Choosing a CNC Machine Service Company: 7 Checks
A practical checklist for engineers and procurement teams comparing machining suppliers. It covers tolerance, machine capacity, certifications, MOQ, quoting detail and lead time. After reading, you can shortlist two or three shops and know which questions to ask before sending a drawing.
In this article
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What matters most when comparing shops
Which supplier profile fits your project
Use this table to map your project type to the checks that carry the most weight. A shop can be strong in one column and weak in another.
| Project signal | What to verify first | Where a shop usually fails | GreatLight position |
|---|---|---|---|
| Tight tolerance, ±0.005 mm | CMM reports and in-process checks | Tolerance quoted, never measured | ±0.005 mm; 100% inspection |
| Large part, over 750 mm | Machine travel and fixturing | Part fits the spec, not the table | Up to 4,000 mm travel |
| Regulated industry | Valid certificate scope | Certificate covers a different plant | ISO 9001, IATF 16949, ISO 13485 |
| One prototype only | MOQ and setup cost policy | Minimum batch of 50 pieces | No MOQ, one piece up |
| Production ramp, 10,000+ parts | Repeatability across batches | First article fine, batch drifts | 99.99% qualification rate |
| Confidential design | NDA and file handling | Drawings forwarded to a broker | ISO 27001, NDA on request |
Tolerance and surface finish: what a CNC machine service company can actually hold
Every shop prints a tolerance on its website. The useful question is narrower: on which feature, in which material, at which machine. A ±0.005 mm callout on a 20 mm aluminum bore is routine. The same number on a 900 mm steel weldment is a different job that needs temperature control and a stable setup. Ask the shop to state the tolerance per drawing feature and name the machine that will run it.
Surface finish follows the same logic. Ra 0.2–0.8 μm is a fine finish that usually needs a finishing pass, a sharp tool and a rigid setup. Ra 0.8–1.6 μm covers most sealing faces and bearing bores. Ra 1.6–3.2 μm is as-machined and fine for brackets and covers. If a quote lists one finish value for the whole part, ask which surfaces actually need it. Polishing a non-functional face only adds cost and time.
- 1Send the functional calloutsMark only the surfaces that seal, slide or locate. Everything else can stay general.
- 2Ask how tolerance is verifiedCMM, micrometer or gauge pin. The method should match the tolerance band.
- 3Watch thin wallsBelow 0.8 mm in aluminum, cutting force deflects the wall even on a good machine.
- 4Materials moveTitanium and 17-4PH shift after roughing. A shop that rough-machines then re-clamps handles this.
Machine capacity: travel, axis count and the geometry you are sending
Capacity is a hardware question, so ask for numbers. Machine travel sets the largest part a shop can cut in one setup. A 4,000 × 400 × 150 mm envelope handles long rails and extrusion profiles. Medium travel around 750 × 1,150 × 550 mm and 600 × 600 × 600 mm covers most housings and brackets. Compact machines at 500 × 500 × 450 mm and 500 × 310 × 200 mm are for small, high-volume parts.
Axis count decides whether your part needs repositioning. A 5-axis machining center cuts angled holes, undercuts and contoured surfaces in one setup, which removes the stacked error of three or four re-clamps. A 4-axis mill with a Ø400 mm rotary table is often enough for cylindrical parts with cross-drilled features. A 3-axis machine is the cheapest and fastest route for flat plates and simple pockets. Send the drawing and let the shop tell you which machine it plans to use.
Ask how many machines of that type are free next week. A shop with 127 CNC machines and 16 simultaneous 5-axis centers has more scheduling room than a workshop with two spindles. Capacity on paper is not capacity on the calendar.
- 1One setup beats threeFewer re-clamps mean fewer datum shifts and less scrap.
- 2Long parts need long tablesA 2,000 mm rail cannot be cut on a 600 mm machine without a second operation.
- 3Mill-turn saves handlingShafts with flats and cross holes often finish in one cycle.
Certifications and inspection: reading the paperwork correctly
Certificates are a filter, not a guarantee. ISO 9001:2015 shows a documented quality system. IATF 16949:2016 is the automotive standard and matters if your part ends up in a vehicle program. ISO 13485:2016 applies to medical devices and brings traceability and validation expectations. ISO 27001:2022 covers information security, which matters when you send CAD files to an overseas supplier.
Two questions expose weak paperwork. First, does the certificate cover the plant that will machine your part? A group can hold a certificate at one site and route your job to another. Second, what is inspected, and at which stage? A useful answer names raw material check, in-process monitoring and final inspection, with reports available on request. A shop that only inspects at the end finds problems after the part is finished.
- 1Ask for the certificate scopeSite name and address should match the factory doing the work.
- 2Request a sample inspection reportIt shows what is measured and how results are recorded.
- 3Traceability for regulated partsMaterial heat numbers and lot tracking are standard for medical and auto work.
MOQ, quoting and lead time: the commercial checks that decide the project
Minimum order quantity kills more prototype projects than machining difficulty does. If a shop requires 50 or 100 pieces before it will quote, your design iteration cycle slows to their production calendar. A supplier that runs one prototype and a 10,000+ part run on the same floor removes that barrier. Ask directly: can you start with a single part?
Read the quote structure, not just the total. A good quote separates material grade, machining time, surface finish, inspection and packaging. It names the alloy: 6061-T6 rather than "aluminum", 316L rather than "stainless". If the quote is one line and a number, you will find the missing items later as change orders.
Lead time has two parts. The first is how fast the shop turns a quote around; a 12-hour quotation with a DFM note lets you fix a design flaw before cutting metal. The second is production: how soon the spindle starts and when parts ship. Production that starts within 24 hours and ships in 3–5 days is a working benchmark for small and medium batches. Ask what happens when a tool breaks, because that is where schedules slip.
- 1No MOQ removes the prototype taxYou can iterate the design without buying a batch you will scrap.
- 2Named materials in the quoteAlloy and temper change machinability and price. Vague lines hide cost.
- 3Ship date, not just cycle timeAsk for the date parts leave the dock.
- 4Confidentiality termsAn NDA and secure file handling should be available before you upload.
Step by step: shortlisting a supplier in one week
Work through these steps in order. Each one removes shops from the list before you spend time on samples.
- 1Send one representative drawingPick the part with the tightest tolerance and the most complex geometry. Ask for a quote with a DFM note, not a price alone.
- 2Ask for the tolerance per featureRequest the value the shop will hold on your critical bore or face, plus the inspection method. Expect ±0.005 mm for precision features on small to medium parts.
- 3Confirm machine and travelAsk which machine will run the job and whether your part fits its travel in one setup. Verify the axis count matches the geometry.
- 4Check certificate scope and inspection planMatch the site address on the certificate to the factory. Ask for raw material check, in-process monitoring and final inspection stages.
- 5Test the MOQ and scheduleAsk for one piece first, then a 500-piece price. Compare the quote turnaround and the ship date, not only the unit cost.
- 6Order a sample and measure itInspect the first article against the drawing yourself. Run two more pieces to see whether the process repeats.
Questions engineers ask before signing off
How do I compare quotes when the line items differ?
Rebuild each quote into the same five lines: material with alloy and temper, machining, surface finish, inspection and packaging. Add the missing lines yourself using your own estimates.
Then compare. A cheap quote that omits anodizing or a CMM report is not cheaper, it just moves the cost to a change order later.
Is a lower unit price worth a longer lead time?
Only when the parts are not on the critical path. For a prototype that feeds a design review, days matter more than dollars per piece.
For a stable production part with a full quarter of stock on hand, a slower and cheaper route can make sense. Decide which case you are in before negotiating.
What should I check on the first article?
Measure the functional features, not every dimension. Bores, mating faces, hole positions and thread depth cover most failures.
Note surface finish on sealing faces and check for tool marks or burrs on edges. If the first article passes, order two more and measure them the same way.
How do I protect my design when sending files overseas?
Ask for an NDA before uploading and confirm how files are stored and who can open them. An ISO 27001:2022 certificate indicates a managed information security system.
Send only what the shop needs. A STEP file with critical dimensions is usually enough for quoting. Keep the full assembly internal until you place an order.
When is 5-axis machining actually necessary?
When the part has angled holes, undercuts or contoured surfaces that would need three or more re-clamps on a 3-axis machine. Each re-clamp adds setup error and labor.
For flat plates, simple pockets and cylindrical parts, a 3-axis or 4-axis machine is faster and cheaper. Do not pay for 5-axis time on geometry that does not need it.
How many parts should I order for a pilot run?
Enough to test assembly, function and one round of process variation. For most mechanical parts that is 10 to 50 pieces.
Ask the shop how the price steps between 1, 50 and 500 pieces. If the first step is steep, the setup cost is being spread over too few parts.
Send your drawing and get a quote with a DFM note
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Quotation in 12 hoursNo MOQ100% inspection before shipmentNDA on request
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