SPC Control Charts



Our SPC software supports the following control charts:

1. Variables Control Charts
1.1. X bar chart using R chart or X bar chart using s chart

The X bar chart indicates the changes that have occured in the central tendency of a process. These changes might be due to such factors as tool wear, or new and stronger materials.

R chart values or s chart values indicate that a gain or loss in dispersion has occured. Such change might be due to worn bearings, a loose tool, an erratic flow of lubricants to a machine or sloppines on the part of the machine operator.

The two types of charts go hand in hand when monitoring variables, because they measure the two critical parameters: central tendency and dispersion.

1.2. X & MR (moving range) chart

An X & MR chart is used when only one observation per subgroup is taken and process variability needs to be determined. The moving range (MR) is used to estimate variation.

Examples include but are not limited to:
- destructive sampling
- testing of the process characteristics is costly
- long production times

2. Attributes Control Charts

2.1. The p chart

The p chart is for the fraction of defective items in a sample.

The fraction defective is the number of defective items in a sample divided by the total number of items in a sample.

The fraction defective chart is used when the sample size varies. If we have a high percentage of good items, say 99%, the fraction defective is small, 0.01. In order to get any defectives in as sample from a high quality population, the sample size must be large. In many cases the sample size is all the daily production. In this situation the sample size will vary from day to day. The only statistical measure of quality would be the fraction rejected.

2.2. The np chart

The np chart is for the number of defective items in a sample.

The number of defective, np, chart shows the number of defective items in samples rather than the fraction of defective items. It requires that the sample size remains constant. It has two benefits over the p chart: there is no calculation required of each sample result; it easier for some people to understand.

2.3. The c chart

The c chart is for the number of defects in an item.

The number of defects, c, chart is based on the Poisson distribution. It is a plot of the number of defects in items. The item may be a given length of steel bar, a welded tank, a bolt of cloth and so on. For the control chart, the size of the item must be constant. If the chart is for the number of defects in a bolt of cloth, all the cloths must be of the same size.

The c chart can also be used for the number of defects in a fixed number of items. The number of defects per 10 bolts of cloth can be plotted on c charts just as well as the number of defects per single roll. The essential factor for using c charts is that each sample has the same opportunity for defects.

2.4. The u chart

The u chart is for the number of defects in a sample.

The symbol u is used to represent defects per unit.

The u chart is used in cases where the samples are of different size. If the sample size varies significantly, each sample value must be plotted within its own u chart limits.

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