Question

Let's say you manufacture 1cm ball bearings and you measure for:

• Diameter must be correct within 0.01%
• Weight must be correct within 0.01%
• Roundness must be correct within 0.01%
• Hardness: must resist permanent deformity due to slow crushing by a specific testing machine

So each part has 4 opportunities to be defective.

Having identified the opportunities for defects, the next task is to take a representative sample of units and measure them against the criteria.

In the above example, there are: 3 non-destructive physical tests; 1 test which may damage the ball bearing. Let's say that 2,000 bearings are tested from various batches.

If we use the variables:

• D = number of Defects observed in the sample
• U = number of Units in the sample
• O = Opportunities per Unit

Then DPMO = 1,000,000 X D / ( U X O )

Suppose you also manufacture a coiled spring that has 12 opportunities (length and diameter of the wire; length and diameter of the coil; weight; resistance to extension and compression along the axis at three operating temperatures; and roundness of the coil).

3. In testing 2000 coils, 8 coils were rejected; each failed different tests, but only one each. Calculate defect DPMO:

4. Calculate the defective DPMO for coils:

5. Which is the better product, ball bearings or coils? Why?

Answer 1