MEANING & SIGNIFICANCE OF STATISTICAL PROCESS CONTROL [SPC] Presented by, JAYA VARATHAN B SANKARAN S SARAVANAN J THANGAVEL S

Transcription

1 MEANING & SIGNIFICANCE OF STATISTICAL PROCESS CONTROL [SPC] Presented by, JAYA VARATHAN B SANKARAN S SARAVANAN J THANGAVEL S

2 PRESENTATION OUTLINE History Of SPC Meaning &Significance Of SPC SPC in TQM SPC in Production SPC Process Variation-Natural&Assignable Using sample in SPC Control charts using SPC Applying SPC to services Advantages of SPC

3 HISTORY OF SPC Foundation for Statistical Process Control was laid by Dr.Walter Shewart In 1920s conducting research on methods to improve quality and lower costs. Concept of control with regard to variation, and came up with Statistical Process Control Charts. Today,SPC is used in both production & services all over the world.

4 MEANING OF SPC Method for achieving quality control in manufacturing processes. An optimisation philosophy concerned with continuous process improvements, using a collection of (statistical) tools for data and process analysis making inferences about process behaviour decision making It Employs control charts to detect whether the process obeserved is under control or not.

5 SIGNIFICANCE OF SPC Detecting error at inspection. More uniform quality of production. Reduces inspection costs. Reduces no of rejects and saves the cost of material. Determining the capability of the manufacturing process. Once the process is stable, provides process capability analysis with comparison to product tolerance.

6 SPC IN TQM SPC Using PDSA CYCLE. Tool for identifying problems and make improvements. Contributes to the TQM goal of continuous improvements.

7 SPC IN PRODUCTION Variation is inherent in every process. Natural or common causes. Special or assignable causes. Four sources of variation - Process, materials, operators & miscellaneous [ includes heat,light,radiation and humidity]. Provides a statistical signal when assignable causes are present. Detect & eliminate assignable causes of variation.

8 PROCESS IN SPC Identify defined process Identify measurable atrributes of process Characterize natural variation of attributes YES Track variation Is process controlled? NO Removes assignable cause Identify assignable causes

9 Common causes NATURAL VARIATION Inherent in a process Cannot be eliminated Like difference in operator,machine vibration, minor variation in raw materials...etc; Output measures follow a probability distribution For any distribution there is a measure of central tendency and dispersion If the distribution of outputs falls within acceptable limits, the process is said in control

10 ASSIGNABLE VARIATION Special causes Larger in magnitude and easily traced Can be eliminated only through improvements in the system Like difference among machines,process, materials, relationship with one another...etc; When assignable causes are present Eliminate the bad causes Incorporate the good causes

11 Frequency USING SAMPLES IN SPC To measure the process, we take samples and analyze the sample statistics following these steps (a) Samples of the product, say five boxes of cereal taken off the filling machine line, vary from each other in weight Each of these represents one sample of five boxes of cereal # # # # # # # # # # # # # # # # # # # # # # # # # # Weight

12 CONTROL CHARTS USING SPC Control charts, also known as Shewhart charts (after Walter A. Shewhart) or process-behavior charts, in statistical process control are tools used to determine if a manufacturing or business process is in a state of statistical control.

13 OBJECTIVES Be able to explain how control charts relate to assigned dimension and tolerance State what value you get from control charts Be able to name several ways that control charts indicate that a process is out of control

14 Reminder: Normal Distribution Defined by two parameters: mean and standard deviation

15 X, R AND S CHART Mean X [avg] - Calculated by summing all of the observations and dividing by the number of observations. Range - Measure of the spread of the data, calculated as highest value minus lowest value SD -Measure of the spread of a set of data from its mean, abbreviated: σ for a population, s for a sample The standard deviation is the square root of the variance.

16 What does the X control chart look like? - First we measure a number of parts as they come off the line. - For eg we might measure 4 parts per hour for 20 hours. Those 80 parts would give us an overall mean and standard deviation that would define the control chart. +3 Time -3

17 Q - How do you know a process is out of control? A When the data aren t normal Out of Control cues include - Points outside of control limits ( 3σ) - 8 consecutive points on one side of center line - 2 of 3 consecutive points outside the 2 limits - 4 of 5 points outside the 1 limits - 7 consecutive points trending up or down

18 Example: Suppose we specify a dimension and tolerance as shown X

19 How does the control chart relate to the tolerances? Assigned Tolerances Measured Variation

20 Defect Prevention When you see signs that the process is out of control you can look for and fix the causes before you make bad parts. The control chart can help you distinguish between common cause & special cause problems.

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23 Applying SPC to Service Nature of defect is different in services Service defect is a failure to meet customer requirements Monitor times, customer satisfaction

24 Hospitals Applying SPC to Service timeliness and quickness of care, staff responses to requests, accuracy of lab tests, cleanliness, courtesy, accuracy of paperwork, speed of admittance and checkouts Grocery Stores waiting time to check out, frequency of out-of-stock items, quality of food items, cleanliness, customer complaints, checkout register errors Airlines flight delays, lost luggage and luggage handling, waiting time at ticket counters and check-in, agent and flight attendant courtesy, accurate flight information, passenger cabin cleanliness and maintenance

25 ADVANTAGES OF SPC Improving product quality Improving productivity Streamlining process Reducing wastage Reducing emissions Improving customer service, etc.

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