Tamara Wilhite is a technical writer, industrial engineer, mother of two, and published sci-fi and horror author.
What Is Quality Control?
Quality is defined as building things and delivering services per the requirements set by the customer. Each metric is a variable that should ideally fall within a desired range.
Quality control is the process of maintaining these variables within accepted target ranges. The job of quality control is to set these acceptable ranges for each variable, identify products that fall outside that range and rework products that fails to meet quality criteria.
How is Quality Control Evaluated?
According to the fifth edition of the book “Maynard's Industrial Engineering Handbook”, quality control has four evaluation standards: quality standards, quality objectives, quality levels and inspection standards. When the quality is measured, the quality standard is the highest and the ideal. The quality standard is what the customer and the designers want to see made.
The inspection standard defines what is allowed to be shipped to the customer and what is sent back as scrap or rework. The quality level is what is established by the internal supply chain. The quality objective is the lowest quality evaluation standard; this is the quality produced by the production line, including out of specification product.
When quality control's results are not as high as management or customers would like, there are steps you can take to improve quality control.
How to Improve Quality Control on the Shop Floor
Quality control is most effective when it takes place early in the manufacturing process, instead of wasting further manufacturing effort on a bad part. Determine where defects are occurring and where they are caught.
Ideally, your quality control process will find defects as soon as they happen instead of letting the product move further downstream. If defects are caused by a particular machine, move inspection to so that products are inspected as soon as they are out of the machine.
Use the inspect and correct method of quality control at each work station, instead of letting defective products reach later manufacturing steps that could result in scrapping the assembly.
Document defects as they occur whenever possible, so that all related information and contributing factors are fresh in the recorder's mind. Use data recording features in shop floor data management software or other online tools to have access to simple defect reports and data analysis.
Ensure that process data is captured properly and in full. "Six Sigma Demystified: A Self-Teaching Guide" by Paul Keller states that process data may be the most prolific and reliable source of information for quality control decision making.
Use standard operating procedures to reduce human sources of variability during manufacturing.
Use automated inspection and test to reduce human oversight during inspection and test.
Production control should be part of quality control. Ensure that personnel have the latest drawings to build from and the correct procedures to follow for the parts they are building. When the product mix shifts, verify that the inspection equipment is following the right program. And ensure that the SPC data and accept/reject standards are for the new product, not the old one.
Maintain your equipment. When tools wear down, they start to produce out-of-spec components. You may be able to improve quality levels more by inspecting and repairing equipment more than you would by inspecting and repairing completed assemblies.
Set up quality circles for each production line or work group. Let employees come up with suggestions that improve product quality, making quality control's job easier.
Balance production goals with quality. The need for speed can hurt quality.
When variables related to quality are monitored real time by equipment, regularly inspect the sensors and equipment to ensure that it is working properly. Ideally, the sensors and hardware are inspected and calibrated by an independent third party so that employees cannot alter the QC equipment's settings deliberately or accidentally, affecting the process' results.
How to Improve Quality Control Through the Supply Chain
Quality should start with the supply chain. Variable raw materials add to the variability of the manufacturing process output.
Periodically check your incoming material’s quality, regardless of the vendor’s quality ratings. You cannot afford to take the supplier’s word for it.
Where possible, work with suppliers to improve their quality level.
Always report problems you trace to the supplier to the suppliers themselves, so that they can address their quality issues.
Just in Time manufacturing relies on immediate delivery of supplies. The production line gets what it needs as soon is needs it. This makes supplier quality paramount, since the production line grinds to a halt when the incoming components do not meet quality standards. If your production line runs on the JIT model, select higher quality suppliers over lower cost competitors, since a lack of quality components leaves production lines idle.
Quality control tests will verify the quality of a product as it is manufactured. Your may want to certify that it is good prior to shipment if there is the possibility that the product could deteriorate while sitting in storage. It may be necessary to ensure the quality is not affected by transportation or handling en route to the customer.
Verify that all suppliers meet new quality standards before implementing them yourself.
How to Improve Quality Control Through Quality Control Plan Management
A quality control plan for new products should be developed as the manufacturing process. Determine how process variables can be reduced and monitored as the manufacturing process is designed. Test the quality control process and inspection methods as the manufacturing line is brought up to speed. Ensure that new assemblers are trained to recognize and log defects as they learn to put together new products.
When you change your manufacturing processes or quality standards, update your quality control plan. Then communicate the new processes – or the fact that nothing has changed – to your employees.
Quality control plans are measured by both the number of bad parts they catch relative to those produced and the number of false positives, the number of good products that are accidentally identified as bad. Regularly check your quality control plan to determine both how many bad products it lets through and how many good products you unnecessarily flag as failing to meet quality standards.
Organizations can improve quality control for the organization as a whole by setting up a quality control system.
What is a Quality Control System?
Quality control systems exist to ensure that product and the processes that create it are defect free. Quality control is an overhead cost that is hopefully offset by reduced rework, returned product, lost sales due to poor product quality and higher prices paid by customers who want a higher quality product.
Quality control is best achieved with a quality system in place, so that quality is a consideration at every step of the manufacturing process – from procurement to product delivery. Putting a quality control system in addition to implementing a number of quality control methods is one of the best ways to improve quality control overall.
How to Improve Quality Control Through Engineering and Design
Engineers should design with quality in mind. Design parts that are easy to make and assemble, so that there are fewer manufacturing and assembly errors. Design assemblies that are easily tested, so that testing can be performed on more assemblies using the same number of inspectors.
Tightening part tolerances can reduce interference problems and loose connections. However, increasing the tolerance of a part by a decimal place is said to increase the manufacturing cost by a factor of ten. For example, a tolerance of a millimeter costs up to ten times more to produce than a part that has a tolerance plus or minus a centimeter.
Balance the cost of more precise tolerances against other methods of improving product quality. And altering design tolerances can alter quality control practices on the shop floor, necessitating the purchase of more accurate sensors or new QC verification tools.
Quality control requirements need to be coordinated between different groups like suppliers and standards organizations. Eliminate redundant inspections and overly precise quality measurements. Determine if different quality control groups can share data from the existing quality control process, instead of relying on secondary inspections. For example, can suppliers inspect process prior to shipping it so that your company does not have to perform incoming QC? Can customers receiving your components accept your quality data instead of inspecting parts again? Do parts need to be checked prior to installation if they recently completed final QC on the assembly line?
Review the quality standards and customer requirements against those variables that are truly important. There is no benefit in monitoring metrics that no longer matter to the customer or are cosmetic instead of functional unless beauty is a customer care-about.
Verify that alternate parts and materials meet quality standards before approving them for use in your production process.
Require engineering approval and thorough testing before allowing brand new substitutions.
How to Improve Quality Control When You Have a Tight Budget
Identify critical to quality, critical to schedule and critical to cost variables as the metrics to document. If your business has limited resources, these are the process variables to improve first. For example, devoting resources to improving a product's testability is not a good if customers are complaining about the poor fit of a protective housing.
When an organization has limited resources, focus on burning platforms before you try to make something perfect.
Focusing on prevention has greater value than inspection, since preventing defects prevents the costs of rework, repair, wasted materials and even greater inspections.
Measure the process capability of your manufacturing processes. How well do the products they produce meet your quality standards? If the normal distribution of the production line generates a significant percentage that falls out of spec, you want to improve the underlying manufacturing process instead of investing in more inspection or testing.
When selecting quality problems to solve, balance the desire to target the defect that occurs most often with the defects that cost the most to fix or those defects that defy customer expectations of your product's quality.
Be careful to identify the real problems and real causes. If the employees with the poorest quality work are all new, the root cause may not be the employees but the training they received. If night shift has worse product quality, is the real root cause working conditions on that shift, environmental variables or something else? If you do not target the true root cause of a problem, you will invest money in an incomplete solution.
How to Improve Quality Control by Using the Right Tools for the Job
According to "Corporate Management, Governance, and Ethics Best Practices" by S. Rao Vallabhaneni, there are seven original quality control tools. These QC tools are check sheets, histograms, scatter diagrams, Pareto charts, flow charts, cause and effect diagrams and control charts.These quality control tools have been around for decades if not centuries.
Vallabhaneni says that there are seven major quality management tools, all of which are more modern inventions. These primary quality management tools are affinity diagrams, process-decision program chart or the PDPC, matrix diagrams like the quality function deployment matrix, interrelationship digraphs, all types of tree diagrams, the prioritization matrix and activity network diagrams. However, by some estimates, there are more than a hundred quality control tools and methods in existence that could be used.
Use the right quality tool for the job. Use check sheets to identify problems as they are identified. Use design of experiments to identify root causes. Mistake proofing only prevents future defects. Control charts and process logs can help you identify major problems, suggest root causes like times at which defect rates are higher and verify improvements in quality control. Process charts reveal current problems and production trends after correction. Statistical process control is one of the few quality tools that support all problem solving cycle steps from problem selection to correction to verification and prevention.
Use run charts instead of bar charts to show process variables, so that variation over time is clearly shown. Run charts will show variation over time, whereas bar charts show totals. Though two production lines may have the same mean, the process with far higher variability is not in control and will produce more out-of-spec product than one with a slightly-off average but low variability.
Automated defect data collection makes data mining a possible tool for improving quality control. However, data mining may not be enough in and of itself. The book "Six Sigma Demystified: A Self-Teaching Guide" by Paul Keller says that data mining can be used to identify opportunities for improvement, but it may not provide enough information to decide how to fix the underlying quality problems.
How to Improve Quality Control Through Changes to Corporate Culture
Recognize that rework is a waste of time and money. It is better than shipping defective product or scrapping an assembly, but the organization needs to focus on the Plan-Do-Check-Act or Define-Measure-Analyze-Improve-Control process improvement methodology to eliminate the root causes of defects and improve overall quality.
Regardless of the quality improvement methodology selected, implement it on a corporate-wide scale to have a lasting impact on product quality. What matters most is selecting a process improvement methodology, training everyone in its use and providing adequate resources to improve quality at all levels of the organization.
Make quality part of the corporate culture. If quality is everyone's job, from the maintenance technician for the production line motors to the CNC machine programmer, employees are more likely to give recommendations to improve product quality and report problems that affect quality.
1. "The Step-by-Step Guide to Measuring Costs and Risks of Poor Quality Information" by Larry P. English
2. "Six Sigma Demystified: A Self-Teaching Guide" by Paul Keller
3. "Six Sigma and Beyond: Problem Solving and Basic Mathematics, Volume II" by D. H. Stamatis
4. "Corporate Management, Governance, and Ethics Best Practices" by S. Rao Vallabhaneni
5. "Variation Risk Management: Focusing Quality Improvements in Product Development and Production" by Anna C. Thornton
6. "Maynard's Industrial Engineering Handbook, Fifth Edition" by Kjell B. Zandin
7. "Project Management, Eighth Edition" by Harold Kerzner
© 2013 Tamara Wilhite