Statistical Process Control

Statistical Process Control

Statistical Process Control is a technique of preventive action. The law of probabilities is the basis of SPC. Therefore, it makes predictions of what your process is going to do based on its immediate past.

The secret to successful use of SPC is having it done by the process operator as the process runs. Plotting data on a SPC chart is the best way to tell whether a process is capable and stable.

Prevention v. Detection

Statistical Process Control (SPC) is an effective method of monitoring a process through the use of control charts. Control charts enable the use of objective criteria for distinguishing common (normal) variation from abnormal variation based on statistical techniques. Much of its power lies in the ability to monitor both process center (x-bar) and its variation about that center (range).

By collecting data from samples at various points within the process, variations in the process that may affect the quality of the end product or service can be detected and corrected before defects happen. This helps in reducing waste as well as the likelihood that problems will be passed on to the customer. With its emphasis on early detection and prevention of problems, SPC has a distinct advantage over standard inspection methods. Inspection detects defects after they have occurred which is more costly than preventing defects.

In addition to reducing waste, SPC can lead to a reduction in the time required to produce the product or service from start to finish. This is due to a reduced probability that the final product will have to be reworked. And, it may also result from using SPC data to identify bottlenecks, WIP, and other sources of delays and costs within the process. Process cycle time reductions coupled with improvements in yield have made SPC a valuable tool from both a cost reduction and a customer satisfaction standpoint.

Statistical Process Control In Manufacturing

In high volume manufacturing the quality of the finished item was traditionally achieved through post-production inspection of the product's features. The product was accepted or rejected based on how well it met its specifications. In contrast, Statistical Process Control uses statistical tools to observe the performance of the production process in order to predict significant deviations that may later result in rejected product.

By using the power of prediction based on the laws of probability and acting on the signals from the control chart potential defects can be avoided.

Two kinds of variations occur in all manufacturing processes: both these process variations cause subsequent variations in the final product. The first are known as natural or common causes of variation and may be variations in temperature, specifications of raw materials or vibration, etc. These variations are small, and are generally near to the average value. The pattern of variation will be similar to those found in nature, and the distribution forms the bell-shaped normal distribution curve. The second kind are known as special causes, and happen less frequently than the first. Special cause of variation is also called abnormal variation.

For example, a machining operation that drills mounting holes in a bracket. Some of the holes will be slightly larger than the nominal and some will be slightly smaller in accordance with a distribution of hole sizes. If the production process, its inputs, or its environment changes (for example, the machines and tooling doing the operations begin to wear) this distribution can change.

For example, as the drill bit begins to dull and fill with material the holes will get bigger. If this change is allowed to continue unchecked, more and more product will be produced that fall outside the design tolerances for form, fit and function, resulting in waste. This eventually results in rework and/or scrap.

By using a SPC control chart the operator can see what's happening in the process. When the process begins to exceed it's normal pattern of variation a correction can be made before scrap or rework is made.

SPC indicates when an action should be taken in a process, but it also indicates when NO action should be taken. An example is a person who would like to maintain a constant body weight and takes weight measurements weekly. A person who does not understand SPC concepts might start dieting every time his or her weight increased, or eat more every time his or her weight decreased. This type of action could be harmful and possibly generate even more variation in body weight. Taking action when it is not needed is called "tampering". Not taking action when it's called for is called "neglect". SPC would account for normal weight variation and better indicate when the person is in fact gaining or losing weight.

Want To Know More?

You can have a SPC class presented at your facility for less than $54.00 per person.

Call or click here and send us a message.

This class is 1 day and includes:

1.         Introduction and overview

2.         Rules for SPC

3.         SPC control charts (variables and attributes)

4.         Applying the knowledge

5.         Process capability

6.         Problem solving techniques using SPC data

7.         Workshops

8.         CEU and RU credits are awarded

AIAG has a good reference guide on SPC for a nominal charge.

 Additional Resources