A Guide to Key CNC Machined Components

A bracket that holds a cable tray can be 0.2 mm off and nobody notices. A hydraulic manifold that is 0.02 mm off leaks. Same material, same machine, very different risk. So the first question is not "how tight can you hold?" but "what happens if this dimension drifts?"

We treat a component as critical when its failure stops the assembly from working, creates a safety risk, or causes a recall. That definition covers a wide range of parts: fuel system housings, surgical instrument bodies, robot joint housings, vacuum chamber fittings, sensor mounts on a moving axis.

Criticality also depends on quantity. A one-off prototype that will be measured on a bench is forgiving. The same part in a 10,000-piece run has to hold the same dimension on the last part as on the first. Process control, not operator skill, carries that.

If you are unsure where your part sits, send the drawing and the function. We will tell you which features drive the cost and which tolerances you can loosen without losing function.

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  Safety or regulatory impactMedical, aerospace, automotive safety items. Documentation matters as much as the cut.
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  Interface fitBearing bores, seal grooves, spigots, dowel holes. Usually the tightest callouts on the print.
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  Downstream cost of a bad partIf a scrapped part means a teardown or a field return, aim for zero defects.

Most cnc machined components can be produced on a 3-axis mill. The question is how many setups that requires. Every extra setup adds a datum shift, and datum shifts are where tolerance stacks go wrong. A part with features on four faces is often cheaper on a 4-axis or 5-axis machine even though the hourly rate is higher.

We run 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. That spread matters because it lets us pick the process that fits the geometry rather than forcing every part onto one platform.

Part size sets the ceiling. Our largest travel is 4,000 × 400 × 150 mm. Medium platforms give 750 × 1,150 × 550 mm and 600 × 600 × 600 mm. Compact platforms cover 500 × 500 × 450 mm and 500 × 310 × 200 mm, and we have a Ø400 mm rotary table for round and prismatic work that needs indexing.

A few signals point away from 5-axis. Simple prismatic plates with one machined face, or parts where a turned feature dominates, are usually faster on a 3-axis mill or a lathe. Five-axis is a tool for reducing setups and reaching angled features, not a default.

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  Use 5-axis whenAngled holes, undercuts, contoured pockets, or four-plus faces in one setup.
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  Use 3-axis whenFlat plates, single-face work, loose tolerances, high volume with simple fixturing.
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  Use mill-turn whenRotational body plus milled flats, ports or slots in one cycle.

Aluminium 6061-T6 is the default for housings, brackets and fixtures. It machines fast, takes anodizing well, and holds ±0.005 mm on a rigid setup. 7075 gives higher strength where weight matters, at the cost of tool wear and a greater tendency to move after roughing.

Stainless 303 and 304 cover most general work. 316L and 17-4PH (SUS630) come up in medical and marine parts where corrosion or post-hardening matters. 17-4PH is worth planning for early, because the heat treat condition changes how the part behaves in the fixture.

For wear surfaces, 4140 and 4340 respond well to hardening. Titanium TC4 (Ti-6Al-4V) and Inconel appear in aerospace and energy work; both need sharp tooling, lower cutting speeds and more attention to heat. Magnesium AZ31B and AZ91D machine quickly but demand careful chip handling.

Plastics behave differently again. POM and PEEK hold tight dimensions but move with temperature. ABS and PC are fine for covers and prototypes. Carbon fibre composites cut cleanly with the right tool geometry and a dust-controlled setup.

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  6061-T6General purpose. Good finish, easy anodizing, predictable.
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  7075High strength aluminium. Expect more distortion on thin walls.
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  316L / 17-4PHCorrosion resistance and hardenable stainless for medical and marine.
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  TC4 / InconelHigh temperature and strength. Slower cutting, higher tool cost.

A critical component cannot be inspected into conformance. If the process is not capable, sorting will only find the bad parts after you have paid for them. So we build the inspection plan around the features that matter, and we check raw material before the first cut.

In-process monitoring catches drift while the part is still in the machine. Final inspection confirms the finished geometry. We inspect 100% of parts before shipment and can supply reports on request, including dimensional results, material certificates and surface finish data.

We hold ±0.005 mm (±0.0002 in) where the drawing calls for it. Our current qualification rate is 99.99%. That number is a result of process control, not of sorting, and it is the reason we ask for a DFM review before quoting a critical part.

For regulated work, our quality system is certified to ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. That covers general industrial, automotive, medical device and information security requirements. If your program needs a specific record format, tell us at the quote stage.

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  Raw material checkCertificates and, where needed, incoming dimensional or hardness checks.
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  In-process monitoringDimensional checks during the run, not only at the end.
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  Final inspection100% inspection before shipment, with reports on request.

Anodizing adds thickness. Hardcoat anodizing can add 25 to 50 μm per surface, which matters on a bore or a thread. If a masked area must stay conductive, say so on the drawing, because a clear anodize film is an insulator.

Electroless nickel gives a uniform coating on complex geometry, which is why it shows up on valve bodies and fluid paths. Zinc, silver and gold plating are usually about corrosion or contact resistance. Powder coating and black oxide cover different needs again.

Bead blasting, tumbling, brushing and polishing change surface texture and can slightly round edges. On a sealing face, that is a functional change, not a cosmetic one. Laser marking is available down to a minimum character height of 1.5 mm.

The practical rule: decide the finish before you finalize tolerances. A ±0.005 mm bore and a 40 μm hardcoat are not automatically compatible. We will flag the conflict during DFM review.

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  AnodizingClear, colour, hardcoat, conductive. Confirm masked areas.
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  PlatingElectroless nickel, zinc, silver, gold. Uniform coverage on complex shapes.
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  Mechanical finishesBead blasting, tumbling, brushing, polishing. Affects edge condition.

A 3D model alone is not enough for a critical part. Send the 2D drawing with tolerances, datums, material, finish and any inspection requirement. If the drawing is incomplete, we will ask rather than guess, because a guess on a critical feature is a scrap risk.

Tell us the function. "This bore carries a bearing" or "this face seals against an O-ring" changes how we plan the process, the fixturing and the inspection. It also tells us which tolerances you can loosen if cost becomes an issue.

Volume matters too. We have no minimum order quantity, so a single prototype and a 10,000+ part run are both workable. The process plan differs, but the starting point is the same review.

Quotation and free DFM analysis come back within 12 hours. Production can start within 24 hours after that, and parts typically ship in 3–5 days. Uploads are secure and confidential, and we can work under an NDA on request.

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  SendSTEP or native model, 2D drawing, material, finish, quantity, target date.
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  SayWhat the part does and which features are functional.
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  AskFor a DFM note if a tolerance or finish looks expensive or risky.