Guide
What Lot Traceability Should You Require on a Machined Parts Purchase Order?
Lot traceability is a supplier's ability to tie every shipped part back to the production lot it was made in and to the material heat it was cut from, so a problem found later can be narrowed to a defined group of parts. The short answer: for a machined part with no safety, regulatory or warranty exposure, require part-to-lot and lot-to-heat traceability, and keep serial numbers for parts where one failure must be traced on its own.
By Saola Manufacturing engineering team · Published · Updated
General information, not legal advice. Have counsel and your quality function review any purchase order clause before you rely on it.
This guide is for a quality or supply chain engineer writing purchase order notes for a supplier that does not work under an aerospace or medical quality system, and who wants to know how much traceability is worth paying for. It defines three levels, shows what each asks of the supplier, explains how a job that spans two bar heats gets split, and ends with sample wording. The documents that prove a heat are covered separately in the guides to reading a mill test report and the EN 10204 certificate types; this guide is about the lot structure inside the supplier.
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What are the levels of lot traceability on a machined part?
There are three: material to heat, part to production lot, and individual serial number. Each level includes the one before it and asks the supplier for more records and more handling.
Material-to-heat traceability means the supplier knows which heat of bar, plate or forging a job was cut from and holds the mill certificate for it. A heat is one batch of melted metal, and its chemistry is reported on the certificate. ASTM A29/A29M-23, the general requirements for hot-wrought carbon and alloy steel bars, covers both heat analysis and product analysis for the bar, which is why a certificate is tied to a heat and not to a part.
Part-to-lot traceability adds a production lot number: each shipped box or tag carries a number that the supplier can link to the job, the heat or heats consumed, and the inspection record. Serial traceability goes further and gives each part its own number. Real purchase order attachments use these levels. In Aerojet Quality Note D03 (2008), the supplier must keep records that give production lot, batch or heat-lot traceability and melt number for each part, with castings, forgings and wrought material traceable to the original mill or melt source, and parts fabricated by the seller identifiable to the lot of material used. A Spectrolab purchase order attachment (Q-3B, revision February 2022) asks for two-way traceability: forward, from a part to the assembly it went into, and backward, from a part to the lot received at the seller's facility.
Both documents come from aerospace buyers and ask for more than a non-regulated part needs, but their structure is a useful menu. You can take the first two levels without taking the paperwork load of the third.
| Level | What the supplier must record | What it adds for the supplier | Usually enough for |
|---|---|---|---|
| 1. Material to heat | Heat number and mill certificate for the bar, plate or forging used on the job | Keep the certificate with the job; stop mixing heats without a record | Commodity parts with no safety, regulatory or warranty exposure |
| 2. Part to production lot | A lot number on each shipped box, linked to the heat or heats consumed and to the inspection record | Label and segregate by lot; split the lot when a job spans two heats; carry the number to the packing list | Parts where a field problem must be narrowed to a lot and a bar heat |
| 3. Serial number | A unique number on each part, linked to its lot, heat and inspection results | Mark every part; record individual measurements; handle rework and scrap by serial | Safety-critical or regulated parts, where the failure of one part must be traced alone |
Which level does a non-regulated part at 10,000 to 20,000 parts a year need?
Usually the second level: a production lot number on every shipment, linked to the bar heat on the certificate. Add serial numbers only where a single part's history matters on its own.
The test is what you will need to know if a part fails in the field or on your line. If the answer is "which group of parts, and which metal", a lot number tied to a heat is enough. It lets you quarantine the right boxes, ask the supplier to check the right records, and ask the mill about the right heat. If the answer is "this one part, and its exact measurements", you are in serial territory, and you should expect the supplier to charge for marking and recording.
Level 1 alone is thinner than it looks. A certificate for a heat tells you what the bar was, but if the supplier cannot say which of your parts came from that bar, the certificate cannot help when a lot goes wrong. Level 2 closes that gap at modest cost, since the supplier already tracks jobs, and it fits a part made in lots of a few thousand.
Parts with safety exposure, a regulated end use or a contractual warranty claim need a more careful conversation, and that decision belongs to your quality function and, where relevant, your customer's requirements. This guide does not replace those requirements.
What does each level cost the supplier, and why does a job split into lots?
Level 1 costs record-keeping; level 2 adds labeling, segregation and splitting a lot when a job spans two heats; level 3 adds marking and per-part records. The lot split is the cost most buyers do not expect.
A turned or milled lot is cut from bar, plate or billet that arrives in lengths. If one heat covers the whole job, the lot has one heat and traceability is simple. If the job needs more bar than one heat supplied, the supplier has two heats in the same job. The honest options are to buy enough from one heat in advance, which locks up material, or to run the job as two lots, each tagged with its own heat and its own number, and to keep the parts physically separate from there on. Aerojet's note asks for exactly this behavior: minimize the number of lot or date codes in a shipment, and segregate each lot in packaging and shipment when several must be used.
That is why a lot is different from the material mill's lot. A material specification defines its own lots at the mill; for example, ASTM A582/A582M-22 for free-machining stainless bars requires at least one hardness test per lot. The supplier's production lot is a separate idea, set by how the machine shop runs the job, and the PO should define it.
For a buyer, the cost shows up as a few extra labels and a rule about not mixing. It does not need to cost much when the part ships in a few lots a year. It gets expensive when lots are small and frequent, because every lot carries its own record.
How should lot numbers be assigned and kept?
Let the supplier assign unique, non-repeating numbers unless you have a format of your own, and require the number to appear on the box label, the packing list and the certificate of conformance. The number should link, in the supplier's records, to the job, the heat and the inspection results.
A lot number is useful only if it is findable later. Ask that it be carried through four places: the shop traveler that follows the job, the label on each box or tag, the packing list, and the certificate of conformance. Then a number on a box in your warehouse leads back to a job and a heat without a phone call.
Keep a rule for rework and nonconforming parts. Spectrolab's attachment says traceability includes parts and materials that are modified, repaired or reworked, and that records are kept for the period the purchase order sets. Reworked parts should keep their original lot number or carry a derived one, never a fresh number that breaks the chain, and the PO should name how long records are kept. Match that period to your product life and any customer requirement, not to the supplier's habit.
If you supply material to the shop yourself, the same logic applies in reverse: the shop should keep the lot or heat number you provide with the parts made from it.
What documents should arrive with the shipment?
A certificate of conformance that lists your part number and revision, the production lot number, the quantity, and the heat number or numbers; the mill certificate for each heat; and the inspection record for the lot. Together they close the chain from part to metal.
The certificate of conformance is the document that connects the supplier's lot to your purchase order, and the mill certificate connects the heat to the metal's chemistry and mechanical properties. If the lot used two heats, both certificates and both heat numbers should be listed against the quantity from each. The guide on certificate of conformance requirements covers the wording, and the guide on incoming inspection shows how to check the paperwork against the boxes on arrival.
On arrival, a short check is worth doing: compare the lot number on the box label with the one on the packing list and the certificate of conformance. A mismatch is the first sign that the supplier's system is looser than its paper.
What sample purchase order wording can you use?
Four short clauses cover most non-regulated parts: material traceability, lot definition, lot marking and records. Adjust the wording to your own quality system and have it reviewed against any customer requirement.
- Material. "Seller shall purchase material for this part number with a mill certificate showing heat number and chemical analysis, and shall keep the certificate with the job record."
- Lot. "A production lot is the quantity made from a single heat on one setup run. If a job uses more than one heat, Seller shall run each heat as its own lot."
- Marking. "Each box or tag shall show the part number, revision, lot number and quantity. The lot number shall be unique and shall appear on the packing list and the certificate of conformance together with the heat number."
- Records. "Seller shall keep records linking each lot number to the job, the heat and the inspection results for the period stated in this order, and shall make them available on request. Reworked parts shall keep their lot identity."
Add the serial-number clause only for parts that need it. For turned and milled parts, the process pages for CNC turning and CNC milling describe what the lead plant runs, and the quote form is the place to state the traceability level you want alongside the drawing.
Frequently asked
Is a mill test report the same as lot traceability?
No. A mill test report is tied to a heat of metal. Lot traceability is the supplier's record that links your shipped parts to that heat and to a production lot. You need both.
What if a job uses two heats of bar?
The supplier should run it as two lots, each with its own heat and lot number, and keep the parts apart. The alternative is to buy enough from one heat before starting, which is a supplier decision that the PO can ask for.
Do I need serial numbers on every part?
Usually not. Serial numbers fit safety-critical or regulated parts where one part must be traced alone. Marking and recording each part adds work for the supplier.
Who assigns the lot number?
The supplier can, unless you specify a format. Require it to be unique and non-repeating, and to appear on the label, the packing list and the certificate of conformance.
How long should records be kept?
Long enough to cover your product life and any customer or regulatory requirement. State the period in the PO instead of leaving it to the supplier's default.
Sources
- Aerojet Quality Note D03, Traceability to Material Lot, revision 0, Aerojet, November 6, 2008
- Purchase Order Attachment Q-3B, Material Traceability, Compliance Evidence and Certification of Special Processes, Spectrolab, revision February 14, 2022
- ASTM A29/A29M-23, General Requirements for Steel Bars, Carbon and Alloy, Hot-Wrought, ASTM International, 2023
- ASTM A582/A582M-22, Free-Machining Stainless Steel Bars, ASTM International, 2022