CNC Screw Machining Services: What to Check Before You Order

• 1
  Six things to settle before you send an RFQ
• 2
  Comparing CNC screw machining services on the checks that move cost
• 3
  What CNC screw machining services actually run on
• 4
  Reading a tolerance callout on a turned part
• 5
  Material choice and how it changes the quote
• 6
  What to verify before you award the order
• 7
  Step by step: how to place a turned part order

The name comes from cam-operated screw machines, the mechanical lathes that made screws by the million in the last century. Modern CNC screw machining services run on CNC lathes and mill-turn centers instead. The work is the same family of parts: round, mostly axisymmetric, often under 60 mm in diameter and made in volume.

The important difference is how many operations the part needs. A plain turned spacer is one setup. A shaft with two cross-holes, a milled flat, and a keyway is four features in four directions. On a lathe with live tooling and a C-axis, all four run in one cycle, and the concentricity between the turned diameter and the cross-hole stays under the machine's positioning error.

Send that same shaft to a shop without live tooling and it becomes three setups: turn, mill, deburr. Each setup adds a fixture, a re-clamp, and a chance to lose the true position callout. The quote looks lower per hour, but the total cost is higher once you count fixtures and scrap.

Bar size matters as much as the machine. Bar feeders are set for a range of diameters, and a Ø20 mm part running from a Ø25 mm bar turns 2.5 mm of material into chips on every cycle. That is machine time and tool wear you pay for. A shop that quotes from a bar list sized to your part is usually cheaper on repeat orders.

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  Best fitShafts, pins, spacers, bushings, and fasteners from Ø2 mm to Ø80 mm, in runs from one piece to 10,000+
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  Also workableParts up to 4,000 mm long on the larger turning centers, though bar-fed work is usually shorter
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  Poor fitThin-wall tubes that deflect under chuck pressure, and parts with deep internal bores that need a boring bar longer than 4× diameter
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  Wrong processFlat plates with no turning axis. That is milling or sheet metal, not screw machining.

A drawing that says ±0.005 mm everywhere is not a better drawing. It is an expensive one. On a lathe, the diameter you turn is controlled by the tool and the slide, and holding ±0.005 mm on a 30 mm diameter is routine. Holding the same band on a length measured from a soft face, or on a slot milled by a small end mill, is a different problem.

The practical split is this: diameters, shoulders, and bores that locate the part get the tight callout. Lengths that only need to clear a mating part get ±0.05 mm or a general tolerance block. Cross-holes get true position, not a diameter tolerance. When you sort features this way, the quote drops and the shop stops asking for tolerance waivers.

Surface finish is the same story. Ra 0.8–1.6 μm comes off a normal turning insert with a correct feed rate. Ra 0.2–0.8 μm needs a wiper insert, a slower feed, or a secondary operation. If the mating surface is a press fit or an O-ring seal, the finish matters. If the surface just needs to look clean after anodizing, Ra 1.6–3.2 μm is enough.

One more item: datum selection. If the drawing has no datum, the shop picks one, and the inspection report will be measured against a setup you did not intend. Mark the datum on the drawing and note which features are inspected from it.

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  Hold tightTurned diameters, bores, and shoulders that set fit or runout
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  Hold looseOverall lengths, chamfer sizes, and non-mating faces
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  Finish by functionSealing and bearing surfaces get Ra 0.2–0.8 μm; cosmetic and clearance surfaces get Ra 1.6–3.2 μm
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  Always call the datumIt sets how the part is clamped, measured, and reported

Free-machining grades exist for a reason. Brass C36000 and stainless 303 cut fast and leave a good finish, which is why they show up in fittings and instrument parts. Stainless 304 and 316 machine slower, work-harden if the feed is too light, and cost more per part even though the bar price is close.

Aluminium is the volume material for housings and spacers. 6061-T6 is the default, 7075 when you need strength, and 2024 when the part flies. Titanium TC4 (Ti-6Al-4V) and Inconel are used for high-temperature or high-strength parts, and both need slower speeds, more tool changes, and a shop that has run them before.

Plastics turn easily but move after machining. POM and PA are dimensionally stable enough for most parts. PEEK holds up at temperature but costs several times more than POM. If the part has a tight tolerance across a wide temperature range, say so on the RFQ, because the shop may need to rough, stress-relieve, and finish.

Volume changes the process, not just the price. A one-piece prototype is turned from bar on a lathe with a manual load. A 10,000-piece run uses a bar feeder, a fixed cycle, and often a second spindle for back-side work. Ask which setup your quote assumes. A quote priced for prototypes will look high on volume, and a volume quote may carry setup you do not need for one piece.

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  Fast and cleanBrass C36000, stainless 303, aluminium 6061-T6
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  Slower but commonStainless 304, 316, 17-4PH, steel 1045 and 4140
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  Special handlingTc4 titanium, Inconel, magnesium AZ31B, beryllium copper
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  Watch the warpPEEK, PA, and thin-wall aluminium parts need stress relief between rough and finish

Start with the certificate scope, not the logo. A supplier can hold ISO 9001:2015 and still have the scope written around assembly. Ask for the certificate and read the scope line. If your parts go into a vehicle, you want IATF 16949:2016. Medical devices need ISO 13485:2016. If you are sending proprietary drawings, ISO 27001:2022 covers information security, and an NDA should be signed either way.

Then look at machine count and mix. A shop with 127 high-precision CNC machines, including 16 simultaneous 5-axis centers, 16 mill-turn centers, and 12 four-axis mills, can move a job between machines when one goes down. A shop with three lathes cannot. That matters most on jobs with a fixed ship date.

Inspection is the next filter. 100% inspection before shipment, with raw material check, in-process monitoring, and final inspection, is the floor for turned parts that go into an assembly. Ask what the report contains: dimensions measured, instrument used, and whether CMM data is available on request.

Finally, ask how the shop handles a change. If a feature cannot be held as drawn, the DFM feedback should come back before the chips fly, not after the first article. Quotation and free DFM analysis within 12 hours is a reasonable target, and it tells you how the rest of the project will go.

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  Scope over logoISO 9001:2015, IATF 16949:2016, ISO 13485:2016, ISO 27001:2022, each read for scope
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  Machine redundancyEnough lathes and mill-turn centers to reroute a job without a schedule slip
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  Inspection data100% inspection before shipment, reports on request, not on demand after a complaint
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  DFM before cuttingFeedback on tolerance and feature risk before the first article, not after