Short Run SPC with Tool Wear



Hi everybody,

Here is a case in SPC, that I try to describe below:

1- It is a process using a perishable tool with high Cpk (2 or more),

2- There is tool wear, that causes the part diameter to gradually increase (indiacted by a slow trend on the X-bar chart),

3- Change of part diameter cannot be helped by readjusting the machine/tool. The only way is to change the tool with a new one.

4- The production is typically a "short-run" production. Completion of each order takes one to maximum 3 days.

5- After completion of the first order the tool is NOT changed, but the production goes on using the same tool for another product having the same diameter, but maybe different length, until USL (upper tolerance) is reached.

Any idea about what kind of Control Chart to use in such a situation?

I thought maybe a Nominal Chart would be suitable?


Rick Goodson


The Nominal chart is appropriate for tool wear, however there are a number of cautions and requirements before you apply the chart. There is certainly considerable debate on what is a 'Save Process Average' which is a necessary part of a nominal tool wear chart. I would recommend reading Don Wheeler's and Davis Bothe's material on SHort Run SPC before applying the concepts.

Hope this helps.



Dear Rick,

Thank you very much for the comment. I'll try to find the material you mentioned.

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