What tolerances can CNC machining hold — and how to spec them
Tolerances define how much a dimension is allowed to vary from nominal. Tighter tolerances require more time, more precise equipment, and more careful inspection — which means higher cost. Understanding what tolerances CNC machining can realistically hold helps you spec parts that are accurate where it matters without over-tolerancing where it doesn't.
What it means: Default for non-critical dimensions. Applied automatically to any dimension on the drawing that doesn't have a tighter callout.
Achievability: Easy. No special processes required. This is the baseline for any competent CNC shop.
Cost impact: Lowest. Standard machining time with normal inspection.
What it means: One thousandth of an inch — the bread and butter of precision machining. This is where CNC really earns its keep.
Achievability: Routinely held on modern CNC equipment with proper tooling and process control. This is our sweet spot.
Cost impact: Moderate. Requires attention to tool wear, temperature, and workholding but doesn't need extraordinary measures.
What it means: Half a thousandth — or five tenths. This is near the practical limit of conventional CNC machining.
Achievability: Achievable on critical features with controlled processes, temperature management, and CMM verification.
Cost impact: Higher. Requires finish passes, careful tool selection, and CMM inspection. Budget accordingly.
Every step tighter roughly doubles the machining and inspection time. A ±0.005" dimension might take one pass. A ±0.001" dimension needs a finish pass and careful measurement. A ±0.0005" dimension may need multiple finish passes, temperature stabilization, and CMM verification. Only tolerance tightly what actually needs to be tight.
Roughness values and what they look like in practice
Visible tool marks. Default finish from standard milling and turning operations. Acceptable for most non-cosmetic, non-sealing surfaces.
Light tool marks visible under close inspection. Achieved with finish passes at higher speeds and lower feeds. Good for mating surfaces and general precision work.
Very smooth with minimal visible tooling marks. Requires optimized feed rates, sharp tooling, and often spring passes. Used for sealing surfaces and bearing surfaces.
Near-mirror finish. Requires slow finish passes with very sharp tooling, often diamond-tipped. May require polishing. Used for optical surfaces and critical seals.
What we see on drawings and what it means for your parts
Geometric Dimensioning and Tolerancing (GD&T) goes beyond simple ± tolerances to control the form, orientation, and location of features. Here are the GD&T callouts we work with most often:
Controls how round and straight a cylindrical surface is. Think of it as a tolerance zone between two concentric cylinders. Critical for bearing bores and shaft journals.
Controls how round a cross-section is at any given point along a cylinder. Important for parts that rotate or seal against round surfaces.
Controls how flat a surface is regardless of its orientation. The surface must lie between two parallel planes separated by the tolerance value. Critical for mating flanges and sealing surfaces.
Controls how square a feature is relative to a datum. Important for mating surfaces, mounting faces, and features that must be exactly 90° to a reference.
Controls how parallel a surface or axis is relative to a datum. Critical when two faces must maintain consistent spacing across their entire area.
Controls where a feature (usually a hole) is located relative to datums. The most commonly used GD&T callout. Defines a cylindrical tolerance zone for the feature's true position.
Controls how much a surface wobbles when the part is rotated about a datum axis. Combines circularity and coaxiality into a single measurement. Essential for rotating components.
Controls how well the center points of a feature align with a datum axis. Tighter than runout and harder to inspect. Used when exact center alignment is critical.
We read and inspect to GD&T callouts every day. If your drawing uses GD&T per ASME Y14.5, we'll interpret it correctly and inspect accordingly. If you're not sure how to tolerance a feature, send us what you have and we'll help you define the right callouts during our drawing review process.
Tolerance stacking occurs when multiple toleranced dimensions accumulate, creating a larger overall variation than any single tolerance suggests. This is one of the most common issues we see on drawings.
Three holes spaced 1.000" apart, each located ±0.005" from the previous one. The first hole is dead-on. But the third hole could be as far as ±0.015" from where you expect it — because the tolerances stack.
Apply ±0.005" as your default block tolerance. Only call out tighter tolerances on features that actually need them.
A clearance hole for a 1/4-20 bolt doesn't need ±0.001". A bearing bore does. Let the part's function drive the tolerance.
When you reference a tolerance to a datum, we know exactly which surface to set up from. No datums = we're guessing at your intent.
Aluminum holds tighter tolerances more easily than stainless steel. Plastics grow with temperature and absorb moisture. Factor material behavior into your tolerance decisions.
If a surface needs to seal, mate, or look good, spec the finish. Otherwise, standard machined finish is assumed and keeps cost down.
A five-minute conversation can save thousands in machining cost. We'll tell you which tolerances are easy to hold and which ones will drive the price up.