CNC Turning Service: How to Pick the Right Supplier

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  What matters most when you buy turning
• 2
  Which turning route fits your part
• 3
  Tolerance: what a cnc turning service can hold
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  Geometry limits: bar size, length and wall thickness
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  Volume and setup cost: where the price steps sit
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  Certifications, inspection and document trail
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  Finishing, secondary operations and hidden cost
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  Step by step: vetting a turning supplier

A cnc turning service quotes tolerance by feature, not by drawing title block. On a stable material like 6061-T6 or 303 stainless, Ø tolerance of ±0.005 mm is realistic on a diameter that is turned in one pass with a finished insert. The same shop will quote ±0.02 mm on a long unsupported shaft, because deflection grows with length. Always mark which dimensions are functional fits and which are reference.

Surface finish and tolerance are linked. A Ra 0.8–1.6 μm finish normally comes off the insert at moderate feed. Going to Ra 0.2–0.8 μm usually means a wiper insert, a slower feed or a second pass. If the drawing calls for mirror finish on an O-ring groove, expect either a polishing step or a higher price. Ask which finish value is measured with a profilometer and which is visual.

Threads and bores are the two areas where tolerance drift shows first. A M12 × 1.75 thread has a class callout; 6g is standard, 6H for internal. If the drawing does not state a class, the shop picks one and you may get a loose fit. For bearing bores, give the fit (for example H7) rather than a single nominal diameter, so the machinist can set the boring bar to the middle of the band.

Runout, not just diameter, decides whether the part works. A Ø20 mm journal with 0.01 mm runout will vibrate even if the diameter is perfect. State the datum and the runout in one line. When a customer sends a drawing with a single datum on a turned face and runout on the far end, we set up between centers or use a drive dog to keep the relationship.

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  Functional diametersGive a fit class (H7, h6) instead of a nominal size.
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  Free dimensionsMark as reference so the shop does not over-inspect.
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  Long shaftsAdd a straightness or runout note; length alone will not hold.
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  InspectionSay which dimensions need a report and which are sampled.

Maximum part diameter on a lathe is set by bar capacity and chuck size, not by the spindle bore number on a brochure. A bar feeder on a Ø32 mm machine cannot run Ø38 mm stock, so the part must be cut from larger bar in a chucking operation, which raises cost. When you quote a turned part, send the bar diameter you intend to use, or ask the shop for the closest standard stock.

Length-to-diameter ratio drives the setup. Up to about 3× diameter, a chuck holds the part well. Between 3× and 6×, expect a tailstock or steady rest and a slower cycle. Past 6×, the part usually needs a traveling steady rest on a Swiss-type machine or a between-centers setup. Thin shafts that pass through a guide bushing run very straight, but soft aluminium can show light bushing marks.

Wall thickness is the other common trap. A Ø60 mm tube with a 1 mm wall will distort when the chuck releases. Soft jaws bored to the finished diameter, or an expanding mandrel inside the bore, spread the clamping force. Even then, a finishing pass of 0.1–0.2 mm radial depth keeps the roundness. If the part is a thin ring, ask for stress-relieved stock.

Undercuts, cross holes and slots push the job toward a mill-turn center. A 2-axis lathe needs a second operation and a fixture, which adds a setup error and a queue. A turn-mill with a subspindle can do both sides in one program. For parts under 200 mm long, the extra machine rate is often paid back by removing one operation and one inspection.

Turning price is mostly setup time plus cycle time. On a simple part, setup is 30–60 minutes. On a part with a fixture, a second operation and a first article inspection, setup can run several hours. That is why the first part is expensive and the hundredth is cheap. When you compare quotes, ask for the setup separately, so you can see what changes if you order 20 instead of 200.

Prototype quantities usually run on a chucking lathe with manual load. At a few hundred parts, a bar feeder with a bar puller makes sense, and the per-part cycle drops. At a few thousand, a Swiss-type machine or a dedicated fixture pays off. The crossover points depend on part length and material, so ask the shop where their step is for your specific part.

No minimum order quantity matters more than it sounds. A shop that will run one part lets you check the design before you commit to a fixture or a die. The same shop can then scale to 10,000+ parts when the design is frozen. If a supplier pushes a large first order, you lose the chance to catch a drawing error cheaply.

Material price moves with the order size and the mill form. Bar stock in standard diameters is cheaper than a custom extrusion. If your part needs a non-standard diameter, consider changing the design to a standard bar size. A 2 mm change in bore diameter can cut material cost and lead time without touching function.

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  Ask for setup separatelyThen you can model the price at 1, 50 and 500 parts.
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  Standard bar sizesRedesign around mill stock where the function allows.
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  Bar feeder thresholdUsually pays off past a few hundred parts on small diameters.
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  One-piece runsUseful for fit checks before a fixture is built.

Certifications filter suppliers by industry. ISO 9001:2015 covers general quality management. IATF 16949:2016 is the automotive standard and comes with PPAP expectations. ISO 13485:2016 is for medical devices and adds traceability and process validation. ISO 27001:2022 covers information security, which matters when you send CAD files and drawings to an outside shop.

Inspection scope should be written into the order. A basic turning order includes raw material check, in-process monitoring and a final inspection. A critical part adds a first article inspection report with actual measured values, not just pass or fail. If your assembly has a stack-up tolerance, ask for the report on the specific dimensions that drive it, so you are not paying for a full CMM report on every feature.

Material traceability is often the missing piece. Ask for the mill certificate with the heat number when the part goes into a regulated product. For 316L stainless or Ti-6Al-4V, the certificate confirms the alloy and the heat treatment condition. Without it, a later failure investigation has no starting point, and you cannot prove what was installed.

Documentation should match the drawing revision. A common failure is a shop running revision B while the customer expects revision C. Send one controlled drawing set, and ask the shop to confirm the revision in the quote. For repeat orders, note the revision on the purchase order. This single habit prevents most rework claims.

As-machined finish is Ra 1.6–3.2 μm and is included in the turning price. Bead blasting, tumbling, brushing and polishing are separate operations with their own queue. Anodizing, plating and powder coating involve a third-party line, so they add days, not hours. If your schedule is tight, split the order: ship the machined parts now and the finished parts later, if the assembly allows it.

Deburring is where cheap quotes hide cost. A sharp edge on a turned thread or a cross hole can fail a functional test or injure an assembler. Specify the edge condition: a 0.2–0.5 mm chamfer, a radius, or a deburr-and-clean note. If the drawing says nothing, some shops will ship a sharp edge, and you will pay for it in rework or in a rejected lot.

Laser marking and engraving need enough space. Minimum character height is 1.5 mm on our machines. If you need a serial number, a date code or a logo in a small area, check the character height before you release the drawing. Engraving on a curved turned surface can look different from a flat test plate, so ask for a sample mark on a scrap part.

Shipping and packaging are part of landed cost. Turned parts with a fine finish need individual wrapping or a compartment tray to avoid nicks in transit. If you buy from a supplier that ships in bulk, budget for touch-up or a higher reject rate at incoming inspection. It is cheaper to pay for a tray than to sort a shipment.