CNC Core Processing Precision: How to Judge a Factory Before You Order
This guide is for engineers and sourcing teams selecting a machining partner for parts where cnc core processing precision decides whether the assembly fits. It covers the checks that separate a capable factory from a well-written quotation, and the points where a job should go somewhere else.
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Seven things worth checking first
What to compare between two factories
Use the same questions on every supplier. The answers are easier to compare than the marketing pages.
| Check | Weak answer | Strong answer |
|---|---|---|
| Tolerance claim | ±0.005 mm, no context | ±0.005 mm on parts up to 500 mm |
| Setup plan | Not discussed | Fixture and datum list in the DFM |
| Inspection | Certificate only | Dimensional report per batch |
| First article | Full run only | One-off prototype accepted |
| Material lot | Generic 'aluminum' | 6061-T6, 7075, 17-4PH named |
| Lead time | 'Depends' | Parts ship in 3–5 days after release |
| Post-processing | Sent out | Anodizing and plating in house |
| Confidentiality | Verbal only | NDA available on request |
What cnc core processing precision actually covers
Core processing is the set of operations that establish the datums, bores, faces and mating features a part is judged by. Everything else, from chamfers to cosmetic blending, hangs off those features. If the core features drift, the assembly fails no matter how good the surface looks.
That is why the tolerance on a drawing is not one number. A Ø40 mm bearing bore at ±0.005 mm and a 900 mm frame at ±0.005 mm are different problems. The first is routine on a mill-turn center. The second needs the right machine travel, a temperature-stable shop and a measurement plan that matches the part size.
For buyers, the practical question is simpler. Which features control fit and function, and can the supplier hold them on the size of part you are sending? A shop that answers that question with machine models and inspection steps is usually the safer partner.
Precision here is also a repeatability claim, not a one-off claim. A single good part proves the operator. A qualification rate of 99.99% across a run proves the process, the fixtures and the in-process checks behind it.
Matching part geometry to the right machine
Three-axis work suits prismatic parts with features on one face: plates, housings, brackets, simple covers. It is fast and cheap because the setup is simple. It stops being cheap when the part needs four or five faces machined, because each new face means another fixture and another chance to lose position.
Four-axis machining adds rotation around one axis. Shafts, pins and cylindrical parts with cross holes fit here. The part turns, the tool stays in one plane, and the operator avoids re-clamping for each hole.
Five-axis machining adds two rotary axes, so the tool reaches almost any direction in one setup. Impellers, turbine components, medical instruments and engine parts with compound angles belong on this type of machine. The gain is not only access. It is fewer datums, tighter true position between features, and shorter cycle time on complex geometry.
GreatLight runs 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers among 127 high-precision CNC machines. Maximum processing size is 4,000 mm, with travel options from 500 × 310 × 200 mm up to 4,000 × 400 × 150 mm.
Material and finish choices that affect precision
Material behavior decides how much of the tolerance you can keep. Aluminum 6061-T6 machines cleanly and holds ±0.005 mm on most features. Thin-wall 7075 parts move after clamping release, so the shop has to plan stock removal and stress relief. Stainless 316L and 17-4PH work harden if the feed is wrong, which shows up as chatter on a finished bore.
Titanium TC4 (Ti-6Al-4V) and Inconel demand low cutting speeds, rigid tooling and patience. Heat stays in the cut. A shop that quotes titanium at aluminum cycle times is guessing, and the first article will show it.
Finish interacts with tolerance too. Anodizing adds a thin oxide layer that changes a bore by a few micrometers. Hardcoat is thicker. If a bore is a press fit, the finish sequence has to be decided before the last cut, not after. Laser marking needs a minimum character height of 1.5 mm to stay legible.
GreatLight stocks or sources aluminum 6061, 2024, 5052, 6063, 6082 and 7075, stainless 303 through 17-4PH, 1018 to 4340 steel, copper and brass grades, magnesium AZ31B and AZ91D, and engineering plastics including POM, PEEK and carbon fibre.
Certifications and records that matter for your audit
Certificates tell you which management systems a factory runs, not how your part will be made. Even so, they narrow the list fast. ISO 9001:2015 covers general quality control. IATF 16949:2016 matters if the part ends up in a vehicle program. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which is relevant when you send CAD files.
The documents that decide a purchase order are usually plainer: a dimensional report per batch, material certificates tied to the heat number, and a first article inspection report on the initial run. GreatLight performs raw material checks, in-process monitoring and a final inspection on 100% of parts before shipment, with reports on request.
Ask how nonconforming parts are handled. A shop that tracks them and tells you before you find out is worth more than one that only reports good news.
Confidentiality belongs on the same list. Uploads stay secure and confidential, and an NDA is available on request when your drawings are sensitive.
Lead time, order size and how quotes are structured
Quotation and free DFM analysis come back within 12 hours. Production can start within 24 hours of release, and parts ship in 3–5 days. Historical late-delivery probability sits below 2%. Treat these as the shop's normal rhythm, then add time for finishes, special material orders and any inspection report you request.
Order size is rarely the blocker. There is no minimum order quantity, and runs span one prototype to 10,000+ parts. That structure suits teams who want to prove a design on a single part before committing to tooling.
Read the quote for what is included. Freight, material certificates, extra inspection and surface finishing are common line items that appear later. A quote that names the material grade, the machine, the finish and the inspection step is easier to compare than a single lump sum.
Payment terms and packaging follow the same logic. Ask how parts are protected in transit, especially on thin walls, long shafts and polished faces.
How to run a first order with a new machining partner
A short sequence that keeps a new supplier honest without slowing the project.
- 1Send the full 3D model and a marked drawingInclude the tolerance block, datum callouts and any finish note. A STEP file alone leaves the critical features open to interpretation.
- 2Read the DFM notes before the priceA useful DFM flags thin walls, deep pockets and features that cannot be reached. If the feedback only repeats your drawing, ask a direct question about the hardest feature.
- 3Order one first articleSingle-part runs are accepted, so use one. Check the core features on a CMM and compare them against the drawing, not against the previous supplier's parts.
- 4Confirm the fixture and datum planAsk which faces are machined in which setup. Fewer setups usually means better true position between features.
- 5Fix the inspection step in writingState which dimensions get measured, on what equipment, and whether a report is required per batch or only on request.
- 6Decide the finish sequence earlyAnodizing, plating and black oxide change dimensions. Name the finish before the final cut so the shop can compensate.
- 7Scale in two stepsMove from the first article to a small batch, then to the full run. Watch the qualification rate and the delivery dates at each step.
- 8Keep the NDA and file trail in placeUse the same file revision throughout. Version drift between the model and the shop's program is a common source of scrap.
Questions buyers ask before awarding the job
Can a factory hold ±0.005 mm on large parts?
Yes, within a stated size range and with the right machine. GreatLight works to ±0.005 mm (±0.0002 in) and runs travel options up to 4,000 mm, but the achievable tolerance on a long frame depends on fixturing and temperature control, not just the machine spec.
Send the drawing and we will say which features can hold that band and which ones need a wider callout.
Is five-axis machining more expensive than three-axis?
The hourly rate is higher, but the total can be lower. One five-axis setup replaces three or four fixtures on a complex part, which cuts handling, datum error and queue time.
On a simple plate with features on one face, three-axis is still the cheaper route. The decision follows the geometry, not the machine class.
What surface finish can be reached on a machined bore?
GreatLight works to Ra 1.6–3.2 μm as machined, Ra 0.8–1.6 μm with a finishing pass, and Ra 0.2–0.8 μm where the geometry allows.
Deep bores and hard materials limit how fine the finish can go. Tell us the function of the bore, and we will pick the pass that meets it.
Do you accept a single prototype order?
Yes. There is no minimum order quantity. The same process runs from one prototype to 10,000+ parts, so a first article is a fair way to test both the design and the supplier.
Rapid prototyping and vacuum casting are also available when the part is not yet ready for machining.
Which certifications should I ask for?
It depends on where the part goes. ISO 9001:2015 for general work, IATF 16949:2016 for automotive programs, ISO 13485:2016 for medical devices, and ISO 27001:2022 when file security matters.
GreatLight holds all four. Ask for the scope statement, not just the certificate number.
How do you handle drawings and confidentiality?
Uploads are secure and confidential. An NDA is available on request before you send files, and we can work to your document control process.
If the design is sensitive, send the NDA first and keep one revision of the model in play.
Send the drawing and get a real answer on precision
Upload your model and receive a quotation plus free DFM analysis within 12 hours.
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