SPC results, limits and rules

Below are the possible results that summarize the SPC test:

Icon

Tool application

Diagnosis

Further action

Can be used

The tool works correctly.

None

Can be used

The average is higher than the upper control limit, but it does not exceed the upper tolerance limit.

Calibrate: decrease the torque

Can be used

The average is lower than the lower control limit, but it does not fall under the lower tolerance limit.

Calibrate: increase the torque

Can be used

Excessive dispersion of the values prevents proper calibration of the tool, but the measured values are still within the tolerance limits.

Repair

CANNOT be used

At least one value is higher than the tolerance limit.

Remove the tool from the production line and calibrate: decrease the torque

CANNOT be used

At least one value is lower than the tolerance limit.

Remove the tool from the production line and calibrate: increase the torque

CANNOT be used

Some measured values are out of tolerance limits. Excessive dispersion of the values PREVENTS proper calibration of the tool.

Remove the tool from the production line and repair

The SPC Rules give the results listed in the above table.

Below are the SPC test limits:

UL

Upper Limit

LWL

Lower Warning Limit

UCL

Upper Control Limit

LCL

Lower Control Limit

UWL

Upper Warning Limit

LL

Lower Limit

T

Target (nominal)

Upper Limit (UL) and Lower Limit (LL) are the SPC test limits.

The SPC test limits are calculated as follows:

  • Upper Control Limit (UCL) = ((UL + LL) / 2) + (A · ((UL - LL) / 6))

  • Lower Control Limit (LCL) = ((UL + LL) / 2) - (A · ((UL - LL) / 6))

  • Upper Warning Limit (UWL) = ((UL + LL) / 2) + (2/3 · (UCL - ((UL + LL) / 2)))

  • Lower Warning Limit (LWL) = ((UL + LL) / 2) - (2/3 · (((UL + LL) / 2) - LCL))

  • Range = D2 · ((UL - LL) / 6)

A and D2 are coefficients that depend on the number of the SPC tests:

Number of SPC tests

A

D2

1

0.000

0.000

2

2.121

3.686

3

1.732

4.358

4

1.500

4.698

5

1.342

4.918

6

1.225

5.078

7

1.134

5.204

8

1.061

5.306

9

1.000

5.393

10

0.949

5.469

11

0.905

5.535

12

0.866

5.594

13

0.832

5.647

14

0.802

5.696

15

0.775

5.741

16

0.750

5.782

17

0.728

5.820

18

0.707

5.856

19

0.688

5.891

20

0.671

5.921

21

0.655

5.951

22

0.640

5.979

23

0.626

6.006

24

0.612

6.031

25

0.600

6.056

Below are the rules applied to the last set of number of samples performed during a SPC test:

  • RULE 1 – The last average is outside the control limits

  • RULE 6 – The dispersion is too large

Dispersion is too big if it is greater than the range.

  • RULE 7 – At least one value outside the tolerance limits

  • RULE 8 - The dispersion is larger than the warning limit

Below are the rules applied to the last averages related to the set of number of samples performed during a SPC test.

The device stores the last seven averages to analyze the trend according to the following rules:

  • RULE 3 – The last 7 averages are over or under the nominal value

  • RULE 4 - The last 7 averages are increasing or decreasing

  • RULE 5 - The last 2 averages are outside the warning limits

  • RULE 2 - The last 4 averages are outside 1/3 of the control limits