Processing CNC Factory: How to Choose One That Holds Tolerance

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  Key takeaways
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  What to verify at each stage of supplier screening
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  Match spindle travel to your part, not to a brochure
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  How to read tolerance and surface finish claims
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  MOQ, lead time and certifications: what each one tells you
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  Material and process fit: where a processing CNC factory earns its keep
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  Finishing options and the paperwork that follows them
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  A six-step screening process for a processing CNC factory

The first question a processing CNC factory should answer is whether your part fits. Not fits on a table, but fits inside the travel envelope with room for the tool holder, the fixture and the tool change. A shop with 127 high-precision CNC machines still cannot cut a 4,000 mm frame if every machine maxes out at 750 mm.

Ask for the travel numbers directly. A capable shop will have several envelopes: 4,000 × 400 × 150 mm for long parts, 750 × 1,150 × 550 mm and 600 × 600 × 600 mm for mid-size work, and 500 × 500 × 450 mm or 500 × 310 × 200 mm for compact parts. If your part sits at the edge of one envelope, the quote should reflect a different machine, not a longer cycle time.

Rotary axes matter as much as linear travel. A Ø400 mm rotary table lets a 5-axis center reach five faces in one setup, which removes the re-fixturing error that shows up as position drift on the second and third setups. For parts with tight hole-to-hole relationships, that single-setup advantage is often the difference between holding ±0.005 mm and chasing it.

When travel is tight, the shop has three options: tilt the part, use a smaller tool, or split the operation. Each adds cost and risk. A good engineering reply will tell you which one they plan to use and what tolerance it protects.

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  Long partsConfirm the 4,000 × 400 × 150 mm envelope and whether the fixture still fits.
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  Mid-size parts750 × 1,150 × 550 mm and 600 × 600 × 600 mm cover most housings and plates.
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  Compact parts500 × 500 × 450 mm and 500 × 310 × 200 mm handle small, high-volume runs.
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  5-axis accessA Ø400 mm rotary table cuts setups and protects position tolerance.

Tolerance is a system, not a number. A processing CNC factory that claims ±0.005 mm (±0.0002 in) should be able to explain how it gets there: which machine, which tool path, what coolant, and how the part is measured. If the answer stops at the number, you are reading marketing, not capability.

Surface finish follows the same logic. As-machined surfaces land around Ra 1.6–3.2 μm. A high-finish pass gets you Ra 0.8–1.6 μm. Fine finishing at Ra 0.2–0.8 μm usually needs a separate operation, sometimes a different machine, and always more time. When a quote lists a fine finish without a price step, ask which operation produces it.

Inspection is the proof layer. A shop that inspects 100% of parts before shipment, checks raw material on arrival, monitors in-process, and runs a final inspection is not doing anything exotic. That is the minimum for parts where a single out-of-tolerance bore scraps an assembly.

Ask for the report format before you place the order. First article inspection, in-process records, and final dimensional reports should be named in the quote if your drawing calls them out. Reports on request are fine for prototypes. For production, they should be standard.

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  ±0.005 mmAsk for the machine, the setup and the measurement instrument behind the number.
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  Ra 1.6–3.2 μmStandard as-machined finish for most structural parts.
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  Ra 0.8–1.6 μmHigh-finish pass, common on sealing surfaces and sliding fits.
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  Ra 0.2–0.8 μmFine finishing, usually a separate operation with added cost.

A processing CNC factory with no minimum order quantity is telling you something about its fixturing. Running one prototype and a 10,000-part run on the same floor means the shop can justify soft jaws and simple workholding for a single part, then move to dedicated fixtures when volume arrives. If a supplier quotes a high MOQ on a prototype, they are optimizing for their schedule, not your design cycle.

Lead time should be broken into three numbers, not one. Quotation and DFM analysis within 12 hours. Production start within 24 hours after PO and material. Parts ship in 3–5 days for standard work. Any supplier who quotes a single vague lead time is hiding which step is the bottleneck.

Certifications are not interchangeable. ISO 9001:2015 covers general quality management. IATF 16949:2016 is the automotive standard and applies to powertrain, chassis and EV parts. ISO 13485:2016 is for medical devices. ISO 27001:2022 covers information security, which matters when you upload CAD files and drawings. Match the certificate to your industry, not to the logo wall.

Confidentiality belongs in the same conversation. Secure uploads and a signed NDA are standard practice for a shop that handles customer designs. If a supplier hesitates on an NDA, treat that as a data point about how they handle drawings.

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  No MOQFrom one prototype to 10,000+ part runs without a setup penalty.
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  12-hour quoteQuotation and free DFM analysis inside half a day.
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  3–5 day shippingStandard production parts, after material and setup.
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  Four certificatesISO 9001, IATF 16949, ISO 13485 and ISO 27001 cover different risks.

Aluminum is the default for prototypes and mid-volume parts. 6061 and 6061-T6 machine cleanly and hold tolerance well. 7075 gives higher strength for aerospace brackets. 2024 is common where fatigue matters. A shop that stocks these grades can start within 24 hours instead of waiting on a mill order.

Stainless and steel change the cutting parameters and the tool wear. 303 and 304 are the everyday grades. 316L shows up in medical and food-contact parts. 17-4PH (SUS630) is used where corrosion resistance and strength both matter. On the steel side, 1018 and 1045 are common for shafts and plates, while 4130, 4140 and 4340 appear in higher-load applications.

Titanium and special alloys test the shop more than the machine. TC4 (Ti-6Al-4V), Inconel and magnesium AZ31B or AZ91D need slower speeds, more coolant and tighter process control. If your part is titanium, ask how many similar jobs the shop has run and what tooling they plan to use.

Plastics round out the list. ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE and carbon fibre all machine differently. PEEK and carbon fibre are abrasive and demand sharp tooling and good extraction. A supplier who treats all plastics the same will give you a part with delamination or a poor finish.

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  Aluminum6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075, ADC12.
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  Stainless303, 304, 316, 316L, 420, 430, 431, 440C, 17-4PH (SUS630).
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  Steel1018, 1045, 4130, 4140, 4340, A36, tool steel.
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  Titanium and specialTA1, TA2, TC4 (Ti-6Al-4V), Inconel, magnesium AZ31B / AZ91D.

Finishing is where a good dimensional part can still fail incoming inspection. Anodizing in clear, colour, hardcoat or conductive versions changes the surface and, in hardcoat, the dimension. If your tolerance is tight, tell the shop which surfaces are masked and which are coated.

Plating options include electroless nickel, zinc, silver and gold. Powder coating and black oxide are common for industrial parts. Bead blasting, tumbling, brushing and polishing handle cosmetic and functional surfaces. Laser marking and engraving need a minimum character height of 1.5 mm to stay legible after coating.

Documentation ties the run together. Material certificates, inspection reports, and finish certificates should be listed in the quote if your quality system requires them. A processing CNC factory that treats paperwork as an afterthought will slow down your incoming inspection.

The practical check is simple: send one drawing with your finish callouts and your report requirements. The reply tells you whether the shop reads the whole print or only the geometry.

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  AnodizingClear, colour, hardcoat and conductive; hardcoat affects dimensions.
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  PlatingElectroless nickel, zinc, silver and gold.
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  CosmeticPowder coating, black oxide, bead blasting, tumbling, brushing, polishing.
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  MarkingLaser marking and engraving, minimum character height 1.5 mm.