QA Report
Tolerances define the boundary between a part that fits and one that does not. For machinists and quality engineers, understanding how tolerances work, when to tighten them, and how to inspect against them is fundamental to shipping parts that function reliably in assembly.
Every machined dimension has a nominal value, the ideal size shown on the drawing. A tolerance is the permissible range of variation around that nominal. A shaft specified as 25.00 mm +0.00 / -0.05 mm can be machined anywhere from 24.95 mm to 25.00 mm and still pass inspection.
Tolerances come in two basic forms:
Tolerances may also be expressed as limits of size (24.95 to 25.00 mm) rather than plus/minus from nominal.
Not every dimension on a drawing needs an explicit tolerance callout. ISO 2768 defines four general tolerance classes that apply to any linear or angular dimension without its own specific tolerance:
When a drawing title block states "General tolerances per ISO 2768-m," every un-toleranced dimension inherits the medium class. Choosing the right class prevents over-tolerancing features that do not need tight control, saving machining time and reducing scrap.
Tightening a tolerance from ±0.1 mm to ±0.01 mm is not a tenfold increase in difficulty; it can be much more. The part may require grinding or honing instead of milling, dedicated fixturing to minimize deflection, and longer inspection times with higher-resolution instruments.
The practical rule: specify only the tolerance the function demands. A clearance hole for an M6 bolt does not need ±0.01 mm, but a bearing bore might. Reviewing your tolerances against actual functional requirements is one of the simplest ways to reduce manufacturing cost without compromising part performance.
When multiple toleranced features contribute to a single assembly fit, their individual tolerances accumulate. This is called a tolerance stack-up, or tolerance chain analysis. In a worst-case analysis, you add the maximum possible deviation of each contributing dimension. If three features each carry ±0.05 mm, the worst-case stack-up is ±0.15 mm at the assembly level.
Statistical stack-up (RSS, root sum of squares) gives a tighter, more realistic estimate by assuming that not every feature will simultaneously sit at its worst limit. Understanding stack-up is essential when designing assemblies with multiple mating parts.
Plus/minus tolerancing controls size but says nothing about how straight, flat, or accurately located a feature is. Geometric Dimensioning and Tolerancing (GD&T), standardized in ASME Y14.5, adds form, orientation, and location controls that describe the geometry of the part, not just its size.
When a position tolerance is applied at Maximum Material Condition (MMC), the part gains extra tolerance as the feature departs from its maximum material size. For a hole with a position tolerance of 0.2 mm at MMC and a size tolerance of 10.0 to 10.1 mm, a hole produced at 10.05 mm (0.05 mm above MMC) receives 0.05 mm of bonus tolerance, making the effective position tolerance 0.25 mm. This reflects physical reality: a larger hole is easier to assemble even if its position shifts slightly.
The general rule of thumb in quality engineering is that the resolution of your measuring instrument should be roughly one-tenth of the tolerance band. For a ±0.05 mm tolerance (0.10 mm total band), you need an instrument with at least 0.01 mm resolution, which is a standard outside micrometer. For a ±0.005 mm tolerance, a standard micrometer is no longer sufficient and you would move to a bore gauge, CMM, or air gauge.
If the instrument resolution is too coarse relative to the tolerance, measurement uncertainty eats into the tolerance band, increasing both false accepts and false rejects.
Manually comparing each measured value against its tolerance limits is slow and error-prone, especially on multi-characteristic inspection reports. QA Report automates this step: you enter the nominal, tolerance, and measured value, and the software instantly validates each measurement. Out-of-tolerance values are highlighted so they stand out in the report. For GD&T callouts with MMC modifiers, QA Report calculates bonus tolerance automatically based on the actual measured size, so inspectors do not need to compute it by hand.
The workflow starts at the drawing. With the free drawing ballooning tool, you can balloon your drawing, assign characteristic numbers, and carry them directly into the report.
Automate tolerance validation, bonus tolerance calculation, and out-of-tolerance flagging with QA Report.
Try QA Report FreeTolerances are the contract between design intent and manufacturing reality. Getting them right means choosing the correct ISO 2768 class, applying GD&T where form and location matter, accounting for stack-ups, and matching each tolerance to an instrument with adequate resolution. With automated validation tools, the inspection step becomes faster and less prone to error, letting quality engineers focus on the measurements that matter most.