CNC Machining of Chinese Parts

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  What actually happens when a shop does CNC machining of Chinese parts
• 2
  Three-axis, four-axis, or five-axis: which one fits the part
• 3
  How material choice changes the cut, the finish, and the risk
• 4
  Where tolerance and surface finish actually come from
• 5
  Certifications and what they mean on the floor
• 6
  Post-processing changes the part after the cut
• 7
  How to judge a quote and a shop before you commit
• 8
  Choosing a process for CNC machining of Chinese parts

A CNC machine does not know where the part is in space. It only knows where the spindle was told to go. So the whole process is a chain of references: the fixture locates the blank, the probe or the operator sets a work offset, the tool cuts relative to that offset, and the CMM measures the result relative to datums on the drawing. Every link in that chain adds or removes error.

In a typical job shop, the first operation is often a vise or a three-jaw chuck. For a part with a 0.05 mm true-position callout on a bolt circle, that is usually fine. Move to ±0.005 mm and the setup starts to matter more than the spindle. Thermal growth in the fixture, chip packing under a locating pad, and a 0.01 mm burr at the datum face will all show up in the final report.

This is why two shops can quote the same drawing and deliver different results. The machine is rarely the limit. The limit is how carefully the shop controls setup, tool wear, and inspection. A 27-machine three-axis floor with a disciplined process will hold ±0.01 mm all day. A five-axis center running without in-process checks will not.

For buyers, the practical takeaway is simple. Ask how the first article is fixtured, what datums are used, and how tool wear is compensated. If the answer is vague, the tolerance on the print is a wish, not a plan.

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  Setup dominates tight toleranceBelow ±0.01 mm, fixture rigidity and thermal stability matter more than spindle accuracy.
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  Datums must be sharedDesign, machining, and inspection should all reference the same A-B-C datum scheme.
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  Tool wear is a moving targetIn-process probing or scheduled offset checks keep a long run inside tolerance.

Three-axis machining moves the table in X and Y and the spindle in Z. The tool always approaches from one direction. Prismatic parts with features on one or two faces are cheap and fast this way. A 500 × 500 × 450 mm envelope covers most brackets, plates, and housings.

Four-axis adds a rotary table, usually Ø400 mm. Now you can cut four faces in one setup. That removes the re-fixturing error that comes with flipping a part. It also cuts cycle time because the operator is not re-zeroing every face. Parts with a repeating pattern around a bore or a shaft are the classic fit.

Five-axis adds two rotary axes and lets the tool tilt. This is how you reach undercuts, cut a compound angle in one pass, and keep a short tool over a deep cavity. Short tools deflect less, so surface finish and dimensional control both improve. A 600 × 600 × 600 mm five-axis envelope handles most medical and aerospace housings we see.

Five-axis is not automatically better. Programming takes longer, the machine hour rate is higher, and fixtures are more complex. For a simple plate with holes on one face, three-axis is the correct answer. For a part with five-sided features and a ±0.01 mm profile, five-axis in one setup usually wins on total cost once you count rework risk.

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  Pick three-axisFlat parts, one or two faces, loose-to-moderate tolerance, high volume.
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  Pick four-axisShafts, bushings, and parts with a repeating pattern around an axis.
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  Pick five-axisUndercuts, compound angles, deep pockets, and tight profile tolerance.

Aluminum 6061-T6 is the default for prototypes and most production parts. It cuts fast, holds ±0.01 mm easily, and takes anodizing well. 7075 is stronger but gummier; it needs sharper tools and more coolant to avoid built-up edge. 2024 machines well but is less corrosion resistant, so it usually gets a coating.

Stainless 303 is free-machining and gives a good finish. 304 and 316 work-harden, so a light feed and a dull tool will rub instead of cut. That rub raises surface hardness and the next pass cuts worse. 17-4PH (SUS630) can be machined in the annealed state and then aged to high strength, which is why it shows up in aerospace and medical brackets.

Titanium Ti-6Al-4V (TC4) has low thermal conductivity, so heat stays at the cutting edge. Tool life drops fast if you push speed. Keep surface speed low, use high-pressure coolant, and expect more time per part. Inconel is worse. It is machinable, but it is a specialty job and should be quoted as one.

Plastics behave differently again. POM and PEEK hold tolerance well but move with temperature. ABS and PC are soft and prone to burrs. Carbon fibre eats tool edges and needs dust control. If your part is plastic, say so at the quote stage; the fixturing and tooling plan changes.

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  Aluminum 6061-T6General purpose, tight tolerance, good anodizing response.
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  Stainless 304 / 316Corrosion resistance, but work-hardening demands a positive feed.
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  Titanium TC4High strength-to-weight, low speed, high coolant pressure.
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  PEEK and POMStable plastics, but control temperature to hold size.

A tolerance number on a drawing is a limit, not a process. To hit ±0.005 mm, the shop needs a machine that can position to a fraction of that, a fixture that does not move, and a measurement system with enough resolution to prove it. A caliper is not enough. You need a micrometer, a bore gauge, or a CMM.

Surface finish follows the same logic. Ra 1.6–3.2 μm is a normal as-machined result. Ra 0.8–1.6 μm needs a controlled finish pass with a sharp tool and a stable setup. Ra 0.2–0.8 μm usually means a fine finishing pass, sometimes with a smaller stepover, and it costs time. Do not call out a fine finish over a whole part if only a sealing face needs it.

Tolerance and finish interact. A very fine finish on a thin wall can distort the part when the tool pressure changes. A tight bore and a rough face on the same part may need two different setups and two different inspection plans. Splitting the print into critical and non-critical features keeps cost sane.

We inspect 100% of parts before shipment, covering incoming material, in-process checks, and final inspection. Reports are available on request. That does not mean every feature is measured on every part; it means the plan is defined and followed, and the paperwork can be shown.

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  As-machinedRa 1.6–3.2 μm, standard tooling, lowest cost.
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  High finishRa 0.8–1.6 μm, finish pass, moderate added time.
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  Fine finishRa 0.2–0.8 μm, controlled pass, highest cost per area.

A certificate is a system, not a part. ISO 9001:2015 says the shop documents its process and audits it. IATF 16949:2016 adds automotive-specific tools like PPAP and control plans. ISO 13485:2016 is the medical device standard and brings more traceability. ISO 27001:2022 covers information security, which matters if you send CAD files.

For a buyer, the useful question is not which certificates the shop holds. It is which one applies to your product and whether the shop can show the records. A medical housing needs device history records. An automotive bracket needs a control plan and a PPAP pack. A one-off prototype usually needs neither.

Traceability is the practical difference. Can the shop tie a delivered part back to a heat number, a machine, and an operator? If yes, a field failure can be investigated. If no, you are guessing. Ask for a sample inspection report before you award the job.

GreatLight holds ISO 9001:2015, IATF 16949:2016, ISO 13485:2016, and ISO 27001:2022. Uploads are treated as confidential, and an NDA is available on request. Those are process commitments, and they are auditable.

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  ISO 9001Baseline quality system, documented process control.
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  IATF 16949Automotive, adds PPAP and control plan discipline.
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  ISO 13485Medical, adds traceability and device records.
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  ISO 27001Information security for CAD and IP files.

Machining leaves tool marks, sharp edges, and sometimes residual stress. Deburring and edge breaking are not optional on a functional part; a 0.1 mm burr on a sliding surface will cause a failure. Tumbling and bead blasting remove marks and give a uniform matte look. Brushing gives a directional grain.

Anodizing adds a hard oxide layer, usually 5–25 μm. It builds on the surface, so a masked dimension can shift. Clear anodize keeps color close to the base metal; color anodize is cosmetic and batch-dependent. Hardcoat is thicker and more wear resistant but more brittle. Conductive anodize keeps electrical contact where grounding matters.

Plating changes dimensions too. Electroless nickel adds a uniform layer, often 5–25 μm, and is good for wear and corrosion. Zinc, silver, and gold plating are used for corrosion, conductivity, or solderability. If a plated bore has a tight tolerance, tell the finisher which dimensions are critical.

Laser marking is common for part numbers and lot codes. Minimum character height is 1.5 mm. Below that, readability on a curved or rough surface drops fast. Powder coating and black oxide are also available when appearance or corrosion resistance is the goal.

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  Deburr firstEdge quality affects function more than appearance.
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  Anodize buildsPlan 5–25 μm growth on critical dimensions.
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  Plating builds tooElectroless nickel adds uniform thickness on all surfaces.

A quote is a set of assumptions. Read the notes. Does it state the material grade, the tolerance standard, the finish, and the inspection level? A price with no notes is a price for something, but not necessarily for your part. Ask for the DFM feedback if it is missing.

Lead time is another place where assumptions hide. A shop can quote a fast ship date and then wait on material. Ask whether the raw stock is on the floor or on order. For a 4,000 mm part or a titanium job, material lead time can exceed machining time.

No minimum order quantity is a real advantage when you are still proving a design. It lets you run one prototype and then scale to a 10,000+ part run without changing suppliers. But confirm that the prototype process and the production process are the same, or the second run will not match the first.

Finally, test communication. Send a question with a drawing and see how fast and how clearly it comes back. Quotation and free DFM analysis within 12 hours is a normal target for us, and production can start within 24 hours once the order is released. If a shop cannot answer a technical question before the sale, it will not answer one after.

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  Read the quote notesMaterial, tolerance, finish, and inspection should be explicit.
• 2
  Check material lead timeStock on hand versus stock on order changes the real date.
• 3
  Keep one processPrototype and production should use the same method and fixtures.