Rugby School Thailand Design & Technology Senior School

2.9.2 Quality Control

Critical path analysis

Critical Path Analysis (CPA) is a project management method used to analyse all individual stages within a project, and to plan the most effective and time efficient completion of each element within the desired schedule.

Using a task analysis, the project/process can be split down and each individual element can be arranged in time order. The basic order is sequential, with each element being completed one after the other.

This method identifies wasted time within the process, when the individual completing the process is waiting unnecessarily to complete the next task.

By analysing dependencies, the process time can be reduced.

Now we will look at analysimg an everyday activity:

Making a cup of tea

It is clear from the list of all tasks that if they were all complete one after the other, there would be a lot of wasted time.

By reorganizing the tasks using dependencies to structure the process, the overall time required to make the tea can be shortened. In this case the process is reduced to 420 seconds; a saving of 90 seconds in the process. Now imagine how much time this could save in a more complex industrial process.

The red line is the critical path (longest time path) and shows the maximum time required to make the cup of tea because the tasks on the bottom line are completed as the kettle boils, which takes just as long as the brew.

Further examples of CPA

To complete a CPA, several elements are required:

a list of all activities within the project
an estimate of the time each stage will take
an understanding of how each stage relates/depends on the completion of other stages in the plan (dependencies)
specific deadlines for individual stages/items

Each stage is given a letter. A circular node, which represents the start of each stage, is included, with the stage letters shown above the joining lines.

The left hand of each circle shows the node number. Upper right is the earliest start time (EST) in hours and bottom right shows the latest finish time (LFT) for the previous stage so as not to impact the overall finish time.

Quality control

Quality control (QC) refers to the monitoring, checking and testing of materials, components, equipment and products throughout production to ensure they conform to acceptable tolerances specified within the QA policies within the company.

QC inspection checks take place throughout the production process and are performed in conjunction with the strict guidance documentation produced by the company and the client.

Material checks

At the beginning of any production process, the materials must be checked for their compliance with the manufacturers specification. These can consist of:

simple visual checks on the materials supplied
chemical analysis on small volumes of the material
colour matching for polymer pigments of paint finishes

Dimensional accuracy checks

have been covered in the previous units and are carried out as part of a QC system.

Digital measuring devices

These were also covered in more detail in the previous unit but it is worth a reminder of some of the digital tools used for measuring accurately. These include digital vernier calipers and micrometers.

Go/no go gauge

When checking dimensional accuracy on a production line, a specific measuring instrument is often used such as a go/no go gauge, which checks whether a single measurement fits within a tolerance range, giving a simple pass/fail reading. This is quicker than using a vernier or micrometer because the operator does not have to check for an accurate reading. It also requires little training to perform this task.

A go/no-go gauge is a precision measuring tool used to verify whether a workpiece meets the specified dimensional tolerances or not. Here are some specific applications where a go/no-go gauge is commonly used:

Machining industry: Go/no-go gauges are widely used in the machining industry to ensure that parts are manufactured to precise tolerances. They are used to check the dimensions of threaded components such as bolts, nuts, and screws.
Automotive industry: In the automotive industry, go/no-go gauges are used to check the dimensions of critical components such as engine parts, transmission gears, and brake components.
Aerospace industry: In the aerospace industry, go/no-go gauges are used to check the dimensions of components that are critical for safety, such as turbine blades, fuel system components, and hydraulic components.

Two reasons why a go/no-go gauge would be used:

To ensure product quality: Go/no-go gauges are designed to verify whether a workpiece meets the specified dimensional tolerances or not. By using these gauges, manufacturers can ensure that their products meet the required quality standards and avoid costly rework or scrap.
To improve efficiency: Go/no-go gauges are quick and easy to use, which makes them ideal for high-volume production environments. By using these gauges, manufacturers can quickly check the dimensions of parts and identify any out-of-tolerance parts before they are assembled, which can help to improve production efficiency and reduce waste.

can you write it as a response to an exam question

Sure! Here's an example response to an exam question about go/no-go gauges:

Question: List some specific applications where a go/no-go gauge is commonly used and give two reasons why it would be used.

Answer: Go/no-go gauges are precision measuring tools used to verify whether a workpiece meets the specified dimensional tolerances or not. They are widely used in various industries, including the machining industry, automotive industry, and aerospace industry. Here are some specific applications where a go/no-go gauge is commonly used:

In the machining industry, go/no-go gauges are used to check the dimensions of threaded components such as bolts, nuts, and screws. In the automotive industry, these gauges are used to check the dimensions of critical components such as engine parts, transmission gears, and brake components. In the aerospace industry, go/no-go gauges are used to check the dimensions of components that are critical for safety, such as turbine blades, fuel system components, and hydraulic components.

Two reasons why a go/no-go gauge would be used are to ensure product quality and to improve efficiency. These gauges are designed to verify whether a workpiece meets the specified dimensional tolerances or not. By using these gauges, manufacturers can ensure that their products meet the required quality standards and avoid costly rework or scrap. Additionally, go/no-go gauges are quick and easy to use, which makes them ideal for high-volume production environments. By using these gauges, manufacturers can quickly check the dimensions of parts and identify any out-of-tolerance parts before they are assembled, which can help to improve production efficiency and reduce waste.

Quality control: Go/no-go gauges are often used in manufacturing and quality control to ensure that the final product meets the required specifications. For example, a go/no-go gauge can be used to verify the diameter of a machined part.
Precision measurement: Go/no-go gauges can be used to measure precise dimensions of parts or components, especially in situations where traditional measuring tools like calipers or micrometers are not appropriate.
Time-saving: Using a go/no-go gauge can be quicker than measuring a part with traditional measuring tools. This can save time, especially when working with large numbers of parts or components.
Standardization: Go/no-go gauges provide a standardized method of measurement, ensuring that measurements are consistent across different operators and production runs.
Ease of use: Go/no-go gauges are relatively simple tools that require minimal training to use, making them accessible to a wide range of operators.
Versatility: Go/no-go gauges can be used to measure a variety of different dimensions, including diameters, thread pitch, and hole depths.

Overall, go/no-go gauges offer a convenient and efficient way to ensure that parts and components meet the required specifications, helping to ensure quality and consistency in manufacturing processes.

Co-ordinate measuring machinery

The use of co-ordinate measuring machinery (CMM’s) such as a probe scanner, allows a manufacturer to check a range of predefined measurements on finished components.

CMM can be used to check tooling for dimensional accuracy during maintenance, and the results may be used to update QA procedures such as setting the regularity of tool changes.

Non-destructive testing (NDT)

refers to methods used to check the internal structure of materials, often after joining through processes such as welding. The two main methods are X-ray and ultrasound analysis.

The material is subjected to radiation or ultrasound waves to check for refraction of the signal which would indicate an internal fault not visible to the naked eye.

Ultrasonic testing is safer than X-ray testing because it uses sound waves rather than radiation. The test records defects where the reflecting signal indicates something other than solid material.

Such testing is necessary for components such as turbine blades and jet engines where failure due to a fault in the welded joints could be catastrophic.

Ultrasound

X-Ray

Case Study

Allied Glass is a world producer of high quality glass containers. Its success over the last 150 years is largely down to its ability to facilitate a wide range of varying designs.

It currently produces in excess of 600 million containers per year for the premium drinks market. To ensure that these containers meet the high level of standards expected by it’s customers, it employs a wide range of QA procedures and QC controls.

Stage 1: customer specification

The initial design of a glass container is developed closely with the prospective customer who will give clear and specific criteria that Allied Glass must follow. These details will include aesthetic elements such as the type and clarity of glass to be used, maximum sizes, details of the market where the product will be sold and the volume of liquid to be held within the container.

stage 2: concept development

Design development is done in-house. This means that the lead time (from initial concept to final production) is reduced because the new product development (NPD) team employs a concurrent approach, guiding the customer through the production limitations during concept generation.

Concepts are modelled in solid forms using 3D CAD software which is used to drive two rapid prototyping machines.

Prototype models are produced as full-size visual representations using PLA polymer, allowing the customer to hold and critique the form of the container.

Stage 3: mould design and sampling

Using 3D CAD software, the mould for the container is developed to fit with the existing production equipment. The testing regime is thorough with some of thes main stages:

working drawings: specification drawings with tolerances used to adjust the production equipment for the specific job
sample prototypes: a sample sets of moulds is used to produce a batch of container for QC testing
sample dimensions checks: measuring equipment is used to check the relatively low volume of sample containers
visual aesthetic checks: sample bottles are checked visually for faults in the forming process such as ‘shorts’ where glass has not filled the mould.
machine tooling and alignment calibration: to maintain accuracy
NDT: samples checked for internal faults and stress to prevent shattering
templates jigs and fixtures produced: as every container is different

stage 4: production

Combined with the QA procedures employed for each individual container, Allied Glass must monitor the overall quality of the glass being used during the production of all containers.

Raw materials are checked before being stored in a silo. Main ingredients are sand, soda ash, limestone and cullets of recycled glass. One issue is the colour of the glass. The higher the iron content, the greener the glass.

As the materials enter the furnace, the temperature is tightly controlled within pre-determined tolerances.

Once glass enters the production line, it is formed through a glass blow moulding process and then fed into an annealing oven to reduce the internal stresses and prevent brittleness.

During production, numerous QC checks continue. These are covered in more detail on the next slide.

When full scale production begins, the new container moulds are installed on a production line and the report from sampling is used to calibrate all machinery and tooling.

qc testing

Every container is checked, using camera analysis, for glass imperfections, which records the individual mould where that container was produced. Any issues with specific moulds will be fed back to the forming end of the production line and investigated.

Every hour, each production line provides a sample for the QC department to check volumes, stresses within the glass, dimensional accuracy and other QC specs. Random sampling also take place using specific gauges. Any faults are recorded with the results feeding back to relevant points on the production line.

summary

When producing a prototype product, it is essential that you plan for accurate production.

The development of the design must allow the concept to be tested against key specification criteria.

Modelling strategies may include virtual CAD simulations.

Effective QA should ensure products are made correctly ‘first time’.

Effective project management is essential in all design and manufacture activities.

QC inspections take place throughout the production process.

Dimensional accuracy is vital for components to be assembled effectively

NDT refers to methods used to check the internal structure of materials, often after joining processes such as welding.

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