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If you are sourcing turned or milled components, this is probably the first question on your mind. The short answer: a well-maintained cnc turning and milling machine routinely holds ±0.05 mm (±0.002") on general features, a precision setup reaches ±0.01 mm, and a high-end turn-mill center running in a temperature-controlled shop can hold ±0.002–0.005 mm on critical dimensions. Which of those numbers applies to your part depends on the process, the material, the geometry, and how the job is set up — so let’s break it down properly.
Before going deeper, here is a realistic reference table. These are the values most machine shops work to every day, not laboratory best cases:
| Process | Standard Tolerance | Precision Tolerance |
|---|---|---|
| CNC turning (lathe) | ±0.02–0.05 mm | ±0.005–0.01 mm |
| CNC milling (3-axis) | ±0.05–0.1 mm | ±0.01–0.02 mm |
| 4-axis / 5-axis milling | ±0.02–0.05 mm | ±0.005–0.01 mm |
| Turn-mill compound machining | ±0.02–0.05 mm | ±0.005–0.01 mm |
| Surface grinding (secondary op) | ±0.005–0.01 mm | ±0.002–0.005 mm |
Anything tighter than roughly ±0.005 mm usually stops being a “machine capability” question and becomes a “process control” question — involving thermal stability, in-process gauging, tool wear compensation, and often a finishing pass on a grinder.
The phrase covers two things, and it is worth knowing the difference. A conventional setup runs a part on a lathe for turned features, then moves it to a milling machine for flats, slots, and cross holes. A turn-mill compound center (also called a mill-turn or multi-tasking machine) combines both in one enclosure: the main spindle turns the part while live tooling mills features, often with a sub-spindle to hand the part off for back-side work.
Why does that matter for tolerance? Because every time a part is re-fixtured, you introduce a small locating error. A turn-mill machine completes the part in a single clamping, so the concentricity between a turned diameter and a milled cross-hole pattern — a classic headache feature — stays far tighter than any two-machine workflow can achieve.
Turning is naturally the more accurate of the two processes. The workpiece rotates around its own axis, so diameters are generated symmetrically and the cutting force is relatively constant. A production lathe holds ±0.02–0.05 mm all day on shaft and bore diameters. With a dedicated finishing tool, sharp inserts, and in-process gauging, ±0.005–0.01 mm is achievable on critical fits.
Length dimensions are a different story. Facing and shoulder positions depend on the machine’s Z-axis repeatability and on how consistently the raw bar stock is cut, so axial tolerances typically run one grade looser than diameters on the same part. Long, slender shafts also deflect under cutting pressure; once the length-to-diameter ratio climbs past about 10:1, expect to loosen the tolerance or add a tailstock/steady rest.
Milling removes material with a rotating cutter on a stationary part, which introduces more variables: tool deflection, spindle runout, and the rigidity of the setup all show up in the final dimension. A 3-axis machining center holds ±0.05–0.1 mm in standard production and ±0.01–0.02 mm when the job is set up for precision work.
Adding a 4th or 5th axis does not automatically mean tighter numbers — it means the machine can reach more faces in one setup. The tolerance gain comes from eliminating re-fixturing: features machined across multiple faces of a part keep their true position and perpendicularity far better when the part never leaves the vise. For complex housings and manifolds, that single-setup advantage is usually worth more than the raw axis accuracy itself.
Two shops with identical machines will quote different achievable tolerances. The difference is almost never the iron — it is everything around it:
A ± tolerance controls size, but not shape. A shaft can measure Ø20.00 mm at every point and still be bowed enough to destroy a bearing. That is why functional parts also carry geometric callouts — flatness on sealing faces, cylindricity on bearing bores, true position on bolt patterns, and runout on anything that spins. On a capable turn-mill machine, geometric tolerances of 0.01–0.03 mm are realistic; below that, you are again in grinder or honing territory.
A practical tip: if your drawing uses ISO 2768-m as the general class, individual critical features still need explicit callouts. The general class covers ±0.1 mm on small features and loosens quickly as dimensions grow — fine for brackets, not fine for a press-fit bore.
Every step tighter costs money, and the climb is steep. Moving from ±0.05 mm to ±0.01 mm might add 20–40% to a part’s price through slower feeds, extra finishing passes, and more inspection time. Going below ±0.005 mm can easily double it, because now the job involves climate control, CMM verification on every piece, and often a grinding operation. Over-tolerancing a non-critical feature is one of the most common ways drawings waste budget.
The sensible approach: apply tight tolerances only where the function demands them — fits, sealing surfaces, alignment features — and let the general class cover everything else. A good machining partner will flag over-toleranced features during the DFM review and suggest where you can relax the spec without any functional risk.
At ANOK Precision Manufacturing, precision cnc machining is built around exactly these principles. Our turning department runs nearly 15 CNC lathes — including turn-mill compound machines for single-clamping parts — holding tolerances down to ±0.002 mm on critical dimensions, on parts up to 520 mm in diameter and 3600 mm long. The milling side covers 3-axis, 4-axis, and 5-axis machining centers with the same ±0.002 mm capability, and our surface grinding and WEDM departments pick up wherever cutting tools reach their limit.
We are ISO 9001:2015 certified and machine everything from aluminum 6061 to titanium Ti-6Al-4V, Inconel, and PEEK, for medical, aerospace, automation, and other demanding industries. If you have a drawing with cnc machining tight tolerance parts and want an honest answer on what is achievable — and where you can save money by relaxing what is not — send it over. Our engineers will come back with a DFM review and a quote, not just a number.