Deviation from Nominal Chart

Summary

An SPC (statistical process control) tool that is a modification of either the I-MR or sometimes the XBar-R/S Chart tool. Use the deviation-from-nominal (delta) chart for low-volume conditions in which a family of parts shares the same process. For example, machining custom shaft diameters between 1.000 and 2.000 inches is all done on the same equipment, but with frequent changes in specified diameters (we might run from 10 to 20 different diameters in a shift). We can measure the individual readings and subtract the nominal (blueprint) diameters from them. Then, we could then use an I-MR chart to plot these deltas. (See I-MR for details.) If each run of a specific diameter is large enough to collect data using rational subgroups, then a delta Xbar-R/S chart is preferred.

• How much common-cause variation does the process exhibit?
• Is the process stable over time?
• Were any special causes apparent during the timeframe of the plotted data?
• Does evidence suggest something has changed or the process performing differently than expected?
When to Use Purpose
Pre-project Assist in project selection by identifying outputs that exhibit high common-cause variation, frequent special causes, unstable variation, or other symptoms that point to the need for improvement.
Start of project Verify process stability when performing a baseline capability analysis.
Mid-project Investigate effects of input variables on the process output over time.
Mid-project Verify process stability when performing confirmation runs after implementing improvements.
End of project Verify process stability after implementing controls to obtain a final assessment of process capability.
Post-project Control inputs to the improved process after the project is complete.
Post-project Monitor the output of the improved process after the project is complete.

Data

Continuous Y data, in which Y is the deviation of each sampled unit from its nominal specification.

How-To

1. Verify the measurement system for the Y data is adequate.
2. Establish a data-collection strategy. Are the runs of parts small enough for you to consider them individually, or large enough for you to use rational subgroups within the runs?
3. In the Minitab worksheet, enter sample measurements into one column and the nominal values for each unit sampled into a second column. Subtract the nominals from the actual measurements; these values are the deltas. Minitab can also directly import data from databases, text files, Microsoft Excel, and so on.
4. If you do not have rational subgroups, run the I-MR Chart with the deltas.
5. If you do have rational subgroups, run the Xbar-R/S Chart with the deltas.

Guidelines

• Verify the operators/staff are actually trying to produce output at the nominal values.
• A critical assumption is the variation is the same for all different nominal values.
• If you have discrete numeric data from which you can obtain every equally spaced value, and you have measured at least 10 possible values, your data are evaluated as though they are continuous.
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