First Edition Dated September, 2000 26
Steps for doing SPC:
1. Determine the process steps where key characteristics are to be
measured
2. Determine the type of data to be analyzed.
3. Select the appropriate control charts.
4. Select the control chart subgroup size and sampling frequency
5. Collect and plot process data
6. Calculate the appropriate control limits
7. Identify out-of-control points and take corrective action to eliminate
the causes
8. Identify the major sources of variation present in the process and
remove or reduce them
9. Calculate process capability indices
10. Continue to reduce variation, as needed
Tooling
Tooling can sometimes provide a method for ensuring consistent product and
a statistically stable process. Measurements must be taken to demonstrate
consistency and stability resulting from the use of tooling.
Two aspects of tool design and manufacture should be considered.
1) Tooling should be designed and built with the goal of minimizing variation
at every stage of part manufacture. This goal often suggests minimizing
variation within certain processes and reducing the amount of variation
introduced by each tool. For example, there may be a need to:
• Reduce warping and twisting during process heating and cooling.
• Ensure adequacy and repeatability of locating features on tools.
• Control flexure during loading, building, or moving of the part.
2) It is important that tooling be designed and built to facilitate the collection
of variables measurements for each key characteristic. Methods should be
developed for indexing the part to the tool so that required key characteristic
measurements can be easily obtained (see Section 6.6).
Standard Processes
A standard process is one that has been developed and documented to be
consistently followed, and is considered the best current method.
Implementing standard processes can be a powerful tool in reducing
variation. A standard process description should include both machine
settings and controls, and work methods. Without standard, repeatable
processes, root cause identification is nearly impossible. Standard processes
