How to ensure consistent quality from a CNC machining provider

19th Feb 2024
by Sam Brown

Are you getting frustrated with a CNC machining supplier who just isn’t delivering when it comes to quality? Not sure where to start with finding a replacement? I get it, it can be a minefield. Funnily enough, no precision machining company ever held up their hands and declared that quality isn’t their strong point. In fact, most suppliers of machined parts will boast ‘excellent’ quality. 

But beyond their claim, how can you determine who really delivers for their customers and who should be avoided? In this blog I’ll break down the key things to look for and the questions to ask when approaching perspective CNC machining partners.


A true commitment to quality: quality standard metrics

A sure-fire way of ascertaining whether a company is serious about quality is by quizzing them about their quality standard metrics. What do they measure and how are they performing?

The most common quality metrics include:

  • DPPM: Defective Parts Per Million (DPPM) quantifies the rate of non-conforming units produced relative to the total output. Lower DPPM values signify a more consistent and reliable quality performance.
  • DPMO/NPMO: Defects Per Million Opportunities (DPMO) or Nonconformities Per Million Opportunities (NPMO) assess the overall effectiveness of the manufacturing process in preventing potential defects. Lower DPMO/NPMO values indicate robust process control and minimised defect risks.
  • NCRs: Non-Conformance Reports (NCRs) document identified deviations from specifications or standards. Regularly reviewing NCRs enables the identification of recurring issues and facilitates continuous quality improvement. A commitment to quality is demonstrably reflected in the clarity and detail maintained in NCRs by the supplier.

Acetal reels



ISO9001 is a globally recognised certification, indicating a specific level of documented processes and traceability. The absence of ISO9001 compliance suggests a company may lack the necessary elements to ensure the consistent quality required for your needs.

In addition to ISO9001, you may wish to dig into industry-specific certifications that can offer an additional lay or assurance. However, a word of caution…don’t assume a company is not up to scratch if they don’t have a certification specific to your industry.

A certification like AS9100 used in the aerospace industry, for example, demonstrates an even higher level of commitment. This rigorous standard signifies unwavering dedication to precision, reliability, and exacting standards – valuable qualities no matter what industry you’re in.  


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Ship to stock?

It’s worth enquiring whether a company has ship-to-stock status with any existing customers. This means their customer trusts them to the extent that they no longer inspect their work – it goes straight into production. That’s not to say you wouldn’t want to inspect their work – at least initially – but knowing they have customers that have this magnitude of trust is very reassuring.

Ship to stock status is a great marker of quality


Robust and repeatable processes

It’s all very well a company producing a quality part once. But the key when it comes to quality is always going to be consistency. Robust, documented, and repeatable processes are another key criteria against which to measure prospective manufacturing suppliers.

Using fixtures helps to ensure repeatable set ups

Here are the main processes that should be documented:

  1. Setup procedures: once the ‘first off’ component has been meticulously inspected and approved, the same level of quality must extend throughout the production process. Rigorously documenting the setup will eliminate human error and ensure absolute repeatability.
  2. File saving procedure: after completing the setup and producing the first component, it’s essential to securely save the machining program and associated data. The file’s easy retrieval and transferability enable the reproduction of the program without recreating the setup, ensuring consistent quality for subsequent components in the batch.
  3. Machine maintenance: reputable suppliers prioritise regular and preventative maintenance procedures to ensure machines operate at peak efficiency and accuracy. Being proactive with this will minimise downtime and prevent errors. 
  4. Equipment calibration: trust suppliers who regularly calibrate tools and instruments, maintain strict equipment integrity procedures, and can offer proof of calibration.
  5. Quality inspection process: the biggy! A company truly committed to quality will have rigorous inspection processes in place which will include inspection at various manufacturing stages to minimise non-conformance risks. Here is an example of a comprehensive inspection process:
  • Order review: Understanding your requirements before machining begins.
  • First-off inspection: Ensuring the initial component meets specifications.
  • Agreed-upon in-process checks: Regularly monitoring quality throughout production.
  • Final comprehensive inspection: Verifying all critical parameters of the finished components.
  • FAIRs and compliance: Providing documentation and adhering to customer requirements.

Another documented process that is often overlooked is a customer service process. While this doesn’t relate specifically to quality, it does pertain to security in your supply chain.

When a supplier can provide evidence of an internal process for managing your order through the business, it gives you peace of mind. Here at Penta, we have a documented customer service process called ‘Penta’s Proven Process’ which you can view here.

First off inspection of a turned component



Communication might not immediately come to mind when thinking about quality. However, in my experience, when talking to prospective customers, what they often deem to be a quality issue, turns out to be a communication issue. This is because poor communication can lead to misunderstandings and ultimately to sub-par components.

The four primary points of communication should be:

  1. At quoting stage: a supplier should always proactively clarify any queries or missing information. Proactivity here will ensure that there are no nasty surprises or additional costs later.
  2. When preparing the order for production: an opportunity to ensure a deeper understanding of the form, fit, and function of the component, ensuring any potential issues have been ironed out.
  3. During the machining process: this is usually minimal if the machining company has been thorough at stages one and two. However, the supplier should regularly update you on the progress of your order and provide notification when the components are ready for delivery.
  4. Addressing unexpected issues: unforeseen challenges do occasionally arise during machining. Seek a supplier who will promptly inform you of issues, letting you know what information they require from your end and outlining a resolution strategy.

Bearing the above in mind, make sure you dig deeper with a prospective supplier on what their communication process looks like. If they look blank, back away.

Define a thorough inspection process with your CNC machining supplier



Get Penta involved

Penta Precision is an ISO 9001:2015 accredited CNC machining company in the UK. We are proud of our exceptional and consistent quality. In fact, we have a 99.8% quality standard and have ship to stock status with an international CAA approved aerospace manufacturer. We’re not afraid to answer questions about quality. We understand its importance and we invite full and frank conversations with prospective customers.

If this is the no-nonsense approach to quality you’re looking for, give me and my team a call on 023 9266 8334.