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FDM vs CNC Machining

FDM and CNC machining are two different ways to make a precision part, one by depositing plastic layer by layer and one by cutting it away from solid stock, and each holds a different accuracy depending on the process, the material and the machine.

How accurate is CNC machining compared to industrial FDM?

At a 6 to 30 mm feature, CNC in metal holds about plus or minus 0.10 mm while industrial FDM's own floor sits at about 0.3 mm. That comparison comes from Protolabs Network's manufacturing standards page, which publishes the ISO 2768 tables for CNC alongside its own FDM and injection molding figures.

What does the full ISO 2768 CNC tolerance table look like, by size?

Protolabs Network publishes the ISO 2768-f table for CNC metal parts and the ISO 2768-m table for CNC plastic parts, both banded by the size of the feature being toleranced. The plastic table is looser than the metal one at every size band shown.

Nominal size band (mm)CNC metal, ISO 2768-f (mm)CNC plastic, ISO 2768-m (mm)Provenance
0.5 to 60.050.1Protolabs Network, Manufacturing Standards
6 to 300.10.1Protolabs Network, Manufacturing Standards
30 to 1200.150.2Protolabs Network, Manufacturing Standards
120 to 4000.20.3Protolabs Network, Manufacturing Standards
400 to 10000.30.5Protolabs Network, Manufacturing Standards
1000 to 20000.50.8Protolabs Network, Manufacturing Standards
ISO 2768-f (CNC metal) and ISO 2768-m (CNC plastic) tolerance by size band, per Protolabs Network

How does that compare with industrial FDM and injection molding on the same page?

The same Protolabs Network page states industrial FDM holds about plus or minus 0.3 percent of the dimension with a floor near 0.3 mm, injection molding holds about plus or minus 0.250 mm, and CNC's own surface finish there is stated as about 3.2 micron Ra. Every one of those three processes carries its own separate tolerance formulation, not a shared one.

What does Protolabs' own CNC machining page state for precision work?

Protolabs states a standard CNC tolerance of plus or minus 0.005 in, a precision tolerance option down to 0.002 in, and reamed holes held to 0.0005 in. Those are Protolabs' own published figures for its own machining process, not a claim about every CNC shop.

Does every bureau publish the same CNC tolerance?

No: Xometry states a flatter, simpler CNC tolerance of plus or minus 0.005 in for metals and 0.010 in for plastics, without the size-banded ISO tables Protolabs Network publishes. Two bureaus, two different ways of stating the same kind of number.

What is the minimum wall CNC machining can hold?

Protolabs Network states a CNC minimum wall of about 0.75 mm, thinner walls risk chatter or breakage during machining before the tool ever finishes the cut.

So which process should I use for a tight-tolerance part?

FDM parts typically hold about plus or minus 0.5 percent, with a floor near plus or minus 0.5 mm. We do not certify a tolerance number on a quote.

No CMM, no dimensional reports, no PPAP, no first article inspection. If your part needs a measured report to be accepted, we are not the right supplier.

What this page does not tell you

  • This page compares published tolerance figures, not a guarantee for a specific part. A feature's actual accuracy also depends on fixturing, tool wear and the machine running the job.
  • The ISO 2768 tables above are size bands for a general feature, not a promise about every geometry, and CNC accuracy on a deep pocket or a thin wall can fall outside them.
  • The bureau figures on this page describe their own published machining processes. They are not a claim about every CNC shop or every FDM printer.

When to print a test coupon instead

Get a real quote for your part before assuming which process meets your tolerance. Send your file and the features that matter and we will tell you what FDM can and cannot hold for that part. A person reviews your request and follows up by email.

Sources

  • Manufacturing Standards, Protolabs Network

    States an FDM prototyping tolerance of 0.5 percent of the dimension with a lower limit of 0.5 mm, and an industrial tolerance of 0.3 percent of the dimension with a lower limit of 0.3 mm.

  • CNC Machining Precision Tolerances, Protolabs

    States a standard CNC tolerance of plus or minus 0.005 in, a precision tolerance option of 0.002 in, and reamed holes held to 0.0005 in.

  • Manufacturing Standards, Xometry

    States FDM tolerance as one build layer thickness for the first inch of a dimension, then 0.002 in for every inch after that. Layer thickness varies by machine: 0.010 in on a Fortus 450, 0.013 in on a Fortus 900, 0.008 in on a desktop machine.

  • 3D Printing vs CNC Machining, Protolabs Network

    States a rule of thumb that 3D printing is the cheaper process under about 10 units, CNC machining wins from about 250 to 500 units and up, and injection molding wins from about 500 identical units and up. Also states a CNC minimum wall of about 0.75 mm.

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Published 5 September 2026.