The Pursuit of Excellence
A Manager's Guide to Quality 		Specifications Revisited


Attempt 1 - The Results

Assemblies Don't Fit Total Accepted Too Loose
1000 120 880 120
120 14 106 14
14 2 12 2
2 0 2 0
1136 136 1000 136

After several levels of assembly and re-assembly, we finally have 1000 finished units. However, about 136 of them will be too loose to function properly. This is not the end of the story, either.

You will recall that the ideal gap was in the range 0.03mm to 0.07mm, shown in green on the figure below.

Distribution of clearance between shaft and bearing, showing ideal range

By calculating the area under the curve between 0.03mm and 0.07mm, we find that roughly 36% of the assemblies can be expected to fall within this range. We can, therefore, calculate the number of assemblies which will be classified as ideal, and add this information to our table.

Assemblies Don't Fit Total Accepted Ideal Too Loose
1000 120 880 360 120
120 14 106 43 14
14 2 12 5 2
2 0 2 1 0
1136 136 1000 409 136

So, in the final reckoning, we have gone through the process of assembling a shaft and bearing together 1136 times, to get 1000 assemblies, of which only about 864 are suitable, and only about 409 are ideal. Surely we could do better than this.