Four different feed rates were investigated in an experiment on a CNC machine producing a component part used in an aircraft auxiliary power unit. The manufacturing engineer in charge of the experiment knows that a critical part dimension of interest may be affected by the feed rate. However, prior experience has indicated that only dispersion effects are likely to be present. That is, changing the feed rate does not affect the average dimension, but it could affect dimensional variability.

The engineer makes five production runs at each feed rate and obtains the standard deviation of the critical dimension

(in 10−3 mm). The data are shown below. Assume that all runs were made in random order.

Feed Rate Production Run

(in/min) 1 2 3 4 5 10 0.09 0.10 0.13 0.08 0.07 12 0.06 0.09 0.12 0.07 0.12 14 0.11 0.08 0.08 0.05 0.06 16 0.19 0.13 0.15 0.20 0.11

(a) Does feed rate have any effect on the standard deviation of this critical dimension?

(b) Use the residuals from this experiment to investigate model adequacy. Are there any problems with experimental validity?