Page 74 - DMGT206_PRODUCTION_AND_OPERATIONS_MANAGEMENT

Unit 4: Quality Control

3. Cycling of data can indicate that temperature or other recurring changes in the environment; Notes
differences between operators or operator techniques; rotation of machines; or differences
in measuring or testing devices being used.
4. Several points near a warning or control limit.
In addition, care needs to be exercised when two of three consecutive points are outside the
2-sigma warning limits but still inside the control limits. This may be the result of a large shift
in the process in the equipment, methods, materials, or operator or a shift in the measurement
system. Even when four of five consecutive points are beyond the 1-sigma limits, it should be a
matter of concern. Vigilance ensures that we prevent defectives and not just identify defectives.

4.2.4 Control Charts for Variables

Many quality characteristics can be expressed in terms of a numerical measurement. A single
measurable quality characteristic, such as a dimension, weight, or volume, is called a variable.
Control charts for variables are the most extensively used control charts in industry. They
usually lead to more efficient control procedures and provide more information about process
performance than do attributes control charts.

Constructing charts for means (µ) and X bar chart using MS Excel: The R chart is developed from
the ranges of each subgroup data, which is calculated by subtracting the maximum and the
minimum value in each subgroup. An example showing the development of the charts is
provided.
Let us take an example showing the procedure for making X bar and R Control Charts. We have
discussed the specifications of the steel shaft in an earlier section. Let us consider the measurements
taken on the diameter of the shaft. The aim is to achieve statistical control of the diameter using
X bar and R charts. Measurements of the diameter of ten samples, each of three units, have been
taken when the process is assumed to be in control.
The data was entered into an Excel worksheet as shown in Table below. The mean in each sample
was calculated by selecting the S symbol on the Excel Toolbar and selecting ‘average’ on the
drop down menu. The averages of each row represent the ‘µ’ value for each set of samples. The
data selected automatically by the program needs to be checked and corrected to include only
the data that represented the observations.

Table 4.1: Data for Measurement on Steel Shaft

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