CNC processing services for custom products: what to check before you commit
This page is for engineers and sourcing staff choosing a machining partner for custom parts. It lists the checks that decide whether a quote is workable: tolerance, machine fit, lead time, MOQ, certification and inspection. Read it and you can tell which supplier is the right fit for your part.
In this article
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Key takeaways
Which CNC processing setup fits your custom product
Match the part to the machine before you compare prices. The wrong machine class is the most expensive mistake in custom CNC work.
| Part type | Best machine class | Typical tolerance | Why |
|---|---|---|---|
| Flat plates, brackets, covers | 3-axis mill | ±0.05 mm | Single setup, no re-fixturing needed |
| Shafts, bushings, fittings | CNC turning or mill-turn | ±0.01 mm | Round features cut in one chucking |
| Pockets on 4 or 5 faces | 4-axis mill | ±0.02 mm | Rotary table indexes the part, one datum |
| Impellers, turbine blades, medical bone plates | 5-axis simultaneous | ±0.005 mm | Contoured surfaces cut without re-clamping |
| Large frames, beams, housings | Large-travel mill | ±0.05 mm | Up to 4,000 mm part length |
| Prototype, one-off, 20 pcs | 3-axis or 5-axis, no MOQ | ±0.01–0.05 mm | Same process scales to 10,000+ parts |
| Thin-wall or flexible parts | 5-axis with light finishing passes | ±0.01 mm | Fewer setups mean less clamping distortion |
The short version
Pick the supplier whose machines, certifications and MOQ match your part, then agree tolerance feature by feature. Price follows from those decisions, not the other way around.
Tolerance and surface finish: what CNC processing services for custom products can hold
Tolerance is the first number to agree on, because it sets the machine class, the fixture, and the inspection method. GreatLight holds ±0.005 mm (±0.0002 in) on 5-axis work. That is not a default. It applies to specific features on specific materials, and it needs the right setup. If your drawing shows ±0.005 mm on a 300 mm aluminium plate with no datum callouts, no shop can hold it reliably, and the quote will either be padded or wrong.
Surface finish follows the same logic. As-machined surfaces land at Ra 1.6–3.2 μm. A fine finish at Ra 0.2–0.8 μm is a separate operation, sometimes a separate machine, and always a separate line on the quote. Tell the supplier which surfaces actually need the fine finish. Putting Ra 0.4 μm on the whole part multiplies cost for no functional gain.
A practical rule: split your drawing into critical and non-critical features. Put tight tolerance only on the mating surfaces, bearing seats, seal grooves and alignment holes. Leave everything else at general tolerance. This single change often cuts a custom CNC quote by 20–40% because it frees the shop to use faster passes on the rest of the part.
- 1±0.005 mmAchievable on 5-axis with probing, on selected features
- 2Ra 0.2–0.8 μmFine finish, usually a secondary operation
- 3Ra 1.6–3.2 μmStandard as-machined finish, no extra cost
- 4Inspection100% before shipment, reports on request
Machine fit: travel, axis count and part size
Ask which machine will run your part, not how many machines the shop owns. A shop with 127 machines still cannot cut a 2 m frame if none of them has the travel. GreatLight runs 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. Maximum processing size is 4,000 mm, with large travel of 4,000 × 400 × 150 mm.
Axis count changes the fixture, not just the toolpath. A 3-axis mill needs one setup per face. Each re-clamping step adds positional error, typically 0.02–0.05 mm, and adds labor. A 5-axis machine holds one datum and cuts angled faces, undercuts and contoured pockets in a single cycle. For parts with features on four or more faces, that difference shows up in both tolerance and price.
There is a limit. Five-axis simultaneous cutting is slower than 3-axis roughing on simple geometry. If your part is a flat plate with drilled holes, a 3-axis mill will finish it faster and cheaper. Use 5-axis where the geometry demands it: impellers, medical bone plates, turbine components, and parts with compound angles.
- 1Ø400 mm rotary tableFor round and indexed parts
- 2750 × 1,150 × 550 mmMedium travel envelope
- 3500 × 500 × 450 mmCompact travel envelope
- 44,000 × 400 × 150 mmLarge travel for frames and beams
Material and post-processing: where custom quotes go wrong
Material choice affects machinability more than most drawings admit. Aluminium 6061 and 6082 cut fast and hold tight tolerance. 7075 is stronger but more prone to stress movement after roughing. Stainless 316L work-hardens, so the shop needs sharp tools and controlled feed, or the surface tears. Titanium TC4 (Ti-6Al-4V) and Inconel need low cutting speeds and plenty of coolant. Each of these changes cycle time, which changes price.
Post-processing is the second place quotes drift. Anodizing, electroless nickel, zinc, silver and gold plating, powder coating and black oxide are all separate vendors and separate lead time. Bead blasting, tumbling, brushing and polishing change dimensions slightly, so they must be sequenced after any tight-tolerance machining. Laser marking needs a minimum character height of 1.5 mm; below that, the mark is unreliable.
Send the full requirement in one package: 3D model, 2D drawing with GD&T, material spec, finish spec and any marking file. Quotes built on a 3D model alone miss the tolerance callouts, and the shop will either guess or ask later. Both cost you days.
- 1Aluminium6061-T6, 7075, 2024, 6082, ADC12 and others
- 2Stainless303, 304, 316L, 17-4PH, 440C
- 3Titanium and specialTC4, Inconel, magnesium AZ31B / AZ91D
- 4PlasticsPOM, PEEK, PC, PA, ABS, carbon fibre
Lead time, MOQ and certification: the commercial filters
Lead time has three parts: quote, production start, and shipping. GreatLight returns a quotation and free DFM analysis within 12 hours. Production can start within 24 hours of approval, and parts ship in 3–5 days. The historical late-delivery probability is below 2%. Ask any supplier to break lead time into these three stages. A single number like "two weeks" hides where the delay actually happens.
MOQ is the filter that kills small projects. There is no minimum order quantity here, from one prototype to 10,000+ part runs. That matters when you need a fit check before tooling. A shop that demands 500 pieces minimum forces you to buy risk. A shop that runs one piece first lets you find the design error while it is still cheap to fix.
Certification must match your industry, not just exist. ISO 9001:2015 covers general quality systems. IATF 16949:2016 is required for automotive and EV parts. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when you upload proprietary CAD. Check the certificate scope, not only the logo on the homepage.
- 1Quote in 12 hoursIncludes free DFM analysis
- 2Production in 24 hoursAfter drawing and material approval
- 3Ship in 3–5 daysStandard machined parts
- 4No MOQOne prototype to 10,000+ parts
Quality control and confidentiality for custom CNC work
Ask what is inspected and when. The useful answer describes three stages: raw material check, in-process monitoring, and final inspection. GreatLight inspects 100% of parts before shipment and provides reports on request. A qualification rate of 99.99% only means something if the shop can show you the inspection record behind it.
For custom products, the drawing itself is often the most valuable thing you send. Uploads are secure and confidential, and an NDA is available on request. If a supplier cannot sign an NDA before you send CAD, treat that as a signal about how they handle customer data.
One more check: ask how first-article inspection is handled on a new part. A shop that measures the first piece, records the numbers, and sends them with the parts is doing what a custom job requires. A shop that ships and waits for your complaint is not.
- 1Raw material checkBefore any cutting starts
- 2In-process monitoringDuring the run, not only at the end
- 3Final inspection100% before shipment, reports on request
- 4NDA availableOn request, before file transfer
7 steps to qualify a CNC partner for custom products
Run these in order. Each step filters out suppliers that will fail later, when the cost of switching is higher.
- 1Send the complete data package3D model, 2D drawing with GD&T, material spec, finish spec, marking file, and target quantity. Missing GD&T is the most common reason a quote comes back wrong.
- 2Ask for the DFM feedback with the quoteA real DFM note points at specific features: a wall too thin for the tool, a corner radius smaller than the cutter, a tolerance that needs a different setup. If the reply is only a price, ask again.
- 3Confirm the machine class for your partAsk which machine will run it and what travel and axis count it has. For parts over 1,000 mm, confirm the envelope covers your full length plus fixturing.
- 4Agree tolerance feature by featureList the critical features and their tolerance. Keep general tolerance on everything else. Confirm the shop will inspect those features and report the numbers.
- 5Check MOQ against your real first orderIf you need a fit check, order one or a few pieces. Confirm the same process will run the volume order so the prototype and the production part match.
- 6Match certification to your industryISO 9001 for general work, IATF 16949 for automotive, ISO 13485 for medical, ISO 27001 when data security is in scope. Ask for the certificate and its scope.
- 7Lock the finish and marking sequenceDecide which surfaces get fine finish, which get anodizing or plating, and where laser marking goes. Put the order of operations in writing so tight features are not machined after coating.
Questions buyers ask about custom CNC processing
What tolerance can I realistically ask for on a custom part?
±0.005 mm (±0.0002 in) is achievable on 5-axis work with probing, but only on selected features and only with a proper datum scheme. General features can sit at ±0.05 mm.
If a drawing puts tight tolerance on every dimension, expect either a high price or a rejected part. Mark the critical features and let the rest run at general tolerance.
Do I need 5-axis machining for my part?
Only if the geometry needs it: contoured surfaces, compound angles, undercuts, or features on four or more faces that would otherwise need multiple setups.
Flat plates, simple brackets and turned parts are usually faster and cheaper on 3-axis or mill-turn machines. Five-axis is a tool for a specific job, not an upgrade that helps every part.
How fast can a custom CNC order start?
The quotation and free DFM analysis come back within 12 hours. Production can start within 24 hours after approval, and parts ship in 3–5 days.
These times assume the material is in stock and the drawing is complete. Missing finish spec or unresolved tolerance callouts are the usual causes of delay.
Is there a minimum order quantity for custom parts?
No. Runs go from one prototype to 10,000+ parts. Small first orders let you check fit and function before committing to volume.
Keeping the prototype and the production run on the same process is what makes that check meaningful. If the shop switches methods between one piece and 1,000 pieces, the prototype proves less than you think.
Which certifications should a CNC supplier hold?
It depends on where the part is used. ISO 9001:2015 covers general quality management. IATF 16949:2016 is the automotive requirement. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security.
Ask for the certificate and read the scope. A certificate that does not cover the process or site doing your work is not evidence of anything.
How do I protect my design when sending CAD files?
Sign an NDA before transferring files, and confirm how uploads are stored and who can access them. An NDA is available on request here, and uploads are treated as secure and confidential.
Send only what the quote needs. A simplified model plus the critical 2D drawing is often enough for pricing, and it limits what leaves your building.
Send your drawing and get a workable answer
Upload your 3D model and 2D drawing. You get a quotation and free DFM analysis within 12 hours, with no minimum order quantity.
12-hour quoteNo MOQ100% inspectionNDA on request
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