Tolerances are the numbers that determine whether a manufactured part actually works. They define how much a dimension can deviate from its nominal value and still be acceptable — and getting them right is one of the most consequential technical decisions in the design-to-manufacturing process. Too tight, and you’ve specified a part that’s expensive or impossible to produce reliably. Too loose, and the assembled product doesn’t fit, function, or perform as intended.
Understanding tolerances isn’t optional for anyone doing serious manufacturing work. Here’s what you need to know.
What Is a Tolerance, and Why Does It Exist?
No manufacturing process produces parts to exact nominal dimensions. Every cut, cast, print, and form introduces some variation — from tool wear, thermal expansion, machine vibration, material inconsistency, and dozens of other sources. A tolerance acknowledges this reality: it defines the acceptable range of variation for a given dimension, within which the part will function as intended.
A dimension might be specified as 25.00 ± 0.05 mm. That means the actual dimension can be anywhere from 24.95 to 25.05 mm and still be acceptable. Parts outside that range are out of specification — they may fail to assemble, perform poorly, or require rework. The ±0.05 mm is the tolerance.
Tolerances aren’t just about individual parts. In an assembly with multiple mating components, the tolerances of each part accumulate — a phenomenon called tolerance stack-up. A design that looks fine when each component is analyzed individually can fail to assemble correctly when multiple parts are at opposite ends of their tolerance ranges simultaneously.
How Tight Should Tolerances Be?
The answer is: exactly as tight as function requires, and no tighter. Tighter tolerances mean more precise — and more expensive — manufacturing. The cost increase isn’t linear: moving from a ±0.5 mm tolerance to ±0.05 mm can multiply machining cost by several times, because it requires slower feeds and speeds, more careful setup, and more frequent inspection. Applying that level of precision to a dimension that doesn’t affect function wastes money without improving the product.
The discipline of tolerance specification is matching the tightness of the tolerance to the functional requirement. Surfaces that don’t mate with other components can usually carry generous tolerances. Bearing bores, mating faces, precision fits, and sealing surfaces need tolerances derived from the actual functional requirements — typically calculated from fits-and-clearances analysis rather than estimated.
Common Tolerance Standards and What They Mean
Tolerances are typically specified according to established standards. In machining, ISO fits and limits define standard shaft and hole tolerances for common precision fits. GD&T (Geometric Dimensioning and Tolerancing) goes further — it specifies not just size tolerances but geometric requirements: flatness, perpendicularity, true position, runout, and others that a simple ± dimension can’t capture. GD&T is the language of precision engineering drawings, and understanding it is essential for anyone communicating dimensional requirements to a machine shop or evaluating inspection reports.
Common precision classes in machining practice range from ±0.25 mm for general fabrication work to ±0.025 mm for precision components to ±0.005 mm and below for ultra-precision work. Each step tighter requires more capable equipment, more controlled conditions, and more extensive inspection.
How Tolerances Are Verified in Production
Specifying a tolerance on a drawing is only useful if you can verify that manufactured parts conform to it. Traditional inspection methods — calipers, micrometers, CMM (coordinate measuring machine) — measure specific features at defined points. They’re accurate and reliable for the features they measure, but they can miss systematic issues across the full part surface.
3D scan-based inspection provides a complete picture. A full-surface scan of a manufactured part is compared against the CAD model, generating a deviation map that shows every point where the part diverges from specification. This reveals systematic issues — warping, sink, tool-path bias — that point measurement misses. Our metrology and inspection services use scan-based deviation analysis for first article inspection, giving clients a comprehensive dimensional report rather than a pass/fail at a limited set of features.
Tolerances in Different Manufacturing Processes
Different manufacturing processes have different achievable tolerance ranges, and designing a part requires understanding what the chosen process can reliably deliver:
- CNC machining is the most dimensionally precise standard manufacturing process, routinely holding tolerances of ±0.025 mm and tighter with appropriate equipment and setup. Our CNC machining services are set up for precision work across metals and engineering plastics.
- 3D printing varies significantly by technology. FDM typically holds ±0.2–0.5 mm. SLA and SLS are tighter, in the ±0.1–0.2 mm range. None approach the precision of machining for functional surfaces that require close fits.
- Casting depends heavily on the process. Investment casting can achieve ±0.1 mm on small features; sand casting is considerably looser. Most cast parts that require precision fits are finish-machined after casting.
- 3D printing followed by machining is an increasingly common approach for complex geometries: print a near-net-shape part, then machine the critical surfaces to the required precision.
Getting Tolerances Right From the Start
The time to think about tolerances is during design, not after the first batch of parts fails incoming inspection. A tolerance analysis as part of the design review — identifying which dimensions are critical to function, calculating the required fits and clearances, and specifying tolerances that reflect both functional requirements and manufacturing capabilities — prevents the expensive downstream problems that come from getting this wrong.
If you’re developing a product that requires precision manufacturing and want to make sure tolerances are specified correctly from the start, talk to our team. We work with clients across the full product development process — from design and engineering through machining and inspection — and we catch tolerance issues before they become production problems.
