SPC Procedure

TITLE: STATISTICAL PROCESS CONTROL 1.0 PURPOSE: This procedure was established to define the proper use of statistical process control needed to ensure monitoring of product/process characteristics results to variation that are statistically in-control and stable and ensure that process demonstrates capability that meets the customer requirements. 2.0 SCOPE: Implementation of this procedure applies to critical product/process characteristics specified in the control plan. Monitoring shall include variable and attribute data gathered during production and inspection processes. 3.0 DEFINITION: 3.1

STATISTICAL PROCESS CONTROL – The use of statistical techniques such as control charts to analyze a process or its outputs so as to take appropriate actions to achieve and maintain a state of statistical control and to improve the process capability.

3.2

VARIATION – The inevitable differences among individual outputs of a process.

3.3

ATTRIBUTE DATA – Qualitative data that can be counted for recording and analysis. Attribute data have only two values (conforming/nonconforming, pass/fail, go/no go)

3.4

VARIABLE DATA – Quantitative data where measurements are used for analysis. Example includes diameter of a bearing in millimeters, the concentration of electrolyte in percent, or the torque of a fastener in Newton-meters.

3.5

COMMON CAUSE – A source of variation that affects all the individual values of the process output being studied; if only common causes of variation are present and do not change, the output of a process is predictable.

3.6

SPECIAL CAUSE – A source of variation that is intermittent, often unpredictable, unstable; sometimes called an assignable cause. If special causes of variation are present, the process output is not stable over time.

3.7

CONTROL CHART – A graphic representation of a characteristic of a process, showing plotted values of some statistic gathered from that characteristic, a central line, and one or two control limits.

3.8

CONTROL LIMIT – A line (or lines) on a control chart used as a basis for judging the stability of a process. Variation beyond a control limit is evidence that special causes are affecting the process.

4.0 REFERENCE:

ISO/TS 16949:2002(e) Standard

5.0 RESPONSIBILITIES: 5.1

QC personnel shall be responsible for gathering the inspection data based on the results of monitoring and measurements performed.

5.2

Production personnel shall be responsible for providing actual and factual measurements data for proper analysis of process controllability and capability.

5.3

QC / Production Supervisor shall be responsible to review the established control chart and assure that analysis and corrective actions are carried out when the data have gone out of the control limits.

6.0 PROCEDURE 6.1

Appropriate statistical techniques will be identified during product quality planning and included in the appropriate sections of the Control Plan.

6.2

Personnel performing statistical process control activities shall be trained or oriented on the use of control charts and interpretation of process capability.

6.3

Specific statistical techniques shall be used during capability studies, incoming and outgoing inspections, and ongoing process control monitoring.

6.4

Capability studies shall be performed on processes that involve characteristics specified in the Control Plan, on characteristics as requested by the customer and for internal problem resolution or continuous improvement as necessary.

6.5

Improvement efforts shall be implemented in accordance with Corrective and Preventive Action, QCD.QSP.005, for machine and processes that do not demonstrate acceptable capability.

6.6

Capability Studies 6.6.1 Initial Process Capability (Short-term) 6.6.1.1 6.6.1.2 6.6.1.3 6.6.1.4 6.6.1.5

6.6.1.6

6.6.1.7

Initial process capability analysis shall be done during product/process quality planning for new product or process. Initial process capability study shall be used to validate measurements collected from one operating run. Before beginning the study, all tool and/or insert changes, adjustments to target, and any necessary preventative maintenance should be done. Make sure nothing abnormal is happening in the process. Once the study starts, no changes should be made to the process. If it is necessary to make adjustments to keep on running out of specification parts, the study should be started over, and the changes documented on the data collection form. Interpret the capability of the machine. 6.6.1.6.1 If Ppk is = or >1.67, the capability of the machine is good. No improvement is necessary. 6.6.1.6.2 If Ppk is = or > 1.33, but < 1.67, the capability of the machine is acceptable. Consider improvement efforts to reduce common causes of variation. 6.6.1.6.3 If Ppk < 1.33, the machine is not capable of running all parts within specification. A request shall be made to the Production Dept. to initiate a corrective action. After Machine Capability has been determined to be at an acceptable level, only then that a Process Capability can be performed.

6.6.2 Continuous Process Capability (Long-term) 6.6.2.1 6.6.2.2

Once a process has been found to be stable and capable of meeting requirements in the initial process capability study, continuous process capability studies are subsequently performed. Continuous process capability studies consist of measurements which are collected over a longer period of time in order to include sources of variation that may not have been observed in the short term studies.

6.6.2.3 6.6.2.4

6.6.2.5

6.7

Capability indices shall be calculated from the data collected from the on-going process control monitoring where process stability and product variation monitoring are being carried out. Interpret the capability value. 6.6.2.4.1 If Cpk = or > 1.33, The process capability is good. Continue charting for long term capability confirmation. No corrective action is necessary. 6.6.2.4.2 If Cpk = or > 1, but < 1.33, the capability of the process is acceptable. Continue to monitor on going capability and consider continuous improvement efforts. 6.6.2.4.3 If Cpk < 1, the process is not acceptable and the process is not capable of running all parts within specification limits. Corrective actions shall be documented and 100 percent inspection for the special characteristics not capable shall be done. After the process capability has been completed and the capability has been determined to be acceptable, continue to monitor using SPC to confirm ongoing capability.

Product/Process Variation Monitoring 6.7.1 Production and inspection personnel shall use appropriate control charts for checking if process is still demonstrating stable conditions. 6.7.2 For economic reasons, the size of subgroups to be collected should be small enough to have a good interpretation of the process performance. 6.7.3 Control charts that indicate out-of-control points shall require investigation of special causes that triggers instability. 6.7.4 Out-of-control indicators include: 6.7.4.1 Points beyond the control limits. 6.7.4.2 Runs. Seven consecutive points that are above or below the process average. Seven consecutive points that are continuously rising or dropping. 6.7.4.3 Obvious non-random patterns. More than 2/3 of the plotted points are inside or outside the inner one-third region of the width of the control limits. 6.7.5 Improvement efforts shall be initiated for processes which demonstrate stability and capability that are not optimal.