Title: Prepare for the Inspection
Course: PW-Course-Import-Test > Section: Module 5: Performing an Inspection

Module 5

In this module we will walk through the sequence of steps from gathering the samples and performing the actual inspection. We will discuss the importance of each step and things you need to look out for.

The flowchart below shows the sequence of steps for the inspection process. The sequence of these steps may vary slightly from company to company but the basic process remains the same.

Product Inspection Flow

Verify Sample Size

Before you gather random samples you need to know how many samples are needed for the inspection. The way this information is provided is different for different companies. Some companies may require that the quality inspector refer to the sampling plan every time they have a lot of parts to inspect.

On the other hand, the sampling plan and acceptance criteria may be provided by the engineer in the drawing. This is usually done in a form of a table. When such a table is present the quality inspector does not have to refer to any sampling plan. All they need to do is know what the lot size is and the table will tell you the quantity of parts to sample and acceptance criteria. Now that makes it very easy. Lets look at an example to better understand this.

Say for example you have received 30,000 metal rings that are used in building a medical device. The drawing of this ring lists the specifications that need to be measured. Along with these specification is a table that lists the sample size requirement for different lot quantities. Such a table would look as follows:

Quality Received | Sample Size | Accept | Reject
10,000 | 200 | 7 | 8
20,000 | 315 | 10 | 11
30,000 | 315 | 10 | 11
40,000 | 500 | 14 | 15

Before constructing this table the engineer knows that the metal rings are always ordered and received in quantities of 10,000, 20,000, 30,000 or 40,000. The engineer has used a general inspection level and AQL 1.5 for the sample size and accept-reject criteria.

So for a lot of 30,000 rings that you have received from the supplier you will randomly inspect 315 parts. You will accept if you find 10 bad parts but will reject the entire lot if you find 11 bad parts out of the 315 you inspected.

In either of the case, one should know what the sampling size is before they can gather random samples.

Gather Random Samples

It is important to use samples for inspection that are selected in a random fashion from the entire lot of population. The statistical sampling plans that we learned about in the previous module are valid only when samples are gathered in a random fashion. So what does ‘gathering parts in a random fashion mean?

For example, there are 1,000 parts shipped by a supplier. Since thier parts are relatively small in size, they all fit in one big bag. Say that the statistical sampling plan requires you to collect 80 parts for using a single sampling table. Random sampling means that these 80 parts cannot be all collected from the top of the bag. Parts throughout the bag should be given equal opportunity of being selected in the random sampling. In this case one needs to empty the contents of the bag into a big tote or bucket and then select these 80 parts randomly.

Now say we received the same number of parts (1,000 parts) in 10 bags of 100 parts each. If using the same sampling plan you would be required to gather 80 parts for these 10 bags. In this case one can decide how they want to do the sampling. They can eight parts each from 10 bags or get 10 parts from 8 bags. Doing this will ensure that the entire lot of parts are sampled randomly.

Gathering random samples is time consuming, but it is required. After all, if the parts are not gathered in a random fashion, it defeats the purpose of the inspection and the results become questionable.

Verify Inspection Tool

Most of the inspections require the use of some kind of tool. Different tools can be used to measure different features on a part. It is always best to be consistent and use the same kind of tool for a particular inspection. Using the same kind of tool for a particular dimension has various advantages. We are not going to discuss these advantages here. However, it is important to know to use the tool that is specified by the drawing or any documentation accompanying the drawing.

Say, for example, there are three dimensions to be measured on a particular part. Each of these three dimensions may have a different type of tool assigned for measurement. So it is essential that the quality inspector use the specified tool.

Once the correct tool has been identified the inspector needs to perform two more checks. First is to verify that the tool has a calibration sticker and is within calibration. Second is to verify that the tool does not show any visual damage on it.

Once these checks are made, the tool is ready to use. Go to the next page to walk through the steps of an actual inspection.