type of problem or defect. The vertical axis (labeled "Number of complaints") shows how often each type of problem or defect occurs. The Pareto chart has bars like a histogram, but it is usually used with attribute data, while a histogram is mainly used for variables data. Page 12 Applying SPC Selecting Parameters

17 Let's assume we have continued keeping track of customer complaints through December, and the results looked like this: Customer Complaints -- January to December 112 Parking problems 73 Rude waiters 58 Cooking problems 12 Noisy 2 Ant in food Applying SPC Selecting Parameters Page 13

18 The Pareto chart for this data would look like this: Number of complaints Parking problems Rude waiters Cooking problems Noisy Ant in food Type of customer complaint At the end of the year, the completed Pareto chart shows us even more problems -- and new categories of problems, too! Let's look at what the Pareto chart tells us: The reasons people don't go to your restaurant in order of importance. The first category of problems accounts for the most complaints, but fixing the parking problems would be very expensive to solve. Page 14 Applying SPC Selecting Parameters

19 Which is the most important reason that you can do something about? The next two categories -- rude waiters and cooking problems -- may be fairly easy to solve, and take care of a relatively large number of complaints. Fishbone diagram The fishbone (or, "cause and effect") diagram graphically shows the elements involved in a process in terms of cause and effect. The diagram helps answer the questions "Where is variation occurring?" and "What are the causes -- and sub-causes?" The SPC team starts by listing effects -- that is, any variation in quality. Then, you'll work backwards and list different causes. You will group them in various categories, such as: The quality of the product(s) produced (What?) The equipment and materials used (Who?) The measurements taken (How well?) The work methods involved (How?) The environment (Where and when?) The SPC team uses the fishbone diagram to talk about what might be causing the variation in quality. Applying SPC Selecting Parameters Page 15

20 Here is an example of a fishbone diagram: Materials Placement Solder paste Raw card Component type Process time Robotic Automatic Vendor A Vapor phase Vendor B Good soldering Paste height IR Measurement Reflow method Step 3: Identify key process parameters Now, identify the key parameters that you think would be good to control. Here are some things to consider: Identify key parameters and the effects of key parameters on the process. Use experience, observation, expert advice, or statistically designed experiments. Focus on the parameters that are known to lead to high scrap or defect rates. Page 16 Applying SPC Selecting Parameters

21 If tooling or other process parameters do not have a: Control standard Standard panel Calibration standard Monitor plate or wafer then consider defining a control standard with the help of management, the SPC team, or the SPC expert. Keep in mind that the parameters from this standard must be defined in terms that can be either measured or counted consistently. Or, let your SPC expert develop a statistically designed experiment to isolate the parameters that are and are not worth charting. These experiments help sort out which parameter has the most impact on results or other parameters. Parameters probably not worth charting include those that: Are constant, or set at fixed values Have a doubtful effect on quality Have a high cost to control Have data that is hard to get Applying SPC Selecting Parameters Page 17

22 To review, find the parameters that are worth controlling by: Separating causes from results Identifying key parameters Developing control standards, if needed Using a statistically designed experiment to identify key parameters, if needed So, in identifying parameters, ask yourself these five questions from the Parameters Test Checklist: 1. How does this parameter affect quality? 2. Can we control it? 3. How expensive would it be to control the parameter? 4. Is data available for the parameter, or is it too costly to collect? 5. Does the data vary? (If the data doesn't vary, there is no need to use a control chart.) Step 4: Select key parameters to monitor Not every parameter you've identified in Step 3 can or should be plotted on a control chart. They can be prioritized according to the following set of monitoring priorities. Page 18 Applying SPC Selecting Parameters

23 Priority 1: Measuring Priority 2: "Making" equipment First is measuring equipment. Since the measurement equipment gives the data we use, it makes sense to ensure that it is in control first. So, we control measurement tools before anything else. Ask yourself, "Are the measurements in control? Can I trust the readings?" Second is equipment that makes product. Control the equipment that makes product. It is better to monitor cause rather than effect. Is the machine in control? Priority 3: The Product Or, control the product or result of the process. If there is nowhere else to start, control the product. Is the product in control? Other considerations Here are some other considerations in selecting which parameters to control. Control many things! If in doubt, control a number of parameters. Over time, you will be able to see which control charts are beneficial and which are not, allowing you to focus on important parameters for control. Perform a statistically designed experiment. Statistically designed experiments are more valuable than anything your SPC team can do to identify parameters to control. These experiments can tell you three things: Applying SPC Selecting Parameters Page 19

24 Which parameters have the most effect on quality. Where to center that parameter to get the best outcome. How two or more parameters influence each other. Minimize known special causes of variation Step 5: Classify data and identify the type of control chart to use Once you have identified the parameters you will initially control and before you collect data, you and your SPC team can take steps to minimize known special causes of variation. If you have an unstable process, now is the time to eliminate known and obvious causes of variation. Special causes of variation known to the operators and the SPC team should be eliminated before actually using the chart. These special causes of variation could have other implications. For example, the process may not be operating as designed. Or, now may be the time for maintenance adjustment or part replacement. Then, decide if the type of data is variables or attribute. If it is variables data, consider X-bar and R ( x and R), X-bar and S ( x and S), or individual and moving range charts (X and MR). Remember, variables data can be measured -- for example, the width of a line. If attribute data, consider C, U, P, and NP charts. Attribute data is counted or classified -- for example, the number of items defective, number of defects, etc. Step 6: Collect data After you collect initial data, you're on your way to learning how control limits are calculated, one of the subjects in the upcoming training. Page 20 Applying SPC Selecting Parameters

25 Words of Advice There are a few summary points to remember before you go through the upcoming training: 1. In selecting priorities, go after "more bang for the buck" -- focus your attention on the most critical parameter first. 2. If you have no clear idea about what parameters to control, it is probably best to start charting data you're familiar with. 3. You may or may not be collecting the correct data today. A new look at the process will help you determine this. Applying SPC Selecting Parameters Page 21

26 Activity 1 Purpose Instructions The purpose of this activity is to give you an opportunity to practice working with a Pareto chart. You own a mail-order business that sells outdoor clothing. For the past year customers have given you a variety of reasons for their dissatisfaction with your business. 1. Read the data presented below on "Reasons for Dissatisfied Customers," and create a Pareto chart on the next page. Draw the bars, and label the causes under the word "Cause." 2. Answer the questions at the bottom of the page. Check your answers with those that follow. The point When you can clearly see the key reasons for dissatisfied customers -- specific areas where your business is not strong -- you can focus your attention on strengthening your weak areas. Reasons for Dissatisfied Customers 16 Wrong Fit 24 Didn't match description in catalogue 1 Received in damaged condition 19 Too few or too many shipped 31 Wrong invoice amount 4 Colors faded after one washing Page 22 Applying SPC Selecting Parameters

27 Cause Cause Cause Cause Cause Cause Questions 1. Which two areas might you focus your attention on first to make your customers more satisfied? 2. Which area might you work on last? Applying SPC Selecting Parameters Page 23

28 Answers to Activity 1 Reasons for Dissatisfied Customers Number of complaints Wrong invoice amount Didn't match descript. Too few/too many shipped Wrong fit Colors faded Rec'vd in damaged condition Type of complaint Questions 1. Which two areas might you focus your attention on first to make your customers more satisfied? Wrong invoice amount, Didn't match description 2. Which area might you work on last? Received in damaged condition Page 24 Applying SPC Selecting Parameters

29 Activity 2 Purpose Instructions The point The purpose of this activity is to give you an opportunity to practice working with a fishbone diagram. Look at the fishbone diagram, which illustrates how to make chocolate-chip cookies. Then, answer the questions. Check your answers with those that follow. Using this simple example, you can see the power of analyzing a process through the use of a fishbone diagram. This tool allows you to see the variables that may be interacting with each other. Applying SPC Selecting Parameters Page 25

30 Questions Ingredients Work Methods Fresh Eggs Sugar Flour Butter Timer Measurement Meas. Spoons Stove Equipment Recipe Quantities Time Cookie sheet Aluminum Steel Temp Glass Good Cookies Color Size Texture Raw Burned Taste Sweet Chocolate Nuts 1. Based on this fishbone diagram, name at least eight things that could go wrong while making good chocolate-chip cookies. 2. If the cookies came out of the oven burned, circle the appropriate category (categories) that could be responsible. Ingredients Work Methods Measurement Equipment Page 26 Applying SPC Selecting Parameters

31 Answers to Activity 2 Questions 1. Based on this fishbone diagram, name at least eight things that could go wrong while making good chocolate-chip cookies. Too much sugar Not enough time in oven Temperature too hot Butter is sour Not enough eggs Too much flour Stove breaks during cooking Measuring spoons mislabeled (other answers are possible) Applying SPC Selecting Parameters Page 27

32 2. If the cookies came out of the oven burned, circle the appropriate category (categories) that could be responsible. Ingredients Work Methods Measurement Equipment The cookies could be burned by any of these. Your job is to carefully examine which parameters of these categories may be the cause. The fishbone diagram makes that easier. Page 28 Applying SPC Selecting Parameters