CNC processing contract: what it covers and where projects go wrong
A CNC processing contract is the document that fixes material, tolerance, inspection method, tooling ownership and change handling before a spindle turns. This page is written for design engineers and sourcing staff who have to sign one. Read it and you will know which clauses to push back on, and which ones are simply standard shop practice.
In this article
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Key takeaways
What a CNC processing contract actually fixes
A CNC processing contract is not mostly about price. It is about defining a part in a way two parties can measure the same way. The commercial sheet says how much and how many. The technical annex says what the part is. When a job goes wrong, the argument is almost never about the rate per hour. It is about whether a bore was Ø10.00 mm or Ø10.02 mm, and who measured it.
Four things have to be pinned down before production starts. Material grade and condition. Dimensional tolerance and datum scheme. Surface finish per feature. Inspection method and sampling level. Leave any of those to a verbal understanding and you are relying on goodwill rather than a document.
The scope clause does the heaviest lifting. It should list the revision of every file you sent, name the CAD format, and state that the 2D drawing governs where it disagrees with the 3D model. Most machine shops program from the model and inspect to the drawing. If the two conflict, the shop follows the drawing unless you say otherwise in writing.
Then there is the boundary of responsibility. Who supplies the raw stock? Who owns the fixture? What happens to the CAM program if you move the job to another supplier? A short paragraph on each of these prevents a long conversation later.
- 1Scope and revisionFile names, revision letters, and which document wins in a conflict.
- 2Technical annexMaterial, tolerance, finish, inspection method per feature.
- 3Responsibility splitStock, tooling, fixtures, program ownership.
- 4Commercial termsQuantity, packaging, delivery basis, payment milestones.
Why tolerance wording causes most disputes
A tolerance callout is a cost instruction, not decoration. On a 6061-T6 aluminum bracket, ±0.10 mm is a three-axis job with standard fixturing. Tighten the same bracket to ±0.005 mm across a 200 mm span and you have added temperature control, a warm-up cycle, and a CMM report. The drawing did not change shape. The process changed completely.
General tolerance blocks are where this gets quiet. A title block note like "unless otherwise specified, ±0.25 mm" is fine for a weldment. It is not fine for a bearing bore that also appears in an assembly stack-up. If only three features matter, dimension only those three and let the rest run loose. Shops price what they see.
Datum selection matters just as much. A datum that exists only in the CAD model, and cannot be touched by a probe or rested on a granite plate, is not a usable datum. Choose a face, a bore, or a slot that the machine can locate from. The contract should say the datum scheme is the one on the drawing, not one invented at the machine.
Finally, decide who absorbs out-of-tolerance parts. The usual arrangement is that the shop reworks or remakes nonconforming parts at its own cost, provided the drawing was unambiguous. That clause only works if the inspection method was agreed up front. Otherwise both sides measure honestly and disagree.
- 1Tighten only what functionsEvery extra tight callout adds machining and inspection time.
- 2Real datums onlyIf a probe cannot touch it, it cannot be a datum.
- 3Agree the gaugeName the instrument and the report format before the first cut.
Inspection, documentation and traceability clauses
Inspection is the part of a CNC processing contract that buyers read last and argue about most. State the method per feature. A bore can be checked with a bore gauge, a plug gauge, or a CMM. Those three tools will not always return the same number at the fourth decimal place. If the contract does not say which one governs, the argument is unresolvable.
Sampling level belongs in the same paragraph. For a one-off prototype, 100% inspection of the critical features is normal. For a 10,000-part run, you need a defined sampling plan and a defined reaction when a sample fails. Say whether the shop holds inspection records, for how long, and in what format they are sent.
Material traceability is separate. If your industry requires mill certificates, the contract should name the material grade and the condition, and require the certificate to be filed against the lot. For medical and automotive work, the certificate set is often the reason a buyer picks one shop over another.
Documentation also covers what leaves the building. First article inspection reports, in-process records, final reports, and any dimensional data you want attached to the shipment. Ask for the format you actually need. A PDF summary is cheap. A full variable-data CMM report takes programming time and is priced accordingly.
- 1One governing methodName the instrument that decides pass or fail.
- 2Sampling planDifferent rules for one-offs and high-volume runs.
- 3CertificatesMill certs filed against the material lot.
- 4Report formatAsk for what your quality system accepts.
Tooling, fixtures, IP and confidentiality
Who owns the fixture is a question that surfaces two years later, usually when a program moves. The common arrangement is that the customer pays for dedicated tooling and owns it, while the shop keeps general-purpose vises, chucks and standard cutters. Write down which is which. A soft jaw machined for your part is dedicated. A set of ER collets is not.
CAM programs are a grayer area. A shop that has spent engineering time on a five-axis setup has real value in that program. Many suppliers keep the program and release the part. If you need the program, negotiate it as a line item rather than assuming it comes with the order.
Confidentiality is usually handled by a separate NDA rather than buried in the contract. That is cleaner, because the NDA survives after the purchase order closes. Uploads should travel over a secure channel, and access to your files should be limited to the people who need them.
If your parts are regulated, add a clause on subcontracting. A shop that sends your anodizing out to a third party should tell you, and the subcontractor should be bound by the same confidentiality terms. Ask for the list of outside processes up front.
- 1Dedicated vs. general toolingList what the customer pays for and keeps.
- 2Program ownershipNegotiate it explicitly if you need the CAM files.
- 3Separate NDAIt outlives the purchase order.
- 4Subcontracting disclosureName the outside processes and who runs them.
Change orders, lead time and delivery terms
A change order is where a smooth project turns into a claim. The mechanism has to be defined before it is needed. Who can authorize a change? What is the cut-off point, at drawing release, at material purchase, or at first cut? How is the price adjusted? A one-page change procedure is enough.
Revision control is the practical half of this. Every file you send should carry a revision letter, and the contract should state that only the latest acknowledged revision is in force. When someone emails a "small tweak" to a STEP file without a revision number, the shop has no way to prove which geometry was quoted.
Lead time should be tied to a trigger, not to a feeling. Typically the clock starts when the purchase order, the drawing revision and the material choice are all confirmed. If material is customer-supplied, the start date depends on arrival and inspection of that stock. Say so.
Delivery terms cover more than the date. Incoterms, packaging, and who pays freight all belong in writing. For repeat work, a rolling forecast helps the shop reserve capacity without locking you into a firm order for every month.
- 1Simple change formOne page: what changed, cost impact, new date.
- 2Revision lettersOnly the acknowledged revision is binding.
- 3Lead time triggerDefine the event that starts the clock.
- 4Delivery and packagingIncoterms, crating, freight responsibility.
Which contract terms matter most by project type
Rows are project types. Columns are the clauses that most often need extra detail.
| Project type | Clause to tighten | Why it matters |
|---|---|---|
| One-off prototype | Revision control | Geometry changes fast; unlabeled files cause rework |
| Bridge production | Tooling ownership | Fixtures paid for now get reused later |
| High-volume run | Sampling plan | 100% inspection on every part is not economic |
| Regulated medical | Traceability | Certificates and records must survive an audit |
| Automotive / EV | Change procedure | PPAP-style documentation needs a defined path |
| Tight-tolerance assembly | Governing gauge | Two honest methods can disagree at ±0.005 mm |
Where we land
If your part is simple and the run is short, spend your negotiation time on revision control and inspection method. If the part is tight, regulated, or headed for volume, spend it on tolerance definition and tooling ownership instead. Get those four right and the rest of the contract is mostly paperwork.
Questions buyers ask before signing
Do we need a separate NDA if the contract already has a confidentiality clause?
A standalone NDA is usually cleaner. It can be signed before any drawings are exchanged, and it stays in force after the purchase order closes. A confidentiality clause buried in the terms and conditions only covers the period of the order.
For most projects, sign the NDA first, then negotiate the technical annex. That order keeps your design data protected during the quoting stage, when several suppliers may be looking at the same files.
How tight a tolerance should we actually specify?
Specify tight where the function needs it and loose everywhere else. A bearing bore, a sealing face, or a mating spigot usually justifies ±0.005 mm to ±0.02 mm. Bolt clearance holes, webs and non-critical bosses rarely do.
Every callout you tighten adds machining time, fixture complexity and inspection effort. On a typical aluminum part, going from ±0.05 mm to ±0.005 mm on a single feature can change the process route, not just the cycle time.
Who owns the CAD model and the CAM program?
You own the design data you send. The CAM program the shop builds from it is a separate question, and it is usually treated as the shop's process knowledge unless you negotiate otherwise.
If you need the program, put it in the contract as a deliverable with a price. Otherwise expect the shop to retain it, which is normal practice and does not affect your ownership of the part design.
What happens if parts measure out of tolerance?
The usual term is that the shop reworks or remakes nonconforming parts at its own cost, as long as the drawing and inspection method were unambiguous. That last condition is doing a lot of work.
If the drawing left a feature undefined, or the contract never named the governing gauge, both sides can measure honestly and still disagree. That is why the technical annex matters more than the liability paragraph.
Can a contract cover both prototyping and volume production?
Yes, and it often should. Use one document with a scope that names the current phase, then add an annex for the volume phase covering sampling, tooling and reporting.
Terms that make sense for a single prototype, such as 100% inspection of every feature, are usually revised before a 10,000-part run. Write the revision path into the agreement so neither side has to renegotiate from zero.
How is lead time usually defined in the agreement?
Tie it to a trigger rather than a calendar date. The common trigger is confirmation of the purchase order, the drawing revision and the material choice. If the customer supplies material, the clock starts when that stock arrives and passes incoming inspection.
Defining the trigger removes most late-delivery arguments. It also makes the shop's planning honest, because the schedule is measured from a real event rather than from the date someone first asked for a quote.
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