± igual Equal bilateral tolerance
Equal bilateral tolerance · Indicadores dimensionales · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “25 ±0.05” states a equal bilateral tolerance requirement.
What controls
Tolerance that allows the same deviation on both sides of the nominal value. It is the most common way to tolerance linear dimensions and the one most often taken for granted. Its practical implication is that the nominal sits at the center of the acceptance zone, which points the manufacturing target at the center; if the function requires the result to lean toward one side, the correct way to express it is an unequal tolerance, not a verbal agreement with the shop.
How to verify
An instrument with resolution and uncertainty suited to the width of the tolerance, measuring the actual value and comparing it with both limits. In production, limit gauges or dedicated fixtures. Record the measured value, not just the pass/fail result, because that is what shows the process trend within the acceptance zone.
Workshop guideline values
- General
- General tolerances of tenths of a millimeter, increasing with the size of the dimension.
- Accuracy
- Tolerances of ±0.02 to ±0.05 mm on explicitly toleranced functional dimensions.
- High precision
- Tolerances of ±0.005 to ±0.01 mm, with grinding and temperature control.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Applying the general tolerance from memory without checking the table, which depends on the size range of the dimension.
- Verbally asking the shop to work toward one side of the tolerance instead of specifying an unequal tolerance.
- Recording only pass/fail results without the measured value, which loses the process trend.
How to get it in the workshop / if it comes out of tolerance
When a dimension has no tolerance of its own, the drawing's general tolerance applies, and it is usually expressed this way in a table by size range: the tolerance grows with the dimension, so the same general callout does not mean the same number on a 10 mm dimension as on a 500 mm one. Find the table or the cited standard before machining, instead of applying another drawing's from memory.
Process centered but with spread that brushes the limits: it is a capability problem, and it is tackled by reducing variation (workholding, temperature, wear), not by re-centering. Process shifted with little spread: correcting the centering with an offset is enough. And if the general tolerance is too loose for the function, the right fix is to tolerance that dimension explicitly, not to tighten it informally at the machine.
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