4 Axis CNC Service: How to Pick a Supplier Without Guesswork
This guide is for engineers and purchasing teams sourcing a 4 axis CNC service in China. It covers the checks that separate a capable shop from a broker with a website: rotary table size, travel limits, tolerance control, certification fit and quote terms. Read it before you send an RFQ.
In this article
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Key takeaways
4 axis CNC service checks against what to ask for
Use the middle column as the question you put in the RFQ. The right column is the range we hold on our own floor.
| Check | What to ask | Reference range |
|---|---|---|
| Rotary table size | Diameter and clamping method | Ø400 mm rotary table |
| Work envelope | X, Y, Z travel and part swing | 4,000 × 400 × 150 mm on the large mill |
| Tolerance | Positional and profile tolerance on a sample | ±0.005 mm (±0.0002 in) |
| Surface finish | Ra value on the sealing face | Ra 0.8–1.6 μm as standard |
| Inspection | What is measured, on which features | 100% inspection before shipment |
| Certification | Which certificate applies to your part | ISO 9001, IATF 16949, ISO 13485, ISO 27001 |
| Minimum order | Prototype quantity and repeat run | No minimum order quantity |
| Lead time | When the first article ships | Parts ship in 3–5 days after release |
When 4 axis is enough and when to move up
A quick way to route a part before you send it out for quote.
| Part feature | Best process | Why |
|---|---|---|
| Holes and flats on four sides of a block | 4 axis | Indexing handles it in one setup |
| Bolt circle on a flange face | 4 axis | Rotary table indexes each hole precisely |
| Helical slot on a shaft | 4 axis with simultaneous feed | Rotation and linear motion run together |
| Compound-angle wall, undercut | 5 axis | Tool must tilt to reach the feature |
| Deep pocket with curved floor | 5 axis or EDM | Short tool cannot reach without tilt |
| Simple plate, two faces only | 3 axis | Extra axis adds cost, not value |
| High-volume turned part with cross holes | Mill-turn | Turning and milling in one cycle |
The short version
Pick the 4 axis CNC service that answers the geometry question first and the price question second. If the shop confirms rotary clearance, names the tolerance on your critical features and puts the inspection plan in the quote, the price is comparable. If it skips those, the quote is not.
What a 4 axis CNC service actually adds to the process
A 4 axis CNC service adds one rotary axis to the usual three linear ones. On most mills the rotary table turns around the X axis, so the part indexes while the tool stays in a fixed orientation. That single motion removes a lot of manual repositioning. Bolt circles, cross holes, flats at 90° to each other and helical slots can often be cut in one setup.
The practical gain is not speed. It is setup count. Every time an operator unclamps a part, resets it and re-probes the datum, you add stack-up error and you add hours. A rotary table that indexes to ±0.001° removes both. For a housing with six faces and four side bores, that is usually the difference between a confident quote and a padded one.
Where it stops. A 4 axis machine cannot tilt the tool. Undercuts, deep pockets with curved floors and features that need to reach around a shoulder stay out of range. Those parts need a fifth axis or a different process. If your drawing has a compound angle on a wall that faces inward, flag it early and let the shop tell you which machine will run it.
- 1Good fitCylindrical parts, valve bodies, manifolds, drive shafts, flanges with bolt patterns
- 2Poor fitUndercuts, compound-angle walls, deep curved pockets that need tool tilt
- 3Watch the swingA long part can hit the table or the enclosure wall when it indexes
Travel, table size and part geometry: the first filter
Before price, ask for the work envelope. A rotary table of Ø400 mm tells you the largest round part that clamps cleanly. It does not tell you how far the spindle can move. On a large 4 axis mill the travel can reach 4,000 × 400 × 150 mm, which suits long shafts and extruded profiles. Medium frames run 750 × 1,150 × 550 mm or 600 × 600 × 600 mm for general housings. Compact frames at 500 × 500 × 450 mm and 500 × 310 × 200 mm handle small precision work.
Then check the swing. A part that fits the table when stationary can still strike the enclosure once it rotates. If the corner radius of your part is large relative to the table, the shop should model the rotation before quoting. A supplier who skips this step will discover the collision on the machine, and your delivery date pays for it.
Clamping matters as much as size. Thin-walled tubes and rings distort when a three-jaw chuck tightens. Ask whether the shop uses a fixture plate, a mandrel or soft jaws. For a 2 mm wall on an aluminium ring, soft jaws bored in place usually hold roundness better than a standard chuck. This is a one-line question that changes the first-article result.
- 1Send the 3D model, not just a drawingRotation clearance can only be checked from solid geometry
- 2State the datumTell the shop which face locates the part so the rotary zero is set the same way every run
- 3Name the critical featuresTwo or three dimensions drive the process. The rest can run to general tolerance
Tolerance, surface finish and inspection evidence
Every supplier claims precision. The question is which feature, measured how, and recorded where. A useful tolerance claim names the feature and the method. ±0.005 mm on a bore diameter measured with a bore gauge and a calibrated micrometer is a different statement from ±0.005 mm on a general note that nobody checks.
Surface finish follows the same logic. As-machined aluminium sits around Ra 1.6–3.2 μm. A sealing face or a bearing seat usually needs Ra 0.8–1.6 μm, and optical or medical contact surfaces can reach Ra 0.2–0.8 μm, which often means a finishing pass with a smaller stepover or a secondary operation. Tell the shop which face needs which value. Blanket finish notes raise the price of the whole part.
Inspection is the tie-breaker in a supplier comparison. Raw material check, in-process monitoring and a final inspection before shipment are the baseline. Reports should be available on request, and a first-article report on the critical dimensions is normal for a new part. If a supplier cannot say what happens between the first cut and the shipping box, the tolerance claim is marketing, not process control.
- 1Ask for the first-article reportIt shows which features were measured and with what instrument
- 2Split the finish calloutsCritical faces tight, cosmetic faces loose. Cost drops fast
- 3Agree on the datum schemeInspection and machining must reference the same faces
Lead time, MOQ and what a quote should contain
A quote that lists only a price and a number of days is not enough to compare. Ask for the quote to state the machine class, the number of setups, the tolerance on critical features, the finish on named faces, the inspection plan and the shipping window. With those six items you can compare two suppliers honestly instead of comparing two guesses.
Lead time has two halves. The front half is quoting and DFM feedback, which should be fast enough to keep your schedule moving, and the back half is production and shipping. Ask when the first article is available and when the full run ships. A shop with a low historical late-delivery rate will usually give you a specific date rather than a range, because it tracks its own on-time performance.
Minimum order quantity matters more than buyers expect. A supplier with a high MOQ pushes you toward a tooling decision before the design is stable. A shop that runs from one prototype up to 10,000+ parts lets you validate the 4 axis process on a real part first. That is cheaper than discovering a fixture problem after a large order is released.
- 1Quote must name the machineThree-axis, four-axis or mill-turn changes the setup count
- 2Ask for the DFM notesA free DFM pass often removes a costly feature before cutting starts
- 3Confirm the shipping termsIncoterms and packing method belong in the quote, not in a later email
Certification, confidentiality and material coverage
Certification is a filter, not a badge. ISO 9001:2015 is the baseline for a general machine shop. IATF 16949:2016 applies to automotive and EV work, where traceability and change control are audited. ISO 13485:2016 applies to medical devices and their components. ISO 27001:2022 covers information security, which matters when you send CAD data and process documents to an overseas supplier.
Match the certificate to the part, not to the company. A shop with four certificates still needs to show that your specific order runs under the right system. Ask which procedure covers incoming material, in-process checks and final release for your part number. That is the question an auditor would ask, and it is the one that protects your product.
Confidentiality usually decides whether the RFQ happens at all. Uploads should be treated as secure and confidential, and a non-disclosure agreement should be available on request before detailed drawings are shared. Material coverage is the other practical filter. If your part is 17-4PH, Ti-6Al-4V, Inconel or PEEK, confirm the shop cuts it regularly. Unusual alloys cut differently, and a supplier learning on your part is a risk you do not need.
- 1Send an NDA firstSign before sharing full drawings, not after
- 2Name the alloy and temper6061-T6 and 6061 cut differently. So do 304 and 316L
- 3Ask about traceabilityMaterial certificates should be linked to your order number
Seven steps to qualify a 4 axis CNC service
Run these in order. Each step either removes a supplier or gives you a number to compare.
- 1Send the model and the drawing together3D geometry shows rotation clearance and tool access. The drawing carries tolerances, datums and finish callouts. If the two disagree, ask which one governs before quoting.
- 2Ask for the work envelope in millimetersRequest X, Y, Z travel and rotary table diameter. Compare against your part plus fixture. A 4,000 mm X travel machine and a Ø400 mm table are different tools from a 500 mm compact frame.
- 3Name the critical featuresMark two to five dimensions that decide function. Ask for the tolerance and the measuring method on each. Everything else can run to general tolerance, which keeps cost down.
- 4Confirm the setup countAsk how many times the part is unclamped. One setup on a 4 axis machine is the goal. Two or three setups mean the price should reflect the extra labor and the added stack-up.
- 5Request the inspection planRaw material check, in-process monitoring, final inspection before shipment. Ask for a first-article report on the critical dimensions and a report on request for the production run.
- 6Match certification to your industryISO 9001:2015 for general work, IATF 16949:2016 for automotive, ISO 13485:2016 for medical, ISO 27001:2022 for data security. Ask which procedure covers your part.
- 7Agree on MOQ and the shipping windowConfirm the prototype quantity, the repeat quantity and the date the parts leave the dock. Keep the quote open until these three are in writing.
Questions buyers ask before releasing a 4 axis order
How do I know whether my part needs 4 axis or 5 axis?
Look at the feature, not the part. If every machined face can be reached by indexing the part around one axis, 4 axis is enough. Bolt circles, cross holes, flats on four sides and helical slots all fall into this group.
If a feature needs the tool to tilt, such as an undercut or a compound-angle wall, you need 5 axis. Send the model and ask the shop to confirm. A good engineering team will tell you when 5 axis is overkill, because the extra axis adds cost without adding value.
What tolerance can a 4 axis CNC service hold in production?
On our floor the working figure is ±0.005 mm (±0.0002 in) on critical features, with 100% inspection before shipment. That number depends on the feature, the material and the fixture, so it should be confirmed per part.
General dimensions can run looser, which lowers cost. Give the shop a clear split between critical and non-critical dimensions. Blanket tight tolerances are the fastest way to inflate a quote.
Is there a minimum order quantity?
We run from a single prototype up to 10,000+ part runs with no minimum order quantity. That lets you prove the 4 axis process on a real part before committing to a larger release.
For repeat orders, keeping the same fixture and the same datum scheme matters more than the quantity. It is what keeps the second run aligned with the first.
How fast can parts ship?
Quotation and a free DFM analysis come back within 12 hours. Production can start within 24 hours of release, and parts ship in 3–5 days after that.
The exact date depends on quantity, material availability and finishing. Anodizing and plating add time, so confirm the finishing route in the quote rather than after machining.
Which materials can be machined on a 4 axis machine?
Aluminium grades such as 6061-T6, 7075 and 6082, stainless steels including 303, 304, 316L and 17-4PH, alloy steels, copper and brass, titanium grades TA1, TA2 and Ti-6Al-4V, Inconel, magnesium, and plastics from POM to PEEK and carbon fibre.
The alloy changes the cutting parameters and sometimes the fixture. Tell the shop the exact grade and temper on the drawing, not just the family name.
Can you sign an NDA before I share drawings?
Yes. Uploads are treated as secure and confidential, and a non-disclosure agreement is available on request. Send the NDA first if your drawings are sensitive.
We also hold ISO 27001:2022 for information security, which covers how design files and process documents are stored and accessed.
Send your 4 axis part for a DFM review
Upload the model and drawing. You get a quotation and a free DFM analysis within 12 hours, with the setup count, tolerance callouts and inspection plan stated in writing.
12-hour quoteFree DFM analysisNo MOQ100% inspection
More from the shop floor
We publish setup notes, tooling trials and inspection data from the factory floor.
