What Tolerances Can CNC Machining Actually Hold, and Why Does It Matter for Your Parts?

When an engineer specifies a tolerance on a drawing, the number usually comes from one of two places: a genuine functional requirement calculated from how the part must fit and perform, or a default carried over from a previous drawing that nobody questioned. The difference matters enormously — not because tight tolerances are hard to hold, but because they cost significantly more to hold, and paying for precision you do not need is a waste that compounds across every part in a production run.

Understanding what CNC machining can realistically achieve, what drives the cost of tighter tolerances, and how to match your tolerance specifications to your actual functional requirements is one of the most practical things a product engineer or procurement manager can do to improve both part quality and total production cost.

What Does "Tolerance" Actually Mean on a CNC Part?

A tolerance is the permitted range of variation around a target dimension. A shaft specified as 20.00mm ±0.05mm can measure anywhere from 19.95mm to 20.05mm and still pass inspection — a total band of 0.10mm. Any part outside that band is non-conforming.

Tolerances appear on drawings in several forms. Bilateral tolerances (±0.05mm) allow equal deviation in both directions. Unilateral tolerances (+0.00/−0.10mm) allow deviation in one direction only — used when a shaft must not exceed a maximum diameter for clearance fit reasons, for example. Limit tolerances state the upper and lower acceptable values directly (19.90/20.00mm). And title block general tolerances — typically ±0.1mm or ±0.2mm — apply to every dimension that is not individually toleranced on the drawing.

For most parts, the majority of dimensions should carry general tolerances. Only the features that directly control fit, function, or assembly — bearing seats, mating bores, sealing surfaces, locating pins, and holes — justify the cost of individually specified tight tolerances.

What Standard CNC Machining Can Hold Routinely

A well-maintained CNC machining center or CNC lathe operated in a temperature-controlled environment, running standard tooling and fixturing, can achieve the following without special processes:

These figures represent what a competent shop can achieve consistently across a production batch — not what is achievable on a single carefully-made prototype with full attention. Consistency across a production run is what actually matters for parts that go into assemblies.

The ISO IT Tolerance Grade System

The ISO system classifies tolerance tightness into standardized grades called IT grades (International Tolerance), numbered from IT01 (the most precise, used in master gauge manufacture) through IT18 (rough structural applications). For CNC-machined parts, the practical range is IT5 through IT11.

The tolerance value for each IT grade scales with the nominal dimension — IT7 on a 10mm shaft is ±0.007mm; IT7 on a 100mm bore is ±0.018mm. The full tables are defined in ISO 286-1 and cover dimensions from 1mm to 3,150mm.

When Do Tighter Tolerances Require Special Processes?

Standard CNC turning and milling can achieve IT7–IT8 on most features. Going tighter than that — into IT5–IT6 on shaft or bore diameters — requires additional operations that add high cost and lead time:

Cylindrical grinding is used for external diameter tolerances of ±0.003–0.008mm (IT5–IT6). The part is rough-turned on the CNC lathe, then transferred to a cylindrical grinding machine for a final abrasive finishing pass that removes very small amounts of material with high geometric precision. Grinding requires dedicated equipment and an extended cycle time compared to turning.

Reaming achieves ±0.008–0.015mm on bore diameters using a multi-edged rotary cutting tool that follows an existing drilled or bored hole and removes a precise amount of material in a single pass. Reaming is faster and less expensive than internal grinding for moderate bore precision requirements and is the standard method for precision hole diameters in the IT6–IT7 range.

Honing uses abrasive stones in a reciprocating and rotating motion inside a bore to achieve ±0.003–0.008mm diameter tolerance with exceptional surface finish (Ra 0.2–0.4 μm). It is the standard process for hydraulic cylinder bores, engine cylinder bores, and any bore where both dimensional precision and low surface roughness are required simultaneously.

Why Tolerances Affect Cost Non-Linearly

The relationship between tolerance tightness and cost is not linear — it escalates rapidly below a certain threshold. Moving from ±0.1mm to ±0.05mm on a turned diameter adds minimal cost: the machine can achieve this with standard operation. Moving from ±0.05mm to ±0.01mm requires slower cutting speeds, more inspection passes, and possibly 100% dimensional measurement instead of sampling — perhaps a 30–50% cost increase for that feature. Moving from ±0.01mm to ±0.003mm requires grinding operations, precision fixturing, temperature-controlled inspection, and calibrated gauging — potentially a 200–400% cost increase for that single diameter dimension.

The practical implication is straightforward: for every tight tolerance on a drawing, ask whether the function of the part genuinely requires that precision. In most parts, the features that truly need tight control are a small fraction of the total dimensions. Applying ±0.01mm to every dimension on a drawing because it "seems precise" increases cost dramatically for no functional benefit. Identifying the three or four dimensions that actually control fit and performance, specifying tight tolerances on those, and leaving everything else at general tolerance (±0.1mm or ±0.2mm) produces a part that performs identically to a tightly-toleranced version at a fraction of the cost.

Geometric Tolerances: Size Is Not Enough

A dimension tolerance controls how big or small a feature is. It does not control its shape. A shaft diameter that measures correctly at every cross-section might still be bent along its length, or oval rather than round at a given cross-section — both of which would cause problems in a bearing seat despite passing a diameter check.

GD&T (Geometric Dimensioning and Tolerancing, per ISO 1101) provides additional controls for shape, orientation, and position:

• Flatness: Controls how flat a surface is — the entire surface must fit between two parallel planes separated by the specified tolerance. Critical for sealing faces, mounting surfaces, and any face that must mate squarely with another component.
• Cylindricity: Controls both the roundness and straightness of a cylindrical surface simultaneously — the most stringent form control for shafts and bores that seat bearings or seals.
• Perpendicularity: Controls the squareness of a surface or axis relative to a datum. Essential for machined flanges, bolt hole patterns, and any feature where angular deviation from 90° causes assembly misalignment.
• True position: Controls the location of a feature (typically a hole or boss) relative to a datum reference system. The most important GD&T control for holes that must align with mating components — bolt hole patterns, dowel holes, bearing housings.
• Runout: Controls the variation of a rotating surface relative to a datum axis. Essential for shafts, hubs, and any rotating component where runout causes vibration, premature bearing wear, or seal leakage.

For buyers without deep GD&T experience, the practical guidance is to identify the features on your part that must align with mating components and ask your machining supplier whether position tolerances on those features would be more effective than relying on individual dimension tolerances alone. A competent CNC machining supplier can advise on GD&T specification as part of the design-for-manufacturing review process.

How to Talk to Your Machining Supplier About Tolerances

The most productive tolerance conversation with a CNC machining supplier starts before the drawing is finalized. Key questions to raise:

• Which tolerances on this drawing are driving high cost, and are there alternative approaches that achieve the same function at lower cost?
• What is your standard general tolerance for milled and turned features? (This tells you what the title block tolerance should specify.)
• For the tightest tolerances on the drawing, what process will you use — standard CNC, reaming, or grinding? Is that process in-house or subcontracted?
• What inspection method will you use for the critical dimensions — CMM, air gauging, or optical comparator? What is the measurement uncertainty?
• Is 100% inspection included in the quoted price for critical dimensions, or is sampling the default?

A supplier who engages substantively with these questions — who can tell you which tolerances are expensive to hold and suggest alternatives — is a supplier who understands their own process capability and will deliver consistent results across a production run.

Precision CNC Machining from Suzhou Heimat

Suzhou Heimat Precision Machinery Co., Ltd. provides CNC precision machining for automation equipment, medical devices, aerospace, and energy industry components. Standard tolerance capability of ±0.01mm on critical features; IT6–IT7 bore and shaft tolerances available with in-house grinding and reaming operations. CMM inspection with full dimensional reporting. Material traceability documentation and first-article inspection are available on request.

Send us your drawing and tolerance requirements to discuss process capability and get a quotation.

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