CNC Machining Tolerances: When ISO 2768 Is Enough

CNC machining tolerances set the cost of a part long before material choice does, so start with ISO 2768-m and tighten only where a feature actually mates. Most plate work, brackets, and housings need nothing finer than ±0.1 mm on milled dimensions; a press-fit bore or bearing seat is where the real money begins.

Typical shop capability is ±0.1 mm on milled features in 6061-T6 and ±0.05 mm on turned diameters in 303 stainless. Going below that means reaming, jig boring, or grinding, plus inspection time on a CMM. Wall thickness under 1 mm in aluminum, or thin POM/Delrin sections, may deflect during clamping and need lighter fixtures and spring passes — that is a process cost, not a tolerance cost alone.

Surface finish follows the same logic. As-machined Ra 3.2 µm suits most non-sealing faces; Ra 0.8 µm needs slower feed and a dedicated finishing pass, and Ra 0.4 µm on 316 stainless often means additional hand or abrasive work. Note the finish and the tolerance on the drawing separately, because a tight Ra callout on an unreachable pocket floor will be quoted either high or not at all.

Watch the tolerance stack, not individual features. A hole pattern that is ±0.05 mm per hole can still fail if each hole sits at opposite limits, so use a common datum and position callouts where parts assemble. Datum choices also change fixture design.

Practical rule: define CNC machining tolerances up front with ISO 2768-m as the general callout, tighter limits only on functional fits, and one clearly defined finish per face. That keeps quotes realistic and lead times predictable for prototypes and repeat batches.

Need a machine shop that can hold these requirements? Explore our CNC machining services or request a quote for your part at CNCdoing.

For more guidance on related work, see: Specify CNC Surface Finish Ra Without Overpaying.