CNC Processing Contract: What It Fixes Before the First Cut

• 1
  What a CNC processing contract actually controls
• 2
  Scope, geometry, and the tolerance clause
• 3
  Material, process, and what the shop can hold
• 4
  Lead time, inspection, and quality clauses
• 5
  Change orders, revisions, and risk split
• 6
  Payment, quantity, and packaging terms
• 7
  Which contract terms matter most by part type

A CNC processing contract is not a purchase order with a longer signature block. It is the record of decisions that were made before cutting started: which revision of the drawing is the authority, what tolerances the shop will hold, which material grade is acceptable, how parts are inspected, and what happens when a dimension turns out to be impossible. Everything downstream, from fixture design to final inspection reports, traces back to these clauses.

The engineering value is traceability. When a 5-axis job runs 10,000 parts over several weeks, a machinist, a programmer and a quality inspector will all touch the same feature. If the contract says only "per drawing," three people can read three different things. If it says the drawing revision, the datum scheme and the inspection method, the argument disappears before it starts.

The commercial value is change control. A revision that arrives after fixtures are cut can add setup time, new tooling and a re-qualification run. A contract that defines how revisions are priced and scheduled turns that into a normal conversation instead of a dispute.

One boundary worth stating early: a contract cannot make an unmanufacturable feature manufacturable. It can only make the risk visible, so both sides decide knowingly.

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  Drawing revisionName the exact revision and the CAD file hash if one exists.
• 2
  Tolerance basisState default tolerance plus any tighter callouts.
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  Inspection methodCMM, gauge, or visual, and who pays for the report.
• 4
  Change pricingHow a post-award revision is quoted and scheduled.

Scope is where most disputes are born. The clause should list part numbers, quantities, the drawing revision, and the operations included. Machining only? Machining plus anodizing plus laser marking? If finishing is subcontracted, say who owns the schedule. A part that leaves the machine on day 4 but waits two weeks at a plating line is still late, and the contract should say whose clock that is.

Tolerance is a cost driver, not a wish. A general block tolerance of ±0.1 mm on a milled aluminum bracket is routine. A ±0.005 mm ( ±0.0002 in ) callout on the same bracket forces slower passes, temperature control and more inspection time. The contract should separate the general tolerance from the critical few dimensions that genuinely need the tight band.

Datums matter as much as numbers. If the drawing datums do not match how the part sits in the fixture, the shop either builds a special fixture or asks for a drawing change. Both are cheaper before the first setup than after.

State the inspection sampling too. For medical and automotive work, 100% inspection before shipment with recorded reports is standard at our plant. For a prototype run of five parts, dimensional reports on the critical features are usually enough.

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  Operations listMachining, finishing, marking, assembly, packaging.
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  Critical dimensionsCap the tight-tolerance list; it drives cost fast.
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  Datum alignmentDrawing datums should match the fixture plan.
• 4
  ReportsSay which parts get a dimensional report and in what format.

Material clauses should name the grade, the condition and the standard. "Aluminum" is not enough. 6061-T6 behaves differently from 7075-T6 in both machinability and distortion after heat. Stainless 316L and 17-4PH need different feeds and different tool life. If the customer supplies material, the contract should say who certifies it and who absorbs scrap from a bad lot.

Process capability sets the real boundary. Our 16 simultaneous 5-axis machining centers hold ±0.005 mm on parts within a 4,000 mm maximum processing size, with large travels of 4,000 × 400 × 150 mm. Fine finishes reach Ra 0.2–0.8 μm when the geometry allows it; a deep pocket with a small corner radius will not, regardless of what the contract says.

Thin walls are the classic trap. A 0.8 mm wall on a 100 mm aluminum housing will move during and after cutting. The contract can require stress-relieved stock and light finishing passes, but it cannot promise a flatness number that physics refuses.

Surface treatment belongs in scope. Anodizing adds a few micrometres and can round a sharp edge. If a mating dimension sits on that edge, say so before coating, not after.

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  Grade and temper6061-T6, 7075-T6, 316L, 17-4PH and so on.
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  Stock sourceCustomer-supplied or shop-supplied, with certs.
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  Coating growthAnodizing and plating shift dimensions slightly.
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  Thin wallsFlag walls under roughly 1 mm for distortion review.

Lead time clauses should be written as milestones, not one date. Quotation and free DFM analysis within 12 hours, production start within 24 hours, parts ship in 3–5 days: those are separate commitments, and each can slip for its own reason. A contract that separates them tells both sides where the delay actually happened.

Inspection is the clause engineers read last and should read first. Ours runs raw material check, in-process monitoring and final inspection, with 100% inspection before shipment and reports on request. The contract should say which of those steps is documented and who receives the documentation. A certificate of conformance with no data behind it is not the same as a dimensional report.

Certification requirements belong here. ISO 9001:2015 covers general quality systems. IATF 16949:2016 applies to automotive production. ISO 13485:2016 covers medical devices. ISO 27001:2022 covers information security, which matters when drawings are confidential. If your program needs one of these, name it in the contract so the paperwork is planned, not improvised.

Confidentiality sits next to quality. Uploads are handled as secure and confidential, and an NDA is available on request.

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  MilestonesQuote, production start and ship date as separate terms.
• 2
  Inspection recordAgree the report format before the first run.
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  CertificatesMatch the certificate to the industry, not the marketing.
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  NDASign before drawings are shared, not after.

A revision after award is normal, not a failure. What matters is the rule. Most shops price a change by the work it actually adds: reprogramming hours, new fixtures, new material, and a re-qualification run if a critical dimension moved. A contract that defines this rule keeps the project moving.

Freeze points help. Agree that the drawing is frozen once fixtures are cut, and that any later change is quoted before it is executed. That single sentence prevents the most common argument in contract machining: work done without an approved price.

Risk split should follow control. The shop controls machining, so it owns scrap from its own process. The customer controls design intent, so it owns whether the feature is needed. Material defects belong to whoever supplied the stock. Write it down once and stop debating it per batch.

Schedule risk follows the same logic. Historical late-delivery probability at our plant is below 2%, but a contract should still say what happens if a plating line or a material mill misses its window. Notice period and remedy are better than silence.

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  Freeze pointDrawing frozen when fixtures are cut.
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  Quote before executeNo unapproved change work, ever.
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  Scrap ownershipFollows whoever controls the cause.
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  Notice periodDefine how fast a slip must be reported.

Quantity clauses should allow both overage and underage within a stated band, because machining a precise count is not always possible when a setup produces a few extras. State the band and the pricing rule for parts inside it.

MOQ is a term worth reading carefully. At our plant there is no minimum order quantity: runs go from one prototype to 10,000+ parts. That matters for contract language, because a single-part order still needs the same scope, tolerance and inspection definitions as a production run.

Payment terms should tie to milestones, not to moods. Deposit on award, balance on shipment, and a clear rule for change orders. Keep it short and unambiguous.

Packaging and shipping belong in the contract too. A 4,000 mm frame needs a crate and a handling plan; a bag of 200 small bushings needs to arrive without dents. Say which side designs the packaging and which side pays for it.

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  Quantity bandState allowed overage and how it is priced.
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  No MOQOne part to 10,000+, same terms apply.
• 3
  Milestone paymentDeposit, shipment balance, change orders.
• 4
  PackagingWho designs it, who pays, what protection level.