A Machine Shop CNC: What It Can Hold and How to Pick One

A machine shop CNC runs on G-code generated from your model. The operator sets the work offset, loads the tool library, and the control repeats the same path on every part. That repeatability is the whole point. Manual machining depends on the skill of the person at the handwheel; CNC depends on the setup, the fixture and the tool data.

The practical answer to "what can a machine shop CNC make for me" starts with travel and axis count. A 3-axis vertical mill cuts from one direction. A 4-axis mill adds an indexer or rotary table, so features on four sides come off in one setup. A 5-axis center tilts the tool or the table, which lets you reach undercuts and blend curved surfaces without re-fixturing.

Most parts never need 5-axis. A bracket with flat faces, through holes and a pocket is cheaper and faster on a 3-axis machine. Reach for simultaneous 5-axis when the part has a curved parting line, deep slender pockets at an angle, impeller blades, or a tolerance stack that would break if you moved the part between operations.

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  3-axisFlat faces, holes, pockets. One setup, lowest cost per part.
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  4-axisFour-sided work, slots, cross holes. Fewer setups, tighter position.
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  5-axisUndercuts, sculpted surfaces, deep angled pockets. Avoids re-fixturing.
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  Mill-turnRound parts with milled flats or cross holes in one cycle.

Ask for the machine list before you ask for a price. Not the brand names, the travel envelope and axis count. At GreatLight we run 127 high-precision CNC machines: 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. That mix matters because the wrong machine either cannot reach the feature or costs more than it should.

Maximum processing size reaches 4,000 mm. Large travel is 4,000 × 400 × 150 mm, medium travel covers 750 × 1,150 × 550 mm and 600 × 600 × 600 mm, and compact travel covers 500 × 500 × 450 mm and 500 × 310 × 200 mm. A Ø400 mm rotary table handles round and four-sided work in the mid range.

Long parts expose a problem that short parts hide: thermal drift. A 4,000 mm rail will move as the spindle warms, so the shop has to rough, cool and finish rather than cutting to size in one pass. If a supplier quotes your long part in a single operation with no mention of temperature, ask how they hold straightness.

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  Envelope firstA feature outside the travel is a redesign, not a quote.
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  Chip clearanceDeep pockets need a spindle small enough to enter.
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  RigidityThin walls deflect under tool pressure. Light passes, sharp tools.

Tolerance is a process decision, not a wish. Our floor holds ±0.005 mm (±0.0002 in) on critical features when the feature, material and fixture support it. That number does not apply to every dimension on the print. If you tolerance a 300 mm aluminium plate to ±0.005 mm across its whole length, the shop will fight thermal expansion instead of cutting metal.

Call out only the dimensions that carry function. A bearing bore, a dowel hole, a sealing face. Let general tolerances cover the rest. This one change usually cuts cost more than any sourcing negotiation, because it lets the shop choose an efficient setup instead of chasing numbers nobody measures.

Surface finish follows the same logic. As-machined is Ra 1.6–3.2 μm, a high-quality machined finish is Ra 0.8–1.6 μm, and fine finish reaches Ra 0.2–0.8 μm. Fine finishes cost time. They need slower feed, sharper tooling and sometimes a separate finishing pass. Specify them where a seal, a sliding fit or an optical surface needs it.

Inspection closes the loop. We check raw material on arrival, monitor in process, and inspect 100% before shipment, with reports on request. Ask any machine shop CNC supplier what they measure with and when. A shop that only measures after the part is finished has already spent the money by the time it finds the error.

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  Functional dimsTolerances on fits and sealing faces, general notes elsewhere.
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  Finish by zoneRa 0.8–1.6 μm on sealing surfaces, as-machined on the rest.
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  First articleMeasure the first part before the run continues.
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  ReportsRequest dimensional and material reports with the shipment.

Material choice drives the process more than most drawings admit. Aluminium 6061-T6 cuts fast and holds tight tolerances, which is why it dominates prototypes and fixtures. 7075 gives higher strength but moves more after machining, so stress relief matters. Stainless 303 machines cleanly; 316L is tougher on tooling and better in corrosive service.

We machine aluminium grades 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12; stainless 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH (SUS630); steels 1018, 1045, 4130, 4140, 4340, A36 and tool steel; copper and brass including C101, C103, C110, beryllium copper, C27400, C28000 and C36000; titanium TA1, TA2 and TC4 (Ti-6Al-4V); Inconel; magnesium AZ31B and AZ91D; and plastics from ABS and POM through PEEK and carbon fibre.

Titanium and Inconel behave differently from aluminium. They conduct heat poorly, so the cutting edge runs hot and tool life drops. Thin ribs on a titanium part can chatter where the same geometry in aluminium is stable. A shop that lists the material on its website but cannot tell you the feeds it runs is not the shop to send titanium to.

Finishing is where one-stop sourcing pays off. Anodizing in clear, colour, hardcoat and conductive types; electroless nickel, zinc, silver and gold plating; powder coating and black oxide; bead blasting, tumbling, brushing and polishing; laser marking and engraving down to 1.5 mm character height. Each finish adds a step where dimensions can shift, so the shop has to plan allowance before the finish, not after.

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  Heat treat timingStress relief before final cuts on moving parts.
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  Finish allowanceAnodizing adds thickness. Plan it into the tolerance.
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  MaskingThreads and bores often need protection during coating.

Start with the certifications that match your industry. ISO 9001:2015 covers a general quality system. IATF 16949:2016 applies to automotive and EV work. ISO 13485:2016 covers medical devices. ISO 27001:2022 covers information security, which matters when you upload proprietary models and drawings.

Then check the response loop. We return a quotation and free DFM analysis within 12 hours, and production can start within 24 hours. Parts ship in 3–5 days on standard work. Historical late-delivery probability is below 2%. Those numbers describe our own floor, so treat them as a reference point when you compare suppliers rather than a promise about anyone else.

Ask about minimum order quantity. We have none, which means a single prototype and a 10,000+ part run use the same quoting path. That matters for engineers who need to validate a design before committing tooling budget.

Finally, ask about confidentiality before you upload anything. Secure uploads and an NDA on request are standard here. If a supplier hesitates on an NDA for a prototype, they will hesitate on your production drawing too.

One more check that costs nothing: send a drawing with a deliberate tight tolerance and see whether the quote flags it. A good machine shop CNC partner will tell you the tolerance is unnecessary or expensive. A weak one will simply quote it and machine it slowly.

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  Cert matchPick the certificate that fits your end market.
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  DFM feedbackA quote without comments is a price, not an opinion.
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  NDASign before the model leaves your network.