CNC Subcontract Precision Service: What Engineers Should Check
This guide is for engineers and sourcing staff who need to hand a finished drawing to an outside machine shop. It covers the seven checks that decide whether a cnc subcontract precision service can actually hold your tolerance, and where the risk sits.
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What matters most before you send a PO
Which subcontract tier fits your part
Pick the lowest row that still covers your tightest tolerance and your annual volume.
| Tier | Typical use | Watch for |
|---|---|---|
| Prototype shop | One-off and short runs | No in-process inspection record |
| General job shop | Simple 3-axis parts | Tolerance drift on long parts |
| Precision subcontract | ±0.005 mm features | Quoted price excludes finishing |
| Full-service partner | Machining plus finish | Longer quoting stage upfront |
Six warning signs in a subcontract quote
Any single sign is a question. Two or more on the same quote is a reason to keep looking.
| Warning sign | What it usually means | What to ask |
|---|---|---|
| One-line price, no breakdown | Costs are hidden or unknown | Separate machining, material and finish |
| No DFM comments | Drawing was never reviewed | Which features are hard to hold? |
| Tolerance stated without method | No CMM plan behind the number | How is this feature measured? |
| Finishing listed as "plus" | Third-party handoff in the chain | Who runs the finish and where? |
| Lead time as one date | Stages are not planned | Quote, start and ship dates |
| No NDA mentioned | Design data is not protected | NDA before files are sent |
Pick the shop that answers the awkward questions
If a supplier will name the machine, the setup count and the measurement method before you send a PO, the tolerance is real. If not, keep looking.
Why a cnc subcontract precision service is judged before the first cut
Most sourcing decisions fail long before metal is cut. They fail at the drawing review. A capable cnc subcontract precision service will read your GD&T, flag a datum that cannot be reached in one setup, and tell you before the quote goes out. A weak one sends a price and waits for you to discover the problem.
The first thing to ask is not the tolerance number. Ask how that number is verified. A shop claiming ±0.005 mm should be able to say which machine holds it, whether the part is fixtured once or twice, and what metrology confirms it. If the answer is vague, the tolerance is marketing, not capability.
Volume changes the answer too. A shop with strong prototype output and a shop built for 10,000-piece runs are different businesses. GreatLight runs both, but many subcontractors only do one. Match the supplier to the run size you actually have, not the one you hope for.
- 1Send GD&T, not just a STEP fileDatum structure and surface callouts drive cost more than geometry.
- 2Ask for the setup countEach extra setup adds stacking error and cycle time.
- 3State the annual volumePrototype, pilot and production may need different suppliers.
Tolerance, surface finish and the measurement behind them
Tolerance and finish are usually quoted together and inspected separately. A feature can sit inside ±0.005 mm and still fail a Ra 0.8–1.6 μm callout if the finishing pass was skipped. Ask the shop to state both, and ask which operations deliver them.
On finish, the numbers are not interchangeable. As-machined surfaces run Ra 1.6–3.2 μm. A tuned finishing pass reaches Ra 0.8–1.6 μm. Fine work goes down to Ra 0.2–0.8 μm, usually on sealing faces, bearing bores or optical housings. Quoting the fine band on a part that does not need it adds cost with no function.
Measurement matters as much as the cutting. A ±0.005 mm callout needs a controlled room, a calibrated CMM and a report you can read. Ask what is measured, how often, and whether the report ships with the parts. GreatLight inspects 100% of parts before shipment with raw material checks, in-process monitoring and final inspection, and shares reports on request.
One caution: thin walls and long slender parts move after machining. If your part has a 1 mm wall or a 300 mm unsupported section, plan a stress-relief step or accept a looser tolerance on those features.
- 1Separate the two calloutsTolerance and Ra are delivered by different operations.
- 2Do not over-specify finishFine Ra on a hidden boss is wasted money.
- 3Request the inspection reportA report you can read is worth more than a certificate on a wall.
Machine capacity and the parts it can actually hold
Machine count tells you throughput. Machine mix tells you what the shop can do. A subcontractor with 127 high-precision CNC machines but no simultaneous 5-axis centers will struggle with a part that has compound angles on five faces.
For five-face work in one setup, simultaneous 5-axis centers are the deciding asset. GreatLight runs 16 of them, plus 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. That mix matters because a turned part with cross-holes is cheaper on a mill-turn center than on a 3-axis mill with two fixtures.
Size sets the ceiling. The largest travel here is 4,000 × 400 × 150 mm, with medium platforms at 750 × 1,150 × 550 mm and 600 × 600 × 600 mm, compact ones at 500 × 500 × 450 mm and 500 × 310 × 200 mm, and a Ø400 mm rotary table for round work. If your part is larger than the envelope, no amount of skill fixes it.
A practical check: send the bounding box and the tightest feature. The shop should tell you which machine it plans to use before you commit.
- 1Ask for the machine planA named machine on the quote is a good sign.
- 2Check the envelope against your bounding box4,000 mm is the ceiling here, not a default.
- 3Mill-turn saves a setupGood for shafts and fittings with cross features.
Certifications and what each one actually covers
Certificates are not decoration. Each one maps to a different risk. ISO 9001:2015 covers the quality management system. IATF 16949:2016 is the automotive standard and adds traceability and change control. ISO 13485:2016 applies to medical device work. ISO 27001:2022 covers information security, which matters when you upload unreleased CAD.
Read the scope line on the certificate, not the logo. A shop can hold IATF 16949 and still run your job on a line that is not in scope. Ask which plant and which processes are covered.
For regulated work, ask what the shop does when a nonconformance appears. The answer should describe containment, notification and a corrective action report, not a promise to redo the parts. That process is what the certification is supposed to guarantee.
If your industry needs none of these, say so. Chasing certificates you do not need slows the quote down.
- 1ISO 9001:2015Baseline quality system and document control.
- 2IATF 16949:2016Automotive traceability and change control.
- 3ISO 13485:2016Medical device process requirements.
- 4ISO 27001:2022Protects your uploaded design data.
Materials, finishing and where subcontract chains break
Material availability drives lead time more than machining does. Aluminium 6061, 7075 and 2024, stainless 303, 304, 316L and 17-4PH, steels like 4140 and 4340, copper alloys and titanium TC4 are routine here. Exotic grades such as Inconel or magnesium AZ31B are workable but need a longer material lead time.
Ask where the material comes from and whether the mill certificate travels with the parts. For aerospace and medical work, an uncertified bar is a scrap risk no matter how good the machining is.
Finishing is the most common place a subcontract chain breaks. Anodizing, plating, powder coating, black oxide, bead blasting and polishing are separate processes, often run by third parties. Every handoff adds days and a chance of damage. A supplier that controls finishing in-house removes one handoff.
Laser marking is a small but frequent request. Minimum character height here is 1.5 mm. If your part number needs to be legible after anodizing, plan the marking before the coating, not after.
- 1Ask for the mill certificateTraceability starts at the raw bar, not the finished part.
- 2Count the handoffsEach outside process adds lead time and handling risk.
- 3Sequence marking before coatingMarks applied after anodizing can flake or fade.
Quotationscope and the numbers that should be on it
A quotation that lists only a unit price is not a quotation. It is an opening position. Ask for a breakdown that separates machining time, material, finishing, tooling if any, and inspection. That breakdown is what lets you compare two suppliers fairly.
Lead time should be stated as stages, not one number. Here the pattern is: quotation and free DFM analysis within 12 hours, production start within 24 hours of PO, and parts shipping in 3–5 days. Historical late-delivery probability sits below 2%. Ask any supplier to state its own stages the same way.
Watch for the phrase that hides cost. "Ex-works, finishing excluded" or "plus anodizing" means the number you see is not the number you pay. For a cnc subcontract precision service, the all-in landed cost is the only figure worth comparing.
Finally, put confidentiality in the paperwork. Uploads are handled as secure and confidential, and an NDA is available on request. For unreleased designs, sign it before the files move, not after.
- 1Demand a cost breakdownUnit price alone cannot be compared between shops.
- 2Stage the lead timeQuote, DFM, production start and shipping are four separate dates.
- 3Check what is excludedFinishing, tooling and freight are common omissions.
How to run a subcontract job without losing a week
Steps 1 to 5 cover the quote stage. Steps 6 and 7 cover the run.
- 1Prepare a quote-ready packageSend STEP plus a 2D drawing with GD&T, material spec, finish callout, critical features and annual volume. Missing any one of these adds a round trip.
- 2Ask for DFM feedback with the priceA serious shop returns a quotation and free DFM analysis within 12 hours. Read the DFM notes first; they often cut cost more than price negotiation does.
- 3Confirm the machine and setup planAsk which machine handles the tightest feature and how many setups are planned. Two setups on a ±0.005 mm datum stack error you will pay for later.
- 4Agree the inspection scope in writingState which features get measured, what report format you want, and whether first-article approval is required before the run continues.
- 5Fix the commercial termsNo minimum order quantity applies here, so a single prototype and a 10,000-piece run can sit on the same PO. Confirm unit, tooling and finishing are all in the price.
- 6Lock the finish and marking sequenceConfirm which operations are in-house and which are subcontracted, then set the marking step before coating.
- 7Track the first articlesProduction can start within 24 hours of PO. Parts normally ship in 3–5 days, so first-article results need to come back fast.
Questions buyers ask before the first PO
How tight a tolerance can a subcontract shop hold on a normal part?
On rigid, well-fixtured parts in aluminium or stainless, ±0.005 mm (about ±0.0002 in) is achievable and is the limit quoted here. That number assumes a single-setup feature with good access.
On thin walls, long slender sections or deep pockets, expect movement after cutting. In those cases a realistic call is ±0.02 mm, or a stress-relief step added to the routing.
Is there a minimum order quantity for a subcontract job?
There is no minimum order quantity here. A single prototype and a 10,000+ part run can both be quoted on the same terms.
That does not make a one-off cheap per piece. Setup, programming and fixturing are spread across just one part, so the unit price reflects that. It does mean you can validate a design without committing to volume.
How is my design data protected during quoting?
Uploads are treated as secure and confidential. An NDA is available on request and can be signed before files are exchanged.
ISO 27001:2022 covers information security at the process level. For unreleased products, ask for the NDA first and send STEP files rather than native CAD where possible.
What should I send with the RFQ to get an accurate price?
Send a STEP file, a 2D drawing with GD&T, the material grade, the finish callout, the features you consider critical, and your annual volume.
Volume changes the process choice, so a quote built on prototype quantities may not reflect a 10,000-piece run. State both if you have them.
Can machining and finishing be handled by one supplier?
Machining, anodizing, plating, powder coating, black oxide, bead blasting, polishing and laser marking can be coordinated under one order.
Ask which steps are in-house and which are subcontracted. Every outside handoff adds transit time and a handling risk, so the split matters for your schedule.
What happens if parts fail inspection?
Ask this before you order. A documented answer covers containment of the suspect lot, notification with measurement data, and a corrective action report describing the process change.
A vague answer is the real warning sign. Rework capability is useful, but the process that prevents a repeat is what protects your next order.
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