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 then 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 these limit numbers in specifications, procedures, drawings etc. Humans create specifications.  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 processes creates the control limits.

Learn SPC in an hour. Train your employees. Improve your processes and products. Prevent defects and save your company money.

SPC Training Video

Control Chart SPC 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

Learn SPC in an hour. Train your employees. Improve your processes and products. Prevent defects and save your company money.

SPC Training Video

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 lets say the tolerance is 3. 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.

Comment Box is loading comments...

More Info

  • Auditing SPC inspection

    Learn to audit your SPC inspection program. This article covers SPC technology keys such as documentation, training, reviewing, and process improvement.

  • SPC definition

    Here we discuss the SPC definition. We describe the charts and the meaning of "special cause variation". Read more...

  • SPC Help Video

    Download Today. The number one mistake company’s make when implementing SPC is not training their employees in SPC. Maximize your SPC efforts! Get SPC help. Use this SPC Training Video to quickly train your staff. $59.00. Satisfaction guaranteed.

  • SPC Calculations

    Find out how to conduct SPC calculations here. Page discusses SPC limits

  • statistical process control

    This article covers a roadmap for statistical process control. Discusses chart structure and implementation mistakes.

  • Control Charts

    Control Charts, Variable data, Attribute data, Variation, Total Quality Management Tool

Quality Assurance Solutions
Robert Broughton
(805) 419-3344
Unique QA Products

All Products

Software, Videos, Manuals, On-Line Certifications

8D Manager

Corrective Action Software

Snap Sampling Plans!

AQL Inspection Software

TrainingKeeper Software

Plan and Track Training

StreamLiner Software

Lean and Continuous Improvement

Statistical Process Control

Training Video

ISO 9001:2015 QA Manual

Editable Template

ISO Calibration Manual

Editable Template

ISO 9001:2015 QMS Kit

Templates, Guides, QA Manual, Audit Checklists

ISO 14001:2015 EMS Kit

EMS Manual, Procedures, Forms, Examples, Audits, Videos

On-Line Accredited Certifications

Six Sigma, Risk Management, SCRUM

All Products

Software, Videos, Manuals, Training Material

Please Recommend Us!