A Step-by-Step Guide to Quality Control for CNC Machining

The world of quality assurance is an alphabet soup of abbreviations: QA, PPAP, APQP, CMM, PSW, etc. 

This article explains many of them. More importantly, it explores how each is vital to CNC machining quality control. It also addresses how a focus on quality standards brings a distinct advantage, especially when running several concurrent projects.

Alone, computer numerical control (CNC) machines cannot achieve the required precision, speed, and repeatability, and quality can’t be “tested in” to a product. Effective quality assurance only happens through a proven process and a highly capable quality assurance (QA) team.

Here’s how it works:

Step 1: Part Production Begins with Design Review

With a purchase order and final drawings approved, the quality control process formally begins with the print review and getting answers to the following questions: 

• Is the customer design manufacturable? 
• Are specified tolerances achievable? 
• Is the current measuring equipment capable of checking all features on the print accurately?

Only then do the QA and engineering teams begin the advanced product quality planning (APQP) process. For new designs, additional upfront work may be needed by both teams who work hand-in-hand with customers and foundry suppliers on new parts. Together, they review the design and manufacturing processes to optimize product function and minimize costs.

Step 2: Quality Leads Production for a Smooth Launch

The APQP process is ongoing as the engineering team designs fixtures, creates programs, and chooses tooling, while the QA team performs analysis and creates checks to verify engineering and production. Both teams work together toward the common goal of accurate production and satisfied customers.
Several tools are used to analyze, control, and document production, including:

• Product Control Plan (PCP) — lists all customer specifications, the manufacturing process steps, and quality controls at each step
• Failure Modes Effect Analysis (FMEA) — used to understand the machining process and controls, plus minimize defect risks
• Process Flow Diagram (PFD) — outlines the manufacturing process for inspections  

These are the most important deliverables:

• First Article Inspection (FAI) — defines all the specifications; checked each time a new job is set up to produce the part
• Operator Audit Sheet (OAS) — lists all the checks the operator completes during production
• New gauges — ordered as needed 
• Coordinate-Measuring Machine (CMM) — a precision measuring instrument programmed to measure and verify positions and features
• Supplier Quality Requirements (SQR) — makes sure any questions and issues are communicated and worked through
• Engineering designs — reviewed by a Quality Engineer (QE)

Step 3: Production Samples

The first production sample gets special attention. After production sets up the job and runs the first piece, the operator completes the FAI, and the part is brought to the QA team who works with the operator and engineering if any features are:

• Out of tolerance — e.g., The need is a 5" diameter bore with a tolerance of +- 0.001". If the bore is 4.9980”, it is too small and out of tolerance 
• In tolerance but off of nominal — e.g., For the same 5" diameter bore, the cut is consistently 4.9991”, 4.9992”, 4.9991”, etc. The goal is to get this dialed closer to 5.000” +- 0.0002” (not 4.9992” +-0.0002”)

After a part passes the FAI and CMM checks, an Initial Sample Inspection Report (ISIR) is completed, which includes a full dimensional layout of the part to verify print specifications. Plus, each dimension on the drawing is measured and recorded.

Next, QA signs off on the part production approval process. Numerous production samples are produced (typically 30 pieces), and three are checked for ISIR (the first piece plus two more samples, typically taken from the middle and end of the run).

Step 4: Part Submission Warrant (PSW)

After the samples pass the ISIR and are complete, a part submission warrant is finalized and submitted. The samples are used to measure the process capability (a process’s ability to create a product within specification limits). At this point, gauge repeatability and reproducibility is measured, ensuring a gauging instrument’s accuracy.

The entire part production approval process (PPAP) documentation package is then completed and submitted with a signed warrant.

A Quality Team Is Key in New Part Production

Too often the quality assurance process in CNC machined parts is seen as “quality assurance vs. production,” but those two can (and should) be complementary. Quality should not hold up production.

An experienced QA team balances those two goals in a smooth PPAP launch. Stecker Machine’s 20-member team includes auditors, technicians, engineers, programmers, and a systems coordinator. The team covers various specialties, but all work together to ensure production happens to customer specifications.

Stecker Machine often launches several new parts concurrently, putting the QA team to the test.  The timelines are typically tight, resources are carefully managed and aligned, and several people step up to complete the many ISIRs with full dimensional layouts. Any schedule and manpower bottlenecks are handled by CMM programmers, technicians, managers, and even a machinist apprentice.

For one particular project, Stecker Machine’s three commonly used CMMs for FAI parts expanded to a fourth (another seven are for in-cycle parts). That required adjusting the production sampling plan, mainly by determining production parts that were stable and could be tested less frequently. There was no increase in production scrap, and the schedule remained unchanged.

With a little creativity and a lot of hard work, Stecker’s QA team produced over 30 new parts concurrently while maintaining quality. That drive and focus ultimately resulted in meeting (and exceeding) customer needs.

Ready to put Stecker’s machining and quality to the test? Reach out with an RFQ (the last of this article’s abbreviations. we promise!), contact us, or call 920-726-4526. Not ready yet? Read this case study highlighting a machining, testing, and quality challenge.