Tolerance of Each CNC Part: How a ±0.05 mm Window Is Held

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  Key takeaways
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  What the tolerance of each CNC part actually means
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  Where the ±0.05 mm window comes from on the machine
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  Which features hold the tolerance and which fight it
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  Material behavior inside a 0.05 mm band
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  Setup, probing and the first-article check
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  Feature by feature: what ±0.05 mm costs you

A tolerance is the allowed spread between the largest and smallest acceptable size of a feature. When a drawing says ±0.05 mm, the total band is 0.10 mm wide. A 20.00 mm bore may measure anywhere from 19.95 mm to 20.05 mm and still pass. That is different from a limit tolerance such as 20.00/19.95 mm, which sets the same band but pins both ends.

Where the band is measured matters. A caliper reading at the top of a bore tells you less than a CMM scan along the full axis. Roundness, taper and straightness live inside the same band. A bore can pass on diameter and still fail on cylindricity, so the drawing needs to say which one rules.

The band applies at 20 °C. Aluminum grows about 23 μm per meter per degree Celsius. A 300 mm part measured at 28 °C reads roughly 0.03 mm larger than it will at 20 °C. On a ±0.05 mm window, that is more than half the budget spent on temperature alone.

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  Bilateral ±0.05 mmBand is 0.10 mm wide, split evenly around nominal.
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  Unilateral +0.05/0 mmBand is 0.05 mm wide, all on one side.
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  GD&T positionA Ø0.10 mm zone controls location and size together.

The number does not come from the control screen. It comes from a stack of small errors: spindle thermal growth, ballscrew pitch error, tool wear, fixture deflection and material springback. Each one is small. Together they decide whether the part lands inside the band.

A 3-axis mill with a good vise can hold ±0.05 mm on a 100 mm aluminum plate without drama. Move to a 4,000 mm long steel rail and the same machine loses the argument. The tool pushes off, the part moves, and the error scales with length.

Our 5-axis centers are rated to ±0.005 mm, ten times tighter than the band being discussed. That headroom is on purpose. It absorbs the errors that appear once real metal, real fixtures and a real shop floor enter the picture.

Roughing and finishing should never share one pass. A 0.3 mm finish pass after a 3 mm roughing cut removes the deflection left behind. Skip it and the wall springs back a few hundredths of a millimeter, right at the edge of the band.

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  Thermal growthLet the spindle warm up 20–30 minutes before the finish pass.
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  Tool wearCompensate every 20–30 parts on long runs.
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  Fixture rigidityThin walls need support, not more clamping force.

Feature geometry decides more than the machine does. A bore machined with a boring head on a rigid setup holds ±0.05 mm reliably. It is a single-point cut, the tool is stiff, and the diameter is set by the head, not by tool pressure.

A thin wall is the opposite case. A 1 mm aluminum wall at 40 mm tall bends under the cutter and springs back after it passes. The measured size depends on when you measure it. Rough it, let it relax, then take a light finish cut from both sides.

Deep pockets and long overhangs behave the same way. A tool sticking out 5× its diameter deflects, and the floor of the pocket comes out tapered. Shorten the gauge length or step down in smaller increments. A 0.2 mm stepover on the floor keeps the load steady.

Holes below Ø1 mm and slots below 0.8 mm wide sit in a different category. Micro tools break before they hold a tight band. Expect to drill undersize, then ream or interpolate, and budget a few extra parts for tool breakage.

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  Holds easilyBores, faces, shoulders, slots over 1 mm wide.
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  Holds with careThin walls, deep pockets, tall ribs.
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  Needs a planMicro holes, thin floors, mirror finishes.

Aluminum 6061 machines clean and holds ±0.05 mm on most features. It also moves more than steel when the temperature shifts, so a part finished hot can measure small the next morning. Let it cool before the final check.

Stainless 304 and 316 work-harden. A dull tool rubs the surface, hardens it, and pushes the next pass off size. Keep the feed per tooth up and the tool sharp. 17-4PH in the H900 condition cuts more predictably than annealed stock, but the heat treat step adds its own size change.

Titanium Ti-6Al-4V and Inconel are the hard cases. They hold heat at the cutting edge, so the tool grows and the part grows with it. Take lighter cuts, use high-pressure coolant, and plan a spring pass. On Inconel, ±0.05 mm is realistic on a bore but optimistic on a long thin wall.

Plastics are a separate world. POM and PEEK move with moisture and temperature far more than any metal. A POM part measured right off the machine can shrink 0.2% as it cools. For a ±0.05 mm band on a 100 mm plastic part, that shrinkage is the whole budget.

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  AluminumFast, stable, but check it after it cools.
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  StainlessSharp tools and constant feed prevent work hardening.
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  Titanium and InconelLighter cuts, more coolant, more time.

Every extra setup adds error. Flip a part three times and the stack-up grows with each datum change. Where the design allows, keep critical features on one face and machine them in one operation.

Probing on the machine catches drift before it becomes scrap. Touch off the datum, cut a test feature, measure it in place, and shift the offset. On a 50-part run this takes minutes and saves the batch.

The first article sets the tone. We cut one part, measure every toleranced feature, and compare the numbers against the drawing before the run starts. If the first article drifts, the offset is corrected then, not after 40 parts are in the bin.

Final inspection happens at 20 °C on calibrated equipment. Every part gets checked before shipment, and reports are available on request. A tolerance that is claimed but never measured is just a number on paper.

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  One face, one operationFewer datums, less stack-up.
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  In-process probingCatch drift on part 3, not part 40.
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  First-article reportNumbers before the run, not after.