Incoming Material Inspection for CNC: The MTC Review Process That Prevents Costly Mistakes

Incoming material inspection at CNC machining shops is rarely as robust as it looks on paper. The aerospace prime sent its supplier quality engineer for a routine survey. The shop had ISO 9001, was working toward AS9100 Rev D, and had what its quality manager described as a "solid" incoming material inspection process. The SQE asked to see how incoming MTCs were handled. He was shown the folder where certificate PDFs were filed. He asked for the incoming inspection records — the logs that document which engineer reviewed which MTC, what was verified, and what the outcome was. The shop quality manager checked the drive. The logs existed for some deliveries, not others. The MTC for a 7075-T7351 aluminum order that had gone into production four weeks earlier was present, but no inspection record existed confirming the chemistry had been checked against the AMS specification. The SQE issued a corrective action request. The shop spent the next two months in formal corrective action, and the prime's new PO placements were on hold during that period.

This is not an unusual scenario. Many shops have certificates. What they lack is the structured receiving inspection record — the formal documented evidence that the certificate was reviewed, verified, and accepted before material entered production. The difference between having files and having a compliant process is that documented evidence of each inspection step.

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What AS9100 Rev D Actually Requires at Receiving

AS9100 Rev D Clause 8.4.3 (Information for External Providers) requires that the organization communicate to external providers its requirements for approval of products and services. Clause 8.6 (Release of Products and Services) requires that documented information be available as evidence that products and services comply with defined acceptance criteria, and that a record of the product or service authorization be retained.

Clause 8.5.2 (Identification and Traceability) requires that the organization maintain documented information to achieve identified product or service traceability, and use suitable means to identify outputs when traceability is required. For aerospace and defense machining, traceability is required by contract, by code, and by customer flow-down — which means the MTC must be traceable to the specific material lot that was used to machine each part.

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The Aerospace Prime Flow-Down Layer

AS9100 establishes the baseline. Aerospace primes add a layer of specific requirements on top of it through their supplier quality requirement documents (SQRDs), purchase order quality clauses, and Approved Supplier List (ASL) conditions.

Common prime-specific requirements that affect incoming MTC inspection:

Material traceability to part number. Many primes require that the finished part be traceable to the specific bar stock lot or plate sheet that was machined, and that the traceability path be documented in the production record. This means the heat/lot number from the MTC must appear on the production traveler, and the traveler must travel with the part through machining, inspection, and shipping.

Approved source restrictions. Primes typically maintain an Approved Manufacturer List (AML) or Qualified Manufacturer List (QML) for specific materials. A shop that machines Ti-6Al-4V for a prime sourcing from a mill not on the AML is non-conforming even if the MTC is perfect. The incoming inspection process must verify that the material source is approved.

Certificate type requirements. Some primes require that the MTC be a direct mill certificate (CMTR — Certified Mill Test Report) rather than a distributor's re-issued certificate. Others accept distributor certificates but require the original mill reference. The prime's SQRD specifies which.

Traceability for dual-use and ITAR material. For defense hardware, some primes require that the material traceability chain support export license compliance — meaning the shop must be able to demonstrate that the material used in a specific controlled part came from an approved domestic source and has a documented chain of custody.

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Building the Incoming Inspection Process: Step by Step

Step 1: Define Requirements Before Purchase

Effective incoming inspection starts at the PO, not at the dock. The purchase order must specify:

• Material grade, specification, and edition (e.g., AMS 4928 Rev D, or ASTM B209 2024 edition)
• Certificate type required (CMTR from producing mill, or distributor certificate with mill traceability)
• Any approved source restriction (specific mills or distributors per prime AML)
• Required test fields beyond the specification minimum (e.g., hardness, fracture toughness, specific trace elements)
• Retention period for records

When these requirements are not on the PO, the distributor will supply whatever is standard, and the incoming inspection step becomes a remediation exercise rather than a verification exercise.

Step 2: Receive the MTC Before Accepting the Shipment

The MTC should arrive with or before the material — ideally electronically, so the receiving review can begin before the truck parks. For time-sensitive deliveries, reviewing the MTC electronically in advance allows non-conformances to be identified while the shipment is still in transit, giving time to resolve documentation issues before the material arrives.

Step 3: Identity and Grade Verification

Compare:

• Material grade on the MTC vs. the PO specification (exact match required — temper, condition, and edition matter)
• Material grade marking on the physical material (label, stencil, or color code on the bar or coil) vs. the MTC
• Certificate type vs. PO requirement

Any mismatch at this step is a hold before the material moves from the dock.

Step 4: Heat or Lot Number Cross-Check

The heat or lot number on the MTC must match the marking on the physical material. For bar stock, this is typically an end-stamped number or a label attached to the bundle. For plate, it may be a stenciled number on the plate face.

If the physical material is unlabeled or the label is damaged, the material cannot be accepted until the supplier confirms which certificate applies — in writing.

Step 5: Chemistry Verification Against AMS or ASTM Limits

Every element in the specification must be within the required limits on the MTC. Common areas where limits are tighter than buyers expect:

• Titanium (AMS 4928 / Ti-6Al-4V): Oxygen ≤ 0.20%, nitrogen ≤ 0.05%, hydrogen ≤ 0.015%, iron ≤ 0.40% for Grade 5; stricter for Grade 23 ELI
• Aluminum 7075-T7351 (AMS 2770 + AMS 2014 + B209/B211): Zinc 5.1–6.1%, magnesium 2.1–2.9%, copper 1.2–2.0% — all checked against AMS 2014
• 17-4 PH Stainless (AMS 5643): Chromium 15.0–17.5%, nickel 3.0–5.0%, copper 3.0–5.0%, niobium+tantalum 0.15–0.45%, carbon ≤ 0.07% — every element checked

A quality engineer verifying by eyeball against a recollected limit is not a reliable process. The limit tables must be available at the inspection step, either on paper or in a software system.

Step 6: Mechanical Properties Verification

Yield strength (0.2% offset), tensile strength, elongation, and reduction of area must all be within specification minimums and maximums. For 7075-T7351 plate, the minimum yield is 434 MPa (63 ksi) and minimum tensile is 496 MPa (72 ksi) in the T7351 temper — values that can only be confirmed against the correct temper specification, not the annealed or T6 temper tables.

For precipitation-hardening stainless steels, the condition designation (A, H900, H925, H1025, etc.) must match the engineering order, and the mechanical properties must be within the condition-specific limits.

Step 7: Hardness Verification

For any material in an application with hardness limits (NACE sour service, aerospace tooling steel), the MTC must report hardness in the correct scale (HRC, HB, or HV) at or within the specified range. For NACE MR0175 applications, HRC ≤ 22 is the limit for most carbon and alloy steels. For 17-4 PH in Condition H900, HRC 40–47 is the acceptance range. These are different lookups; a one-size-fits-all hardness column on an inspection checklist is not sufficient.

Step 8: Create and File the Inspection Record

The inspection record is the deliverable of this process. It must include:

• Delivery date and PO reference
• Material description, grade, and specification
• Heat or lot number(s)
• MTC reference number and issuing organization
• Each check performed (grade, heat number, chemistry elements by element, mechanical properties, hardness)
• Result of each check (pass/fail/deviation)
• Accept, hold, or reject disposition with rationale for holds or rejects
• Name and signature of the inspector
• Date of inspection

This record is what the prime's SQE asks for, what the AS9100 auditor requires, and what the NADCAP auditor cross-checks against the traveler. Without it, the MTC filing is evidence that you received a document — not evidence that you verified it.

Step 9: Link the MTC to the Production Traveler

Before material moves from receiving to storage or staging, the traveler that will follow this material through production is created (or the existing traveler is updated) to reference the specific heat/lot number and MTC reference. From this point, any use of this material in production is traceable back to the specific certificate.

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Common Failures in CNC Shop MTC Review

• Reviewing grade label only, not chemistry values. A distributror can label any bar "7075-T7351." The chemistry on the MTC tells you whether the material actually meets the specification. Grade label ≠ grade conformance.
• Not checking the temper or condition against the PO. T7351 and T6 are different tempers with different mechanical properties. The machining shop that sources T6 when T7351 was specified has a non-conformance regardless of chemistry.
• Accepting a distributor certificate that does not reference the original mill. A distributor's own CoC is not an MTC. The original CMTR from the producing mill is required for prime customers.
• Filing without recording. The MTC arrives, is checked visually, and is filed. No inspection record exists. At audit, this is a finding.
• Not updating the traveler. The heat number never makes it from the receiving record to the traveler. At first article, the prime cannot trace the material.

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How TestCert Closes the Process Gap

TestCert automates the MTC verification steps that CNC shops currently perform manually and inconsistently. Each incoming MTC is ingested, and the AI extraction layer pulls grade, heat/lot number, chemistry by element, mechanical properties, and hardness values regardless of the format or language of the certificate. The verification engine compares each extracted value against the specification limits for the correct AMS or ASTM grade — not manually, but automatically, with a clear pass/hold/fail result per field.

The inspection record is generated automatically: date, inspector (the person who triggered or reviewed the verification), heat/lot number, MTC reference, and a field-level breakdown of every check and its result. This record is stored alongside the original MTC PDF and is linked by heat number to any job that references that material lot. When the prime's SQE asks for the inspection record for a specific batch of Ti-6Al-4V bar, the answer is a query by lot number — not a manual search through email folders.

For NADCAP audits, the record includes the exact test values, the specification limits used for comparison, and the disposition — which is exactly the format auditors expect when cross-checking the traveler against the incoming material acceptance records.

See how AS9100-certified CNC shops use TestCert to eliminate incoming inspection gaps — book a demo at testcert.io.