Leading Custom CNC Machining Services Factories: A 7-Point Checklist
This guide is for design engineers and sourcing managers comparing custom CNC machining services factories before a first order. It covers the checks that decide whether a shop can hold your tolerance at volume: machine mix, inspection method, certification scope, MOQ and quote clarity. Read it before you send the drawing.
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Key takeaways
How custom CNC machining services factories compare on the checks that matter
Use this table to score two or three suppliers on the same row values before you send an RFQ.
| Check | What to ask | Weak answer | Strong answer |
|---|---|---|---|
| Tolerance | Which process holds ±0.005 mm? | We hold everything to ±0.001 mm | 5-axis + CMM, ±0.005 mm on stated features |
| Machine mix | How many 5-axis centers? | We have many advanced machines | 16 simultaneous 5-axis centers, 127 total |
| Max part size | Largest travel available? | We can handle big parts | 4,000 × 400 × 150 mm travel |
| Surface finish | Which Ra range is standard? | Mirror finish on request | Ra 1.6–3.2 μm as machined, Ra 0.2–0.8 μm fine |
| Certifications | Which standards, which scope? | ISO certified | ISO 9001, IATF 16949, ISO 13485, ISO 27001 |
| MOQ | Minimum order quantity? | Depends on the part | None, from one prototype to 10,000+ parts |
| Quote turnaround | How fast is a quote and DFM? | Within a few days | Quote and free DFM analysis within 12 hours |
| Inspection | What is checked before shipment? | We check quality | 100% inspection, reports on request |
Pick the factory that answers the hard questions
If a supplier will name its machines, its tolerance band per feature, its inspection method and its certificate scope, you have enough to make a decision. If it will not, the price is not the problem.
Tolerance and machine mix at custom CNC machining services factories
Tolerance claims are where most supplier comparisons fall apart. A drawing marked ±0.005 mm does not tell you whether the shop can hold it on every feature. Ask which operation produces that number. On a machined housing, the bore position usually comes off a 5-axis setup, while the mounting face may come off a 3-axis mill. Two different processes, two different capability levels.
The second question is what happens when a feature drifts. A shop with in-process monitoring catches the drift during the run. A shop that only inspects at the end catches it after 200 parts are already cut. The difference shows up in your scrap rate, not in the quote.
GreatLight runs 127 high-precision CNC machines: 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. That mix matters because it lets the shop route each feature to the process that holds it cheaply, instead of forcing every part onto one machine.
- 1Stated tolerance±0.005 mm (±0.0002 in) on controlled features.
- 2Finish bandsRa 1.6–3.2 μm as machined, Ra 0.8–1.6 μm high, Ra 0.2–0.8 μm fine.
- 3Size ceilingUp to 4,000 mm maximum processing size.
- 4Rotary workØ400 mm rotary table for cylindrical and indexed features.
Certifications: what each one actually covers
Four certificates come up in almost every RFQ: ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. They are not interchangeable. ISO 9001 is the base quality system. IATF 16949 adds automotive-specific process control, traceability and change management. ISO 13485 adds medical device documentation and validation discipline. ISO 27001 covers information security, which matters when your CAD files and drawings leave your network.
A supplier holding all four is not automatically the right supplier. The question is which scope applies to your part. An EV busbar housing needs the IATF process controls. A surgical instrument prototype needs the ISO 13485 documentation trail. A consumer electronics bracket may need none of that, and paying for it through higher piece price is waste.
Ask for the certificate scope statement, not just the logo. Scope defines which sites and which processes are covered. If your parts are machined at a site outside the scope, the certificate does not apply to your order.
MOQ, quote turnaround and lead time
MOQ is the check that most often kills a good prototype. Some shops quote a low unit price and then add a setup charge that only makes sense at 500 pieces. Others refuse the one-off entirely. Neither is wrong, but you need to know before you commit to a design revision cycle.
GreatLight quotes no minimum order quantity, from one prototype to 10,000+ part runs. Quotation and free DFM analysis come back within 12 hours, and production can start within 24 hours of approval. Parts ship in 3–5 days. Historical late-delivery probability is below 2%.
Read the lead time definition carefully. Does the clock start at PO, at drawing approval, or at material arrival? A 3–5 day ship time assumes material is on hand. If your alloy is 17-4PH or Ti-6Al-4V, ask whether it is stocked or ordered per job. That single question can move your delivery by a week.
Process range: what one factory can finish in-house
Every operation you outsource adds a handoff, and every handoff adds a tolerance stack. A shop that machines, deburrs, anodizes and laser marks under one roof controls the sequence. A shop that brokers finishing to a third party inherits their schedule and their scrap.
GreatLight covers 5-axis, 4-axis and 3-axis machining, CNC milling and turning, rapid prototyping, sheet metal fabrication, custom 3D printing, metal die casting, vacuum casting and part surface finishing. Finishes include anodizing in clear, color, hardcoat and conductive types, electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing and polishing, plus laser marking with a minimum character height of 1.5 mm.
The practical test is to send one drawing with a post-machining requirement and see whether the quote breaks out the finishing step separately. If it does, the shop is planning the whole route. If finishing is bundled into a single line, ask who performs it.
- 1Aluminium6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075, ADC12.
- 2Stainless303, 304, 316, 316L, 420, 430, 431, 440C, 17-4PH (SUS630).
- 3Steel1018, 1045, 4130, 4140, 4340, A36, tool steel.
- 4Titanium and specialTA1, TA2, TC4 (Ti-6Al-4V), Inconel, magnesium AZ31B / AZ91D.
Inspection and documentation before shipment
"We check quality" is not an inspection plan. Ask three things: what is measured, with what instrument, and what document ships with the parts. A useful answer names the CMM, the sampling or 100% rule, and the report format.
GreatLight inspects 100% of parts before shipment, covering raw material check, in-process monitoring and final inspection. Reports are available on request. Qualification rate is 99.99%.
For regulated programs, the report is part of the deliverable. Medical and automotive buyers often need dimensional reports tied to lot numbers. If the shop cannot produce that, the certificate on the wall will not help you at audit.
Confidentiality and the files you upload
Your drawings are the most valuable thing you send a supplier. Ask where they are stored, who can open them, and what happens when the project ends. A supplier without a documented information security process is a risk even if the parts are perfect.
GreatLight holds ISO 27001:2022 for information security and states that uploads are secure and confidential. An NDA is available on request. If your program requires one, request it before the first drawing goes out, not after the quote.
The same applies to subcontractors. If finishing or heat treatment is outsourced, your files and parts travel further than you think. Ask for the full route before you sign.
Scoring suppliers on a single sheet
Put the eight rows from the comparison table on one sheet and score each supplier from 1 to 3. A supplier that scores well on tolerance but cannot state its inspection method is a 2, not a 3. A supplier with no MOQ but a five-day quote turnaround loses points where it matters for prototyping.
Weight the rows for your program. A one-off prototype weights quote turnaround and MOQ. A production run weights process capability, certification scope and inspection reporting. The same factory can be the right answer for one and the wrong answer for the other.
This is why we publish the numbers rather than adjectives. Tolerance, machine counts, travel limits and lead times are checkable. "Leading" and "world-class" are not.
Seven steps to qualify a CNC machining supplier
Run these in order. Each step produces a document you can compare across suppliers.
- 1Send the drawing with a tolerance calloutMark the features that need ±0.005 mm and leave the rest general. Ask the supplier to confirm which process holds each one.
- 2Request the machine list for your part sizeState your envelope. Compare against 4,000 × 400 × 150 mm, 750 × 1,150 × 550 mm and 600 × 600 × 600 mm travel options.
- 3Ask for certificate scope statementsRequest ISO 9001, IATF 16949, ISO 13485 or ISO 27001 scope documents and check that your part type and site are covered.
- 4Confirm MOQ and setup assumptionsAsk whether the quote includes fixture and setup cost, and whether it changes between one piece and 10,000 pieces.
- 5Pin down the lead time definitionAsk when the clock starts and whether your alloy is stocked. 3–5 day shipping assumes material is available.
- 6Request a sample inspection reportAsk for a redacted report from a similar part. Check the instrument, the sampling rule and whether lot traceability is included.
- 7Settle the NDA before files moveIf the program is confidential, sign the NDA first. Ask who inside the shop and which subcontractors can access the files.
Questions buyers ask before the first order
Can a factory really hold ±0.005 mm on a production run, not just a prototype?
It depends on the feature and the process. A 5-axis setup with in-process monitoring can hold ±0.005 mm on controlled features across a run. Features cut on a separate 3-axis operation may sit at a looser band.
Ask the supplier to name the machine and the inspection step behind the number. If the answer is a capability study or a CMM report from a similar part, that is a real answer.
Do I need a supplier with IATF 16949 if my part is not automotive?
No. IATF 16949 adds automotive process control, traceability and change management. If your part is a consumer electronics bracket, that overhead shows up in the piece price without reducing your risk.
Match the certificate to the program. ISO 9001 covers general quality. ISO 13485 matters for medical devices. ISO 27001 matters when file confidentiality is the concern.
What does no MOQ actually mean in practice?
It means the shop will quote a single prototype and a 10,000-part run from the same drawing. Setup and fixture cost are still real, so the unit price at quantity one will be higher.
The useful question is whether the shop carries the fixture over from prototype to production. If it does, your second order does not pay for setup twice.
How early should I involve the factory in the design?
Before you freeze the drawing. A DFM review catches features that force extra setups, deep pockets that need long tools, and tolerances that add cost without adding function.
GreatLight returns quotation and free DFM analysis within 12 hours. That is early enough to change a wall thickness or a corner radius before tooling is cut.
Which materials are hardest to source on short lead times?
Titanium grades such as TA1, TA2 and TC4 (Ti-6Al-4V), Inconel, and stainless 17-4PH (SUS630) are usually ordered per job rather than stocked. Magnesium AZ31B and AZ91D behave the same way.
Common aluminium grades like 6061, 7075 and 6082 plus standard stainless 303, 304 and 316 are typically on hand. If your alloy is on the special list, add material lead time to the machining lead time.
What should be in the inspection report?
At minimum: the measured features, the nominal and actual values, the instrument used, the operator or station, the lot number and the date. A report without lot traceability is hard to use at audit.
Ask whether the report covers 100% of parts or a sample. For tight-tolerance features, 100% inspection is the safer default.
Send one drawing and test the answers
Upload a part with your tolerance callouts. You get a quote and a free DFM analysis within 12 hours, from a factory that will name the machine and the inspection step behind every number.
12-hour quote100% inspectionNo MOQNDA on request
More from the shop floor
We publish setup notes, tooling trials and inspection data from the factory floor.
