Custom CNC Machining Online Service: What to Check Before You Upload
This guide is for design engineers and sourcing engineers who send CAD files to a custom CNC machining online service and need to judge the quote that comes back. It covers tolerance, lead time, MOQ, certification and the details that decide whether a quote is comparable to the next one.
In this article
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Key takeaways
Comparing custom CNC machining online service suppliers
Use this table to score two or three suppliers on the same lines.
| Criterion | Weak signal | Strong signal | How to verify |
|---|---|---|---|
| Quote turnaround | 3–5 business days, no questions | Quote plus DFM notes in 12 hours | Send a part with a thin wall and see if they flag it |
| Tolerance ownership | Claims ±0.005 mm on every feature | States which features hold ±0.005 mm | Ask for the inspection method per feature |
| Machine base | Unnamed machine shop | 127 CNC machines, 16 of them 5-axis | Ask for the machine list under NDA |
| MOQ policy | MOQ 100 pieces | No minimum order quantity | Ask for a one-piece prototype quote |
| Certification | ISO 9001 only | ISO 9001, IATF 16949, ISO 13485, ISO 27001 | Request certificate numbers and scope |
| File handling | Email attachments, no policy | Secure upload, NDA on request | Read the confidentiality terms first |
| Delivery track record | Vague on late orders | Late-delivery probability below 2% | Ask how they measure and report it |
The short version
Pick a custom CNC machining online service that quotes in 12 hours with DFM notes, states which features hold ±0.005 mm, carries the certification your product class needs, and lets you order one part before you commit to volume.
What a custom CNC machining online service actually does
The online layer is the front end: you upload a 3D model, the supplier runs a manufacturability review, and a quote comes back with notes on the features that will be hard to hold. Behind that front end there must be real machines. GreatLight runs 127 high-precision CNC machines across 3 wholly-owned plants, including 16 simultaneous 5-axis machining centers and 16 mill-turn centers. That mix decides which parts you should send.
The useful part of the online model is early feedback. A thin rib, a deep pocket with a 3 mm cutter, or a thread that runs into a fillet will all show up in a DFM review before metal is cut. Catching those at quote stage is cheap. Catching them after the first article is not. Ask whether the DFM analysis is included or billed separately.
Upload formats are usually STEP and IGES, with X_T accepted by most shops. For assemblies, send one ZIP with every component so the reviewer sees fits and clearances, not just single parts. If your model carries only nominal dimensions, say so in the notes. The shop will then ask which features carry the tight tolerance rather than guessing.
- 1Send STEP or IGESOne file per part, one ZIP per assembly.
- 2Mark the critical featuresA short list beats a fully toleranced drawing at quote stage.
- 3State the functionA sealing face and a cosmetic face need different finishes.
Tolerance, finish and the features that decide the price
Tolerance is where most quotes stop being comparable. A shop that quotes ±0.005 mm as a blanket number is either assuming your part is simple or not reading the drawing. In practice, ±0.005 mm (±0.0002 in) is a feature-level call: bores, spigots, bearing seats and mating faces. A 4,000 mm frame will not hold that across its full length, and no honest shop will say it will.
Surface finish follows the same logic. As-machined surfaces run Ra 1.6–3.2 μm. A good general finish is Ra 0.8–1.6 μm, and fine finishing reaches Ra 0.2–0.8 μm. Each step down adds a pass, sometimes a different tool, and often a separate operation. Tell the shop which faces need the fine finish. Marking the whole part is a common way to pay for work you do not need.
Material also moves the number. Aluminium 6061 and 7075 cut fast and hold tolerance well. Stainless 316L and 17-4PH work-harden, so feeds and speeds matter and cycle time goes up. Titanium TC4 (Ti-6Al-4V) and Inconel are slower again. If your part is titanium with a Ra 0.4 μm bore and a ±0.005 mm seat, expect the quote to reflect all three.
- 1Call out critical featuresBores, seats and mating faces, not the whole part.
- 2Separate finish zonesCosmetic faces vs. functional faces.
- 3Name the alloy grade6061-T6 and 6061 are not the same quote.
Lead time, MOQ and what the online quote should tell you
Two numbers matter at the start: how fast you get a quote and how fast you get parts. At GreatLight, quotation and free DFM analysis come back within 12 hours, production can start within 24 hours of order release, and parts ship in 3–5 days for standard work. Those are front-end numbers. They assume the model is clean and the material is in stock.
MOQ is the other early filter. No minimum order quantity means you can order one prototype, check the fit, and only then commit to a 10,000+ part run. That matters when the design is still moving. A shop with a 100-piece MOQ forces you to freeze geometry before you have test data, which is the wrong sequence for most hardware programs.
Read the quote for what is missing as well as what is listed. Does it name the material grade and temper? Does it state the finish and the masking areas? Does it say who pays for inspection reports, and in what format? A quote without those lines is not cheaper. It is just less complete, and the difference shows up as a change order later.
- 1Quote in 12 hoursWith DFM notes, not just a price.
- 2Parts in 3–5 daysStandard geometry and in-stock material.
- 3No MOQOne piece up to 10,000+ runs.
Certifications, confidentiality and inspection reports
Certification is a scope question, not a logo question. ISO 9001:2015 covers general quality management and suits industrial machinery, robotics and electronics work. Automotive and EV parts usually need IATF 16949:2016. Medical devices need ISO 13485:2016. If your data is sensitive, ISO 27001:2022 covers information security. GreatLight holds all four. Ask for the certificate scope, because a certificate that excludes your process is not useful.
Confidentiality is a practical matter for anyone uploading CAD. Uploads are secure and confidential, and an NDA is available on request. If your drawings carry export-controlled content or unreleased product geometry, settle the NDA before the first upload, not after the quote. It takes less time than the review that follows.
Inspection is the last line and often the first thing buyers forget to specify. GreatLight inspects 100% of parts before shipment, covering raw material check, in-process monitoring and final inspection, with reports on request. The qualification rate is 99.99%. Decide up front whether you want a dimensional report, a material certificate, or both, and say which dimensions must appear on it.
- 1Match cert to product classISO 9001, IATF 16949, ISO 13485, ISO 27001.
- 2NDA before uploadSigned on request, no cost to ask.
- 3Name the report formatDimensional report, material cert, or both.
When an online service is the wrong choice
An online custom CNC machining online service fits low to medium volume work: prototypes, bridge tooling, jigs, small production runs and parts with geometry that needs 5-axis access. It fits less well when the part needs a long qualification cycle tied to a specific customer-approved process, or when the geometry is so large that transport and fixturing dominate the cost.
Size is a real boundary. GreatLight handles parts up to 4,000 mm maximum processing size, with travels of 4,000 × 400 × 150 mm on the large platform, 750 × 1,150 × 550 mm and 600 × 600 × 600 mm on the medium platform, and 500 × 500 × 450 mm or 500 × 310 × 200 mm on the compact machines. A Ø400 mm rotary table covers most 5-axis work that needs rotation.
The other boundary is the design stage. If the geometry is not settled and the tolerances are guesses, no shop can quote accurately. Send what you have, mark it as preliminary, and ask for a DFM pass. The notes you get back are often worth more than the price at that point.
- 1Good fitPrototypes, bridge parts, jigs, 5-axis geometry, 1 to 10,000+ pieces.
- 2Poor fitCustomer-locked processes, unfrozen geometry, extreme size.
How to run a first order through an online service
Seven steps from CAD upload to receiving parts.
- 1Prepare the file setExport STEP or IGES at nominal dimensions. Add a short note listing the tight features and their tolerance, for example a Ø20 mm bore at ±0.005 mm.
- 2Upload and request DFMSend the model plus material grade, quantity and finish. Ask explicitly for manufacturability notes, not just a price.
- 3Read the DFM notes before the priceLook for wall thickness below 0.8 mm, pocket depth over 4× cutter diameter, and threads that break into fillets. These drive cost more than material choice.
- 4Confirm material and finish zonesState the alloy and temper, such as 6061-T6 rather than 6061. List which faces get Ra 0.8–1.6 μm and which stay as-machined at Ra 1.6–3.2 μm.
- 5Settle NDA and data handlingIf the design is unreleased, sign the NDA before the upload. Confirm who inside the shop can open the files.
- 6Freeze the inspection planDecide the report type and the dimensions on it. Request reports rather than assuming they ship by default.
- 7Order one piece firstWith no MOQ, run a single part, measure it, and adjust the model before committing to a larger run.
Questions buyers ask before the first order
What file formats does the service accept?
STEP and IGES are the standard formats, and X_T is accepted for most work. For an assembly, send one ZIP file containing every component so the reviewer can check fits and clearances.
If your model is nominal-only, add a short note listing the critical features and their tolerances. That single note usually removes one round of questions from the quote.
How tight a tolerance can I actually ask for?
±0.005 mm (±0.0002 in) is achievable on specific features such as bores, spigots and bearing seats, where the machine, fixture and inspection method are chosen for that feature.
It is not a whole-part number. On a long part, thermal movement and fixture deflection dominate, so tolerance has to be allocated per feature. Ask the shop which features they will hold at ±0.005 mm and how they will measure them.
Is there a minimum order quantity?
No minimum order quantity applies, so you can order a single prototype and scale to 10,000+ part runs on the same process.
Running one piece first is the cheapest way to find out whether the geometry machines as designed. Measure it, adjust the model, then commit to volume.
How fast are quotes and parts?
Quotation and free DFM analysis come back within 12 hours. Production can start within 24 hours of order release, and parts ship in 3–5 days for standard work.
Those timelines assume a clean model, a stated material grade, and material in stock. Specialty alloys and hard coatings add time, so ask for the specific schedule on your part rather than assuming the standard one.
How is my design protected?
Uploads are secure and confidential, and an NDA is available on request. Information security is covered by ISO 27001:2022.
For unreleased products, sign the NDA before the first upload. It costs a few minutes and removes the question of who has seen the geometry.
What inspection comes with the parts?
Every part is inspected before shipment, covering raw material check, in-process monitoring and final inspection, with the qualification rate at 99.99%.
Reports are provided on request. Tell the shop at quote stage whether you need a dimensional report, a material certificate, or both, and which dimensions must be listed.
Send a model and get a real answer
Upload your STEP file and we will return a quote with manufacturability notes, a material and finish recommendation, and an inspection plan within 12 hours.
12-hour quoteNo MOQ100% inspectionNDA on request
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