CNC Machine Service Contracts: What to Check Before You Sign

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  Key takeaways
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  Contract clause checklist: weak vs. usable wording
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  What a CNC machine service contract actually covers
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  Which parts suit a fixed contract and which do not
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  How to write the tolerance and inspection clause
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  Lead time, MOQ and confidentiality clauses that hold up
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  Which certifications belong in the contract
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  Common pitfalls in CNC machine service contracts
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  Step by step: reviewing a CNC machine service contract

A CNC machine service contract is the written scope under which a shop takes your CAD files and returns finished parts. It should state geometry, material grade, tolerance, surface finish, quantity, inspection method and delivery terms. It is not a purchase order with a signature line. The scope section is where most disputes start, because a drawing alone rarely says who owns the risk on a marginal feature.

For machined parts, the drawing is the technical document and the contract is the commercial wrapper. The contract should reference the drawing revision number. If the shop quotes from revision B and you ship revision C, the price and lead time are void. We ask customers to lock a revision at quote stage and note it in the contract, then treat any later change as a written amendment with its own price and date.

The scope should also list what the shop does not do. Heat treatment, plating and inspection by a third party are common exclusions. If your part needs anodizing to a specific color or hardness, that belongs in the finish section, not in an email thread. When the contract is silent, the shop defaults to its own standard, which may be Ra 3.2 μm as machined rather than the Ra 0.8–1.6 μm the assembly needs.

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  Drawing revisionName the exact revision the quote is based on.
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  Deliverable listParts, inspection report, material certs, finish certs.
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  ExclusionsHeat treat, third-party testing, assembly, packaging.

A fixed-scope contract works when the part is stable. A bracket, a housing or a manifold that will run for a year at 2,000 pieces a month is a good fit. The geometry is settled, the material is known and the shop can quote a firm unit price with a tolerance band. The contract can then cover the whole year with quarterly releases.

A fixed contract fits poorly when the design is still moving. Prototype rounds often change wall thickness, hole positions or material after the first article. Locking a price at that stage forces the shop to pad the quote for risk. It is faster to run the first two or three rounds on a simple per-order basis, then sign a longer agreement once the drawing freezes.

Volume is the other divider. If you need one or two pieces of a titanium impeller, a contract with a 500-piece minimum is a waste of everyone's time. Look for a shop that states no minimum order quantity and prices single pieces honestly. If you need 10,000+ parts a year with IATF 16949 traceability, the opposite applies: the contract should carry the traceability clauses, not just a price.

There is a middle case. A part that is stable in design but uncertain in demand, such as a medical instrument housing, benefits from a contract with a fixed unit price and a flexible release schedule. That structure lets you hold the price while ordering in smaller batches, which is usually better than a large blanket order that sits in a warehouse.

Tolerance should be attached to features, not to the whole part. A general note of ±0.1 mm on a machined aluminum housing is normal. A bore that seats a bearing needs its own callout, often ±0.005 mm. The contract should say which features carry the tight tolerance and which inspection method proves it. Without that, the shop can pass a CMM check on the loose features and the assembly still fails.

Inspection method matters as much as the number. A caliper cannot verify ±0.005 mm reliably; a coordinate measuring machine can. For surface finish, the contract should name the scale and range, for example Ra 0.8–1.6 μm on a sealing face. For threads and small bores, go/no-go gauges are often faster and cheaper than CMM time, and they are easier to repeat across batches.

Ask for the inspection stage too. Raw material check, in-process monitoring and final inspection before shipment are three different gates. A contract that only promises final inspection leaves the shop free to scrap a part at the end of a long cycle. For high-value parts, in-process checks after roughing and before finishing catch errors while there is still material to correct.

Finally, decide who pays for the inspection report. Some shops include a dimensional report with every batch; others charge per report. For regulated industries, the report is not optional. For a one-off fixture, a simple dimensional sheet may be enough.

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  Feature-level toleranceTight callouts on mating features only.
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  Named methodCMM, gauge or surface tester, stated in the contract.
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  Stage gatesMaterial, in-process and final checks.
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  Report ownerSay who supplies and who pays for the report.

Lead time is usually written as one number, which hides the real schedule. Break it into stages. The first is quote and DFM feedback, where a shop can reasonably promise a 12-hour turnaround. The second is production start, which depends on material availability and can be as short as 24 hours for common aluminum grades. The third is shipping, often 3–5 days for machined parts once production is complete.

Write each stage with its own condition. If the shop must order 17-4PH stainless, production start depends on the mill's delivery, not the shop's schedule. A contract that ignores material lead time will be late. It is better to list the material grades the shop stocks and the ones it orders in, so you know which parts carry schedule risk.

MOQ is the clause that most often blocks a pilot. A minimum of 500 pieces makes a first-article test impossible. If you are validating a design, look for a supplier that runs from one piece to 10,000+ and prices the low volumes honestly. High MOQ is not a red flag by itself; it just means the shop is built for volume and should be treated as such.

Confidentiality belongs in the same section. An NDA should be available before you send CAD files, not after. The contract should also state where files are stored and who can access them. ISO 27001 is the certification that covers information security management, and it is worth asking whether the shop holds it if your drawings are sensitive.

Certifications are not interchangeable. ISO 9001:2015 covers a general quality management system and is the baseline for any machining supplier. IATF 16949:2016 adds automotive-specific requirements such as traceability and change control, and it matters if your parts go into a vehicle program. ISO 13485:2016 is the medical device standard and brings tighter process validation and record retention. ISO 27001:2022 covers information security, which is relevant when you share proprietary designs.

The contract should name the certificate and the scope. A supplier may hold ISO 9001 for one plant and not for another. If your parts run in a specific facility, the certificate should cover that site. Ask for the certificate number and the expiry date, and keep a copy with the contract. This is a five-minute check that avoids a long argument later.

Material certificates are separate from quality certifications. A mill certificate ties the raw stock to a heat number and a chemistry. For aerospace, medical and automotive parts, the contract should require mill certs for every lot, not just the first. For a simple industrial bracket, a mill cert may be unnecessary overhead.

Do not accept a logo on a website as proof. Certificates are issued to a legal entity with a scope statement. If the scope excludes machining or excludes your industry, the certificate does not cover your order.

The most common mistake is signing a contract with no drawing revision. The shop quotes from an early model, you update the model, and the first batch is wrong. Fix it by attaching the revision to the contract and treating any change as a written amendment. The second mistake is accepting a tolerance note without a method. A number without a measuring tool is an opinion.

Another pitfall is a vague finish call. As machined can mean Ra 3.2 μm or Ra 1.6 μm depending on the shop. If the part is cosmetic or seals against a gasket, write the Ra range and the finish type. For anodizing, name the color and whether it is clear, hardcoat or conductive. Conductive anodizing and hardcoat are different processes with different masking requirements.

Watch for evergreen price clauses. A contract that fixes a unit price for three years without a material index will be renegotiated the first time aluminum spikes. A better structure ties the price to a published material index and reviews it quarterly. That keeps the relationship stable without either side absorbing a shock.

Finally, check the amendment process. A contract that requires a full re-quote for every minor change will slow you down. A simple written change note with a price and a date is enough for most engineering changes. Put that mechanism in the contract before you need it.

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  No revision lockFirst batch built to an old model.
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  Tolerance without methodNumber cannot be verified at the bench.
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  Vague finishAnodize type and Ra range left open.
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  Rigid pricingNo material index, no review window.