From Heat Number to Shipped Lot: Tracing Machined Medical Parts

If a complaint about a machined instrument or implant component shows up two years after shipment, the device maker must answer three questions fast: which material was used, how the parts were made, and which other parts share that history. Medical device machining lot traceability is the set of records and physical identifiers that let you answer those questions without guessing.

This article walks through the chain from the mill heat number to the shipped lot, explains where it usually breaks, and lists the records you should request from a machine shop so it can support your own device history record.

Who owns what

Before the details, a clear split of responsibility. The device maker owns the regulatory file: design controls, process and product validation, biocompatibility, sterilization validation, labeling, UDI, and the regulatory submission. The machine shop supplies parts to the buyer’s drawing and quality requirements and keeps the production records that prove it did. Medical Device Machining sits in front of both: we review the RFQ and route it to a partner shop in southern Vietnam that fits the part. We do not machine parts and we do not hold production records.

In the US, the FDA’s Quality Management System Regulation under 21 CFR Part 820 now incorporates ISO 13485:2016 by reference, and ISO 13485 clauses on identification and traceability apply to the device maker’s system. The device maker then flows the requirements it needs down to suppliers through purchasing controls. That flow-down is what this article is about.

Step 1: Material receipt and certificate check

Traceability starts when bar or plate arrives at the shop. For medical work the material is normally bought with a mill test certificate (often an EN 10204 3.1 type certificate) that states the heat number, chemistry, mechanical properties and the specification the material meets, such as ASTM F136 for Ti-6Al-4V ELI or ASTM F138 for 316L bar.

A proper receiving check covers:

• Heat number on the certificate matches the heat number stamped, etched or tagged on the bar.
• Specification and grade on the certificate match the drawing callout, including the edition if the drawing names one.
• Chemistry and mechanical values fall inside the specification limits, not just “conforms”.
• Dimensions and condition (annealed, solution treated, H900 for 17-4 PH) match the order.
• For PEEK, PEEK-CF and other polymers, the lot number and grade on the supplier certificate match the rod label, and the grade is the one the buyer specified.

The shop logs the receipt with a material receiving number that links the certificate, the purchase order to the mill or distributor, and the storage location.

Step 2: Bar identification in stock

The weak point is often not the certificate but the bar after it leaves the rack. Once a 3 m bar is cut into blanks, the heat marking on the original end is gone. Good practice is to transfer identification at every cut: tag each remnant with the heat and receiving numbers, and keep a job’s blanks in a labeled bin that travels with the traveler. Two heats of Ti Grade 5 look identical, so each heat needs its own rack position.

Step 3: The traveler or router

The traveler (also called a router or job card) is the production record for one lot. It lists every operation in order and gets signed or stamped as each one is completed. For a typical machined medical part it records:

• Part number, drawing revision and lot or job number
• Material receiving number and heat number
• Quantity started at each operation and quantity passed
• Machine used for each operation and the CNC program revision
• Operator and date for each step
• In-process inspection results or a reference to the inspection sheet
• Outside processes (passivation, electropolish, anodizing, heat treatment) with the vendor’s certificate number
• Any nonconformance report, deviation or rework reference

Paper or digital matters less than filling it in at the time of the work, with the program revision recorded so an investigation can tell which toolpath made which parts.

Step 4: Lot numbers

A lot is a group of parts made under the same conditions: same material heat, same setup, same program revision, same run. The lot number is the key that ties the physical parts to the traveler and the inspection records.

Buyers usually define the rules. Common ones are one heat per lot, no lot larger than one continuous production run, and a new lot whenever the program, fixture or machine changes. If you do not state rules, the shop will use its own, and those may not fit your investigation needs.

Step 5: Splitting and merging lots

This is where traceability most often breaks.

Splitting happens when part of a lot goes ahead and part is held: some parts go to passivation while others wait for rework, or you ask for a partial shipment. Each split should get a sub-lot number (for example 24071-A and 24071-B) with its own traveler copy and quantity record. Otherwise the shipped quantity and the inspected quantity will not match later.

Merging is the reverse, and it is riskier. Combining two lots, or two heats, into one shipment loses resolution: if a problem is found, the whole merged group becomes suspect. Many buyers prohibit merging without written approval. If merging is allowed, the packing list and the certificate of conformance should list every source lot and its quantity.

Rework needs the same care. A reworked part should carry its original lot number plus a record of the rework operation, the approval for it, and the re-inspection result.

Step 6: Laser marking

Marking puts the traceability on the part itself, which matters when parts are separated from their packaging during assembly or reprocessing.

UDI marking is the device maker’s decision. The FDA UDI system places requirements on the labeler of the finished device, and direct part marking rules apply to certain reusable devices. The device maker decides whether a component carries a UDI, a lot code, a part number, or nothing, and in what format (human-readable text, a Data Matrix code, or both).

The shop’s job is to laser mark to the buyer’s specification: content, character height, location, depth or contrast, and the verification method. Partner shops we work with can laser mark titanium, 316L and 17-4 PH on request. A few points to specify:

• Mark location relative to datums, and a keep-out zone away from functional surfaces
• Marking method (annealing or dark mark on stainless steel is common where corrosion resistance matters; the buyer should confirm the method against its own cleaning and passivation validation)
• Whether marking happens before or after passivation or electropolish
• How the marked content is verified (visual check, or a code grade if you use a Data Matrix)

Step 7: Certificate of conformance

The certificate of conformance (CoC) is the shop’s signed statement that the lot meets the purchase order and drawing. It should state part number, drawing revision, purchase order number, lot number, quantity, material specification and heat number, and list the outside processes with their certificate references. A CoC that only says “parts conform to drawing” with no lot or heat reference adds little to your file.

What records to request

The records below are the ones buyers most often ask for. Write them into the purchase order or quality agreement, and state which ones ship with each lot and which ones the shop keeps on file.

PPAP-style packages, such as a PPAP Level 3 submission, can be requested in the RFQ. They are quoted separately.

Common traceability failure points

In practice, gaps show up in the same few places:

1. Blanks without identification. Bar is cut, the tagged end is scrapped, and the blanks sit in an unmarked bin. Two weeks later nobody can prove which heat they came from.
2. Unrecorded lot splits. A partial shipment leaves without a sub-lot number, and the remaining parts ship later under the same lot number with a different inspection history.
3. Outside processing without lot linkage. Parts go to a passivation vendor in one bag and come back in two. The vendor certificate lists a quantity but no lot number.
4. Rework outside the traveler. An operator deburrs or re-machines a feature to save a part and does not record it.
5. Program revisions not recorded. The traveler shows the operation was done, but not which program version, so a later toolpath fix cannot be tied to specific parts.
6. Generic certificates of conformance. The CoC carries no heat number or lot number, so it cannot be matched to the physical parts.
7. Mixed heats in one shipment. Two heats end up in one bag because the order quantity was larger than the remaining stock of the first heat.

When we review an RFQ, we look at whether the traceability requirements are stated clearly enough for the shop to plan for them. Mixed-heat rules and marking requirements in particular change how a shop buys material and plans setups, so they belong in the RFQ, not in a later email. More on documentation expectations is on the quality and documentation page.

How to verify it works

Run a trace exercise during an audit or at first shipment: pick one shipped part and ask for the traveler, inspection record, outside process certificate and mill certificate for that lot. Then pick a heat number and ask for every lot made from it. A working system answers both in minutes. Verify any quality system certificate a shop presents directly with the issuing registrar; we do not claim certification on behalf of any shop.

Putting traceability into your RFQ

A clear RFQ gets you an accurate quote and avoids surprises at the first shipment. When you send your drawing through the RFQ form, include:

• Drawing with revision, material specification (for example ASTM F136) and any finish or passivation callout
• Lot rules: one heat per lot, maximum lot size, whether merging is allowed
• Marking specification: content, location, method, and whether it is a lot code or UDI content you supply
• The list of records to ship with each lot and the records to keep on file, with a retention period
• Whether you need an FAI report, CMM data for every lot, or a PPAP-style package

We review the RFQ, return questions or DFM notes with the quote at no charge, and reply within 24 business hours. A mutual NDA can be signed before drawing review, and files go only to the shop assigned to the part.

See also: DMR, DHR, DHF and MDF for where traceability records sit in the device maker’s files.