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## SPC for Turning

The following lesson outline details the tasks needed to develop the necessary skills for machinists and operators to implement an SPC system for turning machines such as CNC Lathes or automatic bar machines. Various combinations of these three lessons can be used to train management, supervisory staff, machine set up and operate personnel, as well as those in quality control and inspection. For example, lessons one and two will provide the machinists with the information and skills they need to effectively implement any one of several SPC techniques. Lesson one can be used to provide an overview of SPC for those not directly involved on a day to day basis. Lessons one and three can be used for QC personnel that will be preparing SPC charts and gauges.

The complex mathematics behind SPC have been avoided. With simple seventh grade math skills, the factor tables provided enable control charts and gauges to be prepared for use at the machine. In order to ease implementation at the machine, several SPC techniques are explained which require no math computations by machine operators. Traditional X bar and R charts are covered as well as median charts and pre-control systems. A Process Control Guide is included to enable QC personnel to determine which SPC method to apply to each job based on a simplified capability ratio calculation performed from capability study data. To learn more the training process, see the Philosophy section.

Lesson 1: Statistical Process Control Concepts [ Top ]

After completing this lesson the operator will be able to:

- Explain the following:
- The purpose of SPC and its value to the trainee.
- The purpose of the capability study.
- When capability studies are performed and why.
- The two S.P.C. systems normally used.
- Subgroups.
- Information gained from the capability study.
- The histogram.
- The bar height of histograms.
- The normal distribution curve.
- The center of the bell curve.
- The width of the bell curve.
- What factors are viewed on a control chart.
- The upper and lower control limits.
- Out-of-control and out-of-tolerance relationships.
- When pre-control is appropriate.

- Perform the following tasks:
- Recognize a control chart.
- Recognize a histogram.
- Recognize a normal distribution curve.
- Recognize the upper and lower control limits.
- Recognize the center or average line.
- Recognize the center of a distribution, and the range of a distribution from a normal distribution curve.

Lesson 2: Using the Control Chart and Pre-Control Systems [ Top ]

After completing this lesson the operator will be able to:

- Explain the following concepts:
- The standard X bar and R chart.
- The meaning of the X and bar symbol.
- The meanings of the scale lines on the X bar chart.
- The meaning of an R chart.
- The meaning of the scale lines on the R chart.
- The effect of increases in R on the bell curve width.
- The relationship between X bar and R changes.
- The meaning of the center line or average line.
- The median chart.
- The advantage of median and range charts.
- The subgroup size for median charts.
- The subgroup size and its relationship to machine spindles.
- The purpose of a range line.
- The type of jobs on which pre-control can be used.
- Out-of-control conditions under pre-control systems.
- The stoplight gauge system.
- Allowing parts to use the full tolerance range in pre-control.

- Perform the following tasks:
- Compute an X bar value.
- Plot an X bar value.
- Compute an R value.
- Plot an R value.
- Recognize the three out-of-control conditions on both X bar and R charts: beyond limits, midpoint shift, and trend.
- Plot points on a median and range chart.
- Determine the average value on a median chart.
- Determine the correct subgroup size for single spindle and multi-spindle machines.
- Use a range line to determine out-of-control range values.
- Write corrective actions taken on charts at the out-of-control points.
- Use a pre-control system.
- Use a stoplight gauge for pre-control.
- Identify out-of-control conditions using pre-control.
- Use either the two-part or five-part pre-control sample.

Lesson 3: Capability Studies and Control Chart Preparation [ Top ]

After completing this lesson the operator will be able to:

- Explain the following concepts:
- The variables that determine the process capability.
- The value of using capability studies before and after machine repair.
- Both full and mini capability studies.
- The R bar and X double bar values.
- The capability ratio.
- The R bar and X double bar and its relationship to tolerance range.
- The Process Control Guide.
- Gauging frequency and inspection frequency.
- Control limit factor tables.
- Control limit computation for X bar, R, and median charts.

- Perform the following tasks:
- Perform a full capability study.
- Perform a mini capability study.
- Record the capability study data.
- Compute X bar values.
- Compute R values.
- Compute R values for mini capability studies.
- Compute X double bar values.
- Compute R bar values.
- Place X double bar relationship on tolerance range.
- Place R bar value on tolerance range.
- Determine the S.P.C. system to use for various capability ratios.
- Determine the gauging frequency from capability ratios.
- Determine the inspection frequency from capability ratios.
- Compute X bar chart control limits from factor table.
- Compute R chart control limits from factor table.
- Compute median chart control limits from factor table.