What Precision Tolerance Can Be Achieved in Various Machine Processing

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  Tolerance Is a Number With Conditions Attached
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  3-Axis Milling: Where Most Tolerance Work Happens
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  4-Axis and 5-Axis: Fewer Setups, Tighter Position
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  Turning and Mill-Turn: Round Parts Hold the Tightest Numbers
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  Practical Tolerance by Process

A drawing that says ±0.05 mm on every dimension is easy to quote and easy to make. A drawing that says ±0.005 mm on every dimension is not, because the tolerance now competes with thermal expansion, tool wear and the repeatability of the machine itself. When engineers ask what precision tolerance can be achieved, the useful answer is always a range plus the conditions that make it real.

Three things set the floor. First, machine geometry: spindle runout, axis straightness and the resolution of the linear scales. Second, the number of setups, because every re-fixture adds its own error stack. Third, the feature type. A bored hole on a lathe holds tighter than a pocket floor milled on a long, thin tool.

At GreatLight we quote ±0.005 mm as the tightest practical shop tolerance on critical features, with ±0.0002 in for imperial drawings. That is not a blanket promise for every dimension on every part. It is the level we can hold when the feature, the setup plan and the inspection method all support it.

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  Process sets the floorGrinding and turning hold tighter than milling on comparable features.
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  Setups set the stackEach re-fixture adds positional error.
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  Feature sets the riskDeep pockets, thin walls and long tools are the hard cases.

A 3-axis mill cuts from one direction. The part sits on a vise or fixture, the spindle moves in X, Y and Z, and every feature that faces the tool is reachable. This geometry is stiff and predictable, which is why 3-axis work is the cheapest way to hit tight numbers on flat and prismatic parts.

Practical tolerance on a 3-axis mill is ±0.01 mm on position and ±0.02 mm on general dimensions, tightening to ±0.005 mm on a boring operation with a rigid setup and a pre-drilled hole. Surface finish lands at Ra 1.6–3.2 μm as machined, and Ra 0.8–1.6 μm with a finishing pass and sharp tooling. Those numbers assume aluminium or mild steel, a tool with reasonable length-to-diameter ratio, and a warm machine.

The failure mode is reach. Five sides of a cube require five setups on a 3-axis machine, and each setup reintroduces the vise, the parallels and the operator's dial indicator. Position error grows with each flip. If a part has features on four or more faces and a true-position callout under ±0.02 mm, 3-axis is the wrong process even though the machine can physically cut every face.

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  Good fitPlates, brackets, housings with features on one or two faces.
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  Holds best onBored holes, flat faces, slots with a rigid setup.
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  Watch out forMany setups, deep cavities, thin floors.

A 4-axis mill adds a rotary table to the 3-axis stack. The part rotates around one axis, so features on the fourth face are cut without re-fixturing. Positional error between those faces drops because there is no second vise setup. A Ø400 mm rotary table is typical, and index positioning repeats well enough for ±0.01 mm true position across the rotated faces.

A 5-axis machine adds a second rotary axis, so the tool can approach the part from almost any direction. That matters for two reasons. Complex geometry with undercuts and curved surfaces can be cut in one setup, and the tool can be kept short and stiff because the table does the reaching. Short tools deflect less, and less deflection means the tolerance on the finished surface is actually achievable rather than theoretical.

GreatLight runs 16 simultaneous 5-axis machining centers, 12 four-axis mills and 27 three-axis machines. The tolerance gain from 5-axis is not that the machine is more accurate on a single straight cut. It is that the whole part is made in one coordinate system, so the stack of setup errors disappears. On a part with angled faces, that is often the difference between ±0.02 mm and ±0.005 mm.

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  4-axis wins onCylindrical parts with milled flats, cross-holes, slots.
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  5-axis wins onAngled faces, undercuts, contoured surfaces, deep pockets.
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  Both cutPosition error by removing setups, not by magic.

Turning is a continuous cut on a rotating workpiece, and the geometry is closed and stiff. There is no interrupted engagement and no long cantilever tool. That is why a lathe holds diameter tolerance more easily than a mill holds a width. Practical turning tolerance is ±0.01 mm on diameter, and ±0.005 mm on a finished bore or journal when the machine is warm and the insert is fresh.

Roundness and concentricity are where turning separates itself. A turned diameter can hold 0.005 mm circularity without special effort. Turned surface finish reaches Ra 0.8–1.6 μm routinely, and Ra 0.2–0.8 μm with a wiper insert, correct feed per revolution and a rigid setup. Feed rate matters more than speed here: halving feed per revolution roughly halves the theoretical roughness.

Mill-turn centers combine both. A shaft with cross-holes, flats and a threaded end can be finished without leaving the spindle, so the concentricity between the turned diameter and the milled feature stays tight. For parts that are mostly round with a few milled details, this is usually the most accurate and fastest path.

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  Best atDiameters, bores, journals, threads, tapers.
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  Hold 0.005 mm onCircularity and concentricity, not just size.
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  Mill-turn forRound parts with off-axis milled features.