What is MTR and CoCs
A shipment of CNC parts arrives with several documents: a CNC Material Certificates, a signed statement, and an inspection report. The paperwork looks complete. But does it prove the parts were made from the material you ordered?
Many buyers are unsure because material certificate can refer to different documents. A mill test report (MTR), a certificate of conformance (CoC), a heat-treatment certificate, and a dimensional inspection report confirm different things. Simply having a certificate does not mean it has been checked and verified.
This guide explains what each document confirms, how to check it before accepting a shipment, and how to state your requirements in an RFQ so the correct paperwork arrives with the parts.
MTR vs. CoC
| MTR / MTC (Mill Test Report) | CoC (Certificate of Conformance) | |
|---|---|---|
| Issued by | The material producer (mill) | The supplier or machine shop |
| What it is | A record of measured test results for the source material | A signed statement that delivered parts meet the purchase order or drawing |
| Typical content | Grade, specification, heat or lot number, chemistry, mechanical properties, condition, country of melt | Supplier name, order reference, part number, drawing revision, quantity, date, authorized signature |
| Contains test data? | Yes | Usually no |
| Proves | What the raw material is and how it tested | That the supplier declares the order conforms to requirements |
| Does not prove | That the finished part meets drawing dimensions | Material chemistry or mechanical properties, unless test data is attached |
| Traceability link | Heat or lot number | Order, part number, and revision |
| Best used for | Verifying material grade, chemistry, and properties | Confirming the shipped order matches the PO and drawing |
| When to require it | Material identity or batch-specific test results matter (request EN 10204 3.1 for batch-specific results) | Formal sign-off on the finished order is required |
| Common mistake | Assuming it proves the parts are dimensionally correct | Accepting it in place of an MTR |
Why CNC Material Certificates Verification Matters
- A certificate for the correct alloy is included, but it is for the wrong heat.
- Leftover bar stock from an earlier job is used, but its certificate is not updated.
- The supplier provides its own statement when the buyer expected the original mill certificate.
- The certificate covers the raw bar, but there is no record of later processes such as heat treatment, plating, anodizing, or passivation.
Documents Buyers Often Confuse
Mill Test Report (MTR or MTC)
- The material grade and specification
- The heat or lot number
- The chemical makeup of the material
- Its mechanical properties, such as strength and elongation, when required
- The material’s form and condition when supplied
- The country where it was melted, when relevant
EN 10204 Certificate Types
Many mills issue certificates under EN 10204, a standard that describes different types of inspection documents. Buyers commonly see these:
| Type | What it means in practice |
|---|---|
| 2.1 | A statement that the material meets the requirements. It has no test results. |
| 2.2 | A test report from the manufacturer. The results may not be from the specific batch you received. |
| 3.1 | Test results for the specific heat or lot delivered. An authorized person outside the production department checks and approves them. |
| 3.2 | Like a 3.1 certificate, but a third party or the buyer’s representative also checks and approves it. |
Certificate of Conformance (CoC)
Distributor or Supplier Material Certificate
A distributor may issue its own certificate linking the material it sold to an incoming lot. That is not necessarily the original mill document. State in your RFQ whether you accept this or require the original mill-issued record.
Process Certificates
If parts are heat treated, plated, anodized, or passivated after machining, each process may need its own certificate. These should reference the process specification, the part or lot, and the results where applicable, such as hardness or coating thickness.
Dimensional Inspection Report
This records measured features of the finished part against the drawing. It confirms geometry, not material identity. A perfect inspection report can accompany the wrong alloy.
How to Verify a Material Certificate
Check who issued it
Check who issued the certificate: the original mill, a distributor, or the machine shop? Make sure this matches your RFQ or purchase order. If you expected a mill certificate but received a supplier statement, ask why. The issuer shows who tested or confirmed the material and is responsible for it.
Check the material details
Check that the material details match your drawing. Confirm the grade, specification, product form, and condition. For aluminum, check both alloy and temper, such as 6061-T6, not just 6061. For steel, confirm the exact grade and supplied condition. These details matter because they affect the part’s strength and performance too.
Find the heat or lot number
Find the heat or lot number on the certificate. This number identifies a batch of metal. Ask the supplier to show the same number in its tracking records and, when required, on package labels. If the numbers do not match, the certificate may not belong to your shipment of parts.
Check the chemical results
If the certificate lists chemical results, compare them with the limits for the specified grade. Each element should be within its allowed range. You need not be an expert. Check the specification or ask your supplier to explain any value that is missing, unclear, or outside the stated limits first.
Check the mechanical results
Check for the mechanical results required by your drawing or material specification. These may include tensile strength, yield strength, elongation, or hardness. Make sure the results are listed and meet the required limits. If a result is missing, do not assume the material passes. Ask the supplier for proof before acceptance.
Confirm the certificate type
Check that the certificate type matches your order. If you requested an EN 10204 Type 3.1 certificate, the document should clearly say Type 3.1. Do not accept a lower type just because the form looks similar. Different types provide different evidence, so confirm the wording before approving the shipment first.
Check the date and signature
Look for the certificate’s issue date and approval from an authorized person. If there is no date or signature, ask the supplier to confirm it is valid. Question signs of editing, such as changed values or inconsistent text. Do not treat a document as verified until the supplier explains it.
Link it to your order
Make sure the certificate can be linked to your order. It should connect to your part number, purchase order, or shipment batch in supplier records. It may show the alloy but belong to another batch. Without a link to your parts, it provides little proof that they meet your requirements.
Check the drawing revision
Check that inspection and process documents show the drawing revision in your order. A part may have been made to an older version with requirements. If the revision is missing or does not match, ask the supplier which drawing was used and request correct records before you accept the shipment.
Consider checking the parts
For high-risk parts, consider checking the material. Require positive material identification, or PMI, or an independent laboratory test. State the test method, sample size, and pass limits in your RFQ before suppliers quote. An MTR is not a PMI test, so it does not prove the material was physically checked.
What to Put in Your RFQ
Material paperwork can affect where suppliers buy the material, order quantities, extra processing, inspection time, delivery time, and price. State your requirements when asking for a quote not after production begins.
- Part number, part name, and drawing revision
- Required material grade, specification, temper, or supplied condition
- Required document, such as the original MTR/MTC, an EN 10204 Type 3.1 certificate, a distributor certificate, or a CoC
- Whether you need the original mill document or will accept a distributor’s certificate
- Required traceability: by material lot, production lot, or individual part
- Any part-marking requirements
- Heat-treatment and final hardness requirements
- Coating, plating, anodizing, or passivation requirements and standards
- Any special testing, such as PMI, independent lab testing, or keeping samples
- First article and final inspection requirements
- Required document format and language
- When and how to send the records: with the parts, by email before shipment, or both
- How long the supplier must keep the records
Conclusion
Frequently Asked Questions
Is a CoC the same as an MTR?
No. An MTR reports measured test results from the material producer. A CoC is a supplier’s statement of conformance and may contain no test values. Specify which one you need.
What is a heat number?
A heat number identifies one batch of metal from a single melt. It appears on the mill certificate and is often stamped on the material. It is the link between a finished part and its source material.
Is an EN 10204 3.1 certificate required for every CNC order?
No. The right level depends on your application, quality system, and regulatory risk. Define the requirement instead of adding documentation by default, or leaving it undefined.
Can an inspection report prove the material grade?
No. A dimensional report confirms measured features only. Material identity needs material documentation or testing.
Does an MTR guarantee the parts are the right material?
Not by itself. It proves what the source batch was. You still need a verified link between that batch and your shipment, and PMI if the risk justifies physical confirmation.
When should I tell the supplier about documentation requirements?
With the RFQ. Late requests can force different raw material, additional testing, new process records, or rework of inspection and shipping documents.
How long should certificates be kept?
A heat number identifies one batch of metal from a single melt. It appears on the mill certificate and is often stamped on the material. It is the link between a finished part and its source material.
