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How to Know the Metal Is What the Drawing Asked For

September 21, 2026 by The Alu CNC Parts editorial desk / how this is checked

A drawing says 6061-T6. Parts arrive. How do you know what they are actually made of?

For most commercial work, the honest answer is that you trust the shop, and that is usually fine. For anything where a failure has consequences, trust needs a paper trail.

The three documents, and what each one proves

Mill test certificate (MTC, or EN 10204 3.1). Produced by the mill that made the metal. It lists the chemical composition and the mechanical test results for a specific heat or lot number. This is the primary evidence of what the material is.

Certificate of Conformance (CoC). Produced by the machine shop. It states that the parts were made to the drawing and revision, from the specified material. It is a declaration, not test data. On its own it proves intent, not composition.

Full traceability. The chain that connects the mill certificate’s lot number to your specific parts, through the shop’s records: which bar was cut, which job it went to, which serial numbers came out. This is what turns a certificate into evidence about your parts.

The distinction matters. A shop can honestly supply a mill certificate for the alloy it bought while having no record of whether your parts came from that lot or from the offcut rack. If the chain matters to you, ask for the chain, not the certificate.

When to require them

Safety critical parts. Anything where failure injures someone. Lifting, pressure, structural, anything overhead.

Regulated industries. Aerospace, medical and defence work usually has traceability written into the contract.

Anything you will qualify. If a part passes a test and becomes the approved design, you need to be able to prove what a future batch is made of and that it matches.

Fatigue critical parts. Where a 2024 or 7075 component is chosen specifically for its properties, an alloy substitution silently undoes the engineering.

Anything where a substitution would be invisible. You cannot tell 6061 from 6082 by looking at it. You cannot tell T6 from T651 by looking at it either.

When not to

Prototypes, fixtures, internal tooling, brackets in a workshop, anything where the consequence of the metal being slightly different is that nothing happens. Requiring paperwork on these adds cost and lead time and buys nothing.

A useful default: require certificates on the parts that go into a product, not on the parts that help you make it.

What to write on the RFQ

Be specific about which of the three things you want, because they are not the same purchase.

MATERIAL: 6061-T651 PER ASTM B209.
SUPPLY MILL TEST CERTIFICATE (EN 10204 3.1) WITH SHIPMENT.
MAINTAIN LOT TRACEABILITY FROM RAW STOCK TO FINISHED PARTS.
MARK EACH PART WITH JOB NUMBER.

The last line is the one people forget. A traceability requirement with no marking on the part means the chain breaks the moment the parts leave the box they shipped in.

What to check when the paperwork arrives

  • Does the alloy and temper on the certificate match the drawing?
  • Does the standard referenced match what you asked for?
  • Is there a heat or lot number, and does the shop’s CoC reference that same number?
  • Are the mechanical test values inside the published range for that alloy and temper?

Most of the time this takes two minutes and everything lines up. The occasions it does not are exactly why you asked.

If certificates matter to you, say so when you request the quote rather than after. It is a real cost for the shop, and a quote that did not include it will not magically absorb it later.

Sources

Figures in this guide are checked against the following. Published values are typical ranges for test material, not a guarantee for your part.

Frequently asked questions

Does a mill certificate prove my part is 6061?

It proves a batch of material with that chemistry existed and was tested. It only connects to your part if the supplier maintained lot traceability from the bar through to the finished piece. A certificate with no traceability chain behind it is a document about someone else's metal.

How much do certificates add to the price?

The certificate itself is usually free from the metal supplier. The cost is in the handling: keeping lots segregated, recording which bar went into which job, and producing the paperwork. Expect a modest addition, more on small batches where the admin does not spread out.

Do I need certs on a prototype?

Usually not, and asking for them on a one-off bracket adds cost for nothing. Require them when the part is safety critical, when a customer or standard demands it, or when you intend to qualify the part and will need to prove later what it was made from.

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