This article explains what incoming quality control is, and how best to use it to gain quality.

What is the purpose of the acceptance test?

The incoming inspection allows the acceptance or rejection of a batch of parts presented by a supplier (internal or external).

 

What should be defined for the acceptance test?

1/ Initially, it is determined that :
• a level of control of the batch (a severity)
• an AQL (Acceptable Quality Level) per characteristic

2/ Depending on the level of control, the AQL and the size of the batch, the standard indicates a number of parts to be sampled according to the normal, reduced and reinforced modes, as well as an « acceptance » and a « rejection » criteria.

3/ Characteristic by characteristic :
• the batch is accepted if there are less than x bad parts in the sample (acceptance criterion)
• the batch is rejected if there are more than x bad coins in the sample (rejection criteria).

 

Here is an example:

I receive a batch of 100 pieces.
Based on a defined inspection level, I am asked to take 20 pieces from this batch.

For a monitored characteristic with an AQL of 4, I get an acceptance criterion of 2 and a rejection criterion of 3 :

• I refuse the lot as soon as I have 3 bad coins (or more) for this rating
• I can accept it if I have no, 1 or 2 bad pieces only

For a more critical characteristic monitored with an AQL of 1, I get an acceptance criterion of 0 and a rejection criterion of 1

• I refuse the lot as soon as I have a bad piece for this rating
• I can only accept it if the 20 pieces checked are in conformity

 

The basic principle is therefore relatively simple.
I only measure a sample of the batch (not 100%), and I have more or less severe criteria depending on the criticality levels of my characteristics.

 

The advanced functions of the incoming quality control

We will see that this basic principle can be quickly completed.

Depending on the history of the batches of parts already received (for a given supplier), we will also manage types of control :

Reduced control

The batches received were accepted, so the number of parts to be checked is reduced and the acceptance and rejection criteria are adjusted.

 

Enhanced control

The last batches received had problems, so the number of parts to be checked is increased.

 

Skip mode

If the quality level received is really excellent and is maintained (i.e. we have been in reduced inspection for a long time), we can even switch to « skip » mode, i.e. we no longer inspect the characteristic concerned in all batches, but only one batch out of 3, 5 or even less.

This transition between the different modes (from reinforced to normal and from reduced to skip) and the conditions that allow it is called dynamisation. It is very important that this dynamisation is characteristic! On the same batch of parts, we can control characteristics in normal mode, others in reduced mode, and others still in reinforced mode.

 

Progressive control

In addition to the simple sampling plan presented above (when the number of pieces requested has been checked, one can only accept or refuse the batch), there are also double, multiple or progressive sampling plans (one can not accept or refuse the « first time », but ask to withdraw a sample to complete the study).

On a practical level, progressive control is certainly the most promising type of control: one can stop the control (by accepting or rejecting the batch) at each new part controlled; one does not have to wait for the end of the control of the defined sample.

Configured with the same statistical risks as a single or double sampling plan, it is the plan that limits the number of parts measured to a minimum, while ensuring the same results!

There is also a particular way of expressing the acceptance and rejection criteria for the measured characteristics. In this case, we will speak of a K coefficient (Ki and Ks).
The acceptance will be the result of a calculation involving the average and the dispersion of the measurements of the parts in the batch.

 

What is the advantage of an incoming quality control software?

Depending on the batch size and acceptable quality levels (AQL), the software configures a table of measurements to be checked, characteristic by characteristic.
Depending on the history of the last measurements for each dimension checked, the software can dynamically change the level of control :

• reinforced (more parts are checked and the acceptance criterion is increased),
• or reduced (fewer parts are checked).

 

Frédéric Henrionnet
CT Infodream

 

Discover Qualaxy Control, the incoming quality control module of the Qualaxy Suite
Discover the advanced functions of Qualaxy Control, the incoming quality control module of the Qualaxy Suite

To find out more, read the article on our blog entitled Some principles of reception control

This article describes some principles of reception control and answers the following questions:

• What is reception control?
• What are the differences with SPC?
• How can reception control be optimised?

 

What is Reception Control?

Incoming or reception control is the process used to validate the quality of a supplied lot of parts or components. The outcome of this process is simply to determine if the lot is « compliant » or not?

If yes, accept the lot, and if no, return it to the supplier.

This « reception control » process is extremely important:
– if we reject a lot and our production system is « just in time », it will de-stabilise the production schedule and could cause other knock-on issues in the workshop (material or component shortages);
– If we accept a « bad’ lot », it will have a direct effect on the quality of future processes and cause manufacturing or assembly problems.

Either way, the effects are immediate…

Given the important nature of this decision there are international standard defining the process, statistical methods and rules. These are found under the term « Control Reception » and the standards are ISO2859 or ISO3951).

 

 

What is the principle of reception control

The principle is to validate a lot by checking a sample of pieces (we can’t check 100% of the parts!). Based on the risk that we decide to accept (depending on the critical nature of the products), we will decide the precise number of parts to be tested (sample size) and the acceptance criteria (e.g. out of a batch of 1000 pieces, we check 20, and if there are 2 or more that are non-compliant, we refuse the lot).

On a practical level, the terms used are:

• AQL: Acceptable Quality Level or
• NQT: Quality Level Tolerated.

There are 2 types of risk :

1. accept a bad lot (which is a risk to the consumer)
2. reject a good  (risk to the producer/supplier)

For a more precise description of the terms and indicators, you can read the article « Reception control vs SPC ».

 

Differences with SPC

When we speak of risks (α, β), sampling and statistics, links with SPC (Statistical Process Control) are obvious.

 

The lack of production sequence on the control charts

The fundamental difference lies in the lack of production sequence on the control charts.

In reception control, we take random samples from the lot, so we have no notion of time between them: it is not known whether sample A was manufactured before the B!

In SPC monitoring: samples are always taken in sequence so process changes can be detected.

 

The interpretation of results

Another difference lies in the interpretation of results and final actions.

Using reception control, the final action is to accept or reject the entire lot!

Using SPC Monitoring the interpretation is dynamic and pro-active. The charts are used to determine when to change or adjust the process to prevent bad quality.

 

 

How to optimise reception control?

As with SPC, there are ways in which we can optimise the reception control process.

Lighter inspection

Have lighter (reduced) or increased (reinforced) inspection based on the history of the supplier results.

Alternative sampling

Apply alternative sampling/decision rules to increase the speed of the process:

• simple or classic sampling: We take all the samples before making the decision, or
• sequential sampling: we stop taking samples as soon as we are sure the lot is sufficiently good (or bad)

 

Dimensional measurements

Use variable charts for dimensional measurements, i.e. not just good / not good.

All these possibilities are related to the initial risk we decide to take: we can change the sampling or the rules of acceptance while having the same statistical confidence.

Optimising the reception control process allows you to reduce the control time by up to 50%!

However, to achieve this, it is essential to use a software that can work according to different standards (attributes / variable measurements), different sampling plans (single, double, etc), and enables the dynamic control of part characteristics.

 

Frédéric Henrionnet
CT Infodream

 

Learn more about Qual@xy SPC software

In this article, we will discuss two concepts in the field of Quality: incoming inspection and SPC. What links exist between the two? What are the common functions? Reception control vs SPC, let’s go!

Reception control and SPC functions

We often separate incoming inspection (incoming inspection of purchased parts and components) and SPC inspection (in-process inspection of manufactured parts). These are not the same references, nor the same people (inspectors in the first case, operators in the second), nor often the same inspection locations (laboratory or workshop). But the finished product manufactured and assembled by the company does include these two types of parts. Should we therefore continue to oppose these two functions, and above all continue to dissociate these two types of measurement data?

 

What is the reception control?

The incoming inspection (or incoming inspection, incoming inspection) makes it possible to accept or reject a batch of parts from a supplier (usually external, but also internal). This stage is very important. Indeed, refusing a batch can lead to a shortage of material in the workshop (especially if you are on a just-in-time basis). Accepting a batch containing potentially non-compliant components can also cause a quality crisis in the rest of the process.

Read also What is the reception control?

 

Incoming inspection: supplier monitoring and reporting

The basic unit of incoming inspection is the batch of parts.

But just as the objective of the SPC is to move from a « part conformity » vision to a « statistical evolution » vision, a Reception Control software must manage the evolution over time of the different acceptances, of the different suppliers, etc. It must therefore propose a complete reporting: summary table of batches, suppliers, evolution of the percentages of refusals per period of time, etc. It must therefore offer complete reporting: summary table of batches, suppliers, evolution of refusal percentages per period of time, etc.

Documentary editing is therefore important. Depending on a batch refusal report, a supplier may have to start quality actions or even be downgraded.

 

Interest of a common SPC and Control database

There are therefore many functions and concepts that are common to both the reception control and the SPC :

• Moving from a conformity vision (batch/piece) to a statistical vision (evolution over time)
• Notions of centring and dispersion (my batch is accepted, of course, but is it well centred? Grouped together? Dispersed? What will be the impact on the final assembly?)
• Punctual reporting (PV on a batch/PV on a part) in parallel with temporal reporting (by time period)
• Definition of inspection plans, incoming inspection and/or SPC
• Notion of SPC or ISO alarms (refusal of a batch)
• Enabling/submission ranges to be compared with green light/production ranges, etc.
• Process and measurement means, repeatability, reproducibility, etc.

But the most important thing is that these purchased components will be used to manufacture or assemble a part or sub-assembly, the final characteristics of which will depend on both the « supplied » and « manufactured » parts. In incoming inspection, there is no notion of real time reaction, to go and adjust the process as soon as possible. But once the measurements have been taken, why not use them to adjust the assembly processes? To shift manufacturing IT when possible? To carry out pairing not by part, but by batch? To detect long-term drifts that are independent of the production itself?

Having a single database means that product references can be defined only once, even if they may have « manufactured » dimensions (and monitored in SPC), and others « purchased » (and monitored in incoming inspection). It will be possible to pool results, changes, make correlations, etc..

In a word, we will be able to better control our process…

Frédéric Henrionnet
Quality Assurance Manager at Infodream

Learn more about Qualaxy Control Software
See the advanced functions of Qualaxy Control software