This article is part of the CNC Tolerances & Quality Control Guide: GD&T, CMM, Cpk, PPAP on CNX Precision.
Every CNC machine shop produces the occasional bad part. A part is nonconforming when it fails to meet any drawing or specification requirement, whether that means out-of-tolerance dimensions, wrong surface finish, damaged threads, incorrect material, or a cosmetic defect on a visible surface. What separates reliable suppliers is what happens next. A disciplined nonconforming part process identifies suspect parts, contains them, documents the deviation, assigns a disposition, and fixes the root cause. CNX Precision runs this nonconforming part process on every job so defective parts never reach customers and repeat failures keep dropping year after year. This guide walks through each step, from the first NCR to verified corrective action and clear buyer communication.
Identify, Contain, and Document the Problem
The moment an inspector rejects a part, containment starts. Suspect parts are tagged, isolated in a clearly marked quarantine area, and never mixed with good inventory. Containment also looks backward and forward. The team checks parts made since the last good inspection and any lots already in downstream processes, so the full affected population is known quickly. Each quarantine tag lists the NCR number, so nobody has to guess why the parts sit there. Nonconformances can surface during first-article inspection, in-process checks, final audit, or even after shipment. Detecting them early is cheap. Detecting them at the customer is expensive, which is why mature shops build inspection gates into every stage of production.
The operator or inspector then opens a nonconformance report, known as an NCR. The NCR records the part number, quantity affected, date, defect description, measured values versus required values, and the inspection point that caught it. Photos and measurement data attach to the report. Clear documentation turns a vague problem into an engineering question with evidence behind it, and it gives reviewers everything they need to choose a disposition without re-measuring the parts. The report also names the machine, shift, and program involved, which speeds later analysis.
Disposition Options: Rework, Repair, Scrap, or Use-As-Is
A quality engineer or material review board reviews the NCR and assigns one of four dispositions. Rework returns the part to full drawing conformance, for example by remachining an oversize bore or chasing a damaged thread. Repair restores function without meeting the original drawing exactly, such as welding and reworking a damaged boss, and usually requires customer approval. Scrap is chosen when the cost of fixing the part exceeds its value. Cost drives the choice. An inexpensive part with a costly fix is scrapped, while an expensive part earns a careful repair review. Accept-as-is, also called use-as-is, is a formal concession that the deviation does not affect fit, form, or function. Only engineering, and often the customer, may approve a use-as-is disposition, and production staff never approve concessions on their own. Every option is recorded with its justification, and reworked or repaired parts always go through inspection again before release. The disposition step is where the nonconforming part process meets the customer most directly, so the decision and its reasoning belong in the NCR where auditors and buyers can review them later.
Root Cause Analysis With 8D and 5-Why
Disposing of parts treats the symptom. Root cause analysis cures the disease. The 5-Why method asks why repeatedly until the true cause appears, for example a worn collet, an uncorrected tool offset, or a missing setup step. The 8D method adds structure: build a team, describe the problem, apply interim containment, verify the root cause, implement permanent corrective action, and prevent recurrence. Typical machining root causes include programming errors, tool wear not caught by the inspection interval, fixture instability, bad raw material, thermal drift, and incorrect measurement technique. The analysis result is written into the NCR so the finding can be tested and verified later instead of living in someone’s memory. For serious or recurring issues, CNX Precision runs a full 8D and shares it with the customer. The choice depends on severity. A one-off, obvious defect needs a quick 5-Why, while a recurring or customer-visible failure deserves full 8D discipline. A strong nonconforming part process treats every root cause as a system defect to fix, not an operator mistake to blame.
Corrective Action, Verification, and Buyer Communication
Once the root cause is confirmed, permanent corrective action follows. Actions may include program updates, revised tooling or workholding, changed inspection frequency, added mistake-proofing, or operator training. The action is verified, not assumed. The shop runs the job again, measures results, and tracks the following lots to confirm the defect stays gone. The NCR closes only when the evidence supports closure, and corrective action records feed back into control plans, setup sheets, and first-article checklists so the fix becomes standard practice instead of tribal knowledge.
Buyers deserve prompt, honest communication throughout. When a deviation affects delivery or interchangeability, CNX Precision notifies the customer immediately and presents the containment plan and proposed disposition. For use-as-is requests we share measured data and engineering rationale so the customer can make an informed decision. Many buyers request an 8D report for serious issues, and we deliver it in their preferred format. Transparent communication builds trust, while hiding problems destroys it. Internally, nonconformance data also feeds management review, supplier scorecards, and process improvement priorities.
What Makes This Nonconforming Part Process Effective
A strong nonconforming part process depends on people as much as paperwork. Operators must be trained to stop and report defects without blame, because fear is the fastest way to drive nonconformances underground. Quality owns the NCR system, engineering owns dispositions and corrective actions, and management reviews trends monthly. Useful metrics include defect rate by part and by cause, NCR closure time, and repeat failure rate. Records stay auditable for customer reviews and quality system audits. CNX Precision reviews this workflow regularly and tightens it whenever the data shows a weak spot. The goal is simple. Every bad part teaches the system something, and the same failure never happens twice.
FAQ
How long does it take to close an NCR?
Simple dispositions such as scrap or rework close within days. Cases that require customer approval, root cause analysis, and verified corrective action typically take longer. We set target dates on every report and track them until closure.
Who can approve a use-as-is disposition?
Use-as-is approval requires engineering review and, in most cases, written customer authorization. Production staff never approve concessions on their own, because only design owners can judge the effect on fit, form, and function.
Does every nonconformance require customer notification?
No. Internal defects caught and corrected in-house are handled through our standard process. We notify the customer when the deviation affects delivery, interchangeability, or any requirement they approved, and we always report escapes that reach the customer.
For related information, see our guide to cnc milling process, and cnc turning process.