What is the difference between dimensional and geometric tolerances in CNC machining?

As engineers, we use both dimensional and geometric tolerances to control how a CNC part fits, functions, and assembles in the real world. They work together—but they are not the same.

Dimensional Tolerances: Controlling Size and Linear Features

Dimensional tolerances define the acceptable variation in size, including length, width, diameter, thickness, hole size, and other dimensions. For example, a Ø10.00 mm shaft with a ±0.02 mm tolerance allows for any actual size between 9.98 mm and 10.02 mm. These limits are typically sufficient for non-critical features or standard fits and are directly tied to machine capability and process stability, as we ensure in our CNC machining, milling, and turning services. When tolerances are tight across multiple features on the same component, we apply dedicated setups and inspection strategies as part of our precision machining service and complex contouring with our multi-axis machining service. Dimensional tolerances answer a basic question: “Is this feature the correct size within a defined range?”

Geometric Tolerances: Controlling Form, Orientation, and Location

Geometric tolerances (GD&T) go beyond size. They control how features relate to each other in 3D space: straightness, flatness, circularity, cylindricity, perpendicularity, parallelism, position, runout, and more. A hole may meet its dimensional tolerance yet still be misaligned; geometric tolerances prevent that. For functional assemblies—such as turbine housings in high-temperature alloys or precision bores in medical and automotive components—geometric controls ensure alignment, sealing, bearing life, and consistent load paths. They define “how good is the geometry” rather than just “how big is the feature.”

How They Work Together in Real Projects

In practice, we combine both: * For aluminum structural parts using alloys like aluminum 6061-T6, dimensional tolerances ensure correct envelope size, while geometric tolerances secure flat mounting surfaces and true hole positions. * In stainless components such as stainless steel SUS304 manifolds, positional and perpendicularity tolerances keep fluid ports aligned. * For critical aerospace or hot-section hardware in Inconel 718 or Ti-6Al-4V, geometric tolerances protect bearing interfaces and sealing lines where thermal and mechanical loads are extreme. * Engineering polymers like PEEK demand proper GD&T to manage creep, assembly stress, and mating with metal components. Downstream performance is also influenced by finishes. An as-machined surface finish may be sufficient for non-critical areas, while precision sealing or sliding interfaces can benefit from controlled roughness via CNC part polishing services, supporting both geometric accuracy and functional life. Industries such as aerospace and aviation, medical device, and automotive rely heavily on precise GD&T application because incorrect orientation, runout, or positional errors can lead to fatigue failures, leaks, noise, or regulatory non-compliance—even when every feature is “dimensionally in tolerance.” Correctly balancing dimensional and geometric tolerances helps optimize manufacturability, inspection cost, and reliability: tight where it matters, lean where it doesn’t.