Control Chart SPC
You can use control chart SPC to determine whether your process is capable or not capable. Capability means your process performs well within specifications. With SPC you compare your control chart limits to your specification limits. You plot both sets of limits on the same chart. SPC software can automatically do this for you.
Control Limits vs Specification Limits
Engineers determine specifications. They document the limit numbers in specifications, procedures, drawings etc. Specifications are created by humans. In many cases, people create these specifications without process knowledge.
The manufacturing process determines the statistical process control limits. These are scientifically limit calculations that come from the data. 99.7% of collected data should fall within these statistical limits.
Specifications differ from control limits. Humans create the specification limits. Manufacturing process creates the control limits.
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Statistical Process Control (SPC) is a scientific visual method used to monitor, control and improve processes by eliminating special cause variation from manufacturing, service and financial processes. It is a key Six Sigma tool.
SPC directs the process champion when to make take action to adjust or investigate the process. Control chart SPC also prevents inappropriate or unnecessary process adjustments. SPC tells you when there is special cause variation or when the process is drifting. SPC can be used to catch machine wear, operator setup issues, raw material changes, and differences between similar machines. Overall, one uses SPC to prevent defects from occurring and tracks process improvement.
You will learn...
What is control chart SPC
Why use SPC
Process vs. product focus SPC
Types of SPC charts
Examples of control chart SPC uses
SPC and normal distribution
Making the SPC chart and the math behind SPC
Setting up the SPC charts
Interpreting the SPC charts
Special Cause Variation
SPC chart rules
Dynamic vs. Static Limits
Spec limits and process capability
Sampling frequency and sample size
Moving Range Chart and Calculations
For more details on the SPC course agenda, click here.
(Course created by Quality Assurance Solutions)
Control Charts limits and Graphs
By graphing the process data, the control limits, the specification limits on a control charts, one can quickly tell if the process is capable of meeting the specification.
In the graph above you see the control limits (UC and LC)are well within the specification limits (US and LS). The process is capable of meeting the specification.
In the graph above you see the control limits (UC and LC) above and below the specification limits. In this case the process is not capable of meeting the specification. When this occurs you have a couple of choices to ship good material.
- Change the manufacturing process
- Change the specification
- Conduct 100% inspection
- Stop shipment of the product
Non-Capable Process Example
If you examine a mechanical drawing you see tolerance callouts for each dimension. These tolerance callouts are specifications.
Many times, these specifications relate to the number of significant decimal points for that dimension on the drawing. For example, if the target dimension is written as 1.000” then the 3 significant decimal points means the allowed tolerance is +/- .003. If the target is written as 1.00” then the allowed tolerance is +/- .03
As described above, when tying significant digits to specifications, there exists a possibility that there is a non-capable process. Why is this?
AutoCad operators normally create the drawings. Although experts with the AutoCad software, they are not experts with the process capability. They have not been trained in control chart SPC. They learned tolerances but not process control and control charts.
The drawing specifications are arbitrary and only based on significant digits. Only the manufacturing process determines the true output measurements for that given dimension. The drawing's specification has no relationship to the manufacturing capability.
After reviewing this page on control chart SPC, see here for more on the SPC definition.
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