CNC machining ISO-based tolerances & finishes

CNC machining is great at making parts that are accurate, repeatable, and ready for assembly. But "accurate" is meaningless without defining acceptable variation, and that's where tolerances come in.

ISO 2768-1 sets baseline CNC machining tolerances for linear and angular dimensions. These standards help prevent parts that don’t fit, leak, or assemble as intended, while keeping costs under control. The goal isn’t “as tight as possible.” It’s as tight as necessary for the part to work.

In this guide, we’ll break down how ISO 2768-1 works, what Protolabs Network applies by default for metals and plastics, when to call out custom tolerances, and how finishing can affect final dimensions. You can also download a printable tolerance and finish chart for quick reference.

What are ISO 2768-1 CNC tolerances?

ISO 2768-1 defines default tolerances for dimensions that don’t have an explicitly specified tolerance.

The standard uses four tolerance classes:

• F (fine): Tightest default limits for smaller, precision-critical features

• M (medium): Common general-purpose default

• C (coarse): More variation allowed for non-critical features

• V (very coarse): Widest default limits, typically for large, forgiving features 

To apply ISO 2768-1, specify the tolerance class (e.g., "ISO 2768-m") in the title block of your technical drawing.

For most engineering applications, ISO 2768-1 provides enough accuracy without the cost premium of tighter specs. It's widely used across industries, making it easier to work with manufacturers globally without extensive documentation for standard features.

Smaller features usually get tighter tolerances, while larger dimensions allow more variation, reflecting the realities of machining at different scales.

Protolabs Network's CNC tolerances

At Protolabs Network, we follow a specific set of manufacturing standards. By default, we machine metal parts to ISO 2768-f (fine) and plastic parts to ISO 2768-m (medium) unless your technical drawing specifies otherwise. Here's what that means in practice:

*For nominal sizes below 0.5mm, please clearly indicate tolerances on your technical drawings.

CNC ISO 2768-1 decision tree

Use this quick guide to decide whether ISO defaults are enough.

• Is the feature fit- or tolerance-critical? (fits, seals, bearing seats, sliding interfaces)

  • No → ISO 2768-1 standards are usually sufficient.

  • Yes → Call out a custom tolerance and attach a technical drawing to your quote request.

• Will the feature be coated or finished? (anodize, plating, powder coat, polishing)

  • Yes → Specify whether the tolerance applies before or after finishing (e.g. “Ø20.00 ±0.02 mm after anodizing”) and include it in your drawing.

• Is a geometric relationship more important than size? (position, runout, perpendicularity)

  • Yes → Consider using GD&T instead of tightening every dimension.

• Not sure which approach is best?

  • Contact your account manager or networksales@protolabs.com.

How surface finishes affect tolerances

Finishing can change dimensions. For fit-critical features (bores, shafts, sealing faces), account for finishing in your tolerance stack and specify whether dimensions apply before or after the process.

Standard CNC finish is “as machined” (Ra 3.2 µm / 126 µin). Additional finishes affect tolerances differently:

• Removal finishes like polishing and brushing remove material.

• Build-up finishes like powder coating, electroless nickel plating, and aluminum anodizing add thickness.

• Abrasive finishes like bead blasting mainly change texture but can still matter on mating/sliding surfaces.

See our surface finishes overview for complete details on how each finish impacts tolerances.

Printable ISO tolerance and finish chart (PDF)

This single-page reference chart includes both ISO 2768-1 tolerance specifications and common surface finish Ra values. Keep it handy at your workstation or share it with your team for quick reference during design reviews.

Download the printable chart

Custom tolerances

ISO 2768-1 defaults cover most features, but you’ll want custom tolerances when function depends on a controlled fit or alignment, such as:

• Fits and sealing surfaces (press/slip/interference, bores/shafts)

• Alignment-critical features (bearing seats, datum-to-datum relationships)

• Precision mating to off-the-shelf components

To request tighter tolerances, attach a technical drawing with your quote request and clearly call out the tolerance-critical features. Tighter tolerances usually mean more process control and inspection (and higher cost), so apply them only where they’re functionally necessary.

For more detail, see this help center page about attaching technical drawings for CNC machined parts.

What to call out

• The specific dimensions that exceed the standard tolerances

• Any finish requirements that affect fit (and whether dimensions are pre- or post-finish)

• Any internal/external threads (threads follow their own ISO standards, so always call them out on the drawing)

For the full standards reference, see our manufacturing standards. If you need help, contact your account manager or networksales@protolabs.com.

Key takeaway: Use ISO 2768-1 defaults for most features, then add custom tolerances only where function depends on fit, finish, or geometric relationships.

Milling vs. turning for tolerance-critical features

The tolerance you can achieve depends partly on the CNC machining process you’re using. Both CNC milling and CNC turning can produce high-accuracy parts, but they’re better suited to different feature types.

Key takeaway: For cylindrical tolerance-critical features (shafts, bores, bearings), turning typically achieves tighter tolerances more cost-effectively. For features requiring positional accuracy across multiple surfaces or complex 3D relationships, milling is the better choice.

Material considerations

Different materials respond differently to machining, which is why we apply ISO 2768-f to metals and ISO 2768-m to plastics.

Metals are more dimensionally stable and predictable during machining, which makes tighter tolerances practical. Plastics are more sensitive to heat and moisture and can warp or creep after machining so tight tolerances are less predictable, and often more expensive.

Where to learn more about standards

• Manufacturing standards: Tolerances, finishes, and process rules across CNC and other manufacturing processes.

• Specifications policy: How we interpret CAD vs. technical drawings, and what happens if they conflict.

• Quality assurance: Inspection methods, documentation, and how quality is handled across the manufacturing network.

Get a quote

Upload your CAD files and get a free, instant quote on CNC parts that hit the exact tolerances you need, whether you’re working to ISO 2768-1 defaults or calling out custom requirements.