Quality assurance in CNC manufacturing: The Ultimate Guide for Procurement Managers

Quality assurance vs. quality control – which is which?

• Quality assurance (QA) includes all planned, preventive measures that ensure that requirements are met (e.g. inspection planning, approval processes, supplier audits or EMPB/PPAP] approvals).
• Quality control (QC), on the other hand, is reactive and checks whether the required quality has actually been achieved – for example through incoming goods inspections, in-process inspections or final inspections.
• Quality management (QM) forms the umbrella over both disciplines and ensures targets, reporting and continuous improvement.

👉 The differences at a glance:

Modern quality philosophies in CNC production

• Zero-defect thinking: errors should be ruled out from the outset. In CNC production, this can be achieved through precise process controls, clean tools and stable machines.
• TQM (Total Quality Management): All stakeholders – including suppliers – are integrated into a uniform quality system. This is particularly important for global CNC supply chains.
• Six Sigma:Target value ~3.4 defects per million possibilities ([PPM]). The relevant parameter for this is the [Cpk] value (process capability index) – it shows how stable a process produces within the tolerance limits. The following usually applies:

Business impact and ROI

Quality costs – but poor quality costs more. Prevention and testing cause measurable costs, but the cost of errors is many times higher (rework, rejects, express deliveries, loss of customers).

The PAF model:

The PAF model is often used in industry to systematize quality costs. It distinguishes between three types of costs:

• Failure costs: Costs incurred due to defective parts – internally (rejects, rework) or externally (complaints, customer losses).
• Prevention costs: Costs for preventive measures designed to avoid errors from the outset.
• Appraisal costs (testing costs): Costs for tests and inspections to ensure that products meet the requirements.

Investment in quality assurance follows a clear logic: preventive measures such as planning, training and supplier audits as well as inspection activities in incoming goods, during production and at final acceptance do incur costs, but these are significantly lower than the consequential costs of quality defects. Rework, rejects, express delivery, delays and customer dissatisfaction quickly add up to considerable sums.

A tried-and-tested rule of thumb is that every error prevented at an early stage saves on average many times more than it would cost to rectify the error later on. For a realistic ROI calculation, you should therefore not rely on blanket industry assumptions, but use specific complaint data from your company – such as PPM rates, documented 8D cases, actual rework minutes and logistics costs incurred.

Incoming goods inspection: risk-oriented inspection strategies

Not every component requires the same inspection intensity; a blanket 100% inspection would be uneconomical, while an inspection that is too lax entails considerable risks. The AQL methodology (Acceptable Quality Limit) and the ABC/XYZ analysis help to achieve an optimum balance between inspection effort and quality risk. While the ABC analysis classifies components according to their value share, the XYZ analysis evaluates the consumption regularity; together, the two provide a differentiated picture that enables risk-adequate inspection strategies. This allows you to focus your resources on critical parts without having to check non-critical components.

Key quality indicators (KPIs)

In order to evaluate quality objectively, measurable parameters are required. Key performance indicators (KPIs) make it possible to compare supplier performance transparently, identify trends and initiate targeted measures. For procurement managers, they are indispensable for assessing the actual supplier quality beyond pure prices and for making fact-based strategic decisions.

The most important key figures in CNC procurement are: